Abstract
Amphibian populations are declining globally, however, the contribution of reduced reproduction to declines is unknown. We investigated associations between morphological (weight/snout-vent length, nuptial pad colour/size, forelimb width/size) and physiological (nuptial pad/testis histomorphology, plasma hormones, gene expression) features with reproductive success in males as measured by amplexus success and fertility rate (% eggs fertilised) in laboratory maintained Silurana/Xenopus tropicalis. We explored the robustness of these features to predict amplexus success/fertility rate by investigating these associations within a sub-set of frogs exposed to anti-androgens (flutamide (50 μg/L)/linuron (9 or 45 μg/L)). In unexposed males, nuptial pad features (size/colour/number of hooks/androgen receptor mRNA) were positively associated with amplexus success, but not with fertility rate. In exposed males, many of the associations with amplexus success differed from untreated animals (they were either reversed or absent). In the exposed males forelimb width/nuptial pad morphology were also associated with fertility rate. However, a more darkly coloured nuptial pad was positively associated with amplexus success across all groups and was indicative of androgen status. Our findings demonstrate the central role for nuptial pad morphology in reproductive success in S. tropicalis, however, the lack of concordance between unexposed/exposed frogs complicates understanding of the utility of features of nuptial pad morphology as biomarkers in wild populations. In conclusion, our work has indicated that nuptial pad and forelimb morphology have potential for development as biomarkers of reproductive health in wild anurans, however, further research is needed to establish this.
Highlights
One of the most critical wildlife conservation issues today is the global amphibian decline
In this study we investigated the relationships between frog size (body weight, snout-vent length (SVL)), secondary sexual characteristics, testicular histomorphology, gene expression and hormone levels with reproductive success in males, as defined by males gaining amplexus with a female under competition and the fertility rate of winning frogs
In this study we set out to investigate the relationships between features of external morphology and physiology in male S. tropicalis with reproductive success, measured in terms of achieving amplexus and fertility rate
Summary
One of the most critical wildlife conservation issues today is the global amphibian decline. In the last comprehensive global assessment (2004), one third of amphibian species were reported as threatened with extinction and more than 40% were experiencing population declines, a level greater than for any other vertebrate taxa [1]. Species producing fewer offspring over their lifespan are at increased risk of decline [2] and reduced reproduction is thought to contribute to amphibian population declines [3]. The delineation between reduced survival and reduced reproduction is important for the development of effective mitigation strategies, since reproduction occurs in the aquatic environment for the vast majority of anuran amphibians, which is distinct from the terrestrial environment they inhabit as adults [5]
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