Testis size depends on social status and the presence of male helpers in the cooperatively breeding cichlid Julidochromis ornatus

Cooperatively breeding animals, in which helpers may participate in reproduction with dominant breeders, are ideal species for examining intraspecific variation in testis size because they often exhibit both monogamous breeding (low risk of sperm competition) and polyandrous breeding (high risk) within a population. However, little is known about testis investment as a result of sperm competition in these animals. The substrate-brooding cichlid fish Julidochromis ornatus has a cooperatively breeding system, in which some males mate monogamously and other males reproduce as dominant breeders or helpers within cooperatively breeding groups, in which male helpers frequently sire young. We examined the relationship between testis investment and male social status in relation to the risk of sperm competition. As predicted from sperm competition models, in groups with male helpers, both the male breeders and the male helpers invested more in testes mass, compared to breeding males without male helpers. We also found a positive relationship between the testes mass of male breeders and their male helpers, suggesting that males increase their investment in reproductive capability under the risk of sperm competition. Sperm competition models also predict that larger testes are associated with increased siring success. Our paternity analysis supported this prediction; we found a positive relationship between testis investment by male helpers and the number of offspring they sired.