Betta fish show incipient cooperative breeding, study finds

In some group-living animals, individuals other than the parents help raise offspring, a breeding system known as cooperative breeding. This system is thought to have evolved from monogamous family groups in which offspring remain in their natal group and engage in cooperative behaviours, including caring for younger siblings. Cooperative breeding is well documented in mammals and birds, and among fishes it has been recorded in roughly 30 species of cichlids inhabiting Lake Tanganyika in East Africa, where helpers perform tasks such as egg care, nest maintenance and territorial defence. Whether the phenomenon occurs in fishes outside the cichlid family, however, has remained an open question.

A new observational study examined Betta brownorum, a freshwater fish native to Southeast Asia, under captive conditions. The species had previously been suggested to show cooperative breeding based on anecdotal observations that non-parental individuals attack intruders of other species entering their territory, but this idea had never been quantitatively tested, and the extent of cooperation in the species was unclear. The research is published in the journal Behavioral Ecology and Sociobiology.

The researchers carried out detailed observations of family groups of B. brownorum in captivity. In intrusion experiments, they introduced an individual of another species into the territories of the family groups. Both breeding adults and subordinate individuals, which were retained offspring from earlier breeding events, frequently attacked the intruders, indicating that the subordinates contribute to territorial defence. During these experiments, the team also observed one instance in which a subordinate individual remained at the bubble nest while the breeding male was absent.

To probe the role of the subordinates more directly, the researchers ran experiments in which subordinate individuals were temporarily removed from their family groups. Attacks on heterospecific intruders by the breeding adults increased while the subordinates were absent and decreased again after the subordinates were returned to their groups. These results suggest that the retained offspring reduce the burden of territorial defence on the breeding adults, a form of helping behaviour consistent with the early stages of cooperative breeding.

Despite this evidence of cooperation, the study did not observe subordinate individuals directly caring for eggs or fry. The authors therefore cannot conclusively classify B. brownorum as a cooperative breeder. Instead, they describe the species as exhibiting an incipient form of cooperative breeding, in which family members cooperate in some ways, such as shared territorial defence, but do not yet show the direct offspring care that defines full cooperative breeding systems.

Similar intermediate forms have been reported in a few species of birds and reptiles, but the researchers note that, to their knowledge, such an intermediate stage had not previously been documented in fishes. Because cooperative breeding is thought to have evolved gradually from family living, documenting an intermediate case in a fish lineage outside the well-studied cichlids of Lake Tanganyika offers a new comparative point for understanding how cooperative societies may have arisen across different animal groups.

The study was conducted entirely under captive conditions, and the authors describe their work as an observational study of animal behaviour, so the findings characterise the behaviour of this species in captivity rather than establishing how widespread the pattern is in wild populations. The work also leaves open the question of whether the helping behaviour of subordinate B. brownorum would extend to direct care of eggs and fry under other circumstances, which would be needed to classify the species as a full cooperative breeder.

By combining intrusion experiments with temporary removal of subordinates, the study provides quantitative support for the idea that family living in B. brownorum involves measurable cooperation, and it identifies the species as a candidate for studying the gradual evolution of cooperative breeding in fishes.

Subject of Research: Incipient cooperative breeding behaviour in the family-living fish Betta brownorum

Article Title: Cooperation in fish living in families

Article References: Cooperation in fish living in families. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: cooperative breeding, Betta brownorum, fish behaviour, territorial defence, family living, Lake Tanganyika cichlids, Behavioral Ecology and Sociobiology, observational study, Southeast Asia, evolution of cooperation, Cooperation, fish