Context-dependent relationships between swimming, terrestrial jumping and body composition in the amphibious fish Kryptolebias marmoratus

Understanding the mechanisms that create phenotypic variation within and among populations is a major goal of physiological ecology. Variation may be a consequence of functional trade-offs (i.e. improvement in one trait comes at the expense of another trait) or alternatively may reflect the intrinsic quality of an organism (i.e. some individuals are simply better overall performers than others). There is evidence for both ideas in the literature, suggesting that environmental context may mediate whether variation results from trade-offs or differences in individual quality. We tested this overarching 'context dependence' hypothesis by comparing the aquatic and terrestrial athletic performance of the amphibious fish Kryptolebias marmoratus captured from two contrasting habitats, a large pond and small burrows. Overall, pond fish were superior terrestrial athletes but burrow fish were better burst swimmers, suggestive of a performance trade-off at the population level. Within each population, however, there was no evidence of a performance trade-off. In burrow fish, athletic performance was positively correlated with muscle content and body condition, consistent with the individual quality hypothesis. In pond fish, there was only a relationship between glycolytic white muscle and aquatic burst performance. Notably, pond fish were in better body condition, which may mask relationships between condition and athletic performance. Overall, our data highlight that population-level trends are insufficient evidence for the existence of phenotypic trade-offs in the absence of similar within-population pattems. Furthermore, we only found evidence for the individual quality hypothesis in one population, suggesting that patterns of phenotypic covariance are context dependent.

Automated summary

Understanding the mechanisms of phenotypic variation within and among populations is an important goal in physiological ecology. This study explores the potential trade-offs and differences in individual quality as the underlying reasons for the variation. By comparing the aquatic and terrestrial athletic performance of amphibious fish captured from different habitats, the researchers find evidence for both trade-offs and individual quality, suggesting that environmental context may mediate the variation. These findings emphasize the importance of considering within-population patterns in addition to population-level trends when studying phenotypic trade-offs.