Sperm production characteristics vary with level of sperm competition in Cataglyphis desert ants

Under polyandry, males are selected to produce more competitive ejaculates. Theoretical models have explored how the mechanism of sperm competition drives males to partition investment within an ejaculate between sperm quantity and quality. The raffle-based competition model predicts that increased level of sperm competition selects for larger numbers of sperm in ejaculates. Sperm competition is also thought to promote the evolution of longer sperm, because longer sperm could be faster. In eusocial Hymenoptera, the mating system imposes unique selective pressures on male ejaculates. Males are short lived; they reach adulthood with a finite amount of spermatozoa, and they mate typically with a single or a few females and die. The actual number of spermatozoa stored in their accessory testes at emergence is thus a reliable measure of total investment into sperm production. In a comparative study of 15 species of Cataglyphis desert ants, we used phylogenetically controlled analyses to inv...