The males of the most advanced eusocial Hymenoptera live short protected lives inside colonies where climatic conditions are meticulously controlled, and where disease pressure is low due to the social immunity provided by their worker sisters. Mature males only leave the colony to mate and die soon after, whereas queens can live for years without ever re-mating but only using the stored sperm they have received from one or several males. The number of live sperm received and stored during mating therefore ultimately limits queen lifetime reproductive success, and males are consequently under strong selection to produce high quality ejaculates.
In multiply mated queens, ejaculates from different males arrive as discrete units, but genetic diversity among workers has been documented to provide several colony benefits and this diversity can only be maximized if sperm becomes completely mixed after storage. Queens are also expected to use all the sperm they receive and not bias sperm use away from certain males, yet sperm mixing and sperm use in eusocial insects is poorly investigated. Using molecular techniques, I quantified the relative ejaculate contributions from males to the queen’s sperm stores and compared this with paternity shares in offspring. I also examined whether patriline distributions in worker cohorts remain stable over time, which is expected to yield the highest colony benefits in terms of worker genetic variation.
Selection for extreme sperm viability in males is likely costly and expected to trade off with investments into other traits. I tested this by measuring sperm viability in honeybee males after exposure to mild external stress factors. Additionally, I infected leaf-cutting ant males with a pathogenic fungus and measured their subsequent immune response and sperm viability. I hypothesise that male fertility is easily compromised but that male immune defences are not under selection to respond to challenges that they are unlikely to experience under natural conditions.
I found that sperm use in a long-lived eusocial ant species follows a ‘fair raffle’, where paternity shares in offspring can be accurately predicted from relative number of sperm stored. Furthermore, sperm was completely mixed inside the spermatheca and sperm used is highly consistent over time in leaf-cutting ants, supporting expectations if polyandrous queens are to obtain the highest maximal fitness benefits through a high degree of colony genetic diversity.
My results support that males of advanced eusocial Hymenoptera are under strong selection to perform a very specialized set of tasks and that they might be operating at their physiological limit. This indicates that in the most organismal eusocial societies, males have become completely dependent on the protective environment in which they are raised and are not adapted to any significant form of independent life.