Short-term increases in rival number improves single mating productivity in male <i>Drosophila</i>

In variable environments, animals can change their reproductive behaviors and physiology to maximize reproductive returns. Natural environments vary in multifaceted ways, and animals may need to integrate multiple social or physical cues to adopt the most effective behavioral strategy. In a fully factorial 2 × 2 × 2 experiment, we exposed males to three factors: the number of rivals (10 or 30), food availability (present/absent) and mechanical shaking (present/absent). After 60 min of exposure, we recorded the male's mating latency, copulation duration and the number of offspring produced after a single mating. We also noted the latency of the males partner to remate with a stock male 24 h later. When rival number was increased from 10 per vial to 30 per vial, males sired more offspring. Males also varied their copulation duration and mating latency in response to the number of rivals, but in a condition-dependent manner. In the absence of vortexing, males mated for a shorter time when kept with 30 rivals, but the opposite was observed when males were vortexed. When males were fed and held in groups of 30, they took longer to begin mating compared to the other treatments. Our findings are consistent with the idea that male <i>Drosophila</i> integrate social cues to respond to levels of sperm competition and plastically allocate their ejaculate, but we have demonstrated that they can occur more rapidly (1 h) than previously thought (>24 h). Overall, our data highlight that combinatorial approaches can reveal new relationships between environment and behavior.