The effects of age and mating history on male reproductive success
Creators: name: Aich, Upama
Publisher: name: The Australian National University
Year: 2021

description: Old age in males is commonly associated with negative effects on reproduction and offspring traits. Even so, how a male's age affects reproductive traits under sexual selection is poorly understood. In Chapter 1 I note that a problem in quantifying male age effect is that age and mating history are highly confounded: older males usually have more mating experience. This begs the question: to what extent does mating history rather than male age explain variation in traits under sexual selection? The answer requires an experimental approach. In this thesis, I disentangle the effects of male age and mating history to test for their independent and interactive effects on reproduction in eastern mosquitofish (Gambusia holbrooki). Chapter 2 investigates the effect of paternal lifetime mating history on offspring traits. I found that the daughters, but not the sons, of 'no-mating' treatment males matured significantly sooner, and at a significantly smaller size, than those of 'lifetime-mating' treatment males. Here I discuss sex-specific epigenetic effects of male mating history. Chapter 3 investigates the effect of male age and mating history on female choice. Females did not show a clear preference for males based on either characteristic. This was not due to a lack of female preference: females spent significantly more time with larger males. In addition, female choice was significantly repeatable across four trials: twice as a virgin and twice as a non-virgin. Although female mating status (virgin or non-virgin) did not affect her choice of mate, virgin females spent significantly more time associating with males. In Chapter 4 I tested if mating history rather than age explained variation in male mating behaviour, and determines paternity success when males competed for mates and fertilizations. Both old males, and naive males, spent significantly more time approaching females and attempting to mate, than did young males and those with greater mating experience. Male age and mating history interacted to affect paternity: old males benefited from having previous mating experience, but young males did not. In chapter 5 I tested if variation in the mating history of old and young males affects sperm traits under sexual selection, and paternity success under sperm competition. Old, naive males had more sperm than old, experienced males, while the reverse was true for young males. In contrast, neither male age nor mating history affected sperm velocity. Finally, old males sired significantly more offspring than young males independently of their mating history under sperm competition. The results from chapter 4 and 5 highlight that age-related changes in male reproductive traits, and even paternity, that have been described in many taxa may often be partly attributable to male mating history, and not only to age itself. In chapter 6 I manipulated male mating history to tease apart its effects from those of paternal age on female fertility and offspring traits. Male age did not affect female fertility. However, experienced males produced significantly larger broods. Intriguingly, sons of old, naive males were larger than those of old, experienced males. However, sons sired by old, naive males produced fewer sperm than those sired by old, experienced males, indicating a potential trade-off in beneficial paternal effects. Like their older fathers, sons of older males made significantly more mating attempts than those sired by young males. I discuss how inter-generational transfer of paternal ageing could partly arise due to a father's mating history, and not simply to his age. Overall, my results reveal the complexity of testing male age effects on reproductive traits and offspring traits. They bolster my claim that it is worthwhile to experimentally separate male mating history from age to test for male age effects under sexual selection.