Comparison of prenatal and postnatal ontogeny: cranial allometry in the African striped mouse (Rhabdomys pumilio)

Postnatal Skull Development Reveals a Conservative Pattern in Living and Fossil Vizcachas Genus Lagostomus (Rodentia, Chinchillidae)

The plains vizcacha, Lagostomus maximus, is the only living species in the genus, being notably larger than fossil congeneric species, such as Lagostomus incisus, from the Pliocene of Argentina and Uruguay. Here, we compare the skull growth allometric pattern and sexual dimorphism of L. maximus and L. incisus, relating shape and size changes with skull function. We also test whether the ontogenetic trajectories and allometric trends between both sexes of L. maximus follow the same pattern. A common allometric pattern between both species was the elongation of the skull, a product of the lengthening of rostrum, and chondrogenesis on the spheno-occipitalis synchondrosis and coronalis suture. We also detected a low proportion of skull suture fusion. In some variables, older male specimens did not represent a simple linear extension of female trajectory, and all dimorphic traits were related to the development of the masticatory muscles. Sexual dimorphism previously attributed to L. incisus would indicate that this phenomenon was present in the genus since the early Pliocene and suggests social behaviors such as polygyny and male-male competition. Ontogenetic changes in L. incisus were similar to L. maximus, showing a conservative condition of the genus. Only two changes were different in the ontogeny of both species, which appeared earlier in L. incisus compared to L. maximus: the development of the frontal process of the nasals in a square shape, and the straight shape of the occipital bone in lateral view. Juveniles of L. maximus were close to adult L. incisus in the morphospace, suggesting a peramorphic process. The sequence of suture and synchondroses fusion showed minor differences in temporozygomatica and frontonasalis sutures, indicating major mechanical stress in L. maximus related to size. We suggest a generalized growth path in Chinchillidae, but further analyses are necessary at an evolutionary level, including Lagidium and Chinchilla.

Ontogenetic and static allometry in the skull and cranial units of nine-banded armadillos (Cingulata: Dasypodidae: Dasypus novemcinctus)

A large part of extant and past mammalian morphological diversity is related to variation in size through allometric effects. Previous studies suggested that craniofacial allometry is the dominant pattern underlying mammalian skull shape variation, but cranial allometries were rarely characterized within cranial units such as individual bones. Here, we used 3D geometric morphometric methods to study allometric patterns of the whole skull (global) and of cranial units (local) in a postnatal developmental series of nine-banded armadillos (Dasypus novemcinctus ssp.). Analyses were conducted at the ontogenetic and static levels, and for successive developmental stages. Our results support craniofacial allometry as the global pattern along with more local allometric trends, such as the relative posterior elongation of the infraorbital canal, the tooth row reduction on the maxillary, and the marked development of nuchal crests on the supraoccipital with increasing skull size. Our study also reports allometric proportions of shape variation varying substantially among cranial units and across ontogenetic stages. The multi-scale approach advocated here allowed unveiling previously unnoticed allometric variations, indicating an untapped complexity of cranial allometric patterns to further explain mammalian morphological evolution.

Postcranial heterochrony, modularity, integration and disparity in the prenatal ossification in bats (Chiroptera)

BACKGROUND

Self-powered flight is one of the most energy-intensive types of locomotion found in vertebrates. It is also associated with a range of extreme morpho-physiological adaptations that evolved independently in three different vertebrate groups. Considering that development acts as a bridge between the genotype and phenotype on which selection acts, studying the ossification of the postcranium can potentially illuminate our understanding of bat flight evolution. However, the ontogenetic basis of vertebrate flight remains largely understudied. Advances in quantitative analysis of sequence heterochrony and morphogenetic growth have created novel approaches to study the developmental basis of diversification and the evolvability of skeletal morphogenesis. Assessing the presence of ontogenetic disparity, integration and modularity from an evolutionary approach allows assessing whether flight may have resulted in evolutionary differences in the magnitude and mode of development in bats.

RESULTS

We quantitatively compared the prenatal ossification of the postcranium (24 bones) between bats (14 species), non-volant mammals (11 species) and birds (14 species), combining for the first time prenatal sequence heterochrony and developmental growth data. Sequence heterochrony was found across groups, showing that bat postcranial development shares patterns found in other flying vertebrates but also those in non-volant mammals. In bats, modularity was found as an axial-appendicular partition, resembling a mammalian pattern of developmental modularity and suggesting flight did not repattern prenatal postcranial covariance in bats.

CONCLUSIONS

Combining prenatal data from 14 bat species, this study represents the most comprehensive quantitative analysis of chiropteran ossification to date. Heterochrony between the wing and leg in bats could reflect functional needs of the newborn, rather than ecological aspects of the adult. Bats share similarities with birds in the development of structures involved in flight (i.e. handwing and sternum), suggesting that flight altriciality and early ossification of pedal phalanges and sternum are common across flying vertebrates. These results indicate that the developmental modularity found in bats facilitates intramodular phenotypic diversification of the skeleton. Integration and disparity increased across developmental time in bats. We also found a delay in the ossification of highly adaptable and evolvable regions (e.g. handwing and sternum) that are directly associated with flight performance.

On the lack of a universal pattern associated with mammalian domestication: differences in skull growth trajectories across phylogeny

As shown in a taxonomically broad study, domestication modifies postnatal growth. Skull shape across 1128 individuals was characterized by 14 linear measurements, comparing 13 pairs of wild versus domesticated forms. Among wild forms, the boar, the rabbit and the wolf have the highest proportion of allometric growth, explaining in part the great morphological diversity of the domesticated forms of these species. Wild forms exhibit more isometric growth than their domesticated counterparts. Multivariate comparisons show that dogs and llamas exhibit the greatest amount of differences in trajectories with their wild counterparts. The least amount is recorded in the pig-boar, and camel and horse pairs. Bivariate analyses reveal that most domesticated forms have growth trajectories different from their respective wild counterparts with regard to the slopes. In pigs and camels slopes are shared and intercepts are different. There is a trajectory extension in most domesticated herbivores and the contrary pattern in carnivorous forms. However, there is no single, universal and global pattern of paedomorphosis or any other kind of heterochrony behind the morphological diversification that accompanies domestication.

On the development of the chondrocranium and the histological anatomy of the head in perinatal stages of marsupial mammals

An overview of the literature on the chondrocranium of marsupial mammals reveals a relative conservatism in shape and structures. We document the histological cranial anatomy of individuals representing Monodelphis domestica, Dromiciops gliroides, Perameles sp. and Macropus eugenii. The marsupial chondrocranium is generally characterized by the great breadth of the lamina basalis, absence of pila metoptica and large otic capsules. Its most anterior portion (cupula nasi anterior) is robust, and anterior to it there are well-developed tactile sensory structures, functionally important in the neonate. Investigations of ossification centers at and around the nasal septum are needed to trace the presence of certain bones (e.g., mesethmoid, parasphenoid) across marsupial taxa. In many adult marsupials, the tympanic floor is formed by at least three bones: alisphenoid (alisphenoid tympanic process), ectotympanic and petrosal (rostral and caudal tympanic processes); the squamosal also contributes in some diprotodontians. The presence of an entotympanic in marsupials has not been convincingly demonstrated. The tubal element surrounding the auditory tube in most marsupials is fibrous connective tissue rather than cartilage; the latter is the case in most placentals recorded to date. However, we detected fibrocartilage in a late juvenile of Dromiciops, and a similar tissue has been reported for Tarsipes. Contradictory reports on the presence of the tegmen tympani can be found in the literature. We describe a small tegmen tympani in Macropus. Several heterochronic shifts in the timing of development of the chondocranium and associated structures (e.g., nerves, muscles) and in the ossification sequence have been interpreted as largely being influenced by functional requirements related to the altriciality of the newborn marsupial during early postnatal life. Comparative studies of chondocranial development of mammals can benefit from a solid phylogenetic framework, research on non-classical model organisms, and integration with imaging and sectional data derived from computer-tomography.

Do Developmental Constraints and High Integration Limit the Evolution of the Marsupial Oral Apparatus?

Developmental constraints can have significant influence on the magnitude and direction of evolutionary change, and many studies have demonstrated that these effects are manifested on macroevolutionary scales. Phenotypic integration, or the strong interactions among traits, has been similarly invoked as a major influence on morphological variation, and many studies have demonstrated that trait integration changes through ontogeny, in many cases decreasing with age. Here, we unify these perspectives in a case study of the ontogeny of the mammalian cranium, focusing on a comparison between marsupials and placentals. Marsupials are born at an extremely altricial state, requiring, in most cases, the use of the forelimbs to climb to the pouch, and, in all cases, an extended period of continuous suckling, during which most of their development occurs. Previous work has shown that marsupials are less disparate in adult cranial form than are placentals, particularly in the oral apparatus, and in forelimb ontogeny and adult morphology, presumably due to functional selection pressures on these two systems during early postnatal development. Using phenotypic trajectory analysis to quantify prenatal and early postnatal cranial ontogeny in 10 species of therian mammals, we demonstrate that this pattern of limited variation is also apparent in the development of the oral apparatus of marsupials, relative to placentals, but not in the skull more generally. Combined with the observation that marsupials show extremely high integration of the oral apparatus in early postnatal ontogeny, while other cranial regions show similar levels of integration to that observed in placentals, we suggest that high integration may compound the effects of the functional constraints for continuous suckling to ultimately limit the ontogenetic and adult disparity of the marsupial oral apparatus throughout their evolutionary history.

Ontogenetic allometry in the foot size of Oligoryzomys flavescens (Waterhouse, 1837) (Rodentia, Sigmodontinae)

Ontogenetic allometry is the study of how the size or shape of certain structures changes over the course of an animal's development. In this study, using Huxley's formula of allometric growth (1932), we assessed the changes in the rate of growth of the feet size of the sigmodontine rodent Oligoryzomys flavescens during its ontogeny and compared differences between males and females. We find evidence of a change of polarity during the ontogenetic development of the species, with the presence of positive allometry during pregnancy and negative allometry in adulthood. Moreover, we note the presence of sexual dimorphism in the size of the feet, in which males of the species have a higher rate of growth than females. This growth pattern is positively related to escape from predators in childhood in both sexes and, in adulthood, provides a higher encounter rate of females by males, due to the larger displacement of the latter. We suggest that both the forces of natural selection and sexual selection have acted to shape the evolution of foot size in this species.

A three-dimensional skull ontogeny in the bobcat (Lynx rufus) (Carnivora: Felidae): a comparison with other carnivores

The maturation of mammalian carnivores from a lactating juvenile to a predatory adult requires a suite of changes in both morphology and behaviour. Bobcats (Lynx rufus (Schreber, 1777)) are medium-sized cats with well-developed skulls to process large prey that can exceed their body mass. An integrated view of the skull ontogeny in the bobcat was developed to detect the relationship between shape, size (on the basis of three-dimensional geometric morphometric analysis), and life history. Dietary changes from juvenile to adults were taken into account and compared with other carnivores. Newborns were different from the remaining age stages in the behavioral and morphological characters examined, which allows us to relate them to the terminal morphology reached during the prenatal period. All findings were related to the reinforcement of the skull and the enhancement of predatory skills in adult bobcats. The final cranial shape is reached in A2 age class, after 2 years of age, and once sexual maturity has been reached. This is a pattern not followed for the rest of carnivores previously studied, which might be related to the capacity of subduing prey that exceed them in size, a behavior not common in felids of the body size of bobcats.