Evidence for alternative male morphs in a Tanganyikan cichlid fish

Abstract Teleost fish show tremendous variation in breeding systems. In particular, indeterminate growth and external fertilization create great disparities in reproductive success among males, which drive the evolution of male reproductive polymorphisms. Here, we present evidence for alternative male reproductive tactics in N eolamprologus modestus , a substrate‐spawning A frican cichlid fish. We observed two types of males in our study site: (1) males that guarded large territories comprised of smaller subterritories of several females and (2) males that did not hold territories and were vigorously attacked by the guarding males upon intrusion into the guarding male's territory. We hypothesized that these intruder males constitute an alternative male phenotype. To test this hypothesis, we collected both territorial males and these intruder males to determine if there were differences in overall body size, gonad size, age or hormone profiles between the two male phenotypes that would be consistent with alternative reproductive tactics in this species. We also collected guarded offspring from territorial male and female pairs to determine if there was any extra‐pair paternity in N . modestus that could indicate the possibility of alternative reproductive tactics. We found that intruder males were significantly smaller in body size and had significantly larger testes in absolute and relative mass compared with territorial paired males. While we assigned no paternity to any collected intruder males, we found that extra‐pair paternity occurred in eight of the 12 collected broods and accounted for c . 27% of all offspring across all broods. Finally, the two sets of males did not differ significantly in age or in circulating androgen levels, suggesting that individuals may not change morphs within their lifetime, but instead may adopt distinct life history strategies. Collectively, we provide strong evidence that intruder N . modestus males represent an alternative small‐bodied male morph that may practice alternative reproductive tactics.