The effects of male age, sperm age and mating history on ejaculate senescence

1. In polyandrous species, a male's reproductive success depends on his ability to fertilize females which, in turn, depends on his mating ability and his ability to produce competitive ejaculates. In many species, sperm traits differ between old and young males in ways that are likely to decrease the sperm competitiveness and fertility of older males. This age-ejaculate quality relationship is attributed to male ageing (i.e. senescence).
2. In a natural setting, male age and mating history are usually confounded: older males have usually mated and replenished their sperm supplies more often, so they have made a greater lifetime reproductive effort. In principle, the costs of reproduction, independent of any causal effect of male age, could generate an age-related decline in ejaculate quality.
3. To date, only a handful of studies have determined how male age, reproductive effort, or their interaction affect ejaculate quality. Here, we experimentally manipulated the long-term mating...