Wen Bo Liao

Andrew meets Rensch: sexual size dimorphism and the inverse of Rensch’s rule in Andrew’s toad (Bufo andrewsi)

Variation in sexual size dimorphism (SSD) is a widespread phenomenon and is commonly attributed to variation in sex-specific patterns of selection. According to Rensch’s rule, SSD increases with increasing body size when males are the larger sex, and decreases when females are the larger sex. Using data from 17 populations of Andrew’s toad (Bufo andrewsi), we tested whether the patterns of SSD conform to Rensch’s rule. Using field experiments, we also evaluated the hypothesis that sexual selection favours large male body size and that fecundity selection favours large female body size. The results revealed that the degree of SSD increased with increasing mean size in females, consistent with the inverse of Rensch’s rule. Although experiments revealed evidence for a large-male mating advantage, selection for large male size was weak at best, and hence unlikely to be an important source of variation in SSD. However, fecundity selection favouring large females was evident, and likely to explain the observed inverse of Rensch’s rule. After correcting male and female body size for age differences, the patterns of SSD remained the same, suggesting that the intra- and interpopulational variation in SSD is not driven by sex differences in age structure. Hence, these findings suggest that the strong fecundity selection favouring large females drives the evolution of female-biased SSD in B. andrewsi, providing an explanation for the inverse of Rensch’s rule. As such, the study provides an important addition to the small body of literature that uses an intraspecific approach to demonstrate the inverse of Rensch’s rule.

Sexual size dimorphism in anurans fails to obey Rensch's rule

BackgroundSexual size dimorphism (SSD) is related to ecology, behaviour and life history of organisms. Rensch’s rule states that SSD increases with overall body size in species where males are the larger sex, while decreasing with body size when females are larger. To test this rule, we analysed literature as well as own data on male and female body size in anurans (39 species and 17 genera). We also tested the hypothesis that SSD is largely a function of age difference between the sexes.ResultsOur data set encompassed 36 species with female-biased SSD, and three species with male-biased SSD. All considered species failed to support Rensch’s rule, also when the analyses were phylogenetically corrected. However, SSD was significantly correlated with Sexual Age Difference (SAD) across species. We also found a significant correlation between SSD contrasts and SAD contrasts.ConclusionsOur study suggests that Rensch’s rule does not accurately describe macroevolutionary patterns of SSD in anurans. That SAD can explain most of the variation in SSD among species when controlling for phylogenetic effects suggests that phylogeny is not responsible for the broad relationship between age and size across the sexes.

A skeletochronological estimation of age and body size by the Sichuan torrent frog ( Amolops mantzorum ) between two populations at different altitudes

The age and body size of Amolops mantzorum between two populations distributed in western China were estimated using skeletochronology. The age at sexual mature of individuals was be estimated 2 yrs old in males for both populations while females reached sexual maturity at 2 yrs old at the low-altitude site and 3 yrs old at the high-altitude site. The oldest males and females from the high-altitude site were 7 yrs and 10 yrs old while longevity of males reached 6 yrs and 7 yrs in females at the low-altitude site. Average age between males and females differed significantly at the high-altitude site, but it did no differ significantly at the low-altitude site. For both sexes, average age did not differ significantly between the populations. On average, adult females had significantly larger body sizes than adult males for both populations. Sexual size dimorphism from the low-altitude site and the high-altitude site was 0.280 and 0.282, respectively. Body size between the populations differed significantly within each sex. Positive correlations were found between age and body size for both sexes within each site. The growth coefficient did not differ significantly within a population both sexes or in a sex between the populations. Our findings suggest that inter-population difference in body size of the frogs seems to be related to longevity of individuals, ambient temperature and construction of dams for electricity.

Large Brains, Small Guts: The Expensive Tissue Hypothesis Supported within Anurans

Brain size differs substantially among species, and several hypotheses have been proposed to explain the evolution of brain size. Because the brain is among the most energetically expensive organs in the vertebrate body, trade-offs have been hypothesized to exert constraints on brain size evolution. Prominently, the expensive tissue hypothesis (ETH) proposes that reducing the size of another expensive organ, such as the gut, should compensate for the cost of a large brain. But energetic constraints may also drive covariation between the brain and other costly traits—such as body maintenance, locomotion, or reproduction—as formulated in the energy trade-off hypothesis. To date, these hypotheses have mainly been tested in homeothermic animals and within the ectothermic animals, primarily in fishes. Here, we undertake a comparative test of the interplay between energetic limitations and brain size evolution within amphibians. After controlling for phylogenetic relationships and body size, we find a negative correlation between brain mass and the length of the digestive tract within 30 species of anurans. We further find that the evolution of large brain size is accompanied by an increase in female reproductive investment into egg size. Our results suggest that the evolution of brain size follows general patterns across vertebrate clades.

Geographic variation in life-history traits: growth season affects age structure, egg size and clutch size in Andrew’s toad (Bufo andrewsi)

BackgroundEnvironmental variation associated with season length is likely to promote differentiation in life-history traits, but has been little studied in natural populations of ectotherms. We investigated patterns of variation in egg size, clutch size, age at sexual maturity, maximum age, mean age, growth rate and adult body size in relation to growth season length among 17 populations of Andrew’s toad (Bufo andrewsi) at different latitudes and altitudes in the Hengduan Mountains, western China.ResultsWe found that egg size, age at sexual maturity, and mean age increased with decreasing length of the growth season, whereas clutch size showed a converse cline. Body size did not increase with decreasing length of the growth season, but was tightly linked to lifetime activity (i.e. the estimated number of active days during lifetime). Males and females differed in their patterns of geographic variation in growth rates, which may be the result of forces shaping the trade-off between growth and reproduction in different environments.ConclusionsOur findings suggest that growth season plays an important role in shaping variation in life-history traits in B. andrewsi across geographical gradients.

Evolution of sperm morphology in anurans: insights into the roles of mating system and spawning location

BackgroundThe degree of postcopulatory sexual selection, comprising variable degrees of sperm competition and cryptic female choice, is an important evolutionary force to influence sperm form and function. Here we investigated the effects of mating system and spawning location on the evolution of sperm morphology in 67 species of Chinese anurans. We also examined how relative testes size as an indicator of the level of sperm competition affected variation in sperm morphology across a subset of 29 species.ResultsWe found a significant association of mating system and spawning location with sperm morphology. However, when removing the effects of body mass or absolute testes mass for species for which such data were available, this effect became non-significant. Consistent with predictions from sperm competition theory, we found a positive correlation between sperm morphology and relative testes size after taking phylogeny into account.ConclusionsOur findings suggest that sexual selection in Chinese anurans favors longer sperm when the level of sperm competition is high. Pre-copulatory male-male competition and spawning location, on the other hand, do not affect the evolution of sperm morphology after taking body mass and absolute testes mass into account.

Evolution of anuran brains: disentangling ecological and phylogenetic sources of variation.

Variation in ecological selection pressures has been implicated to explain variation in brain size and architecture in fishes, birds and mammals, but little is known in this respect about amphibians. Likewise, the relative importance of constraint vs. mosaic hypotheses of brain evolution in explaining variation in brain size and architecture remains contentious. Using phylogenetic comparative methods, we studied interspecific variation in brain size and size of different brain parts among 43 Chinese anuran frogs and explored how much of this variation was explainable by variation in ecological factors (viz. habitat type, diet and predation risk). We also evaluated which of the two above‐mentioned hypotheses best explains the observed patterns. Although variation in brain size explained on average 80.5% of the variation in size of different brain parts (supporting the constraint hypothesis), none of the three ecological factors were found to explain variation in overall brain size. However, habitat and diet type explained a significant amount of variation in telencephalon size, as well in three composite measures of brain architecture. Likewise, predation risk explained a significant amount of variation in bulbus olfactorius and optic tecta size. Our results show that evolution of anuran brain accommodates features compatible with both constraint (viz. strong allometry among brain parts) and mosaic (viz. independent size changes in response to ecological factors in certain brain parts) models of brain size evolution.

Relative testis size and mating systems in anurans: large testis in multiple-male mating in foam-nesting frogs

Comparative studies of the relative testes size in animals show that promiscuous species have relatively larger testes than monogamous species. Sperm competition favours the evolution of larger ejaculates in many animals – they give bigger testes. In the view, we presented data on relative testis mass for 17 Chinese species including 3 polyandrous species. We analyzed relative testis mass within the Chinese data set and combining those data with published data sets on Japanese and African frogs. We found that polyandrous foam nesting species have relatively large testes, suggesting that sperm competition was an important factor affecting the evolution of relative testes size. For 4 polyandrous species testes mass is positively correlated with intensity (males/mating) but not with risk (frequency of polyandrous matings) of sperm competition.

Seasonality and Age is Positively Related to Brain Size in Andrew’s Toad (Bufo andrewsi)

The Expensive Brain Framework (EBF) and the Cognitive Buffer Hypothesis are commonly used to explain inter- and intraspecific variation in brain size and brain architecture. Using Andrew’s toad (Bufo andrewsi) as a model species, we investigated whether brain attributes in amphibians are shaped by the interplay between age and the length of the activity season as determined by altitude and latitude. We found significant differences in absolute and relative brain volume across 12 populations. Supporting the EBF, we observed a positive correlation between absolute and relative brain volume with season length. In particular, populations experiencing long seasons were characterized by having relatively large olfactory nerve and optic tecta. Relative, but not absolute, brain volume was also positively correlated with individual age. When accounting for the effects of body size and brain volume, however, the size of different brain structures (olfactory nerve, olfactory bulbs, telencephalon, optic tecta and cerebellum) was independent from age. Taken together, our findings are largely in agreement with the EBF to determine brain size variation in Andrew’s toad.

Sexual selection impacts brain anatomy in frogs and toads

Abstract Natural selection is a major force in the evolution of vertebrate brain size, but the role of sexual selection in brain size evolution remains enigmatic. At least two opposing schools of thought predict a relationship between sexual selection and brain size. Sexual selection should facilitate the evolution of larger brains because better cognitive abilities may aid the competition for mates. However, it may also restrict brain size evolution due to energetic trade‐offs between brain tissue and sexually selected traits. Here, we examined the patterns of selection on brain size and brain anatomy in male anurans (frogs and toads), a group where the strength of sexual selection differs markedly among species, using a phylogenetically controlled generalized least‐squared (PGLS) regression analyses. The analysis revealed that in 43 Chinese anuran species, neither mating system, nor type of courtship, or testes mass was significantly associated with relative brain size. While none of those factors related to the relative size of olfactory nerves, optic tecta, telencephalon, and cerebellum, the olfactory bulbs were relatively larger in monogamous species and those using calls during courtship. Our findings support the mosaic model of brain evolution and suggest that while the investigated aspects of sexual selection do not seem to play a prominent role in the evolution of brain size of anurans, they do impact their brain anatomy.