Honey bee (Apis mellifera) sperm competition in vitro - Two are no less viable than one

Sperm competition is the competition between sperm of different males for the fertilization of an ovum. Queen honey bees mate with many males over a short period, establishing ideal conditions in which sperm competition might occur. One hypothesized mechanism by which sperm competition may occur is via sperm incapacitation (SI), which involves the killing and/or inhibition of function of sperm from one male by sperm (or seminal fluid) of another male. However, there is very little empirical support for SI in any animal. We tested whether reported increases in mortality of honey bee spermatozoa when semen from several drones is mixed can be attributed to SI. We found that when the collection method involves minimal manipulation, sperm viability is not reduced in samples of mixed semen from two drones relative to those of a single drone. Our results do not support the existence of SI by killing of sperm (during early encounter in vitro) between semen from unrelated drones, and suggest that reported reductions in sperm viability in mixed samples arise from mechanical damage during semen collection.