The evolution of expenditure on testes

Evolutionary theory for expenditure on gonads attracted little attention until studies in the past 3-4 decades of allocation to male and female function in hermaphrodites, and of relative testes size (RTS) in animals with separate sexes. RTS appears to have varied enormously over evolutionary time, from extremely high (over 40%) in some broadcast spawners to less than 1% in some taxa with copulation and internal fertilization. Reasons for this wide evolutionary diversity in testes expenditure deserve attention. Sperm demand (the product of expenditure per ejaculate and mating rate) increases expenditure on testes, and is moulded by a number of selective forces: sperm competition, mating rate and sperm limitation are identified as the three typically most powerful. Trade-offs between somatic and testes expenditures can be complex, particularly those between pre- and post-copulatory expenditure. Theoretical models predict that RTS should correlate positively with sperm demand, and there is much evidence from many taxa that RTS increases with sperm competition level. The relation between RTS and expenditure on each ejaculate is predicted to be positive over low sperm competition (risk) levels, and negative over high (intensity) sperm competition levels. Sexual cascade logic suggests that gonad expenditure should be high and equal for the two sexes soon after the early evolution of anisogamy, but should diverge widely through evolutionary time, with RTS reducing notably in those taxa that have evolved advanced mobility and internal fertilization.