Altitudinal variation in organ mass from three mountain systems: The case of mesquite lizard Sceloporus grammicus

High altitude environments provide a fertile ground for investigating the benefits of phenotypic adjustments at several levels of biological organization. Low oxygen partial pressure and low environmental temperature are the main limiting factors that promote phenotypic variation in different organs, such as the lung and heart. Although high-altitude environments act like natural laboratories, most morphological studies conducted to date lack replication. Here, we evaluated organ mass variation in nine populations of Sceloporus grammicus, throughout three altitudinal gradients (mountains) from the Trans-Mexican volcanic belt. A total of 84 individuals from three different altitudes at three different mountains were collected. Then, we used generalized linear models to analyze the pattern of variation in internal organs mass as a function of altitude and temperature. We observed a striking pattern of altitudinal variation in the size of cardiorespiratory organs: while heart mass increased with altitude and decreased with temperature, the lung showed a significant statistical interaction between mountain transect and temperature. Overall, our results support the hypothesis that cardiorespiratory organs should be bigger in populations occurring at higher altitudes. Moreover, the study of different mountain systems allowed us to observe some differences in one mountain in relation to the other two.

Multiple forms of hotspots of tetrapod biodiversity and the challenges of open-access data scarcity

The uneven spatial distribution of biodiversity is a defining feature of nature. In fact, the implementation of conservation actions both locally and globally has progressively been guided by the identification of biodiversity 'hotspots' (areas with exceptional biodiversity). However, different regions of the world differ drastically in the availability of fine-scale data on the diversity and distribution of species, thus limiting the potential to assess their local environmental priorities. Within South America-a megadiverse continent-Uruguay represents a peculiar area where multiple tropical and non-tropical eco-regions converge, creating highly heterogeneous ecosystems, but where the systematic quantification of biodiversity remains largely anecdotal. To investigate the constraints posed by the limited access to biodiversity data, we employ the most comprehensive database for tetrapod vertebrates in Uruguay (spanning 664 species) assembled to date, to identify hotspots of species-richness, endemism and threatened species for the first time. Our results reveal negligible spatial congruence among biodiversity hotspots, and that tetrapod sampling has historically concentrated in only a few areas. Collectively, our study provides a detailed account of the areas where urgent biodiversity monitoring efforts are needed to develop more accurate knowledge on biodiversity patterns, offering government and environmental bodies a critical scientific resource for future planning.

Biodiversidata: An Open-Access Biodiversity Database for Uruguay

Background

The continental and marine territories of Uruguay are characterised by a rich convergence of multiple biogeographic ecoregions of the Neotropics, making this country a peculiar biodiversity spot. However, despite the biological significance of Uruguay for the South American subcontinent, the distribution of biodiversity patterns in this country remain poorly understood, given the severe gaps in available records of geographic species distributions. Currently, national biodiversity datasets are not openly available and, thus, a dominant proportion of the primary biodiversity data produced by researchers and institutions across Uruguay remains highly dispersed and difficult to access for the wider scientific and environmental community. In this paper, we aim to fill this gap by developing the first comprehensive, open-access database of biodiversity records for Uruguay (Biodiversidata), which is the result of a large-scale collaboration involving experts working across the entire range of taxonomic diversity found in the country.

New information

As part of the first phase of Biodiversidata, we here present a comprehensive database of tetrapod occurrence records native from Uruguay, with the latest taxonomic updates. The database provides primary biodiversity data on extant Amphibia, Reptilia, Aves and Mammalia species recorded within the country. The total number of records collated is 69,380, spanning 673 species and it is available at the Zenodo repository: https://doi.org/10.5281/zenodo.2650169. This is the largest and most geographically and taxonomically comprehensive database of Uruguayan tetrapod species available to date and it represents the first open repository for the country.

Testing the metabolic homeostasis hypothesis in amphibians

A number of hypotheses about compensatory mechanisms that allow ectothermic animals to cope with the latitudinal decrease in ambient temperature ( T_A) have been proposed during the last century. One of these hypotheses, the 'metabolic homeostasis' hypothesis (MHH), states that species should show the highest thermal sensitivity of the metabolic rate ( Q_10-SMR) at the colder end of the range of T_As they usually experience in nature. This way, species should be able to minimize maintenance costs during the colder hours of the day, but quickly take advantage of increases in T_A during the warmer parts of the day. Here, we created a dataset that includes Q_10-SMR values for 58 amphibian species, assessed at four thermal ranges, to evaluate three predictions derived from the MHH. In line with this hypothesis, we found that: (i) Q_10-SMR values tended to be positively correlated with latitude when measured at lower T_As, but negative correlated with latitude when measured at higher T_As, (ii) Q_10-SMR measured at lower T_As were higher in temperate species, whereas Q_10-SMR measured at higher T_As were higher in tropical species, and (iii) the experimental T_A at which Q_10-SMR was maximal for each species decreased with latitude. This is the first study to our knowledge showing that the relationship between Q_10-SMR and latitude in ectotherms changes with the T_A at which Q_10-SMR is assessed, as predicted from an adaptive hypothesis. This article is part of the theme issue 'Physiological diversity, biodiversity patterns and global climate change: testing key hypotheses involving temperature and oxygen'.

Seasonal changes in digestive enzymes in five bird species

Most animals must cope with seasonal fluctuations in environmental conditions, including variations in food availability and composition. Accordingly, it is expected that most species should exhibit reversible seasonal phenotypic adjustments in their physiology. Here, we assessed seasonal variation in the activity of three digestive enzymes (sucrase, maltase, and aminopeptidase-N) in one omniviorous bird species (Rufous-collared Sparrow (Zonotrichia capensis (P. L. Statius Müller, 1776))), three granivorous bird species (Black-chinned Siskin (Carduelis barbata (Molina, 1782)), Common Diuca Finch (Diuca diuca (Molina, 1782)), and Mourning Sierra Finch (Phrygilus fruticeti (Kittlitz, 1833))), and one insectivorous bird species (Plain-mantled Tit-Spinetail (Leptasthenura aegithaloides (Kittlitz, 1830))). Based on the adaptive modulation hypothesis, we predicted that the omnivorous species should exhibit the largest seasonal variation in the activity of its digestive enzymes in relation to granivorous and insectivorous species. We found that Z. capensis adjusts total activities of disaccharidases, total sucrase activity varied between seasons in C. barbata, and total activity of aminopeptidase-N only changed seasonally in L. aegithaloides. Moreover, this last species modified the tissue-specific activity of both disaccharidases as well as the wet mass of its intestine. Taken together, our results suggest that seasonal dietary changes occur in most of the species, regardless of the trophic categories in which they belong. Consequently, a better knowledge of the diet and its seasonal variation is necessary to better account for the results recorded in this study.

An association between differential expression and genetic divergence in the Patagonian olive mouse (Abrothrix olivacea)

Recent molecular studies have found striking differences between desert-adapted species and model mammals regarding water conservation. In particular, aquaporin 4, a classical gene involved in water regulation of model species, is absent or not expressed in the kidneys of desert-adapted species. To further understand the molecular response to water availability, we studied the Patagonian olive mouse Abrothrix olivacea, a species with an unusually broad ecological tolerance that exhibits a great urine concentration capability. The species is able to occupy both the arid Patagonian steppe and the Valdivian and Magellanic forests. We sampled 95 olive mouse specimens from four localities (two in the steppe and two in the forests) and analysed both phenotypic variables and transcriptomic data to investigate the response of this species to the contrasting environmental conditions. The relative size of the kidney and the ratio of urine to plasma concentrations were, as expected, negatively correlated with annual rainfall. Expression analyses uncovered nearly 3,000 genes that were differentially expressed between steppe and forest samples and indicated that this species resorts to the "classical" gene pathways for water regulation. Differential expression across biomes also involves genes that involved in immune and detoxification functions. Overall, genes that were differentially expressed showed a slight tendency to be more divergent and to display an excess of intermediate allele frequencies, relative to the remaining loci. Our results indicate that both differential expression in pathways involved in water conservation and geographical allelic variation are important in the occupation of contrasting habitats by the Patagonian olive mouse.

Climate change and body size trends in aquatic and terrestrial endotherms: Does habitat matter?

Several studies have claimed that reduction in body size comprises a nearly universal response to global warming; however, doubts about the validity of this pattern for endothermic species have been raised recently. Accordingly, we assessed temporal changes in body mass for 27 bird and 17 mammal species, to evaluate if a reduction in body size during the 20^th century is a widespread phenomenon among endothermic vertebrates. In addition, we tested if there are differences in the temporal change in size between birds and mammals, aquatic and terrestrial species, and the first and second half of the 20^th century. Overall, six species increased their body mass, 21 species showed no significant changes in size, and 17 species decreased their body mass during the 20th century. Temporal changes in body mass were similar for birds and mammals, but strongly differ between aquatic and terrestrial species: while most of the aquatic species increased or did not change in body mass, most terrestrial species decreased in size. In addition, we found that, at least in terrestrial birds, the mean value of the correlation between body mass and year of collection differs between the first half and the second half of the 20^th century, being close to zero for the former period but negative for the later one. To our knowledge, this is the first study showing that temporal changes in body mass differ between aquatic and terrestrial species in both mammals and birds.

Understanding evolutionary variation in basal metabolic rate: An analysis in subterranean rodents

Understanding how evolutionary variation in energetic metabolism arises is central to several theories in animal biology. Basal metabolic rate (BMR) -i.e., the minimum rate of energy necessary to maintain thermal homeostasis in endotherms- is a highly informative measure to increase our understanding, because it is determined under highly standardized conditions. In this study we evaluate the relationship between taxa- and mass-independent (residual) BMR and ten environmental factors for 34 subterranean rodent species. Both conventional and phylogenetically informed analyses indicate that ambient temperature is the major determinant of residual BMR, with both variables inversely correlated. By contrast, other environmental factors that have been shown to affect residual BMR in endotherms, such as habitat productivity and rainfall, were not significant predictors of residual BMR in this group of species. Then, the results for subterranean rodents appear to support a central prediction of the obligatory heat model (OHM), which is a mechanistic model aimed to explain the evolution of residual BMR. Specifically, OHM proposes that during the colonization of colder environments, individuals with greater masses of metabolically expensive tissues (and thus with greater BMR) are favored by natural selection due to the link between greater masses of metabolically expensive tissues and physiological capacities. This way, natural selection should establishes a negative correlation between ambient temperature and both internal organ size and residual BMR.

The effects of phenotypic plasticity and local adaptation on forecasts of species range shifts under climate change

Species are the unit of analysis in many global change and conservation biology studies; however, species are not uniform entities but are composed of different, sometimes locally adapted, populations differing in plasticity. We examined how intraspecific variation in thermal niches and phenotypic plasticity will affect species distributions in a warming climate. We first developed a conceptual model linking plasticity and niche breadth, providing five alternative intraspecific scenarios that are consistent with existing literature. Secondly, we used ecological niche-modeling techniques to quantify the impact of each intraspecific scenario on the distribution of a virtual species across a geographically realistic setting. Finally, we performed an analogous modeling exercise using real data on the climatic niches of different tree provenances. We show that when population differentiation is accounted for and dispersal is restricted, forecasts of species range shifts under climate change are even more pessimistic than those using the conventional assumption of homogeneously high plasticity across a species' range. Suitable population-level data are not available for most species so identifying general patterns of population differentiation could fill this gap. However, the literature review revealed contrasting patterns among species, urging greater levels of integration among empirical, modeling and theoretical research on intraspecific phenotypic variation.

Characterization of the kidney transcriptome of the South American olive mouse Abrothrix olivacea

Background

The olive mouse Abrothrix olivacea is a cricetid rodent of the subfamily Sigmodontinae that inhabits a wide range of contrasting environments in southern South America, from aridlands to temperate rainforests. Along its distribution, it presents different geographic forms that make the olive mouse a good focal case for the study of geographical variation in response to environmental variation. We chose to characterize the kidney transcriptome because this organ has been shown to be associated with multiple physiological processes, including water reabsorption.

Results

Transcriptomes of thirteen kidneys from individuals from Argentina and Chile were sequenced using Illumina technology in order to obtain a kidney reference transcriptome. After combining the reads produced for each sample, we explored three assembly strategies to obtain the best reconstruction of transcripts, TrinityNorm and DigiNorm, which include its own normalization algorithms for redundant reads removal, and Multireads, which simply consist on the assembly of the joined reads. We found that Multireads strategy produces a less fragmented assembly than normalization algorithms but recovers fewer number of genes. In general, about 15000 genes were annotated, of which almost half had at least one coding sequence reconstructed at 99% of its length. We also built a list of highly expressed genes, of which several are involved in water conservation under laboratory conditions using mouse models.

Conclusion

Based on our assembly results, Trinity's in silico normalization is the best algorithm in terms of cost-benefit returns; however, our results also indicate that normalization should be avoided if complete or nearly complete coding sequences of genes are desired. Given that this work is the first to characterize the transcriptome of any member of Sigmodontinae, a subfamily of cricetid rodents with about 400 living species, it will provide valuable resources for future ecological and evolutionary genomic analyses.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-446) contains supplementary material, which is available to authorized users.

Are the most plastic species the most abundant ones? An assessment using a fish assemblage

Few studies have evaluated phenotypic plasticity at the community level, considering, for example, plastic responses in an entire species assemblage. In addition, none of these studies have addressed the relationship between phenotypic plasticity and community structure. Within this context, here we assessed the magnitude of seasonal changes in digestive traits (seasonal flexibility), and of changes during short-term fasting (flexibility during fasting), occurring in an entire fish assemblage, comprising ten species, four trophic levels, and a 37-fold range in body mass. In addition, we analyzed the relationship between estimates of digestive flexibility and three basic assemblage structure attributes, i.e., species trophic position, body size, and relative abundance. We found that: (1) Seasonal digestive flexibility was not related with species trophic position or with body size; (2) Digestive flexibility during fasting tended to be inversely correlated with body size, as expected from scaling relationships; (3) Digestive flexibility, both seasonal and during fasting, was positively correlated with species relative abundance. In conclusion, the present study identified two trends in digestive flexibility in relation to assemblage structure, which represents an encouraging departure point in the search of general patterns in phenotypic plasticity at the local community scale.

Thermal conductance and basal metabolic rate are part of a coordinated system for heat transfer regulation

Thermal conductance measures the ease with which heat leaves or enters an organism's body. Although the analysis of this physiological variable in relation to climatic and ecological factors can be traced to studies by Scholander and colleagues, only small advances have occurred ever since. Here, we analyse the relationship between minimal thermal conductance estimated during summer (Cmin) and several ecological, climatic and geographical factors for 127 rodent species, in order to identify the exogenous factors that have potentially affected the evolution of thermal conductance. In addition, we evaluate whether there is compensation between Cmin and basal metabolic rate (BMR)-in such a way that a scale-invariant ratio between both variables is equal to one-as could be expected from the Scholander-Irving model of heat transfer. Our major findings are (i) annual mean temperature is the best single predictor of mass-independent Cmin. (ii) After controlling for the effect of body mass, there is a strong positive correlation between log10 (Cmin) and log10 (BMR). Further, the slope of this correlation is close to one, indicating an almost perfect compensation between both physiological variables. (iii) Structural equation modelling indicated that Cmin values are adjusted to BMR values and not the other way around. Thus, our results strongly suggest that BMR and thermal conductance integrate a coordinated system for heat regulation in endothermic animals and that summer conductance values are adjusted (in an evolutionary sense) to track changes in BMRs.

Contrasting patterns of morphological variation with dietary preferences in Micropogonias furnieri: insights from stable-isotope and digestive-trait analyses

The dietary preferences of populations of whitemouth croaker Micropogonias furnieri, which commonly inhabit estuarine and oceanic environments of the south-western Atlantic Ocean, were investigated using stable-isotope analysis and digestive traits, and compared with previous genetic and morphometric surveys of this species. Isotopic and C:N-derived data suggested that individuals from coastal lagoons are the most differentiated from the remaining localities surveyed. In contrast, the analysis of the digestive traits did not show the same differentiation pattern. The overall correlation between isotopic, molecular and morphological variations suggests that genetic and phenotypic differences among populations are accompanied by differential resource use, supporting the idea that selective forces could be playing an important role in population differentiation.

How does evolutionary variation in Basal metabolic rates arise? A statistical assessment and a mechanistic model

Metabolic rates are related to the pace of life. Hence, research into their variability at global scales is of vital importance for several contemporary theories in physiology, ecology, and evolution. Here we evaluated the effect of latitude, climate, primary productivity, habitat aridity, and species trophic habits, on mass-independent basal metabolic rates (BMRs) for 195 rodent species. The aims of this article were twofold. First, we evaluated the predictive power of different statistical models (via a model selection approach), using a dimensional reduction technique on the exogenous factor matrix to achieve a clear interpretation of the selected models. Second, we evaluated three specific predictions derived from a recently proposed hypothesis, herein called the "obligatory heat" model (OHM), for the evolution of BMR. Obtained results indicate that mean/minimum environmental temperature, rainfall/primary productivity and, finally, species trophic habits are, in this order, the major determinants of mass-independent BMR. Concerning the mechanistic causes behind this variation, obtained data agree with the predictions of the OHM: (1) mean annual environmental temperature was the best single predictor of residual variation in BMR, (2) herbivorous species have greater mass-independent metabolic rates, and tend to be present at high-latitude cold environments, than species in other trophic categories.

Latitudinal patterns in phenotypic plasticity and fitness-related traits: assessing the climatic variability hypothesis (CVH) with an invasive plant species

Phenotypic plasticity has been suggested as the main mechanism for species persistence under a global change scenario, and also as one of the main mechanisms that alien species use to tolerate and invade broad geographic areas. However, contrasting with this central role of phenotypic plasticity, standard models aimed to predict the effect of climatic change on species distributions do not allow for the inclusion of differences in plastic responses among populations. In this context, the climatic variability hypothesis (CVH), which states that higher thermal variability at higher latitudes should determine an increase in phenotypic plasticity with latitude, could be considered a timely and promising hypothesis. Accordingly, in this study we evaluated, for the first time in a plant species (Taraxacum officinale), the prediction of the CVH. Specifically, we measured plastic responses at different environmental temperatures (5 and 20°C), in several ecophysiological and fitness-related traits for five populations distributed along a broad latitudinal gradient. Overall, phenotypic plasticity increased with latitude for all six traits analyzed, and mean trait values increased with latitude at both experimental temperatures, the change was noticeably greater at 20° than at 5°C. Our results suggest that the positive relationship found between phenotypic plasticity and geographic latitude could have very deep implications on future species persistence and invasion processes under a scenario of climate change.

Latitudinal patterns in rodent metabolic flexibility

Macrophysiology is defined as the study of variation in physiological traits-including physiological trait flexibility-over large geographical and temporal scales, and the ecological implications of this variation. A classic example of a macrophysiological trend is the one emerging from the climatic variability hypothesis, which states that as the range of climatic fluctuation experienced by terrestrial animals increases with latitude, individuals at higher latitudes should be more plastic than individuals inhabiting lower latitudes. In this context, we evaluate the correlation between absolute metabolic scope during cold exposure (an instantaneous measure of metabolic flexibility) and different geographic and climatic variables for 48 rodent species. Conventional and phylogenetic informed analyses indicated a positive correlation between metabolic scope and geographic latitude. These findings, together with previous reports on latitudinal pattern in phenotypic flexibility, suggest that an increase in physiological flexibility with latitude may hold for many phenotypic traits.

Between-population differences in digestive flexibility in the olivaceous field mouse

The flexibility of digestive traits characterizes a standard model of physiological flexibility, demonstrating that animals adjust their digestive attributes in order to maximize overall energy return. Using an intraspecific experimental study, we evaluated the amount of flexibility in digestive tract mass and length in individuals from field mouse populations inhabiting semi-arid and temperate rain forest habitats and acclimated for six months to diets of different qualities. In accordance with the predictions of the theory of digestion, we observed a highly significant relationship between dietary variability and digestive flexibility in both specific digestive chambers and in the total digestive tract mass and length. Specifically, we found higher digestive plasticity in response to diet quality in rodents inhabiting southern temperate ecosystems with higher dietary variability in comparison to individuals from northern semi-arid habitats.

The effect of short- and long-term fasting on digestive and metabolic flexibility in the Andean toad, Bufo spinulosus

Hibernation in ectothermic animals was historically considered as a simple cold-induced torpor state resulting from the inability to maintain a high body temperature at low ambient temperatures. During the last decades this vision changed and nowadays there is a myriad of studies showing that hibernation implies different adjustments at the genetic, molecular, biochemical and cellular levels. However, studies oriented to evaluate changes of whole organism structure and physiology still are scarce, which is particularly true for amphibians that hibernate on land. Accordingly, in the Andean toad(Bufo spinulosus), we investigated the effect of short-term fasting and hibernation on the hydrolytic activity of digestive enzymes, histology of the small intestine, gross morphology of digestive and other internal organs and standard metabolic rate. Based on the pattern of size variation, internal organs may be grouped into those that were affected by both season and feeding condition (small intestine, stomach and liver), those that were only affected by season (fat bodies), those that were only affected by feeding condition(kidneys) and, finally, those that did not change between the three groups(large intestine, heart and lungs). Hydrolytic activity of maltase, trehalase and aminopeptidase-N followed the same pattern of variation(feeding>fasting>hibernating toads), although the change for the latter enzyme was less noticeable than for the disaccharidases. Enzymatic adjustments were correlated with changes in small intestine histology: villus and enterocyte height increased from hibernating to fasting and more markedly from fasting to feeding toads. Metabolic rate decreased during hibernation to 7.8%(at 5°C) and 13.6% (at 15°C) of summer values, which is one of the highest metabolic depressions reported for any ectothermic vertebrate. Our results suggest that amphibian persistence in highly seasonal environments is related to a large capacity of phenotypic flexibility at different organisational levels; an ability that may be related to the extensive ranges of temporal existence and geographic distribution of these vertebrates.

Physiological flexibility in the Andean lizard Liolaemus bellii: seasonal changes in energy acquisition, storage and expenditure

According to the "barrel model", an organism may be represented by a container, with input energy constraints (foraging, digestion, and absorption) symbolized by funnels connected in tandem, and energy outputs (maintenance, growth, and reproduction) symbolized by a series of spouts arranged in parallel. Animals can respond to changes in environmental conditions, through adjustments in the size of the funnels, the fluid stored inside the barrel, or the output flow through the spouts. In the present study, we investigate the interplay among these processes through the analysis of seasonal changes in organ size and metabolic rate in a lizard species (Liolaemus bellii) that inhabits extremely seasonal environments in the Andes range. We found that digestive organ size showed the greatest values during spring and summer, that is, during the foraging seasons. Energy reserves were larger during summer and autumn, and then decreased through winter and spring, which was correlated with overwintering maintenance and reproductive costs. Standard metabolic rate was greater during the high-activity seasons (spring and summer), but this increase was only noticeable at higher environmental temperatures. The ability of many lizard species to reduce their maintenance cost during the cold months of the year, beyond what is expected from temperature decrease, is probably related to their success in coping with highly fluctuating environments. Here, we demonstrate that this ability is correlated with high physiological flexibility, which allows animals to adjust energy acquisition, storing and expenditure processes according to current environmental conditions.

A new species of Melanophryniscus (Anura, Bufonidae) from Uruguay

A new species of bufonid toad of the genus Melanophryniscus from northern Uruguay is described. It is included in the M. moreirae group and its external morphology is similar to Melanophryniscus sanmartini. Melanophryniscus sp. nov. is distinguished by having a light brown dorsal coloration with six darker longitudinal glandular ridges on the dorsal surface of the body and the gular region uniformly black.

The effect of diet quality on physiological and life-history traits in the harvestman Pachylus paessleri

The balance between energy acquisition and expenditure is critical to the survival and reproductive success of animals. Here we investigate the long-term effects of diet quality on physiological and life-history flexibility in the harvestman, Pachylus paessleri. We used cow meal as a protein-rich diet and potatoes as a carbohydrate-rich diet in order to reproduce two extreme conditions regarding food quality in harvestmen natural habitat. As proxy variables of the energy expenditure process, we quantified standard metabolic rate (maintenance), changes in body mass (somatic condition), and fecundity (reproduction). We found that animals consuming the protein-rich diet were able to increase both their body condition and fecundity. However, the increment in these two life-history traits was correlated with higher maintenance costs. In contrast, the carbohydrate-rich diet did not provide enough specific nutrients for reproductive events, although it may have allowed animals to survive for a long time. Thus, according to the quality of the diet available in the environment, harvestman females can adopt different life-history strategies correlated with phenotypic adjustments at anatomical and physiological levels. In the Mediterranean region, spatial and temporal changes in food quality are typical, so greater phenotypic flexibility is expected to cope with this kind of environmental variation.

Adjusting energy expenditures to energy supply: food availability regulates torpor use and organ size in the Chilean mouse-opossum Thylamys elegans

We studied how food abundance and consumption regulates torpor use and internal organ size in the Chilean mouse-opossum Thylamys elegans (Dielphidae), a small nocturnal marsupial, endemic in southern South America. We predicted that exposure to food rations at or above the minimum energy levels necessary for maintenance would not lead to any signs of torpor, while reducing food supply to energy levels below maintenance would lead to marked increases in frequency, duration and depth of torpor bouts. We also analyzed the relationship between food availability and internal organ mass. We predicted a positive relationship between food availability and internal organ size once the effect of body size is removed. Animals were randomly assigned to one of two groups and fed either 70, 100 or 130% of their daily energy requirement (DER). We found a positive and significant correlation between %DER and body temperature, and also between %DER and minimum body temperature. In contrast, for torpor frequency, duration and depth, we found a significant negative correlation with %DER. Finally, we found a significant positive correlation between the %DER and small intestine and ceacum dry mass. We demonstrate that when food availability is limited, T. elegans has the capacity to reduce their maintenance cost by two different mechanisms, that is, increasing the use of torpor and reducing organ mass.

Some vaguely explored (but not trivial) costs of tail autotomy in lizards

Lizard tail autotomy is considered an efficient anti-predator strategy that allows animals to escape from a predator attack. However, since the tail also is involved in many alternative functions, tailless animals must cope with several costs following autotomy. Here we explicitly evaluate the consequences of tail autotomy for two costs that have been virtually unexplored: 1. we test whether the anatomical change that occurs after tail loss causes a reduction in the role of the tail as a distraction mechanism to predators; 2. we analyzed whether tail synthesis comprises an energetically costly process in itself, by directly comparing the cost of maintenance before and after autotomy. We found that original tails displace further and at greater velocity than regenerated tails, indicating that the anti-predation responses of a lizard probably changes according to whether its tail is original or regenerated. With regard to the energetic cost of tail synthesis, we observed a significant increase in the standard metabolic rate, which rose 36% in relation to the value recorded prior to tail loss. This result suggests that the energetic cost of tail synthesis itself could be enough to affect lizard fitness.

Digestive morphology and enzyme activity in the Andean toad Bufo spinulosus: hard-wired or flexible physiology?

Gut plasticity is a trait with implications on animal performance. However, and despite their importance as study models in physiology, research on gut flexibility in amphibians is scarce. In the present work, we analyse digestive adjustments of Bufo spinulosus adult individuals to cope with changes in diet quality and quantity at two organizational levels (i.e., digestive morphology and enzymes). We found that changes in gut size are related to the amount of food ingested, but not to diet composition. This is in agreement with "the gut seasonal change" hypothesis and offers a proximal explanation for this change. Digestive enzymatic activity (maltase and aminopeptidase-N) did not change with diet quality or quantity, which agrees with the hypothesis of "hard-wired physiology in adult amphibians". Both hypotheses are in agreement with the general theoretical framework of gut phenotypic flexibility when interpreted in light of amphibian natural history. In addition, our results indicate that the correlation between feeding frequency and the level of gut up-regulation proposed for interspecific comparisons may also be found at the intraspecific level.

Dynamic digestive responses to increased energy demands in the leaf-eared mouse (Phyllotis darwini)

A major area of interest in comparative physiology has been to understand how animals cope with changing environmental demands in time and space. The digestive system has been identified as one of the more sensitive systems to changes in environmental conditions. However, most research on this topic has evaluated these effects during peak energetic demands, which do not allow for evaluation of the dynamics of the digestive response along a more natural continuous gradient of environmental conditions. We examined phenotypic flexibility in digestive responses of the leaf-eared mouse Phyllotis darwini to increments in total energy demands (via sequential exposure to 26, 12 and 0 degrees C). Additionally, we evaluated the effect of a moderate energy demand (12 degrees C) over three different time periods (7, 17 and 27 days) on digestive traits. Moderate increases in energy demand were associated with changes in the distribution of digesta in the gut, whereas higher increases in energy demand involved increases in the tissue mass of digestive organs. Time-course analysis showed that at 12 degrees C practically all digestive variables reached stable values within 7 days, which is in agreement with empirical data and theoretical deductions from cellular turnover rates. We conclude that although the input of energy and nutrients into the digestive tract is typically periodic, many aspects of digestive physiology are likely to be flexible in response to environmental variability over both short-term (daily) and long-term (seasonal) time scales.

Digestive Phenotypic Flexibility in Post-Metamorphic Amphibians: Studies on a Model Organism

Studies of phenotypic flexibility are central to the understanding of evolutionary and comparative physiology. Research conducted on many vertebrate species has shown that the digestive system is highly responsive and sensitive to environmental cues. However, amphibians, which are a standard and classic model organism for the study of many physiological processes, have been poorly considered in the study of ecological consequences on digestive flexibility. Here we review and analyze the current information on this topic for amphibians. We identify three major bodies of empirical evidence: a) seasonal changes in gut development, b) lack of dietary modulation of gut attributes in adult individuals, c) a relationship between feeding habits and the magnitude of digestive performance regulation. Once the natural history characteristics of the species under study are taken into account, all the evidence is in full agreement with the predictions of digestive theory. We propose that evolutionary and comparative physiology could benefit greatly from the study of phenotypic flexibility in amphibians.

Pinniped diets inferred from scats: analysis of biases in prey occurrence

The diets of pinnipeds have often been used to study their ecology and resource dynamics and in ecosystem monitoring. Scat analysis is now the most widely used method of inferring the diets of pinnipeds. Using a mathematical model the present study explores the expected biases in prey occurrence that are related to prey mass, proportion of loss of remains, predator assimilation efficiency, size of the scats collected, and meal size. With the exception of meal size, variation in parameters implied variation in the probability of biases in prey occurrence. Biases were asymmetric among prey sizes, but in contrast with previous ideas, the results indicate that small prey have smaller biases than large ones do. Furthermore, the number of scats required to detect a large prey was notably larger than that needed to detect a small prey with the same occurrence. The present study is not limited to a particular pinniped or prey species, so it has the potential to represent a general framework for interpreting the results of scat analysis in pinnipeds. The model complements empirical studies, advancing our comprehension of biases associated with prey occurrence in dietary studies of pinnipeds.