Large donors transfer more sperm,but depletion is faster in a promiscuous hermaphrodite

Males are limited in sperm and seminal fluid, which can lead to prudent sperm allocation across consecutive matings. While sperm depletion is usually considered a population characteristic, we investigated an individual size-related variation in sperm depletion in the promiscuous sea slug Chelidonura sandrana. We found that sperm counts substantially declined across four successive copulations, indicating sperm depletion. Sperm depletion occurred at a disproportionally fast rate in larger sperm donors. Based on our finding of body size-dependent sperm depletion, we further investigated whether this translates into prudent sperm allocation across ejaculates. We found no evidence that slugs adjusted the number of delivered sperm in response to partner body weight, a tight proxy of fecundity. Instead, sperm counts increased with sperm donor body weight. Sperm donation independent of partner body size and—as found in previous work—partner novelty leaves scope for currently unidentified traits like the sperm competition environment to affect male sperm allocation. Alternatively, we propose that, in C. sandrana, a low optimal male mating rate close to the female optimum as indicated by our data may ultimately render indiscriminate sperm allocation the most beneficial male mating strategy.