Animals that exhibit indeterminate growth obey such a functional relationship: adult body size = f (initial size ? growth rate 9 age). Using this framework, we investigated how and why body sizes of a toad species (Bufo andrewsi) covaried across six altitudes (760-2,100 m) in western China. Towards high altitudes, toads tended to produce large eggs, attain large sizes at metamorphism and have great average age, but grow slowly. This indicated that the former three variables contributed more to the observed altitudinal increase in body size than did the last one. The altitudinal variation in these lifehistory traits should be adaptive to increased climate harshness and decreased predation risks at higher altitudes. We suggest that the relative significance of responses of these size-related parameters to local environments may provide critical cues to explaining considerable variability in geographic size pattern among ectothermic vertebrates.
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.
Being able to accurately estimate the persistence time of populations of endangered plants and animals is central to conservation biology and is of considerable importance in informing land-use decisions. Genetic deterioration (due to inbreeding and random genetic drift) and environmental deterioration (e.g. climate change, pollution and introduced species) clearly contribute to population extinction, however, considerable recent evidence suggests that interactions between genetic deterioration and environmental stress are ubiquitous. The importance of these interactions for potentially reducing persistence times has not been quantified and has not been taken into account by major conservation organizations. Using a computer simulation, we determined that including reasonable estimates of the inbreeding-environment interaction reduces persistence times by 17.5-28.5% (mean = 23%) for a wide range of carrying capacities, assumptions concerning the number of lethal equivalents and different regimes for the frequency and magnitude of the stressful environment. We note that the proportional decrease in persistence time with inclusion of the interactions becomes larger (i.e. the interaction becomes more important) as absolute time to extinction gets larger. Thus, inclusion of the interaction is important and surprisingly may be most needed when populations are of intermediate size and are considered relatively safe from environmental and genetic stresses acting independently.
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.
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