Separating the effects of paternal age and mating history: evidence for sex-specific paternal effect in eastern mosquitofish

Paternal age and past mating effort by males are often confounded, which
can affect our understanding of a father’s age effects. To our knowledge,
only a few studies have standardized mating history when testing for
effects of paternal age, and none has simultaneously disentangled how
paternal age and mating history might jointly influence offspring traits.
Here, we experimentally manipulated male mating history to tease apart its
effects from those of paternal age on female fertility and offspring
traits in the eastern mosquitofish (Gambusia holbrooki). Male age did not
affect female fertility. However, males with greater past mating effort
produced significantly larger broods. Paternal age and mating history
interacted to affect sons' body size: sons sired by old-virgin males
were larger than those sired by old-mated males, but this was not the case
for younger fathers. Intriguingly, however, sons sired by old-virgin males
tended to produce fewer sperms than those sired by old-mated males,
indicating a potential trade-off in beneficial paternal effects. Finally,
neither paternal age nor mating history affected daughter’s fitness. Our
results highlight that variation in offspring traits attributed to
paternal age effect could partly arise due to a father’s mating history,
and not simply to his chronological age.