True individual recognition (TIR), the ability to distinguish multiple familiar members individually, is more elaborate than class-level recognition, and evidence for the ability to perform TIR is reported from primates, some other social mammals, birds and lizard in vertebrates. These animals exhibit a highly social structure, wherein TIR is essential for their social interactions. Such high sociality has been documented in fish, but clear evidence of TIR has been limited. The cichlid,Neolamprologus pulcher, a cooperative breeder that guards a territory, exhibits the dear enemy relationship. Here, we show that this fish distinguishes two familiar neighbours individually, i.e., TIR ability, using one-way mirrors in experimental tanks. Focal fish established the dear enemy relationship with two neighbours, NAand NB, and displayed limited aggression towards these familiar neighbours. However, their aggressiveness towards neighbour NBincreased when they were shifted from the original side of the tank after NAwas removed, suggesting that they distinguished NBfrom NAor regarded NBas a stranger. Interestingly, this aggression level against the shifted neighbour NBlargely decreased within 1 min. This decrease contrasted with the longer and more frequent aggressiveness towards unfamiliar strangers. These results suggest that focal fish recognised neighbour NBas a familiar stranger but probably punished NBthat moved beyond its territory, that is, betrayed the dear enemy relationship. We prevented the effects of the behavioural reactions of exposed individuals using a one-way mirror. Thus, we conclude that this fish species displays TIR and discuss that TIR is prevalent in territorial animals in which the dear enemy effect is common.

Siblings often compete for limited resources, such as food provided by their parents. However, although several functions of nonlethal sibling (nonsiblicidal) aggression have been proposed, there is currently little empirical evidence for these, apart from food monopolization. Here, we investigated the functions of nonlethal sibling aggression in the biparental-caring territorial herbivorous cichlid Varibilichromis moorii. We found that the juveniles of this species are highly aggressive and that larger juveniles are more aggressive toward their smaller siblings. Larger juveniles feed on algae more frequently than smaller siblings, thereby indicating a dominance hierarchy. Sibling aggression decreased when algae in the nest was experimentally removed. Furthermore, the removal of smaller juveniles decreased sibling aggression among the remaining larger juveniles, whereas the removal of larger juveniles increased aggression among smaller juveniles. The algal feeding rate of juveniles only increased when larger individuals were removed from the nest. Moreover, larger juveniles attained higher growth rates and remained in natal nests longer than smaller individuals. Our results indicate that sibling aggression may facilitate the monopolization of resources by larger juveniles and extend the parental care period. Interestingly, a small subset of juveniles was observed to migrate to other nests. These juveniles were larger than those of the host brood, and their growth rate increased within the new nests. We suggest that subordinate juveniles may disperse from natal nests and sneak into new nests to enhance their rank, which may represent a novel example of a “best of a bad job” strategy associated with sibling competition.

Cooperative breeding, a social system in which offspring receive care from other group members as well as their parents, occurs in insects, fish, birds and mammals. In this study, we report a new example of cooperative breeding in the cichlid fish (Neolamprologus bifasciatus). This species, endemic to Lake Tanganyika, Africa, inhabits rocky areas at depths below 30 m. Observations of eight nests using underwater video cameras showed that one to six subordinates were present in the nests of breeding pairs. The subordinates shared shelters with juveniles and breeders and engaged in brood/territorial defence and nest maintenance (removing sand from the nest), albeit at a lower frequency than female breeders. The frequency of sand digging by subordinates and female breeders decreased as the number of subordinates in the group increased, suggesting that subordinates reduce the costs of nest maintenance for breeders. The male breeders seldom showed territorial defence and nest maintenance. Subordinates exhibited submissive behaviours towards dominant breeders and larger subordinates within their breeding group, indicating a dominance hierarchy among group members. Gonad examination revealed that the subordinates of both sexes were sexually immature, suggesting that they may not participate in reproduction. An aquarium experiment showed that juveniles become subordinates in their natal nest and participate in territory defence. We conclude that N. bifasciatus is a cooperative breeder, with social systems and subordinate roles similar to those observed in other cooperatively breeding cichlids.

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