What doesn't kill you makes you stronger? Effects of paternal age at conception on fathers and sons.

Advancing male age is often hypothesised to reduce both, male fertility and offspring quality due to reproductive senescence. However, the effects of advancing male age on reproductive output and offspring quality are not always deleterious. For example, older fathers might buffer effects of reproductive senescence by terminally investing in reproduction. Similarly, males that survive to reproduce at an old age, might carry alleles that confer high viability (viability selection) which are then inherited by offspring, or might have high reproductive potential (selective disappearance). Differentiating these mechanisms requires an integrated experimental study of paternal survival and reproductive performance, as well as offspring quality, which is currently lacking. Using a cross-sectional study in Drosophila melanogaster, we test the effects of paternal age at conception (PAC) on paternal survival and reproductive success, and on the lifespans of sons. We discover that mating at an old age is linked with decreased future male survival, suggesting that mating-induced mortality is possibly due to old fathers being frail. We find no evidence for terminal investment, and show that reproductive senescence in fathers does not onset until their late-adult life. Additionally, we find that as a father's lifespan increases, his probability of siring offspring increases, for older PAC treatments only. Lastly, we show that sons born to older fathers live longer than those born to younger fathers, due to viability selection. Collectively, our results suggest that advancing paternal age is not necessarily associated with deleterious effects for offspring, and may even lead to older fathers producing longer-lived offspring.

Meta-analysis shows no consistent evidence for senescence in ejaculate traits across animals.

Male reproductive traits such as ejaculate size and quality, are expected to decline with advancing age due to senescence. It is however unclear whether this expectation is upheld across taxa. We perform a meta-analysis on 379 studies, to quantify the effects of advancing male age on ejaculate traits across 157 species of non-human animals. Contrary to predictions, we find no consistent pattern of age-dependent changes in ejaculate traits. This result partly reflects methodological limitations, such as studies sampling a low proportion of adult lifespan, or the inability of meta-analytical approaches to document non-linear ageing trajectories of ejaculate traits; which could potentially lead to an underestimation of senescence. Yet, we find taxon-specific differences in patterns of ejaculate senescence. For instance, older males produce less motile and slower sperm in ray-finned fishes, but larger ejaculates in insects, compared to younger males. Notably, lab rodents show senescence in most ejaculate traits measured. Our study challenges the notion of universal reproductive senescence, highlighting the need for controlled methodologies and a more nuanced understanding of reproductive senescence, cognisant of taxon-specific biology, experimental design, selection pressures, and life-history.

Effects of developmental and adult environments on ageing.

Developmental and adult environments can interact in complex ways to influence the fitness of individuals. Most studies investigating effects of the environment on fitness focus on environments experienced and traits expressed at a single point in an organism's life. However, environments vary with time, so the effects of the environments that organisms experience at different ages may interact to affect how traits change throughout life. Here, we test whether thermal stress experienced during development leads individuals to cope better with thermal stress as adults. We manipulated temperature during both development and adulthood and measured a range of life-history traits, including senescence, in male and female seed beetles (Callosobruchus maculatus). We found that thermal stress during development reduced adult reproductive performance of females. In contrast, life span and age-dependent mortality were affected more by adult than developmental environments, with high adult temperatures decreasing longevity and increasing age-dependent mortality. Aside from an interaction between developmental and adult environments to affect age-dependent changes in male weight, we did not find any evidence of a beneficial acclimation response to developmental thermal stress. Overall, our results show that effects of developmental and adult environments can be both sex and trait specific, and that a full understanding of how environments interact to affect fitness and ageing requires the integrated study of conditions experienced during different stages of ontogeny.