Terminal reproductive effort in a marsupial

Abstract

Life history theory predicts that as organisms approach the end of their lifespan they should increase their reproductive effort (RE). In mammals, reproduction involves large transfers of energy from mothers to young, so the theory predicts an increase in energy transfer by older mothers. However, this measure of RE is typically constant with maternal age. This might be because in mammals the offspring has some control over energy transfer and this constrains adaptive variation in RE by mothers. Such an effect might be strong in placental mammals because of the offspring’s control of placental function, but in marsupials energy transfer is overwhelmingly by lactation and is firmly under maternal control, leaving marsupial mothers free to vary RE. Here we provide the first analysis of age-specific RE in a marsupial. We measure RE as the proportion of maternal mass lost during lactation, which was strongly correlated with offspring mass at weaning, and we show that it increased during the last three years of life. These results provide the clearest support yet for terminal reproductive investment in a mammal.