Sperm competition, mating rate and the evolution of testis and ejaculate sizes: a population model

Abstract

There are various ways to estimate ejaculate expenditure. Ejaculate size or sperm number ( s ) is an absolute number of units of ejaculate. Relative ejaculate expenditure ( E ) is the expenditure on the ejaculate as the proportion of the total expenditure on all aspects of the mating, including finding and acquiring a female, and so on. Relative testis size or gonadosomatic index ( σ ) is testes mass divided by body mass; it is assumed to reflect the product of mating rate ( M ) and ejaculate mass ( s ). In a new model, where mating rate, sperm competition and sperm allocation interact, and where the female's inter-clutch interval is assumed to be independent of s or M , we show that σ is directly proportional to the mean E for a species; across species σ and E increase monotonically with sperm competition. However, the relation between s and sperm competition across species depends on the range of sperm competition (low risk or high intensity): s increases with sperm competition at low risk levels, but decreases with sperm competition at high intensity levels. This situation arises because s ∝ E / M ; both E and M increase with sperm competition, but E increases differently with sperm competition in its two ranges.