Springhares, flying and flightless scaly-tailed squirrels (Anomaluromorpha, Rodentia) are the squirrely mouse: comparative anatomy of the masticatory musculature and its implications on the evolution of hystricomorphy in rodents

Anomaluromorpha is a particularly puzzling suborder of Rodentia. Endemic to Africa, this clade includes the extant genera Idiurus, Anomalurus, Zenkerella, and Pedetes. These rodents present an hystricomorphous condition of the skull, characterized by a large infraorbital foramen, which evolved independently within the mouse-related clade over a span of approximately 57 million years. They exhibit a high disparity in craniomandibular and dental morphology that has kept their phylogenetic affinities disputed for a long time. Given the past significance of masticatory morphotypes in establishing the classification of Rodentia, we propose to explore variations in the masticatory apparatus of Anomaluromorpha in order to evaluate whether its related features can offer additional data for systematics and contribute to our understanding of the complexity of hystricomorphy. In order to do so, we used traditional dissection and diffusible iodine-based contrast-enhanced computed tomography (diceCT) to accurately describe and compare the anatomy of the specimens. We found that the muscle morphology displays clear differentiation among each anomaluromorph taxonomic unit. Specifically, the masseteric complex of Anomaluromorpha exhibits distinctive synapomorphies such as the infraorbital part of the zygomaticomandibularis muscle being separated into a rostral and orbital part and an absence of a posterior part of the zygomaticomandibularis. Additionally, the orbital portion of the infraorbital part originates from a well-marked ridge and fossa at the level of its area of origin on the anteromedial wall of the orbital cavity, a feature that is absent in other members of the mouse-related clade. This evident bony feature, among others, is strongly associated with muscular anatomy and can contribute to ascertaining the taxonomic status of extinct representatives of the clade. Finally, we showed that the hystricomorphy of Anomaluromorpha largely differs from those of Ctenohystrica and Dipodoidea and that the definition of this morphotype is complex and cannot be reduced simply to the size of the opening of the infraorbital foramen.

The impact of locomotion on the brain evolution of squirrels and close relatives

How do brain size and proportions relate to ecology and evolutionary history? Here, we use virtual endocasts from 38 extinct and extant rodent species spanning 50+ million years of evolution to assess the impact of locomotion, body mass, and phylogeny on the size of the brain, olfactory bulbs, petrosal lobules, and neocortex. We find that body mass and phylogeny are highly correlated with relative brain and brain component size, and that locomotion strongly influences brain, petrosal lobule, and neocortical sizes. Notably, species living in trees have greater relative overall brain, petrosal lobule, and neocortical sizes compared to other locomotor categories, especially fossorial taxa. Across millions of years of Eocene-Recent environmental change, arboreality played a major role in the early evolution of squirrels and closely related aplodontiids, promoting the expansion of the neocortex and petrosal lobules. Fossoriality in aplodontiids had an opposing effect by reducing the need for large brains.

Morphology of the Lingual Papillae of the Japanese lesser flying squirrel and four-toed hedgehog

Author examined the dorsal lingual surfaces of the adult Japanese lesser flying squirrel (Pteromys momonga) and four-toed hedgehog (Atelerix albiventris) by scanning electron microscopy. In the Japanese lesser flying squirrel, the filiform papilla of the lingual body consisted of a large conical papilla. The filiform papilla of the lingual prominence was spoon in shape. The fungiform papillae were round in shape and scattered among the filiform papillae. Many foliate papillae were observed on the posterolateral regions of the lingual body. The foliate papillae had some ridges separated by deep grooves. The vallate papilla was located between lingual body and root. Several long conical papillae derived from the posterolateral margin of the tongue. In the four-toed hedgehog, the filiform papilla of the lingual apex had a conical process. The filiform papilla of the lingual body had some processes. The fungiform papillae were round in shape. The foliate papillae were observed on the posterolateral regions of the lingual body. The papilla was separated from each other by a furrow. The vallate papilla consisted of a central papilla and an annular pad. These findings suggest that in the structure of the lingual papillae of the Japanese lesser flying squirrel there is similar to that of the sugar glider and the lingual papillae of the four-toed hedgehog is different from that of the Japanese lesser flying squirrel.

The Taxonomic Status of Spermophilus in the Plague Area of Dingbian County, Shaanxi Province, China

This study was conducted to define the taxonomic status of Spermophilus in the plague area of Dingbian County in Shaanxi Province, China, through the two-factor variance analysis of morphological characteristics, DNA barcoding, and chromosome karyotype analysis. The Spermophilus samples collected from Dingbian and Zhengxiang Baiqi Counties exhibited significant differences in their morphological measurements. All Spermophilus samples form two distinct branches in neighbor-joining (NJ) tree. One branch included the Spermophilus samples collected from Inner Mongolia, and the other branch included samples collected from the plague foci of Shaanxi Province and the Ningxia Region. The Spermophilus samples collected from Dingbian County had a chromosome number of 2n = 38 in 84.40% of all their cells. The Spermophilus species collected from the plague area of Dingbian County was categorized as Spermophilus alashanicus (S.alashamicus). The findings reported in this study are epidemiologically significant for monitoring plague in this region of west-central China.

Virtual endocast of the early Oligocene Cedromus wilsoni (Cedromurinae) and brain evolution in squirrels

Extant squirrels exhibit extensive variation in brain size and shape, but published endocranial data for living squirrels are limited, and no study has ever examined brain evolution in Sciuridae from the perspective of the fossil record to understand how this diversity emerged. We describe the first virtual endocast for a fossil sciurid, Cedromus wilsoni, which is known from a complete cranium from Wyoming (Orellan, Oligocene), and make comparisons to a diverse sample of virtual endocasts for living sciurids (N = 20). The virtual endocasts were obtained from high-resolution X-ray micro-computed tomography data. Comparisons were also made with endocasts of extinct ischyromyid rodents, the most primitive rodents known from an endocranial record, which provide the opportunity to study the neuroanatomical changes occurring near the base of Sciuridae. The encephalization quotient of C. wilsoni is higher than that of Ischyromys typus from the same epoch, and falls within the range of modern terrestrial squirrel variation, but below the range of extant scansorial, arboreal and gliding sciurids when using cheek-tooth area for the estimation of body mass. In a principal components analysis, the shape of the endocast of C. wilsoni is found to be intermediate between that of primitive fossil taxa and the modern sample. Cedromus wilsoni has a more expanded neocortical surface area, especially the caudal region of the cerebrum, compared with ischyromyid rodents. Furthermore, C. wilsoni had proportionally larger paraflocculi and a more complex cerebellar morphology compared with ischyromyid rodents. These neurological differences may be associated with improvements in vision, although it is worth noting that the size of the parts of the brain most directly involved with vision [the rostral (superior) colliculi and the primary visual cortex] cannot be directly assessed on endocasts. The changes observed could also relate to balance and limb coordination. Ultimately, the available evidence suggests that early squirrels were more agile and visually oriented animals compared with more primitive rodents, which may relate to the process of becoming arboreal. Extant sciurids have an even more expanded neocortical surface area, while exhibiting proportionally smaller paraflocculi, compared with C. wilsoni. This suggests that the neocortex may continue increasing in size in more recent sciurid rodents in relation to other factors than arboreality. Despite the fact that both Primates and Rodentia exhibit neocortical expansion through time, since the adoption of arboreality preceded major increases in the neocortex in Primates, those neurological changes may be related to different ecological factors, underlining the complexity of the inter-relationship between time and ecology in shaping the brain in even closely related clades.

Anatomy and Disorders of the Oral Cavity of Rat-like and Squirrel-like Rodents

The order Rodentia comprises more than 2000 species divided into 3 groups based on anatomic and functional differences of the masseter muscle. Myomorph and sciuromorph species have elodont incisors and anelodont cheek teeth, unlike hystrichomorph species which have full anelodont dentition. Diseases of incisors and cheek teeth of rat-like and squirrel-like rodents result in a wide variety of symptoms and clinical signs. Appropriate diagnostic testing and imaging techniques are required to obtain a definitive diagnosis, formulate a prognosis, and develop a treatment plan. A thorough review of elodontoma, odontoma, and pseudo-odontoma is provided, including treatment of pseudo-odontomas in prairie dogs.

Differential Effects of Roads and Traffic on Space Use and Movements of Native Forest-Dependent and Introduced Edge-Tolerant Species

Anthropogenic infrastructure such as roads and non-native species are major causes of species endangerment. Understanding animal behavioral responses to roads and traffic provides insight into causes and mechanisms of effects of linear development on wildlife and aids effective mitigation and conservation. We investigated effects of roads and traffic on space use and movements of two forest-dwelling species: endemic, forest-dependent Mount Graham red squirrels (Tamiasciurus hudsonicus grahamensis) and introduced, edge-tolerant Abert’s squirrels (Sciurus aberti). To assess the effects of roads on space use and movement patterns, we compared the probability that a squirrel home range included roads and random lines in forests, and assessed effects of traffic intensity on rate of road crossing and movement patterns. Red squirrels avoided areas adjacent to roads and rarely crossed roads. In contrast, Abert’s squirrels were more likely to include roads in their home ranges compared to random lines in forests. Both red squirrels and Abert’s squirrels increased speed when crossing roads, compared to before and after road crossings. Increased hourly traffic volume reduced the rate of road crossings by both species. Behavioral responses of red squirrels to roads and traffic resemble responses to elevated predation risk, including reduced speed near roads and increased tortuosity of movement paths with increased traffic volume. In contrast, Abert’s squirrels appeared little affected by roads and traffic with tortuosity of movement paths reduced as distance to roads decreased. We found that species with similar body size category (<1 kg) but different habitat preference and foraging strategy responded to roads differently and demonstrated that behavior and ecology are important when considering effects of roads on wildlife. Our results indicate that roads restricted movements and space use of a native forest-dependent species while creating habitat preferred by an introduced, edge-tolerant species.

The size of non-hippocampal brain regions varies by season and sex in Richardson's ground squirrel

Sex- and season-specific modulation of hippocampal size and function is observed across multiple species, including rodents. Other non-hippocampal-dependent behaviors exhibit season and sex differences, and whether the associated brain regions exhibit similar variation with sex and season remains to be fully characterized. As such, we examined the brains of wild-caught Richardson's ground squirrels (RGS; Urocitellus richardsonii) for seasonal (breeding, non-breeding) and sex differences in the volumes of specific brain areas, including: total brain volume, corpus callosum (CC), anterior commissure (AC), medial prefrontal cortex (mPFC), total neocortex (NC), entorhinal cortex (EC), and superior colliculus (SC). Analyses of variance and covariance revealed significant interactions between season and sex for almost all areas studied, primarily resulting from females captured during the breeding season exhibiting larger volumes than females captured during the non-breeding season. This was observed for volumes of the AC, mPFC, NC, EC, and SC. Where simple main effects of season were observed for males (the NC and the SC), the volume advantage favoured males captured during the NBr season. Only two simple main effects of sex were observed: males captured in the non-breeding season had significantly larger total brain volume than females captured in the non-breeding season, and females captured during the breeding season had larger volumes of the mPFC and EC than males captured in the breeding season. These results indicate that females have more pronounced seasonal differences in brain and brain region sizes. The extent to which seasonal differences in brain region volumes vary with behaviour is unclear, but our data do suggest that seasonal plasticity is not limited to the hippocampus and that RGS is a useful mammalian species for understanding seasonal plasticity in an ecologically relevant context.

Differentiation in cranial variables among six species of Hylopetes (Sciurinae : Pteromyini)

There is some discrepancy in the classification of different species of Hylopetes, particularly regarding systematic status of H. electilis and H. phayrei and their relationship to other species. In the present study, for the first time we have brought together six of the nine Hylopetes species and performed statistical analysis of 14 measurable cranial variables, analyzing in total 89 specimens, including H. electilis, H. alboniger, H. phayrei, H. lepidus, H. spadiceus, and H. nigripes. Both univariate and multivariate analysis results indicate that H. electilis can not only be obviously distinguished from H. phayrei, but also clearly differs from the other four Hylopetes species. These results sustain the contention that H. electilis is neither a synonym nor subspecies of H. phayrei,but should be considered a distinct and valid species. Subsequently, a straightforward discussion on the biogeography of Hylopetes in southeastern Asia gives further insight into the differentiation and variety of species belonging to this genus. There is some discrepancy in the classification of different species of Hylopetes, particularly regarding systematic status of H. electilis and H. phayrei and their relationship to other species. In the present study, for the first time we have brought together six of the nine Hylopetes speciesand performed statistical analysis of 14 measurable cranial variables, analyzing in total 89 specimens, including H. electilis, H. alboniger, H. phayrei, H. lepidus, H. spadiceus, and H. nigripes. Both univariate and multivariate analysis results indicate that H. electilis can not only be obviously distinguished from H. phayrei, but also clearly differs from the other four Hylopetes species. These results sustain the contention that H. electilis is neither a synonym nor subspecies of H. phayrei,but should be considered a distinct and valid species. Subsequently, a straightforward discussion on the biogeography of Hylopetes in southeastern Asia gives further insight into the differentiation and variety of species belonging to this genus.

Conservatism and adaptability during squirrel radiation: what is mandible shape telling us?

Both functional adaptation and phylogeny shape the morphology of taxa within clades. Herein we explore these two factors in an integrated way by analyzing shape and size variation in the mandible of extant squirrels using landmark-based geometric morphometrics in combination with a comparative phylogenetic analysis. Dietary specialization and locomotion were found to be reliable predictors of mandible shape, with the prediction by locomotion probably reflecting the underlying diet. In addition a weak but significant allometric effect could be demonstrated. Our results found a strong phylogenetic signal in the family as a whole as well as in the main clades, which is in agreement with the general notion of squirrels being a conservative group. This fact does not preclude functional explanations for mandible shape, but rather indicates that ancient adaptations kept a prominent role, with most genera having diverged little from their ancestral clade morphologies. Nevertheless, certain groups have evolved conspicuous adaptations that allow them to specialize on unique dietary resources. Such adaptations mostly occurred in the Callosciurinae and probably reflect their radiation into the numerous ecological niches of the tropical and subtropical forests of Southeastern Asia. Our dietary reconstruction for the oldest known fossil squirrels (Eocene, 36 million years ago) show a specialization on nuts and seeds, implying that the development from protrogomorphous to sciuromorphous skulls was not necessarily related to a change in diet.

Estimating lean mass over a wide range of body composition: a calibration of deuterium dilution in the arctic ground squirrel

Calculating body water through isotope dilution has become a useful way to nondestructively estimate body composition in many species. The most accurate estimates using this method require calibration against proximate chemical analysis of body composition for individual species, but no studies to our knowledge have calibrated this method on a hibernating mammal that seasonally undergoes dramatic changes in body composition. We use deuterium oxide to estimate total body water in captive arctic ground squirrels, Urocitellus parryii, and compare two approaches of calculating lean mass from total body water, both calibrated against lean mass based on proximate analysis. The first method uses a single tissue hydration constant to calculate lean mass from total body water; the second method uses a predictive equation to calculate lean mass from total body water and body mass. We found that the predictive equation performs better over the large range of body composition common to this species. Distillation of blood samples did not affect lean mass estimates from either calculation method. These findings indicate that isotope dilution using a predictive equation should work well as an alternative to destructive methods in other small mammals that undergo radical changes in body composition across their annual cycle.

Physiologic reference ranges for captive black-tailed prairie dogs (Cynomys ludovicianus)

The black-tailed prairie dog (Cynomys ludovicianus) is a member of the order Rodentia and the family Sciuridae. Ecologically, prairie dogs are a keystone species in prairie ecology. This species is used as an animal model for human gallbladder disease and diseases caused by infection with Clostridium difficile, Yersinia pestis, Francisella tularensis, and most recently, Orthopoxvirus. Despite increasing numbers of prairie dogs used in research and kept as pets, few data are available on their baseline physiology in animal facility housing conditions. To establish baseline physiologic reference ranges, we designed a study using 18 wild-caught black-tailed prairie dogs. Telemetry data were analyzed to establish circadian rhythms for activity and temperature. In addition, hematologic and serum chemistry analyses were performed. Baseline measurements were used to establish the mean for each animal, which then were compiled and analyzed to determine the reference ranges. Here we present physiologic data on serum chemistry and hematology profiles, as well as weight, core body temperature, and daily activity patterns for black-tailed prairie dogs. These results reflect the use of multiple measurements from species- and age-matched prairie dogs and likely will be useful to ecologists, scientists interested in using this animal model in research, and veterinarians caring for pet prairie dogs.

[Disproportionate eruption of maxillary and mandibular incisors in the long-tailed ground squirrel]

Abstract-The surface of the maxillary and mandibular incisors of Spermophilus undulatus long-tailed ground squirrels, including those born in the current year and those that have hibernated (trapped one month or later after hibernation) is studied. The presence of daily growth on the incisors' surface allows the evaluation of their eruption rate; a specific change in the character of the incisors' eruption corresponds to winter hibernation (hibernation zone), which serves as the time mark. Correlation between the eruption rates of the maxillary and mandibular incisors typical for rodents is found in yearlings and some animals after hibernation. The eruption rate of the mandibular incisors is higher than the eruption rate of the maxillary incisors and can be taken as proportional to their length. In individuals that have hibernated and show proportional eruption of the incisors, the proportion of the total length of the incisor formed before and after hibernation is equal for the maxillary and mandibular incisors. In the individuals that have hibernated and show the correlation of the total length of the maxillary and mandibular incisors typical for rodents, the eruption rate of the mandibular incisor is equal to or less than the eruption rate of the maxillary incisor and the proportion of the incisor formed before hibernation is greater in the mandibular incisor than in the maxillary. This disproportionate pattern of incisor eruption is not typical for rodents and is a result of inequal grinding of the maxillary and mandibular incisors, which ultimately results in the normal ratio of the total length of the maxillary and mandibular incisors.

[Temperature and cardiotropic effects of kyotorphin and neokyotorphin in hibernating and nonhibernating animals]

Physiological effects of kiotorphyn (KT) and neokiotorphyn (NKT) has been reviewed. These peptides were isolated from the brain of hibernating ground squirrel Citellus undulates (CU) and assumed to be endogenous regulators of hibernation NKT does not alter the process of entering to hibernation, but induces arousal from the sleeping. KT administration in squirrel delays the rise of heart rate and body temperature associeated with awakening provoked in the middle but not at the final stage of hibernation bout. Neither KT nor NKT induce physiological effects in rats and mice under normal environmental conditions. In mice with induced hypothermia (17-18 degrees C) NKT accelerates restoration of body temperature, whereas KT induces progressive hypothermia. In addition, NKT modulates cardiotropic effects of nucleotides found in physiologically active fractions of hibernating squirrel tissues. NKT as well as KT modify sensitivity of cardiovascular system to adrenaline and AMP in rats one day after hypothermia.

Architectonic subdivisions of neocortex in the gray squirrel (Sciurus carolinensis)

Squirrels are highly visual mammals with an expanded cortical visual system and a number of well-differentiated architectonic fields. To describe and delimit cortical fields, subdivisions of cortex were reconstructed from serial brain sections cut in the coronal, sagittal, or horizontal planes. Architectonic characteristics of cortical areas were visualized after brain sections were processed with immunohistochemical and histochemical procedures for revealing parvalbumin, calbindin, neurofilament protein, vesicle glutamate transporter 2, limbic-associated membrane protein, synaptic zinc, cytochrome oxidase, myelin or Nissl substance. In general, these different procedures revealed similar boundaries between areas, suggesting that functionally relevant borders were being detected. The results allowed a more precise demarcation of previously identified areas as well as the identification of areas that had not been previously described. Primary sensory cortical areas were characterized by sparse zinc staining of layer 4, as thalamocortical terminations lack zinc, as well as by layer 4 terminations rich in parvalbumin and vesicle glutamate transporter 2. Primary areas also expressed higher levels of cytochrome oxidase and myelin. Primary motor cortex was associated with large SMI-32 labeled pyramidal cells in layers 3 and 5. Our proposed organization of cortex in gray squirrels includes both similarities and differences to the proposed of cortex in other rodents such as mice and rats. The presence of a number of well-differentiated cortical areas in squirrels may serve as a guide to the identification of homologous fields in other rodents, as well as a useful guide in further studies of cortical organization and function.

Multigenic and morphometric differentiation of ground squirrels (Spermophilus, Scuiridae, Rodentia) in Turkey, with a description of a new species

This study reports on the molecular phylogenetics of ground squirrels, genus Spermophilus, in Turkey using cytochrome b (1140bp), part of the D-loop and flanking tRNAs (572bp), X chromosome (867-1051bp) and Y chromosome (983-989bp) DNA sequences. Individuals also were characterized by karyotype and with geometric morphometric analyses of mandibles and skulls. Two hundred fourteen individuals from 91 localities were studied. All the data support the recognition of a new species in SW Anatolia: the Taurus ground squirrel Spermophilus taurensis sp. nov. The new species has a small distribution in the Taurus Mountains in an area that is a hotspot for biodiversity. Molecular clock analysis suggests that the new species diverged from the European ground squirrel, Spermophilus citellus, about 2.5 million years ago and that the ancestor of these two species diverged from the widespread Anatolian ground squirrel, Spermophilus xanthoprymnus, about 5 million years ago. Morphometric differentiation in skull and mandible shape among the three species is incomplete, but statistically significant. S. xanthoprymnus is subdivided into five cytochrome b phylogroups and we use these data to infer the location of glacial refugia where the species lived during the last glacial maximum. This study illustrates the potential of combined molecular and morphometric studies to uncover new Anatolian species and to reconstruct their phylogeographic history. The new species is important for squirrel taxonomy and for understanding Eurasian mammal evolution.

Geographical Variation in Skull Morphology of Gray-Bellied Squirrel Callosciurus caniceps

The geographical variation of the gray-bellied squirrel (Callosciurus caniceps) was examined using osteometry of skull in Southeast Asia. From the principal component analysis (PCA), the plots of the northern localities from Nan to Kanchanaburi and those of the southern localities from Narathiwat to Kuala Lumpur in male were completely separated. In female, the plots of the locality from Uttradit to Kanchanaburi and those of the locality from Pattani to Negri Sembilan were completely separated. We called these northern localities and southern localities which are distinguished by the PCA as N group and S group. The size and shape of the skulls of these squirrels indicated the differences between N group and S group from t-test and U-test. These results may be influenced by the two transitions of the phytogeography around the southernmost locality in N group and the northernmost locality in S group in the peninsular Thailand and Malay Peninsula. Localities which are located between N and S groups were called the Middle (M) group. From the PCA among N, S groups and each locality of M group, the plots of localities such as Prachuap Khiri Khan, Chumphon, Krabi, Nakhon Si Thammarat and Trang in both sexes of M group could not be separated from those of N and S groups. We suggest that the sympatric distribution of N and S groups and the hybrid of N and S populations may be seen in these localities of M group.

Conjunctival Lymphoid Follicles in New World Rodents

We report for the first time, the detection of conjunctival lymphoid follicles (CLF) in the eyes of New World rodents. CLF were found in 7 of the 15 species examined, 6 of the 10 genera, and in at least one individual in four families of rodents. These follicles are dense collections of leukocytes in the conjunctival substantia propria with a thinned overlying epithelium lacking in goblet cells. Although the precise location of CLF within the conjunctiva varied from species to species, all CLF were found in the fornix of the conjunctival sac. In general, size and complexity of CLF varied with the size of the eye; the larger the eye, the larger and more complex the CLF. Our findings also reveal that some species of New World rodents, like the majority of Old World rodents examined in this and previous studies might lack CLF. However, until larger samples are examined, this is difficult to state with certainty. Consequently, the presence/absence of CLF at this point might not be informative for phylogenetic comparisons. Our findings also suggest the deer mouse, Peromyscus maniculatus, might serve as a useful model species for studying ocular infections and immunology of the eye.

The squirrel as a rodent model of the human visual system

Over the last 50 years, studies of receptive fields in the early mammalian visual system have identified many classes of response properties in brain areas such as retina, lateral geniculate nucleus (LGN), and primary visual cortex (V1). Recently, there has been significant interest in understanding the cellular and network mechanisms that underlie these visual responses and their functional architecture. Small mammals like rodents offer many advantages for such studies, because they are appropriate for a wide variety of experimental techniques. However, the traditional rodent models, mice and rats, do not rely heavily on vision and have small visual brain areas. Squirrels are highly visual rodents that may be excellent model preparations for understanding mechanisms of function and disease in the human visual system. They use vision for navigating in their environment, predator avoidance, and foraging for food. Visual brain areas such as LGN, V1, superior colliculus, and pulvinar are particularly large and well elaborated in the squirrel, and the squirrel has several extrastriate cortical areas lateral to V1. Unlike many mammals, most squirrel species are diurnal with cone-dominated retinas, similar to the primate fovea, and have excellent dichromatic color vision that is mediated by green and blue cones. Owing to their larger size, squirrels are physiologically more robust than mice and rats under anesthesia, and some hibernating species are particularly tolerant of hypoxia that occurs during procedures such as brain slicing. Finally, many basic anatomical and physiological properties in the early visual system of squirrel have now been described, permitting investigations of cellular mechanisms. In this article, we review four decades of anatomical, behavioral, and physiological studies in squirrel and make comparisons with other species.

Distribution of NMDA receptor subunit NR1 in arctic ground squirrel central nervous system

Hibernation is a natural model of neuroprotection and adult synaptic plasticity. NMDA receptors (NMDAR), which play key roles in excitotoxicity and synaptic plasticity, have not been characterized in a hibernating species. Tolerance to excitotoxicity and cognitive enhancement in Arctic ground squirrels (AGS, Spermophilus parryii) suggests that NMDAR expression may decrease in hibernation and increase upon arousal. NMDAR consist of at least one NMDAR1 (NR1) subunit, which is required for receptor function. Localization of NR1 reflects localization of the majority, if not all, NMDAR complexes. The purpose of this study, therefore, was to characterize the distribution of NR1 subunits in AGS central nervous system using immunohistochemistry. In addition, we compare NR1 expression in hippocampus of hibernating AGS (hAGS) and inter-bout euthermic AGS (ibeAGS) and assess changes in cell somata size using NR1 stained sections in three hippocampal sub-regions (CA1, CA3, and dentate gyrus). For the first time, we report that immunoreactivity of anti-NR1 is widely distributed throughout the central nervous system in AGS and is similar to other species. No differences exist in the expression and distribution of NR1 in hAGS and ibeAGS. However, we report a significant decrease in size of hippocampal CA1 and dentate gyrus NR1-expressing neuronal somata during hibernation torpor.

Lack of patchy horizontal connectivity in primary visual cortex of a mammal without orientation maps

In the cerebral cortex of mammals, horizontal connections link cells up to several millimeters apart. In primary visual cortex (V1) of mammals with orientation maps, horizontal connections ramify in periodic patches across the cortical surface, connecting cells with similar orientation preferences. Rodents have orientation-selective cells but lack orientation maps, raising questions about relationships of horizontal connections to functional maps and receptive field properties. To address these questions, we studied anatomy of horizontal connections and characterized horizontal functional interactions in V1 of the gray squirrel, a highly visual rodent. Long-range intrinsic connections in squirrel V1 extended 1-2 mm but were not patchy or periodic. This result suggests that periodic and patchy connectivity is not a universal organizing principle of cortex, and the existence of patchy and periodic connectivity and functional maps may be linked. In multielectrode and intracellular recordings, we found evidence of unselective local interactions among cells, similar to pinwheel centers of carnivores. These data suggest that, in mammals with and without orientation maps, local connections link near neighbors without regard to orientation selectivity. In single-unit recordings, we found length-summing and end-stopped cells that were similar to those in other mammals. Length-summing cell surrounds were orientation selective, whereas surrounds of end-stopped cells were not. Receptive field response classes are quite similar across mammals, and therefore patchy and columnar connectivity may not be essential for these properties.

Visual method for evaluating the state of sexual development in male grey squirrels (Sciurus carolinensis)

Stages of sexual development in grey squirrels were classified by scoring the presence of periscrotal hair and staining, as well as the position, size and colour of testes, to develop a visual breeding score (VBS). The VBS was a highly significant predictor of the presence and concentration of epididymal spermatozoa, which were produced from 12 months of age. Two classes of producers were identified: 'high' with spermatozoa levels > 10 x 10(6) mL(-1) and 'low' with spermatozoa levels < 10 x 10(6) mL(-1). Sperm motility was variable across all levels of productive males but was generally higher in the 'high' group, indicating that these were 'functional' males. Sexual regression was observed in two autumn periods. The VBS was less effective in determining states of regression and redevelopment; the confidence of classification improved when age class (adult or prepubertal) and date of capture was known. The use of the VBS to classify stages of sexual development in grey squirrels will improve selection of animals for studies of population biology and fertility control, and has potential application in captive breeding studies of rare Sciuridae species.

Laminar organization of response properties in primary visual cortex of the gray squirrel (Sciurus carolinensis)

The gray squirrel (Sciurus carolinensis) is a diurnal highly visual rodent with a cone-rich retina. To determine which features of visual cortex are common to highly visual mammals and which are restricted to non-rodent species, we studied the laminar organization of response properties in primary visual area V1 of isoflurane-anesthetized squirrels using extra-cellular single-unit recording and sinusoidal grating stimuli. Of the responsive cells, 75% were tuned for orientation. Only 10% were directionally selective, almost all in layer 6, a layer receiving direct input from the dorsal lateral geniculate nucleus (LGN). Cone opponency was widespread but almost absent from layer 6. Median optimal spatial frequency tuning was 0.21 cycles/ degrees . Median optimal temporal frequency a high 5.3 Hz. Layer 4 had the highest percentage of simple cells and shortest latency (26 ms). Layers 2/3 had the lowest spontaneous activity and highest temporal frequency tuning. Layer 5 had the broadest spatial frequency tuning and most spontaneous activity. At the layer 4/5 border were sustained cells with high cone opponency. Simple cells, determined by modulation to drifting sinusoidal gratings, responded with shorter latencies, were more selective for orientation and direction, and were tuned to lower spatial frequencies. A comparison with other mammals shows that although the laminar organization of orientation selectivity is variable, the cortical input layers contain more linear cells in most mammals. Nocturnal mammals appear to have more orientation-selective neurons in V1 than diurnal mammals of similar size.

Functional cell classes and functional architecture in the early visual system of a highly visual rodent

Over the last 50 years, studies of receptive field properties in mammalian visual brain structures such as lateral geniculate nucleus (LGN) and primary visual cortex (V1) have suggested the existence of cell classes with unique functional response properties, and in visual cortex of many mammals these functional response properties show considerable spatial organization termed functional architecture. In recent years, there has been considerable interest in understanding the cellular mechanisms that underlie visual responses and plasticity in intact animals, and studies of individual neurons in brain slices have identified distinct cell classes on the basis of anatomical features, synaptic connectivity, or gene expression. However, the relationships between cell classes identified in studies of brain slices and those in the intact animal remain largely unclear. Rodents offer many advantages for investigating these relationships, as they are appropriate for a wide variety of experimental techniques and genetically modified mice are relatively easy to obtain or produce. Unfortunately, a barrier to using these animals in vision research is a lack of understanding of the relationship of rodent visual systems to the visual systems in more commonly studied mammals such as carnivores and non-human primates. Here we review recent comparative studies of functional response properties in LGN and V1 of a highly visual diurnal rodent, the gray squirrel. In the LGN, our data are consistent with the idea that all mammals have a class of LGN neurons that is sustained, another class that is transient, and a third class of more heterogeneous cells, but some response properties such as linearity of spatial summation, contrast gain, and dependence of receptive field size on eccentricity vary from species to species. In V1, the squirrel has many orientation-selective neurons, and these orientation-selective cells can be further subdivided into simple and complex cells. Despite the fact that squirrel has greater visual acuity and a physically larger V1 than some mammals that have orientation maps in V1, we do not find orientation maps in V1 of squirrel, which is similar to results in other less visual rodents. We suggest that orientation maps are not necessary for high acuity vision or orientation selectivity and that cortical functional architecture can vary greatly from species to species.

Where's my dinner? Adult neurogenesis in free-living food-storing rodents

Postnatal hippocampal neurogenesis in wild mammals may play an essential role in spatial memory. We compared two species that differ in their reliance on memory to locate stored food. Yellow-pine chipmunks use a single cache to store winter food; eastern gray squirrels use multiple storage sites. Gray squirrels had three times the density of proliferating cells in the dentate gyrus (determined by Ki-67 immunostaining) than that found in chipmunks, but similar density of young neurons (determined by doublecortin immunostaining). Three explanations may account for these results. First, the larger population of young cells in squirrels may increase the flexibility of the spatial memory system by providing a larger pool of cells from which new neurons can be recruited. Second, squirrels may have a more rapid cell turnover rate. Third, many young cells in the squirrels may mature into glia rather than neurons. The densities of young neurons were higher in juveniles than in adults of both species. The relationship between adult age and cell density was more complex than that has been found in captive populations. In adult squirrels, the density of proliferating cells decreased exponentially with age, whereas in adult chipmunks the density of young neurons decreased exponentially with age.

Histo-anatomical Studies on the Accessory Reproductive Glands of the Anatolian Souslik (Spermophilus xanthoprymnus) (Mammalia: Sciuridae)

In this study, the histo-anatomy of the accessory reproductive glands of the Anatolian souslik (Spermophilus xanthoprymnus) is investigated, using 102 males of the vouchers which were captured from central Anatolia and eastern Mediterranean regions. Averages of measurements taken from each part of the accessory reproductive glands were as follows: length of vesicular gland [right: 3.96 (+/-0.48) mm; left: 3.70 (+/-0.55) mm]; prostate gland [length: 2.55 (+/-0.38) mm; width: 2.63 (+/-0.45) mm]; length of bulbourethral gland [right: 2.32 (+/-0.33) mm; left: 2.37 (+/-0.39) mm], and the length of the pars pelvina of the urethra was 12.31 (+/-0.76) mm. Histological sections showed that vesicular gland was composed of typically branched tubulo-alveolar glands. As the fibrous connective tissue that was found among alveoles in the prostate was dense and relatively common, only few tubulo-alveolar glands occurred. The bulbourethral gland (Cowper's gland) was a tubulo-alveolar gland with a large and serrated lumen and septa between the alveoles consisted of dense fibro-muscular fibres.

Optic nerve head morphology of the Eastern gray squirrel, Sciurus carolinensis

The objective of this study was to characterize the morphology of the Eastern gray squirrel optic nerve head, using histochemical techniques and scanning electron microscopy, in order to make comparisons with other mammalian species. Eyes of three Eastern gray squirrels were enucleated, fixed in 10% neutral buffered formalin, and embedded in paraffin. Sections through the optic nerve head were made in the three different planes, and staining was accomplished with hematoxylin and eosin and Gomori's trichrome stain. Stained sections were examined using light microscopy. The surface of the optic nerve head and the peripapillary retina were evaluated using field emission scanning electron microscopy. The anterior optic nerve head of the Eastern gray squirrel was found to be horizontally elongated and tapers to a more conventional round shape after it exits the eye. Retinal nerve fiber axons converge on the optic nerve head from all directions. Anterior to the lamina cribrosa, the mean (+/- SD) optic nerve axon bundle diameter was 27 +/- 11 microm. The lamina cribrosa measured 363 microm dorsal to ventral and 3.60 mm nasal to temporal. The thickness of the lamina was 125 micro m anterior to posterior with a mean (+/- SD) laminar pore size of 45 +/- 12 microm. The optic nerve head measured 3.65 mm nasal to temporal and 330 microm dorsal to ventral. Trichrome staining indicated the presence of collagen in the lamina cribrosa. This is the first detailed description of the squirrel optic nerve head. It has an unusual shape among mammals with a dramatic horizontal elongation. The purpose of this shape is not apparent. The presence of collagen in the scleral lamina cribrosa indicates a similar extracellular matrix composition as in other species.

The neurons of the ground squirrel retina as revealed by immunostains for calcium binding proteins and neurotransmitters

Ground squirrel retinas were immunostained with antibodies against calcium binding proteins (CBPs) and classical neurotransmitters in order to describe neuronal phenotypes in a diurnal mammalian retina and to then compare these neurons with those of more commonly studied nocturnal retinas like cats' and rabbits'. Double immunostained tissue was examined by confocal microscopy using antibodies against the following: rhodopsin and the CBPs, calbindin, calretinin, parvalbumin, calmodulin and recoverin (CB, CR, PV, CM, RV), glycine, GABA, choline acetyltransferase (CHAT) and tyrosine hydroxylase (TOH). In ground squirrel retina, the traditional cholinergic mirror symmetric amacrine cells colocalize CHAT with PV and GABA and faintly with glycine. A second cholinergic amacrine cell type colocalizes glycine alone. CR is found in at least 3 different amacrine cell types. The CR-immunoreactive (IR) cell population is a mixture of glycinergic and GABAergic types. The dopamine cell type IR to tyrosine hydroxylase has the typical morphology of a wide field cell with dendrites in S1 but the "rings" seen in cat or rabbit retina are not as numerous. TOH-IR amacrine cells send large club-shaped processes to the outer plexiform layer. CB and CR are in bipolar cells, A- and B-type horizontal cells and several amacrine cell types. Anti-rhodopsin labels the low density rod photoreceptor population in this species. Anti-recoverin labels cones and some bipolar cells while PKC is found in several different bipolar cell types. One ganglion cell with dendritic branching in S3 is strongly CR-IR. We find no evidence for an AII amacrine cell in the ground squirrel, with either anti-CR or anti-PV. An amacrine cell with similarity to the DAP1-3 cell of rabbit is CR-IR and glycine-IR. We discuss this labeling pattern in relationship to other mammalian species. The differences in staining patterns and phenotypes revealed suggest a functional diversity in the populations of amacrine cells according to whether the retinas are rod or cone dominated.

Geographical variation of skull morphology and its functional significances in the red-cheeked squirrel

Skulls of the red-cheeked squirrel (Dremomys rufigenis) from various geographical locations: Malaysia (peninsular area), Vietnam (south district)-Laos, and Thailand (north district) were osteometrically examined. The skull size of the squirrels in the southern (Malaysia) population was fundamentally larger than that in the northern (Vietnam, Laos and Thailand) populations. The proportion indices indicated that the splanchnocranium was relatively longer in the Malaysia population, and that the interorbital space was narrower in Vietnam-Laos, and Thailand populations. We suggest that the long nose and laterally-oriented orbits in the skull may be better adapted for terrestrial-insectivorous life in the Malaysia population and the binocular sense facilitated by rostrally-oriented eyes contributes to the arboreal-fruit eating behavior in the two northern populations. The Malaysia population was clearly distinguished from the other populations by the principal component analysis. We suggest that the geographical barrier of the Isthmus of Kra influences the morphological variation of the skull among the squirrel populations.

Phylogeography and introgressive hybridization: chipmunks (genus Tamias) in the northern Rocky Mountains

If phylogeographic studies are to be broadly used for assessing population-level processes relevant to speciation and systematics, the ability to identify and incorporate instances of hybridization into the analytical framework is essential. Here, we examine the evolutionary history of two chipmunk species, Tamias ruficaudus and Tamias amoenus, in the northern Rocky Mountains by integrating multivariate morphometrics of bacular (os penis) variation, phylogenetic estimation, and nested clade analysis with regional biogeography. Our results indicate multiple examples of mitochondrial DNA introgression layered within the evolutionary history of these nonsister species. Three of these events are most consistent with recent and/or ongoing asymmetric introgression of mitochondrial DNA across morphologically defined secondary contact zones. In addition, we find preliminary evidence where a fourth instance of nonconcordant characters may represent complete fixation of introgressed mitochondrial DNA via a more ancient hybridization event, although alternative explanations of convergence or incomplete sorting of ancestral polymorphisms cannot be dismissed with these data. The demonstration of hybridization among chipmunks with strongly differentiated bacular morphology contradicts long-standing assumptions that variation within this character is diagnostic of complete reproductive isolation within Tamias. Our results illustrate the utility of phylogeographic analyses for detecting instances of reticulate evolution and for incorporating this and other information in the inference of the evolutionary history of species.

Geographical and Functional-Morphological Variations of the Skull in the Gray-Bellied Squirrel

The geographical variations of the skulls were osteometrically examined in the gray-bellied squirrel (Callosciurus caniceps) from the populations of Korat, Ranong, southernmost Thailand, and Terutau Island. The skull size was larger in northern population than in the southern population in the continental mainland. The zoogeographical influences of the Isthmus of Kra remained unclear, since the plots from Korat population were intermingled with those from southernmost Thailand population in the principal component charts. Although Korat population has been thought to belong to north group, we suggest that Ranong and southernmost Thailand populations may contain individuals from both north and south groups separated by the ancient Kra barrier. Terutau Island population was similar to southernmost Thailand population in skull size, although Terutau population has been isolated in the island and separated from the south group of the Isthmus of Kra. In the proportional analysis the interorbital space was narrower and the binocular sense has been well-developed in Terutau population. It suggests that this population has been highly adapted to arboreal behavior. In contrast, the skull with larger interorbital space was more adaptive for terrestrial life in Korat population. The canonical discriminant analysis could clearly separate the four populations in the scattergrams of discriminant scores.

Geographical Variation of Skull Size and Shape in Various Populations in the Black Giant Squirrel

We osteometrically examined the skulls of the black giant squirrel (Ratufa bicolor) from three mainland populations (M. Malayan Peninsula, V. South Vietnam, and B. Burma, India and North Thailand) and from two island populations (T. Tioman, and S. Sumatra Islands). The skull in the Malayan peninsula population was significantly smaller than that of the two other mainland populations. It is consistent with Bergmann's rule as shown in the gray-bellied squirrel. The two island populations did not show obvious differences in comparison with the Malayan population in many measurements. In the proportion analysis eliminating the size factor, the differences among populations were not easily confirmed and we concluded that the osteological characters peculiar to each population could not be shown in this species. The first and second principal component scores of M, S, and T populations were intermingled, whereas the V and B populations of V and B were not separated in the chart. We pointed out that the morphological differences were demonstrated between northern and southern groups of the Isthmus of Kra in the mainland populations, and that the two island populations did not show the island-isolation effect in comparison with the M population. The adaptational variation related to feeding and locomotion could not be confirmed among populations of the black giant squirrel as shown in the proportion analysis.

Changes in hippocampal histamine receptors across the hibernation cycle in ground squirrels

Hibernation is a physiological state characterized by a dramatic reduction in various functions, such as body temperature, heart rate, and metabolism. The hippocampus is thought to be important for regulation of the hibernation bout because it remains electrophysiologically active throughout this extremely depressed state. The question arises as to what neuronal influences act within the hippocampus during hibernation to sustain its activity. We hypothesized that histaminergic input might be an important contributor. Brain histamine is involved in functions relevant to hibernation, such as the regulation of diurnal rhythms, body temperature, and energy metabolism. Furthermore, we have previously shown that the histaminergic system appears to be activated during the hibernating state. In this study, we used receptor binding autoradiography, in situ hybridization, and GTP-gamma-S binding autoradiography to study changes in histamine receptors across the hibernation bout. We were able to demonstrate an increase in histamine H1 and H2 receptors in the hippocampus during hibernation, whereas the mRNA expression and receptor density of the inhibitory H3 receptor decreased. Histamine H3 receptors were shown to exhibit both histamine-activated and constitutive GTP-gamma-S-binding activity in the ground squirrel hippocampus, both of which decreased during hibernation, indicating a decrease in H3 receptor G-protein activation. Taken together, our results indicate that histamine may be involved in maintaining hibernation by sustaining hippocampal activity, possibly through H1 and H2 receptor activity and decreased inhibition by H3 receptors. The involvement of brain histamine, which is generally thought of as an arousal molecule, in maintaining a depressed state of the brain suggests a more general role for the amine in controlling arousal state.

Pattern of retinal projections in the California ground squirrel (Spermophilus beecheyi): anterograde tracing study using cholera toxin

The retinofugal pathways in the California ground squirrel, Spermophilus beecheyi, were mapped after intravitreal injections of cholera toxin B-subunit. The results of the current study are consistent with work in other mammals and provide new details relevant to the organization and evolution of the visual system. All retinorecipient nuclei received bilateral input, with a contralateral predominance. The suprachiasmatic nucleus is heavily innervated, and sparse terminals were noted in other hypothalamic areas. In addition to the interstitial, medial, lateral, and dorsal terminal nuclei, a few fibers of the accessory optic tract may enter the ventral lateral geniculate and the nucleus of the optic tract, though this innervation may not derive from the same ganglion cells innervating the accessory optic nuclei. Retinal terminals are found in the intergeniculate leaflet and the "dorsal cap" of the ventral lateral geniculate. Retinal fibers pass rostrally from the dorsal cap toward the anterodorsal thalamus, confirming a projection described in the tree shrew and monkeys. Retinal termination patterns in the dorsal lateral geniculate reveal a hexilaminate organization of alternating ipsilateral and contralateral input. Variations in terminal morphology suggest that sublayers receive input from distinct ganglion cell types and that laminar comparisons can be made with primates. Finally, terminal patterns in the superior colliculus reveal a dense, highly ordered columnar organization supporting functional properties of tectal receptive fields. All the visual structures in the ground squirrel are large and well differentiated, making the sciurid visual system an accessible rodent model for comparing visual processing with that in other diurnal vertebrates.

[The alimentary canal of the Siberian chipmunk (Eutamias sibiricus, Laxm. 1769): macroscopic and light microscopic examination]

The canalis alimentarius of the Burunduk (Eutamias sibiricus), a rodent belonging to the family Sciuridae, were examined macroscopically (12 animals) and light microscopically (three animals). The esophagus is lined with a stratified non-keratinized squamous epithelium. The one-chambered stomach is of a simple type covered with a glandular mucous membrane. When empty and moderately filled, the stomach looks like a curved sack and lies intrathoracally. The filled stomach extends to the left and ventrally into the regio abdominis media. The greater omentum covers incompletely ventrally and in a part laterally the intestinal mass. The intestinal canal averages about 780 mm in length, that is 6.5 as long as the whole body. The relative length of the small intestine compared with the large intestine is 36-64%. The U-shaped Duodenum is composed of a Pars cranialis, descendens and ascendens and possesses a Plica duodenocolica as well as a Plica duodenocolica accessoria. The Jejunum averages about 420 mm length and is mainly located in the right and ventral part of the regio abdominis media. As in other rodents, the cecum is well developed. The length of the ascending colon averages about the body length and forms an Ansa proximalis and two parallel loops, Ansa media and Ansa distalis coli, both lying in the right Cavum abdominis. Peculiar for the colon are Noduli lymphatici solitarii surrounding cavities lined by a surface epithelium. According to the anatomical structure of the gut and based on physiological diet facts the Burunduk is not a mere herbivore but has to be classified as an omnivore depending upon uptake of animal food protein.

The effects of Cenozoic global change on squirrel phylogeny

By modifying habitats and creating bridges and barriers between landmasses, climate change and tectonic events are believed to have important consequences for diversification of terrestrial organisms. Such consequences should be most evident in phylogenetic histories of groups that are ancient, widespread, and diverse. The squirrel family (Sciuridae) is one of very few mammalian families endemic to Eurasia, Africa, and North and South America and is ideal for examining these issues. Through phylogenetic and molecular-clock analyses, we infer that arrival and diversification of squirrels in Africa, on Sunda Shelf islands, across Beringea, and across the Panamanian isthmus coincide in timing and location with multiple well-documented sea-level, tectonic, and paleontological events. These precise correspondences point to an important role for global change in the diversification of a major group of mammals.

[Effect of temperature on the heart function of the five-strip eyebrow squirrel]

This is an in vitro study designed to assess the effect of temperature variation on the heart function of the five-strip eyebrow squirrel--amammal of the hibernating kind. A heart model of exsomatizing action and for determining the physiological parameters with four physiological recorders was used to measure the LVSP, LVEDP and (+/- dP/dtmax), respectively. It was found that the curves of the squirrel's heart mechanics property in relation to temperature variation are similar to the curves of the non-hibernating animals'. But, the squirrels' heart contraction function changes with in a much wider range, suggesting that their hearts are more tolerant of temperature variation.

Choline acetyltransferase is expressed by non-starburst amacrine cells in the ground squirrel retina

We have used immunostaining techniques to reveal a new type of amacrine cell that is immunoreactive for choline acetyltransferase (ChAT), the acetylcholine synthesizing enzyme, in the Ground Squirrel (Spermophilus beecheyi) retina. Cryostat sections and double immunostained wholemount preparations were examined by confocal microscopy. This new ChAT type III cell is distinct in morphology and neurotransmitter content from the well know 'starburst' amacrine cells (types I and II) that are so well represented in the ground squirrel retina [J. Comp. Neurol. 365 (1996) 173-216]. The type III cell colocalizes glycine with the acetylcholine and does not appear to be GABAergic or exhibit calcium-binding proteins like the well-known starburst type. As well, type III cells do not occur as a mirror-symmetric pair with normally placed and displaced varieties. The type III cell is probably a small field amacrine type branching broadly in upper sublamina b of the inner plexiform layer, and is most likely A6 of the Ground Squirrel retina [J. Comp. Neurol. 365 (1996) 173-216]. Type III cells are ideally placed in the architecture of the Ground Squirrel retina to influence ON directionally selective ganglion cell types.

[Structure of the developing and definitive eye retina in the little suslik]

Using general histological, neurohistological and cytochemical methods, the processes of maturation and structural peculiarities of retina were studied in pre- and postnatal periods of ontogenesis in the ground squirrel--the mammal with day activity. It was shown that in a ground squirrel, similar to what has been noted in all immature-born animals, maturation of the retina took place from its center to periphery and was realized from the innermost layers towards the outermost ones. Differentiation of retinal neurons was finished by the moment of eyes' opening--at the age of 12 days. The dynamics of protein metabolism was studied in ganglion neurons of the retina in the process of ontogenesis.

Female-biased sexual size dimorphism in the yellow-pine chipmunk (Tamias amoenus): sex-specific patterns of annual reproductive success and survival

Sexual size dimorphism is ultimately the result of independent, sex-specific selection on body size. In mammals, male-biased sexual size dimorphism is the predominant pattern, and it is usually attributed to the polygynous mating system prevalent in most mammals. This sole explanation is unsatisfying because selection acts on both sexes simultaneously, therefore any explanation of sexual size dimorphism should explain why one sex is relatively large and the other is small. Using mark-recapture techniques and DNA microsatellite loci to assign parentage, we examined sex-specific patterns of annual reproductive success and survival in the yellow-pine chipmunk (Tamias amoenus), a small mammal with female-biased sexual size dimorphism, to test the hypothesis that the dimorphism was related to sex differences in the relationship between body size and fitness. Chipmunks were monitored and body size components measured over three years in the Kananaskis Valley, Alberta, Canada. Male reproductive success was independent of body size perhaps due to trade-offs in body size associated with behavioral components of male mating success: dominance and running speed. Male survival was consistent with stabilizing selection for overall body size and body size components. The relationship between reproductive success and female body size fluctuated. In two of three years the relationship was positive, whereas in one year the relationship was negative. This may have been the result of differences in environmental conditions among years. Large females require more energy to maintain their soma than small females and may be unable to maintain lactation in the face of challenging environmental conditions. Female survival was positively related to body size, with little evidence for stabilizing selection. Sex differences in the relationship between body size and fitness (reproductive success and survival) were the result of different processes, but were ultimately consistent with female-biased sexual size dimorphism evident in this species.

Elevated arylalkylamine-N-acetyltransferase (AA-NAT) gene expression in medial habenular and suprachiasmatic nuclei of hibernating ground squirrels

Hibernation, an adaptive response for energy conservation in mammals, involves a variety of physiological changes. Melatonin is linked with the regulation of core body temperature and intervenes in generating circadian cycles; its role in seasonal (circannual) rhythms of hibernation is explored here. Melatonin is primarily produced in the pineal gland. Since arylalkylamine-N-acetyltransferase (AA-NAT) is the rate-limiting enzyme for synthesizing melatonin, AA-NAT gene expression was investigated to assess the possible role of melatonin in hibernation. The findings presented here utilized combined in situ hybridization and immunohistochemistry methodologies to evaluate the AA-NAT mRNA expression in brains of both hibernating and non-hibernating ground squirrels. Brains were examined for the expression of AA-NAT mRNA using a oligonucleotide AA-NAT probe; antibody against neurofilament-70 (NF-70) was used as a neuronal marker. All hibernating animals expressed significantly (P<0.01) elevated levels of AA-NAT mRNA in both the epithalamic medial habenular nuclei (MHb) area and the hypothalamic suprachiasmatic nuclei (SCN), which is also known as the master biologic clock. These findings represent the first demonstration of the expression of mRNA encoding for AA-NAT in the extra-pineal (i.e. SCN and MHb) sites of thirteen-lined ground squirrels and indicate that the habenular nucleus may be an important supplementary location for melatonin biosynthesis. The data presented here indicate that AA-NAT gene is one of the few specific genes up-regulated during hibernation and suggest that elevation of its expression in SCN and MHb may play an essential role in the generation and maintenance of hibernation.

Phylogeography of the red-tailed chipmunk (Tamias ruficaudus), a northern Rocky Mountain endemic

The northern Rocky Mountains have experienced a complex history of geological events and environmental fluctuation, including Pleistocene glaciation. To provide an initial assessment of the genetic impact of this history on the regional biota we estimated phylogenetic relationships within Tamias ruficaudus, a regional endemic, from cytochrome b sequence variation using parsimony, maximum likelihood, and nested clade analysis. Analyses of sequence variation in 187 individuals from 43 localities across the distribution of T. ruficaudus indicate a history of vicariance events and range fluctuation consistent with successive periods of extensive Pleistocene glaciation in the northern Rocky Mountains. Intraspecific divergence levels (c. 4.7% uncorrected) and phylogenetic structure are consistent with a genealogical vicariance initiated prior to the Late Pleistocene, whereas nested clade analyses indicate more recent population history structured by both fragmentation and range expansion. A comparison of sequence variation with bacular morphology indicates that the two genetically and morphologically differentiated entities exhibit a zone of differential character introgression. Sequence data support a multiple refugia hypothesis and provide a phylogeographical case study for the ongoing synthesis of regional biogeography for northern Rocky Mountain endemics.

Anatomy of the squirrel wrist: bones, ligaments, and muscles

Anatomical differences among squirrels are usually most evident in the comparison of flying squirrels and nongliding squirrels. This is true of wrist anatomy, probably reflecting the specializations of flying squirrels for the extension of the wing tip and control of it during gliding. In the proximal row of carpals of most squirrels, the pisiform articulates only with the triquetrum, but in flying squirrels there is also a prominent articulation between the pisiform and the scapholunate, providing a more stable base for the styliform cartilage, which supports the wing tip. In the proximal wrist joint, between these carpals and the radius and ulna, differences in curvature of articular surfaces and in the location of ligaments also correlate with differences in degree and kind of movement occurring at this joint, principally reflecting the extreme dorsal flexion and radial deviation of the wrist in flying squirrels when gliding. The distal wrist joint, between the proximal and distal rows of carpals, also shows most variation among flying squirrels, principally in the articulations of the centrale with the other carpal bones, probably causing the distal row of carpal bones to function more like a single unit in some animals. There is little variation in wrist musculature, suggesting only minor evolutionary modification since the tribal radiation of squirrels, probably in the early Oligocene. Variation in the carpal bones, particularly the articulation of the pisiform with the triquetrum and the scapholunate, suggests a different suprageneric grouping of flying squirrels than previously proposed by McKenna (1962) and Mein (1970). J. Morphol. 246:85-102, 2000. Published 2000 Wiley-Liss, Inc.

mRNA stability and polysome loss in hibernating Arctic ground squirrels (Spermophilus parryii)

All small mammalian hibernators periodically rewarm from torpor to high, euthermic body temperatures for brief intervals throughout the hibernating season. The functional significance of these arousal episodes is unknown, but one suggestion is that rewarming may be related to replacement of gene products lost during torpor due to degradation of mRNA. To assess the stability of mRNA as a function of the hibernation state, we examined the poly(A) tail lengths of liver mRNA from arctic ground squirrels sacrificed during four hibernation states (early and late during a torpor bout and early and late following arousal from torpor) and from active ground squirrels sacrificed in the summer. Poly(A) tail lengths were not altered during torpor, suggesting either that mRNA is stabilized or that transcription continues during torpor. In mRNA isolated from torpid ground squirrels, we observed a pattern of 12 poly(A) residues at greater densities approximately every 27 nucleotides along the poly(A) tail, which is a pattern consistent with binding of poly(A)-binding protein. The intensity of this pattern was significantly reduced following arousal from torpor and undetectable in mRNA obtained from summer ground squirrels. Analyses of polysome profiles revealed a significant reduction in polyribosomes in torpid animals, indicating that translation is depressed during torpor.

Bottlebrush dendritic endings and large dendritic fields: motion-detecting neurons in the mammalian tectum

The widefield vertical neurons of the lower stratum griseum superficiale (SGS3) and upper stratum opticum (SO) of the superior colliculus provide an extrageniculate route for visual information to reach the pulvinar. Previous physiological studies indicate that SGS3/SO neurons have large receptive fields and respond to small moving stimuli. We sought to better characterize the dendritic morphology of SGS3/SO neurons with intracellular filling in slice preparations of the ground squirrel superior colliculus. We found that dendrites of widefield vertical cells end in monostratified arrays of spiny terminal specializations called "bottlebrush" dendritic endings. Two major subtypes of neurons are described. Type I neurons have somata restricted to the SGS3 and bottlebrush endings in the most superficial sublayer of the SGS. Type II neurons are found at the base of the SGS and in the upper SO, and have bottlebrush endings arrayed within the middle sublayers of the SGS. Bottlebrush endings may sample and integrate laminated afferents to the superior colliculus, and cellular subtypes may underlie multiple information streams within the tectopulvinar pathway. A similar dendritic morphology and projection pattern can be found in cells of the avian optic tectum that project upon the nucleus rotundus, a thalamic nucleus homologous to the mammalian caudal/inferior pulvinar. Because motion processing is a dominant feature of the avian tectorotundal pathway, the current results suggest that both dendritic morphology and motion processing are conserved features of widefield vertical cells in the tectopulvinar pathway of vertebrates.

State-dependent effects of some neuropeptides and neurotransmitters on neuronal activity of the medial septal area in brain slices of the ground squirrel, Citellus undulatus

Neuronal activity of the medial septal area was recorded extracellularly in brain slices taken from hibernating (winter) and waking (summer) ground squirrels. The effects of neuropeptides identified in the brain tissue of hibernators (Thr-Ser-Lys-Tyr, Thr-Ser-Lys-Tyr-Arg and Asp-Tyr) on the background activity and responses to electrical stimulation of the median forebrain bundle were analysed. For comparison, the effects of bath application of noradrenaline and serotonin were also tested. Spontaneous activity in half of all neurons (47-56%) was changed under the influence of neuropeptides in hibernating ground squirrels, while in waking ground squirrels the proportion of responsive neurons was significantly lower (25-30%). The tendency for higher efficacy in hibernating ground squirrels was observed for serotonin; only noradrenaline was equally effective in both groups of animals. Electrically evoked responses of the medial septal nucleus-nucleus of the diagonal band neurons were also strongly modulated by neuropeptides; their changes could occur in the absence of shifts in the level and pattern of spontaneous activity. All three neuropeptides had differential action on the level of spontaneous activity, as well as on inhibitory and excitatory components of electrically evoked responses. Thus, the character and distribution of the effects were state dependent and differed greatly in hibernating and waking ground squirrels. The experiments confirmed that medial septal nucleus-nucleus of the diagonal band neurons have higher excitability and responsiveness to some neuropeptides and neurotransmitters in hibernating ground squirrels.The data obtained suggest an increased latent excitability and responsiveness of septal neurons during hibernation and their possible active participation in urgent arousal under the influence of sensory signals.

The seasonal pattern of cell proliferation and neuron number in the dentate gyrus of wild adult eastern grey squirrels

The dentate gyrus is one of two areas in the mammalian brain that produces neurons in adulthood. Neurogenesis (proliferation, survival, and differentiation of new neurons) is regulated by experience, and increased neurogenesis appears to be correlated with improved spatial learning in mammals and birds. We tested the hypothesis that in long-lived mammals that scatter-hoard food, seasonal variations in spatial memory processing (i.e. increased processing during caching season in the autumn) might correlate with changes in neurogenesis and neuron number in the granule cell layer of the dentate gyrus (gcl DG). We investigated the rate of cell proliferation and the total number of neurons in the granule cell layer of wild adult eastern grey squirrels (Sciurus carolinensis) at three different times of the year (October, January and June). We found no seasonal differences in cell proliferation rate or in total neuron number in the granule cell layer. Our findings are in agreement with those of previous studies in laboratory mice and rats, and in free-ranging, food-caching, black-capped chickadees, as well as with current hypotheses regarding the relationship between neurogenesis and learning. Our results, however, are also in agreement with the hypothesis that neurogenesis in the dentate gyrus represents a maintenance system that may be regulated by environmental factors, and that changes in total neuron number previously reported in rodents represent developmental changes rather than adult plasticity. The patterns observed in mature wild rodents, such as free-ranging squirrels, may represent more accurately the extent of hippocampal plasticity in adult mammals.

SEM study on the dorsal lingual surface of the flying squirrel, Petaurista leucogenys

The dorsal lingual surface of the flying squirrel was examined by scanning electron microscopy (SEM). Filiform (FI), fungiform (FU), foliate (FO) and vallate papillae (VA) were observed. The FI were distributed over the entire dorsal surface of the tongue. In the region of the VA, the FI appeared as giant cones. The FU were present as rounded bodies scattered over the dorsal surface of the tongue. They were relatively scarce and appear to be concentrated around the edges of the tongue. The FO were observed on the posterolateral regions of the tongue. The flying squirrel showed the triangular arrangement of the three VA, with the apex of the triangle directed posteriorly.