Seasonality and brain size are negatively associated in frogs: evidence for the expensive brain framework

Salinization and low-dose levels of pesticides alter brain shape of larval amphibians

Wetland communities are increasingly threatened by multiple stressors simultaneously, such as pesticides and salinization. We examined the effects of ecologically-relevant exposures to broad-spectrum insecticides and salinization on amphibian neurodevelopment, which is strongly linked to how organisms respond behaviorally to environmental change. Prior research showed that exposure to trace concentrations of an organophosphate pesticide (chlorpyrifos) altered the brain shape and behavior of larval and metamorphic amphibians. It is unknown whether brain shape is altered by additional pesticides and road salt. Using outdoor mesocosms, we tested whether salt (NaCl) and representatives from three pesticide families (organophosphates, pyrethroids, and neonicotinoids) altered tadpole (Lithobates pipiens) brain shape. Of the two organophosphates, chlorpyrifos induced relatively longer telencephalon lengths relative to body mass, consistent with previous studies, but malathion had no effect on brain shape. Of the two pyrethroids, permethrin, but not cypermethrin, increased telencephalon length. For the neonicotinoids, there were marginally significant effects of imidacloprid and thiamethoxam on telencephalon length. Thus, the impacts of pesticides on brain shape was not dictated by pesticide family. Exposure to relatively high concentrations of salt resulted in brains that were less wide but had longer optic tecta. Although we failed to find strong interactive effect of salt with pesticides, there was some weak, nonsignificant, evidence that exposure to salt masked responses to pesticides. Together, our results indicate that environmentally realistic levels of pesticides and salinization can alter larval brain shape. Our study highlights the importance of studying the impacts of naturally-occurring levels of pesticides and salinization on vertebrate neural development.

Does the expensive brain hypothesis apply to amphibians and reptiles?

Vertebrate brains show extensive variation in relative size. The expensive brain hypothesis argues that one important source of this variation is linked to a species' ability to generate the energy required to sustain the brain, especially during periods of unavoidable food scarcity. Here we ask whether this hypothesis, tested so far in endothermic vertebrates, also applies to ectotherms, where ambient temperature is an additional major aspect of energy balance. Phylogenetic comparative analyses of reptiles and amphibians support the hypothesis. First, relative brain size increases with higher body temperature in those species active during the day that can gain free energy by basking. Second, relative brain size is smaller among nocturnal species, which generally face less favorable energy budgets, especially when maintaining high body temperature. However, we do not find an effect of seasonal variation in ambient temperature or food on brain size, unlike in endotherms. We conclude that the factors affecting energy balance in ectotherms and endotherms are overlapping but not identical. We therefore discuss the idea that when body temperatures are seasonally very low, cognitive benefits may be thwarted and selection on larger brain size may be rare. Indeed, mammalian hibernators may show similarities to ectotherms.

How hibernation in frogs drives brain and reproductive evolution in opposite directions

Environmental seasonality can promote the evolution of larger brains through cognitive and behavioral flexibility but can also hamper it when temporary food shortage is buffered by stored energy. Multiple hypotheses linking brain evolution with resource acquisition and allocation have been proposed for warm-blooded organisms, but it remains unclear how these extend to cold-blooded taxa whose metabolism is tightly linked to ambient temperature. Here, we integrated these hypotheses across frogs and toads in the context of varying brumation (hibernation) durations and their environmental correlates. We showed that protracted brumation covaried negatively with brain size but positively with reproductive investment, likely in response to brumation-dependent changes in the socio-ecological context and associated selection on different tissues. Our results provide novel insights into resource allocation strategies and possible constraints in trait diversification, which may have important implications for the adaptability of species under sustained environmental change.

Geographic Variation in Organ Size in a Toad (Duttaphrynus melanostictus)

Adaptive evolution is the process by which organisms change their morphological, physiological and biochemical characteristics to adapt to different environments during long-term natural selection. Especially, researching variation in organ size can provide important insights into morphological adaptation in amphibians. In this study, we comparatively studied differences in organ sizes (heart, lungs, liver, gallbladder, kidneys, spleen, digestive tract, testes and brain) among five geographical populations of the Asian common toad Duttaphrynus melanostictus. Our results revealed significant variations in the size of these nine specific organs among the populations. Notably, we observed a significant positive correlation between the relative size of the testes and latitude and/or altitude. However, no correlation was found between the relative size of the heart and the length of the digestive tract with altitude across populations, respectively, contradicting Hesse's rule and the digestion theory. These findings suggest that our study does not provide substantial theoretical support for the adaptive evolution of organ size in this particular toad species, but rather contributes to the understanding of the evolution and adaptations of species' different environmental conditions. Further research is warranted to delve deeper into the factors influencing organ size in amphibian populations.

Exploring potential drivers of brain size variation in the electric fish Brachyhypopomus occidentalis

Characterizing the factors that shape variation in brain size in natural populations is crucial to understanding the evolution of brain size in animals. Here, we explore how relative brain size and brain allometry vary with drainage, predation risk and sex in natural populations of the electric knifefish Brachyhypopomus occidentalis. Fish were sampled from high and low predation risk sites within two independent river drainages in eastern and central Panamá. Overall, we observed low variation in brain-body size allometric slopes associated with drainage, predation risk and sex category. However, we observed significant differences in allometric intercepts between predation risk sites. We also found significant differences in relative brain mass associated with drainage, as well as significant differences in absolute brain mass associated with drainage, predation risk and sex category. Our results suggest potential constraints in brain-body allometry across populations of B. occidentalis. However, both drainage and predation risk may be playing a role in brain mass variation among populations. We suggest that variation in brain mass in electric fishes is affected by multiple extrinsic and intrinsic factors, including geography, environmental complexity, social interaction and developmental or functional constraints.

Testis Size Variation and Its Environmental Correlates in Andrew's Toad (Bufo andrewsi)

Reproductive investments influenced by environmental conditions vary extensively among geographically distinct populations. However, investigations of patterns of intraspecific variation in male reproductive investments and the mechanisms shaping this variation in anurans remain scarce. Here, we focused on the variation in testis size in 14 populations of the Andrew's toad Bufo andrewsi, a species with weak dispersal ability but wide distribution in southwestern China, to establish whether male reproductive investment varies on an environmental gradient. Our analysis revealed a significant variation in relative testis size across populations, and a positive correlation between testis size and body condition. We, however, found no geographic trends explaining the variability in the testis size. The relative testis size did not increase with increasing latitude or altitude. We also found no relationship between relative testis size and rainfall, but a negative correlation with the coefficient of variation of temperature, with larger testes under stable environments. These findings suggest that the decreased male reproductive investment of this species may be a consequence of harsher or fluctuating environmental conditions.

Learning with lacertids: Studying the link between ecology and cognition within a comparative framework

Cognition is an essential tool for animals to deal with environmental challenges. Nonetheless, the ecological forces driving the evolution of cognition throughout the animal kingdom remain enigmatic. Large-scale comparative studies on multiple species and cognitive traits have been advanced as the best way to facilitate our understanding of cognitive evolution, but such studies are rare. Here, we tested 13 species of lacertid lizards (Reptilia: Lacertidae) using a battery of cognitive tests measuring inhibitory control, problem-solving, and spatial and reversal learning. Next, we tested the relationship between species' performance and (a) resource availability (temperature and precipitation), habitat complexity (Normalized Difference Vegetation Index), and habitat variability (seasonality) in their natural habitat and (b) their life history (size at hatching and maturity, clutch size, and frequency). Although species differed markedly in their cognitive abilities, such variation was mostly unrelated to their ecology and life history. Yet, species living in more variable environments exhibited lower behavioral flexibility, likely due to energetic constrains in such habitats. Our standardized protocols provide opportunities for collaborative research, allowing increased sample sizes and replication, essential for moving forward in the field of comparative cognition. Follow-up studies could include more detailed measures of habitat structure and look at other potential selective drivers such as predation.

Relationship between brain size and digestive tract length support the expensive-tissue hypothesis in Feirana quadranus

The brain is among the most energetically costly organs in the vertebrate body, while the size of the brain varies within species. The expensive-tissue hypothesis (ETH) predicts that increasing the size of another costly organ, such as the gut, should compensate for the cost of a small brain. Here, the ETH was tested by analyzing the relationship between brain size variation and digestive tract length in a Swelled-vented frog (Feirana quadranus). A total of 125 individuals across 10 populations ranging from 586 to 1,702 m a.s.l. from the Qinling-Daba Mountains were sampled. With the increase in altitude, the brain size decreases and the digestive tract length increases. Different brain regions do not change their relative size in a consistent manner. The sizes of telencephalon and cerebellum decrease with the increase in altitude, while the olfactory nerve increases its size at high altitudes. However, the olfactory bulb and optic tectum have no significant relationship with altitude. After controlling for snout-vent length (SVL), a significant negative correlation could be found between brain size and digestive tract length in F. quadranus. Therefore, the intraspecific variation of brain size follows the general patterns of ETH in this species. The results suggest that annual mean temperature and annual precipitation are environmental factors influencing the adaptive evolution of brain size and digestive tract length. This study also suggests that food composition, activity times, and habitat complexity are the potential reasons driving the adaptive evolution of brain size and digestive tract length.

The impact of environmental factors on the evolution of brain size in carnivorans

The reasons why some animals have developed larger brains has long been a subject of debate. Yet, it remains unclear which selective pressures may favour the encephalization and how it may act during evolution at different taxonomic scales. Here we studied the patterns and tempo of brain evolution within the order Carnivora and present large-scale comparative analysis of the effect of ecological, environmental, social, and physiological variables on relative brain size in a sample of 174 extant carnivoran species. We found a complex pattern of brain size change between carnivoran families with differences in both the rate and diversity of encephalization. Our findings suggest that during carnivorans’ evolution, a trade-off have occurred between the cognitive advantages of acquiring a relatively large brain allowing to adapt to specific environments, and the metabolic costs of the brain which may constitute a disadvantage when facing the need to colonize new environments.

The brain size of carnivores has evolved to balance a trade-off between increased cognitive function and increased metabolic cost.

Down a Rabbit Hole: Burrowing Behaviour and Larger Home Ranges are Related to Larger Brains in Leporids

Studies on the evolution of brain size variation usually focus on large clades encompassing broad phylogenetic groups. This risks introducing ‘noise’ in the results, often obscuring effects that might be detected in less inclusive clades. Here, we focus on a sample of endocranial volumes (endocasts) of 18 species of rabbits and hares (Lagomorpha: Leporidae), which are a discrete radiation of mammals with a suitably large range of body sizes. Using 60 individuals, we test five popular hypotheses on brain size and olfactory bulb evolution in mammals. We also address the pervasive issue of missing data, using multiple phylogenetic imputations as to conserve the full sample size for all analyses. Our analyses show that home range and burrowing behaviour are the only predictors of leporid brain size variation. Litter size, which is one of the most widely reported constraints on brain size, was unexpectedly not associated with brain size. However, a constraining effect may be masked by a strong association of litter size with temperature seasonality, warranting further study. Lastly, we show that unreasonable estimations of phylogenetic signal (Pagel’s lamba) warrant additional caution when using small sample sizes, such as ours, in comparative studies.

Geographical variation of organ size in Andrew’s toad (Bufo andrewsi)

Phenotypic variation of morphological and physiological traits is assumed to be generated from spatial heterogeneity in environments, and it has been regarded as an important concern domain in evolutionary biology. Organs display markedly size variation among populations along environmental gradients and this variation is associated with changes in oxygen supply and energy demands. Here, we investigated geographical variation in the relative size of organs (i.e., brain, heart, lung, gallbladder, livers, spleen, kidneys, and digestive tract) among 14 populations of Andrew’s toad (Bufo andrewsi) transcending an elevational range from 864 to 2,367 m, and spanning 8° latitude. We found that although the relative sizes of the eight specific organs varied significantly among populations, none organ size was affected by altitude and latitude. However, based on the combined the new data and published data we found a negative relationship between the relative size of the heart and latitude, contrasting to the Hesse’s rule. We also found that the relative size of livers was positively linked to latitude, suggesting that more energy demands and intakes due to slower metabolism in high latitude shaped the evolution of larger livers.

Allometric model of brain morphology of Hemiculter leucisculus and its variation along climatic gradients

Prior studies on Hemiculter leucisculus, which is a widespread native fish in China, mainly focused on its growth, feeding habits, and individual fecundity, but few have investigated the brain. In this research, we explored the developmental patterns of the Hemiculter leucisculus brain and found the brain showed allometry through sample time points and three age groups. At the same time, we found that the brain varied along climatic gradients. The volumes of the olfactory bulbs, telencephalic lobes, optic tectum, corpus cerebelli, and total brain in the south were larger than those in the north, while the volume of the hypothalamus in the north was larger than in the south. This study provides a view for the in-depth study of the acclimatized mechanism of the teleost brain, lays a foundation for the further study of evolutionary ecology, and provides a reference for the phenotypic plasticity of the teleost brain.

The economics of brain size evolution in vertebrates

Across the animal kingdom, we see remarkable variation in brain size. This variation has even increased over evolutionary time. Traditionally, studies aiming to explain brain size evolution have looked at the fitness benefits of increased brain size in relation to its increased cognitive performance in the social and/or ecological domain. However, brains are among the most energetically expensive tissues in the body and also require an uninterrupted energy supply. If not compensated, these energetic demands inevitably lead to a reduction in energy allocation to other vital functions. In this review, we summarize how an increasing number of studies show that to fully comprehend brain size evolution and the large variation in brain size across lineages, it is important to look at the economics of brains, including the different pathways through which the high energetic costs of brains can be offset. We further show how numerous studies converge on the conclusion that cognitive abilities can only drive brain size evolution in vertebrate lineages where they result in an improved energy balance through favourable ecological preconditions. Cognitive benefits that do not directly improve the organism's energy balance can only be selectively favoured when they produce such large improvements in reproduction or survival that they outweigh the negative energetic effects of the large brain.

Testing the Role of Environmental Harshness and Sexual Selection in Limb Muscle Mass in Anurans

Sexual dimorphism is regarded as the consequence of differential responses by males and females to selection pressures. Limb muscle plays a very important role during amplexus, which is likely to be under both natural and sexual selection in anurans. Here, we studied the effects of natural and sexual selection on limb muscle mass in males and females across 64 species of anurans. The results showed that there were non-significant differences in relative limb muscle mass between the sexes among species, exhibiting no sexual dimorphism in limb muscle. Absolute and relative limb muscle mass positively displayed correlations with snout-vent length (SVL)for both sexes. However, neither male-biased operational sex ratio (OSR) nor environmental harshness [e.g., coefficient of variation (CV) in temperature and CV in rainfall] can explain relative limb muscle mass (e.g., forelimb, hindlimb, and total limb muscle) within each sex. The findings suggest that environmental harshness and sexual selection cannot play important roles in promoting variations in limb muscle among anuran species.

Color Discrimination Provides Insight into the Relationship between Personality Cognition and Brain Morphology in the Western Mosquitofish (Gambusia affinis)

Recent studies on the differences in cognitive ability between individuals focused on two aspects: one is whether the individual differences in cognitive ability are related to brain size, the other is whether they pertain to certain personality traits. To explore these two hypotheses, we tested the personality traits, cognitive abilities, and brain volumes of western mosquitofish (Gambusia affinis). First, a color preference test was conducted to select two unbiased colors for G. affinis for subsequent cognitive tests. The results showed that G. affinis had a great preference for red and green to yellow and blue; therefore, the red-green combination was selected for the study of cognitive abilities. Then, we explored the relationship among cognition, personality, and brain morphology through cognitive abilities tests, personality traits, and brain volume measurements. We found that there was a trade-off among cognition, personality, and brain morphology. For example, more active individuals found food faster, but had also poor memory; Those individuals with larger corpus cerebelli were bolder while they were less likely to find food; The individuals that found food faster were more active and had a smaller inferior lobe. The color preference test provides a reliable way for selecting unbiased colors for behavioral studies in G. affinis. Meanwhile, our study indicates that there exists a balance mechanism among cognition, personality, and brain morphology.

Soldier neural architecture is temporarily modality-specialized but poorly predicted by repertoire size in the stingless bee Tetragonisca angustula

Individual heterogeneity within societies provides opportunities to test hypotheses about adaptive neural investment in the context of group cooperation. Here we explore neural investment in defense specialist soldiers of the eusocial stingless bee (Tetragonisca angustula) which are age sub-specialized on distinct defense tasks and have an overall higher lifetime task repertoire than other sterile workers within the colony. Consistent with predicted behavioral demands, soldiers had higher relative visual (optic lobe) investment than non-soldiers but only during the period when they were performing the most visually demanding defense task (hovering guarding). As soldiers aged into the less visually demanding task of standing guarding this difference disappeared. Neural investment was otherwise similar across all colony members. Despite having larger task repertoires, soldiers had similar absolute brain size and smaller relative brain size compared to other workers, meaning that lifetime task repertoire size was a poor predictor of brain size. Both high behavioral specialization in stable environmental conditions and reassignment across task groups during a crisis occur in T. angustula. The differences in neurobiology we report here are consistent with these specialized but flexible defense strategies. This work broadens our understanding of how neurobiology mediates age and morphological task specialization in highly cooperative societies. This article is protected by copyright. All rights reserved.

Neuroanatomical differentiation associated with alternative reproductive tactics in male arid land bees, Centris pallida and Amegilla dawsoni

Alternative reproductive tactics (ARTs) occur when there is categorical variation in the reproductive strategies of a sex within a population. These different behavioral phenotypes can expose animals to distinct cognitive challenges, which may be addressed through neuroanatomical differentiation. The dramatic phenotypic plasticity underlying ARTs provides a powerful opportunity to study how intraspecific nervous system variation can support distinct cognitive abilities. We hypothesized that conspecific animals pursuing ARTs would exhibit dissimilar brain architecture. Dimorphic males of the bee species Centris pallida and Amegilla dawsoni use alternative mate location strategies that rely primarily on either olfaction (large-morph) or vision (small-morph) to find females. This variation in behavior led us to predict increased volumes of the brain regions supporting their primarily chemosensory or visual mate location strategies. Large-morph males relying mainly on olfaction had relatively larger antennal lobes and relatively smaller optic lobes than small-morph males relying primarily on visual cues. In both species, as relative volumes of the optic lobe increased, the relative volume of the antennal lobe decreased. In addition, A. dawsoni large males had relatively larger mushroom body lips, which process olfactory inputs. Our results suggest that the divergent behavioral strategies in ART systems can be associated with neuroanatomical differentiation.

Brain size evolution in small mammals: test of the expensive tissue hypothesis

Brain size exhibits significant changes within and between species. Evolution of large brains can be explained by the need to improve cognitive ability for processing more information in changing environments. However, brains are among the most energetically expensive organs. Enlarged brains can impose energetic demands that limit brain size evolution. The expensive tissue hypothesis (ETH) states that a decrease in the size of another expensive tissue, such as the gut, should compensate for the cost of a large brain. We studied the interplay between energetic limitations and brain size evolution in small mammals using phylogenetically generalized least squares (PGLS) regression analysis. Brain mass was not correlated with the length of the digestive tract in 37 species of small mammals after correcting for phylogenetic relationships and body size effects. We further found that the evolution of a large brain was not accompanied by a decrease in male reproductive investments into testes mass and in female reproductive investment into offspring number. The evolution of brain size in small mammals is inconsistent with the prediction of the ETH.

Brain size variation along altitudinal gradients in the Asiatic Toad (Bufo gargarizans)

Size changes in brain and brain regions along altitudinal gradients provide insight into the trade-off between energetic expenditure and cognitive capacity. We investigated the brain size variations of the Asiatic Toad (Bufo gargarizans) across altitudes from 700 m to 3,200 m. A total of 325 individuals from 11 sites and two transects were sampled. To reduce confounding factors, all sampling sites within each transect were within a maximum distance of 85 km and an altitudinal difference close to 2,000 m. Brains were dissected, and five regions were both measured directly and with 3D CT scan. There is a significant negative correlation between the relative whole-brain volume (to snout-vent length) and altitude. Furthermore, the relative volumes (to whole-brain volume) of optic tectum and cerebellum also decrease along the altitudinal gradients, while the telencephalon increases its relative volume along the gradients. Therefore, our results are mostly consistent with the expensive brain hypothesis and the functional constraint hypothesis. We suggest that most current hypotheses are not mutually exclusive and data supporting one hypothesis are often partially consistent with others. More studies on mechanisms are needed to explain the brain size evolution in natural populations.

Testing hypotheses of marsupial brain size variation using phylogenetic multiple imputations and a Bayesian comparative framework

Considerable controversy exists about which hypotheses and variables best explain mammalian brain size variation. We use a new, high-coverage dataset of marsupial brain and body sizes, and the first phylogenetically imputed full datasets of 16 predictor variables, to model the prevalent hypotheses explaining brain size evolution using phylogenetically corrected Bayesian generalized linear mixed-effects modelling. Despite this comprehensive analysis, litter size emerges as the only significant predictor. Marsupials differ from the more frequently studied placentals in displaying a much lower diversity of reproductive traits, which are known to interact extensively with many behavioural and ecological predictors of brain size. Our results therefore suggest that studies of relative brain size evolution in placental mammals may require targeted co-analysis or adjustment of reproductive parameters like litter size, weaning age or gestation length. This supports suggestions that significant associations between behavioural or ecological variables with relative brain size may be due to a confounding influence of the extensive reproductive diversity of placental mammals.

Effects of weather and season on human brain volume

We present an exploratory cross-sectional analysis of the effect of season and weather on Freesurfer-derived brain volumes from a sample of 3,279 healthy individuals collected on two MRI scanners in Hartford, CT, USA over a 15 year period. Weather and seasonal effects were analyzed using a single linear regression model with age, sex, motion, scan sequence, time-of-day, month of the year, and the deviation from average barometric pressure, air temperature, and humidity, as covariates. FDR correction for multiple comparisons was applied to groups of non-overlapping ROIs. Significant negative relationships were found between the left- and right- cerebellum cortex and pressure (t = -2.25, p = 0.049; t = -2.771, p = 0.017). Significant positive relationships were found between left- and right- cerebellum cortex and white matter between the comparisons of January/June and January/September. Significant negative relationships were found between several subcortical ROIs for the summer months compared to January. An opposing effect was observed between the supra- and infra-tentorium, with opposite effect directions in winter and summer. Cohen’s d effect sizes from monthly comparisons were similar to those reported in recent psychiatric big-data publications, raising the possibility that seasonal changes and weather may be confounds in large cohort studies. Additionally, changes in brain volume due to natural environmental variation have not been reported before and may have implications for weather-related and seasonal ailments.

Brain Reconstruction Across the Fish-Tetrapod Transition; Insights From Modern Amphibians

The fish-tetrapod transition (which incorporates the related fin-limb and water-land transitions) is celebrated as one of the most important junctions in vertebrate evolution. Sarcopterygian fishes (the “lobe-fins”) are today represented by lungfishes and coelacanths, but during the Paleozoic they were much more diverse. It was some of these sarcopterygians, a lineage of the tetrapodomorph fishes, that gave rise to tetrapods (terrestrial vertebrates with limbs bearing digits). This spectacular leap took place during the Devonian Period. Due to the nature of preservation, it is the hard parts of an animal’s body that are most likely to fossilize, while soft tissues such as muscular and brain tissues, typically fail to do so. Thus, our understanding of the adaptations of the hard skeletal structures of vertebrates is considerably greater than that of the soft tissue systems. Fortunately, the braincases of early vertebrates are often ossified and thereby have the potential to provide detailed morphological information. However, the correspondence between brain and endocast (an internal mold of the cavity) has historically been considered poor in most “lower” vertebrates and consequently neglected in such studies of brain evolution. Despite this, recent work documenting the spatial relationship in extant basal sarcopterygians (coelacanth, lungfish, axolotl, and salamander) has highlighted that this is not uniformly the case. Herein, we quantify and illustrate the brain-endocast relationship in four additional extant basal tetrapod exemplars: neobatrachian anurans (frogs) Breviceps poweri and Ceratophrys ornata; and gymnophionans (caecilians) Gegeneophis ramaswamii and Rhinatrema bivittatum. We show that anurans and caecilians appear to have brains that fill their endocasts to a similar degree to that of lungfishes and salamanders, but not coelacanth. Ceratophrys has considerably lower correspondence between the brain and endocast in the olfactory tract and mesencephalic regions, while Breviceps has low correspondence along its ventral endocranial margin. The brains of caecilians reflect their endocasts most closely (vol. ∼70%). The telencephalon is tightly fitted within the endocast in all four taxa. Our findings highlight the need to adequately assess the brain-endocast relationship in a broad range of vertebrates, in order to inform neural reconstructions of fossil taxa using the Extant Phylogenetic Bracket approach and future studies of brain evolution.

Mandibular musculature constrains brain-endocast disparity between sarcopterygians

The transition from water to land by the earliest tetrapods in the Devonian Period is seen as one of the greatest steps in evolution. However, little is understood concerning changes in brain morphology over this transition. Here, we determine the brain-braincase relationship in fishes and basal lissamphibians as a proxy to elucidate the changes that occurred over the fish-tetrapod transition. We investigate six basal extant sarcopterygians spanning coelacanths to salamanders (Latimeria chalumnae, Neoceratodus, Protopterus aethiopicus, P. dolloi, Cynops, Ambystoma mexicanum) using micro-CT and MRI and quantify the brain-braincase relationship in these extant taxa. Our results show that regions of lowest brain-endocast disparity are associated with regions of bony reinforcement directly adjacent to masticatory musculature for the mandible except in Neoceratodus and Latimeria. In Latimeria this deviation from the trend can be accounted for by the possession of an intracranial joint and basicranial muscles, whereas in Neoceratodus difference is attributed to dermal bones contributing to the overall neurocranial reinforcement. Besides Neoceratodus and Latimeria, regions of low brain-endocast disparity occur where there is less reinforcement away from high mandibular muscle mass, where the trigeminal nerve complex exits the braincase and where endolymphatic sacs occupy space between the brain and braincase wall. Despite basal tetrapods possessing reduced adductor muscle mass and a different biting mechanism to piscine sarcopterygians, regions of the neurocranium lacking osteological reinforcement in the basal tetrapods Lethiscus and Brachydectes broadly correspond to regions of high brain-endocast disparity seen in extant taxa.

Relative Brain Size Is Predicted by the Intensity of Intrasexual Competition in Frogs

Competition over mates is a powerful force shaping trait evolution. For instance, better cognitive abilities may be beneficial in male-male competition and thus be selected for by intrasexual selection. Alternatively, investment in physical attributes favoring male performance in competition for mates may lower the resources available for brain development, and more intense male mate competition would coincide with smaller brains. To date, only indirect evidence for such relationships exists, and most studies are heavily biased toward primates and other homoeothermic vertebrates. We tested the association between male brain size (relative to body size) and male-male competition across N=30 species of Chinese anurans. Three indicators of the intensity of male mate competition-operational sex ratio (OSR), spawning-site density, and male forelimb muscle mass-were positively associated with relative brain size, whereas the absolute spawning group size was not. The relationship with the OSR and male forelimb muscle mass was stronger for the male than for the female brains. Taken together, our findings suggest that the increased cognitive abilities of larger brains are beneficial in male-male competition. This study adds taxonomic breadth to the mounting evidence for a prominent role of sexual selection in vertebrate brain evolution.

Body mass variation is negatively associated with brain size: Evidence for the fat-brain trade-off in anurans

Species can evolve diverse strategies to survive periods of uncertainty. Animals may either invest in energy storage, allowing them to decrease foraging costs, such as locomotion or risk of predation, or they may invest in better cognitive abilities helping them to flexibly adapt their behavior to meet novel challenges. Here, we test this idea of a fat-brain trade-off in 38 species of Chinese anurans by relating the coefficient of variation of body mass (CV_bodymass ; as an indicator of how much animals invest into storage over the season) to brain anatomical features. After correcting for shared ancestry and body mass, we found a negative relationship between relative brain size and CV_bodymass . This indicates that anurans seem to trade-off physiological and cognitive buffering during energy shortages. As similar patterns have been reported in arboreal mammals and primates our findings suggest that the fat-brain trade-off, where animals either invest into physiological or cognitive strategies to survive harsh conditions, may be a general pattern across vertebrates.

Frogs with denser group-spawning mature later and live longer

The understanding of the intrinsic and extrinsic causes of longevity variation has deservedly received much attention in evolutionary ecologist. Here we tested the association between longevity and spawning-site groups across 38 species of Chinese anurans. As indicators of group-spawning we used spawning-site group size and spawning-site density, which we measured at 152 spawning sites in the field. We found that both spawning-site density and group size were positively associated with longevity. Male group-spawning (e.g., male spawning-site density and male spawning-site group size) was also positively correlated with longevity. A phylogenetic path analysis further revealed that longevity seems directly associated with spawning-site density and group size, and that the association in part depend on the 'groups-spawning-age at first reproduction' association. Our findings suggest that the increased group-spawning are likely to benefit in declining extrinsic mortality rates and living longer through improving total anti-predator behaviour under predation pressure.

Hibernation constrains brain size evolution in mammals

The expensive brain hypothesis predicts that the lowest stable level of steady energy input acts as a strong constraint on a species' brain size, and thus, that periodic troughs in net energy intake should select for reduced brain size relative to body mass. Here, we test this prediction for the extreme case of hibernation. Hibernators drastically reduce food intake for up to several months and are therefore expected to have smaller relative brain sizes than nonhibernating species. Using a comparative phylogenetic approach on brain size estimates of 1104 mammalian species, and controlling for possible confounding variables, we indeed found that the presence of hibernation in mammals is correlated with decreased relative brain size. This result adds to recent comparative work across mammals and amphibians supporting the idea that environmental seasonality (where in extremis hibernation is necessary for survival) imposes an energetic challenge and thus acts as an evolutionary constraint on relative brain size.

Modulation of Gene Expression in Liver of Hibernating Asiatic Toads (Bufo gargarizans)

Hibernation is an effective energy conservation strategy that has been widely adopted by animals to cope with unpredictable environmental conditions. The liver, in particular, plays an important role in adaptive metabolic adjustment during hibernation. Mammalian studies have revealed that many genes involved in metabolism are differentially expressed during the hibernation period. However, the differentiation in global gene expression between active and torpid states in amphibians remains largely unknown. We analyzed gene expression in the liver of active and torpid Asiatic toads (Bufo gargarizans) using RNA-sequencing. In addition, we evaluated the differential expression of genes between females and males. A total of 1399 genes were identified as differentially expressed between active and torpid females. Of these, the expressions of 395 genes were significantly elevated in torpid females and involved genes responding to stresses, as well as contractile proteins. The expression of 1004 genes were significantly down-regulated in torpid females, most which were involved in metabolic depression and shifts in the energy utilization. Of the 715 differentially expressed genes between active and torpid males, 337 were up-regulated and 378 down-regulated. A total of 695 genes were differentially expressed between active females and males, of which 655 genes were significantly down-regulated in males. Similarly, 374 differentially expressed genes were identified between torpid females and males, with the expression of 252 genes (mostly contractile proteins) being significantly down-regulated in males. Our findings suggest that expression of many genes in the liver of B. gargarizans are down-regulated during hibernation. Furthermore, there are marked sex differences in the levels of gene expression, with females showing elevated levels of gene expression as compared to males, as well as more marked down-regulation of gene-expression in torpid males than females.

Latitudinal variation in body size in Fejervarya limnocharis supports the inverse of Bergmann’s rule

Bergmann’s rule states that within a species of endotherms smaller individuals are found in warmer conditions, which is consistent for nearly all endotherms, while in ectotherms body size patterns are less consistent. As ectothermic vertebrates, the morphology of amphibians is likely impacted by climatic conditions. Here, we examined latitudinal variation in body size in the ranid frog, Fejervarya limnocharis, based on literature and our own data on mean body size of 3637 individuals from 50 populations and average age of 2873 individuals from 40 populations in China. The results showed that body size was positively correlated with environmental temperature, but not with precipitation. Body size was negatively correlated with latitude among populations in this species, which supported the inverse of Bergmann’s rule. Our findings suggest that a larger body size in low-latitude populations is associated with a longer growing season related to the higher environmental temperature.

Cerebellum size is positively correlated with geographic distribution range in anurans

The ‘cognitive buffer’ hypothesis predicts that the costs of relatively large brains are compensated for later in life by the increased benefits of large brains providing a higher chance of survival under changing environments through flexible behaviors in the animal kingdom. Thus, animals that live in a larger range (with a higher probability of environmental variation) are expected to have larger brains than those that live in a restricted geographic range. Here, to test the prediction of the ‘cognitive buffer’ hypothesis that larger brains should be expected to occur in species living in geographic ranges of larger size, we analyzed the relationship between the size of the geographic range and brain size and the size of various brain regions among 42 species of anurans using phylogenetic comparative methods. The results show that there is no correlation between relative brain size and size of the species’ geographic range when correcting for phylogenetic effects and body size. Our findings suggest that the effects of the cognitive buffer and the energetic constraints on brains result in non-significant variation in overall brain size. However, the geographic range is positively correlated with cerebellum size, but not with optic tecta, suggesting that species distributed in a wider geographic range do not exhibit larger optic tecta which would provide behavioral flexibility to allow for an early escape from potential predators and discovery of new food resources in unpredictable environments.

No evidence for the expensive-tissue hypothesis in Fejervarya limnocharis

Because the brain is one of the energetically most expensive organs of animals, trade-offs have been hypothesized to exert constraints on brain size evolution. The expensive-tissue hypothesis predicts that the cost of a large brain should be compensated by decreasing size of other metabolically costly tissues, such as the gut. Here, we analyzed the relationships between relative brain size and the size of other metabolically costly tissues (i.e., gut, heart, lung, kidney, liver, spleen or limb muscles) among four Fejervarya limnocharis populations to test the predictions of the expensive-tissue hypothesis. We did not find that relative brain size was negatively correlated with relative gut length after controlling for body size, which was inconsistent with the prediction of the expensive-tissue hypothesis. We also did not find negative correlations between relative brain mass and relative size of the other energetically expensive organs. Our findings suggest that the cost of large brains in F. limnocharis cannot be compensated by decreasing size in other metabolically costly tissues.