Stable isotope tracers reveal a trade-off between reproduction and immunity in a reptile with competing needs

Abstract

Trade-offs between the reproductive and immune systems are predicted when resources are limited, but are difficult to measure. We used 15N-labelled amino acids to directly compare protein allocation by lizards to eggs and healing wounds. We showed that these two demands compete for the same resource, and that the ratio of protein allocation between them was related to body size, reproductive stage and plasma corticosterone levels. On average, rates of leucine deposition into eggs (0.022 μg/day) were an order of magnitude greater than those into scabs (0.002 μg/day), similar to leucine excretion rates (0.027 μg/day), and about half as large as leucine egestion rates (0.054 μg/day). We found that earlier reproductive stages and greater amounts of stored energy, but not manipulated food availability or the amount of food consumed, increased the absolute and relative allocation of protein to self-maintenance. We provide direct evidence for amino acid trade-offs between reproduction and wound healing. A plain language summary is available for this article.