Risk of Sperm Competition Mediates Copulation Duration, but not Paternity, of Male Burying Beetles

Abstract

Males should increase their investment in ejaculates whenever they are faced with an increased risk of sperm competition. Burying beetles (Nicrophorus vespilloides), insects that breed on small vertebrate carcasses, offer an ideal model with which to examine sperm allocation tactics because females typically mate with many males prior to laying eggs. Males compete directly for control of carcasses, and males losing such contests often become satellites, lurking in the vicinity of the carcass and attempting surreptitious copulations with the resident female. We predicted that both the dominant resident male and the satellite male would increase their sperm allocation in the presence of the other, but that relative to dominant males, satellite males would allocate a greater number of sperm per ejaculate. We employed a repeated-measures design in which two full-sib rival males, differing only in their dominance status, were each mated a single time to a previously-inseminated female under two conditions, once in the absence of their rival and once in the presence of their rival. Satellite males exhibited longer copulation durations than dominant resident males when both males were present on a carcass. Copulation durations of dominant males did not differ in the presence or absence of satellite males. Contrary to expectation, the increased copulation durations of satellite males did not result in a greater share of paternity relative to dominant males. The absence of any discernible effect of increased copulation durations on paternity in satellite males could be due to post-copulatory preferences of females or, alternatively, satellite males may require longer durations of copulation to transfer the same amount of sperm as dominant males.