The social evolution of the butterflyfish, Chaetodon multicinctus: Monogamy, sexual selection, juvenile behavior, and territorial aggression

Monogamy has many meanings. The term has variously been used to describe exclusive mating (genetic monogamy), two organisms living together (social monogamy), and the nature of the relationship between those two organisms (pairbonding). These various aspects of monogamy can be thought of in terms of shared genes, shared space, and shared time respectively. The butterflyfish, Chaetodon multicinctus, breeds exclusively with its mate though sneakers are occasionally present (genetically monogamous), defends a male-female territory (socially monogamous), and pairmates spend most of their time in very close proximity. Social monogamy is proximately the result of mate guarding by both sexes. Females defend males' territorial defense abilities whereas males defend females for spawning access. Pairbonding is evolutionarily favorable because both sexes feed at higher rates when with their pairmate. Males, but not females, show a preference for larger mates in tank studies. This may be the result of a female preference for a male's territory quality or some characteristic other than size or be a case of sex role reversal. Males and females pair assortatively by size; this is likely due to intra-cohort pairing rather than sexually selected means, though it may be reinforced by intrasexual competition among females. Juvenile C. multicinctus settle near adult territories. Most do not survive. Pairing behavior occurs shortly after settlement and seems to co-occur with territorial behavior. Adult populations appear to be saturated; juveniles recruit to the adult population either by pairing up when small and successfully defending a larger and larger territory or they become a floater. All ages of C. multicinctus feed primarily on coral, but the percentage of non-coral items in the diet decreases with age. Adults show seemingly more intense displays toward juvenile intruders. The more intense display may be the result of a greater threat to the territory posed by juveniles, avoidance of injury in fights with other adults, or a lack of pairmate recognition behavior toward juveniles.