In some group-living animals, individuals other than the parents help care for offspring. This breeding system, known as cooperative breeding, is thought to have evolved from monogamous family groups in which offspring remain in their natal group and engage in various cooperative behaviours, including helping to care for younger siblings. Cooperative breeding has been documented in mammals and birds. Among fishes, it is known in approximately 30 species of cichlids inhabiting Lake Tanganyika in East Africa. In these species, helpers engage in a variety of cooperative behaviours, including egg care, nest maintenance, and territorial defence. However, it remains unclear whether cooperative breeding occurs in fishes outside the family Cichlidae.
In this study, we investigated Betta brownorum, a freshwater fish native to Southeast Asia, under captive conditions. B. brownorum has previously been suggested to exhibit cooperative breeding based on anecdotal observations that non-parental individuals attack heterospecific intruders entering their territory. However, this idea has not been quantitatively tested, and the extent of cooperation in this species remains unclear.
We conducted detailed observations of B. brownorum in captivity. In intrusion experiments, we introduced an individual of another species into the territories of B. brownorum family groups. We found that both breeding adults (parents) and subordinate individuals (retained offspring) frequently attacked the intruders, indicating that subordinate individuals contribute to territorial defence. During these experiments, we also observed one instance in which a subordinate individual remained at the bubble nest while the breeding male was absent. To further investigate the role of subordinate individuals, we conducted experiments in which they were temporarily removed from their family groups. Attacks on heterospecific intruders by breeding adults increased during subordinate removal and decreased after the subordinates were returned to their family groups. These results suggest that subordinate individuals reduce the burden of territorial defence on breeding adults.
Together, these findings demonstrate substantial cooperation among family members in B. brownorum. However, we did not observe subordinate individuals directly caring for eggs or fry. Therefore, we cannot conclusively classify B. brownorum as a cooperative breeder. Instead, this species appears to exhibit an incipient form of cooperative breeding. Similar intermediate forms have been reported in a few species of birds and reptiles but, to our knowledge, have not previously been documented in fishes. Our findings provide new insights into the gradual evolution of cooperative societies across diverse animal groups.
