Kyoto, Japan -- "It takes a village to raise a child" doesn't apply merely to humans. Many species of mammals, birds, fish, and various invertebrates have evolved complex social care systems known as cooperative breeding. In these animal societies, offspring receive attention not only from their parents but also from other group members called helpers.
Such social systems have evolved independently multiple times across various taxa, yet most studies have focused on birds and mammals. After studying fish in Lake Tanganyika, one of Africa's great lakes bordering four countries, a team of researchers at Kyoto University was inspired to investigate the forces behind cooperative breeding in lamprologine cichlid, a fish variety endemic to the lake. Specifically, they aimed to elucidate the evolutionary history of cooperative breeding and its correlation with the life history traits of several of these species.
"I have long been interested in how animals cooperate with other individuals," says first author Shun Satoh. "Even when social systems appear superficially similar, the environmental factors that promoted increasing social complexity may have differed among mammals, birds, and fish, and I find that especially fascinating."
To estimate how many times cooperative breeding evolved in Lake Tanganyika, the team used molecular phylogeny to reconstruct the ancestral states of 73 species of lamprologine cichlids in the lake. They then used their own field data and other published literature to compile data on factors such as body size and breeding environment, as well as statistical models to account for phylogenetic relationships testing their associations with cooperative breeding. They also used phylogenetic path analyses to test evolutionary causal hypotheses.
Their results suggest that cooperative breeding first appeared in the lake around 4 million years ago, and that it evolved independently at least seven times. Compared to non-cooperatively breeding species, the researchers observed that cooperatively breeding species had smaller body sizes and produced fewer eggs per clutch, or group of eggs laid at one time. Analyses of the stomach contents of predatory fish revealed most prey to be small cichlids around the same size as those that have evolved cooperative breeding.
These findings support a scenario in which small-bodied species more vulnerable to predation evolved cooperative breeding as a means of maintaining nests and territories and protecting offspring through cooperation among group members. Then, reduced clutch size subsequently arose as a consequence of the constraints associated with group living.
This study provides a basis for re-examining the evolutionary generality of cooperative breeding, and offers a new perspective on the relationship between the ecological factors that drive the evolution of complex societies and the changes in life-history that follow.
Next, the team hopes to connect these findings to bioinformatic approaches, and take on the challenge of identifying the genetic basis underlying the evolution of cooperation and sociality.
