Detecting life history trade-offs: measuring energy stores in 'capital' breeders reveals costs of reproduction

Life history trade-offs should be detectable as negative correlations between the relevant traits (e.g. reproductive output versus energy storage), but may be masked by variation in resource levels among individuals. One way to detect underlying trade-offs, at least in organisms that rely on stored energy for reproduction ('capital breeders'), may be to monitor an individual's energy stores before and after reproduction. We analysed energy stores and reproductive output in Eulamprus tympanum, a viviparous scincid lizard that stores energy for reproduction in its tail. One predicted trade-off (that between the size and number of offspring in a litter) is consistently observed, and is detectable with minimal information. Another predicted trade-off (that between offspring size and subsequent energy reserves) is not apparent in our data, perhaps because of constraints imposed by correlations among other traits. Finally, tradeoffs between reproductive output and subsequent energy stores are evident in this species, but are only detectable with information on the extent of pre-reproductive as well as post-reproductive energy stores. For 'capital breeders', non-destructive measurement of pre- and post-reproductive energy stores may greatly enhance our ability to detect significant life history trade-offs.