Intergenerational costs of reproduction: Comparative and empirical studies in mammals

Abstract

Life-history theory is built upon the principles of resource allocation and trade-offs. While intraindividual trade-offs have received the most attention, intergenerational trade-offs could be equally essential to our understanding of life-history evolution. One of the most well-studied trade-offs is that of reproduction and survival, known as the ‘cost of reproduction’. Currently, oxidative stress is thought to explain this cost but could be paid for by offspring as well as mothers and possibly transferred via several pathways. If oxidative stress can have negative intergenerational implications, then mothers should make some effort to mitigate against this to maximise their fitness. In this thesis, I explore that possibility using a comparative approach examining the evolutionary transitions and life-history associations of one organ that may be responsible for perpetuating intergenerational effects – the eutherian placenta. Repeated evolutionary transitions away from high levels of placentation and associations between high levels of placentation and smaller body masses/shorter gestation lengths provide some evidence for a protective function of the placenta against negative intergenerational effects. To complement this study, I examined the potentially negative intergenerational consequences of the oxidative cost of reproduction. This was done by testing the oxidative cost, constraint and shielding hypotheses in parallel using a supplementary feeding experiment in wild banded mongooses, Mungos mungo. While we found that supplementary feeding did not influence oxidative state, we were able to find some support for each of these hypotheses. Of particular interest was the finding that a specific marker for oxidative damage was lower in pregnant females compared to non-breeders, and negatively associated with pup survival to one year. This is suggestive of a deleterious intergenerational effect as a consequence of oxidative stress and emphasises a need to mitigate against such effects to maximise fitness through mechanisms such as oxidative constraint and shielding. The wide diversity of placentation that we see across mammals may reflect some of the other strategies used by mammals to further minimise negative intergenerational effects and maximise fitness. Ultimately, placentation could play a significant role in the transfer of intergenerational effects and facilitate life-history evolution.