Monogamy and sex role reversal in the pipefish Corythoichthys haematopterus

We observed the mating pattern and social behaviour of the pipefish Corytho ichthys haematopterus in temperate waters of Japan during three successive breeding seasons. Males cared for a clutch in their brood pouch for 9-19 da ys until hatching and had several broods in the season with nonbrooding int ervals of only 1 or 2 days. The population sex ratio was female biased and some females were always excluded from reproduction. Although males were so metimes courted by unmated females together with their regular partners, th ey always mated with the latter. The pair bond was maintained until the nex t season if both members survived. When males lost their partners, they rem ated with neighbouring unmated females within a few days. In contrast, wido wed females remained unmated for a long time. Females had larger home range s and were more active in courtship displays than males. This pipefish prov ides the first example of sex role reversal among monogamous syngnathid fis h. We suggest that mate guarding by females is a primary proximate factor f or maintenance of monogamy in this fish. (C) 2001 The Association for the S tudy of Animal Behaviour.