Altitude underlies variation in the mating system, somatic condition, and investment in reproductive traits in male Asian grass frogs (Fejervarya limnocharis)
There is substantial comparative and growing experimental evidence that the competition for fertilization among sperm from different males can drive variation in male reproductive investments. However, less is known about the extent of natural variation in these investments relative to environmental variables affecting resource availability and mating system dynamics, which would allow insights into the mechanisms shaping reproductive allocation. Here, we studied interpopulation variation in male investments in testis size and sperm length across 25 populations of the Asian grass frog Fejervarya limnocharis along a 1550-km latitudinal and 1403-m altitudinal transect in China. We found relative testis mass and sperm length, male somatic condition, and the male/female sex ratio to increase with elevation but not latitude or longitude. Our results suggest that environmental variation may underlie local adaptations to reproductive investments among natural populations, mediated by differences in the availability of both resources and sexual partners (including the resulting male–male competition). These findings contrast with previous predictions that increasing latitude and/or elevation should lead to declining reproductive investments in male anurans due to shortening breeding seasons, declining resource availability, and lowering (rather than increasing) male/female sex ratios. We discuss these species differences in the context of differential resource allocation strategies, breeding ecology, and patterns of male–male competition. These differences show the need for future work on reproductive investments in anurans beyond the few model systems and for potential extension of the theoretical framework to species with different mating systems and strategies.
Despite a broad theoretical framework of how environmental parameters can shape mating systems and these, in turn, drive the evolution of reproductive traits, there is limited empirical evidence from natural populations. We examined differences in the size of testes and sperm between males between natural populations along latitudinal and altitudinal transects. We found that at high elevation, males were relatively heavier and had longer sperm and disproportionately large testes, and the sex ratio was more male-biased. These interpopulation differences may reflect local adaptations to variation in the resource availability and temporal patterns of female availability, which affects the breeding synchrony, sperm demand, and male–male competition for mates and paternity. However, our results differ from previous studies of frogs, possibly due to different breeding ecology, which highlights the need for further work and potential extension of theoretical predictions to non-model anurans.
KeywordsGeographic variation Relative testis mass Sperm length Sperm competition Operational sex ratio Anura
We thank Shang Ling Lou, Mao Jun Zhong, and Cheng Chen for assistance with lab work, and M. Schäfer and two anonymous reviewers for their constructive comments.
Compliance with ethical standards
Financial support was provided by the Sichuan Province Outstanding Youth Academic Technology Leaders Program (2013JQ0016) and Sichuan Province Department of Education Innovation Team Project (14TD0015; 15TD003519) to WBL and by the Swiss National Science Foundation (PZ00P3_154767) to SL.
All experiments involving the sacrifice of live animals were approved by the Animal Ethics Committee at China West Normal University. We declare that all animals used in the study were treated humanely and ethically following all applicable institutional animal care guidelines in China.
Conflict of interest
The authors declare that they have no conflict of interest.
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