Cooperatively breeding cichlid fish adjust their testis size but not sperm traits in relation to sperm competition risk

Abstract

Theoretical models predict that in external fertilisers, sperm competition risk selects for larger testes and larger sperm with higher velocity, at the expense of sperm longevity. Although several empirical studies have tested this prediction within a fish species, there has been little evidence that an individual male can adjust his testis size and sperm traits according to the risk of sperm competition. Using the cooperatively breeding cichlid Julidochromis transcriptus, we examined testis size and sperm traits (sperm size, velocity and longevity) of males that were assigned to monogamous pairs (without risk of sperm competition) or cooperatively polyandrous trios composed of two males and one female (with risk); the groups were reared for breeding for 2 or more months. Our experiments showed that the polyandrous males, both siring eggs in a clutch, had larger relative testis mass than the monogamous males. However, sperm traits were unrelated to sperm competition. These results suggest that individual males of J. transcriptus can adjust their testis size but not sperm traits in response to sperm competition risk. We also examined the effect of testis investment on growth rate and the somatic condition of males. Increased investment in testes slowed growth rates, whereas testis investment did not affect somatic condition, indicating a trade-off between investment in testes and growth. We conclude that testis investment, in response to perceived sperm competition risks, occurs at the expense of growth and may be a conditional reproductive strategy that is determined by the social status (monogamous or polyandrous) of each J. transcriptus male.