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

Abstract

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). When ejaculates are energetically costly, males should strategically adjust sperm production in response to relevant social cues such as female availability. Using four socio-sexual scenarios, we demonstrate that male mosquitofish G. holbrooki do not produce less sperm when housed alone, compared to being exposed only to chemical cues from females, or to chemical and visual cues, or even when allowed full access to a female. The absence of plasticity in sperm production, when compared to that reported in other Poeciliid fishes, suggests that the relationship between sperm number and mating success might depend upon the mating system. If increased sperm number has a small effect on male reproductive success, there might not be selection for plastic shifts in sperm production. We also showed that ablation surgery on the male’s gonopodium allows us to let males interact with females without ejaculating.