Sex allocation, terminal investment and the effects of environmental constraints on maternal investment in subtropical antechinus

Abstract

School of Biological Sciences iiCover photo: Female subtropical antechinus named Olive, born in 2012, hand-reared by Daniela Parra-Faundes and released back to the wild on mid-January 2013. This picture was taken three weeks after she was released. Photo: Daniela Parra-Faundes iii AbstractThis thesis addresses questions on sex allocation, life history strategies and costs of reproduction using experimental manipulations of litter sex ratios and field data on ecology and behaviour of the subtropical antechinus (Antechinus subtropicus).In chapter 2, I investigate two major adaptive hypotheses to explain sex ratio bias at birth: the Trivers-Willard Hypothesis (TWH) and the Local Resource Competition Hypothesis (LRCH). I show that sons are more costly to produce than daughters because they have fast growth rates and there are greater survival costs to mothers when they wean more sons. Mothers that naturally produced male-biased litters were slightly heavier than mothers that gave birth to female-biased litters. These results are consistent with the TWH, which states that mothers with more resources to invest benefit by producing high quality competitive sons that will reproduce. However, after increasing the natural bias of litter sex ratios, mothers were able to increase investment to meet demands of rearing more sons than they had naturally produced, without compromising offspring growth.These results are inconsistent with a key prediction of the TWH, that females give birth to the number of sons that they can afford to raise. Also inconsistent with the TWH, malebiased litters grew more quickly after the sex ratio manipulation and, were more likely to survive to weaning. The LRCH predicts that mothers in poor condition should reduce competition from the sex that competes the most, by allocating more to sons than to daughters, as females often remain in their natal home range after weaning. In support of the LRCH, large litter size was associated with slower growth rate in daughters, but not sons. These results differ from previous cross fostering manipulations to test sex allocation in mammals, which have unequivocally supported the TWH.In chapter 3 I examine changes in reproductive performance and survival with age in females. Senescence and terminal investment are two major models to understand effects of age on reproduction. Reproductive investment and success declines with age if senescence occurs, and investment in young increases near the end of life if terminal investment occurs, improving offspring performance at a cost to mothers. I show that older subtropical antechinuses females are not reproductively senescent. On the contrary, females had a greater investment ability and an overall improvement in reproductive performance with age. Older mothers increased investment in their second litters, and were able to produce high quality, large, fast growing offspring that were also more likely iv to survive that the offspring from younger females. However, this greater maternal reduced their own survival. These...