Novel ablation technique shows no sperm priming response by male eastern mosquitofish to cues of female availability

Changes in mate availability and sperm competition should generate selection to adjust investments into different pre- and post-
copulatory traits so that the product of mating and fertilization success maximize net male reproductive success. Given costly
sperm production and the risk of sperm depletion, males should invest strategically in ejaculates. Here, we use the eastern
mosquitofish (Gambusia holbrooki), where males have a single coercive mating tactic, to test whether the number of cues
indicative of female availability affects the rate of sperm available for mating (so-called sperm priming). We also tested whether
plasticity in sperm production varies with male body size. We created four socio-sexual treatments that differ in the number of
female-derived cues: none, chemical, chemical and visual, and full access to a female. We used ablation surgery, removing the tip
of the male gonopodium (intromittent organ), to prevent males from mating with a female in the treatment where they interacted
with females. We hypothesized that elevated sperm priming would be associated with more cues about female availability, and be
more apparent in smaller, subordinate males due to their lower baseline sperm count (higher risk of sperm depletion) and their
potential disadvantage during premating competition (leading to higher marginal benefits from sperm investment). There was,
however, no evidence for sperm priming. The rate of sperm availability for mating and the baseline sperm reserves were,
however, dependent on male body size. We discuss possible reasons for our findings. We also note that our study provides novel
insights into the proximate mechanisms associated with sperm release in Poeciliids. Our confirmation of the fact that removing
the gonopodium tip prevents a male from releasing sperm when housed with a female has many potential applications (e.g., in the
study of effects of ejaculate investment and mating effort on male mating success and longevity).