Even Fish Societies Show Signs of Social Control and Nepotism

Cichlids, fresh water fish known for their aggressive behavior, tend to turn a blind eye to their relatives who shirk responsibilities, like caring for vulnerable eggs. Members of the group that are not biologically related to the dominant fish don’t seem to be offered the same lenient treatment. Researchers at Columbia University and the University of Bern have now been able to prove the existence of this form of “nepotism” in fish for the first time in experiments. 

Cooperatively breeding fish such as African cichlids divide different tasks among the various members of the group. These tasks include, for example, caring for vulnerable fish in the group, digging out caves to provide safe shelter, or defending the territory against competitors and predators. So how do they make sure that all this work doesn’t simply fall to an unlucky few? Social control is an effective tool: Dominant group members can punish others if they do not do their fair share of the workload. In social cichlids, this happens through physical attacks directed against “lazy” group members. If this does not bring about any improvement, they are expelled from the group, which drastically reduces their chances of survival.

But what if it’s their own offspring who get lazy? Will they be punished equally severely? The experiment showed that these fish are treated more leniently, as a severe punishment would jeopardize the dominants’ own reproductive success. 

Whether the behavior of social animals adheres to this basic principle has been investigated by Irene García-Ruiz, a postdoctoral fellow in the Department of Ecology, Evolution and Environmental Biology at Columbia University, and Michael Taborsky from the Institute of Ecology and Evolution at the University of Bern. A central model system for this is the African cichlid species “Princess of Lake Tanganyika.” This species exhibits a sophisticated social structure, the complexity of which is no less than that of cooperatively breeding birds and mammals—right up to our closest animal “relatives” in the primates. In the current study, García-Ruiz and Taborsky have shown that cichlids’ own descendants are punished less harshly if they do not “help out”—that is, that fish engage in “nepotism.” The results of the study were published in the journal iScience.

The researchers studied the behavior of cichlids at the Ethological Station Hasli in Switzerland. In a first step, they manipulated the behavior of the subordinate group members so that they could not take part in the care of the brood of the breeding couple, essentially making them appear lazy. (The eggs were placed in an area where the fish could not access them in order to care for them; fish were also prevented from another helping activity, digging sand from the shelter that protected the eggs). “We tested how the breeding pairs responded. As expected, the “lazy” brood care helpers were punished more severely,” explains García-Ruiz, lead author of the study. In the second step, the researchers observed whether the helpers who were so punished improved their work performance, which they indeed did; only when the breeding pair could physically attack a “lazy” helper did it increase its work performance. 

The crucial question in this experiment was whether the breeding pair expected their own offspring to participate in the necessary care for the eggs. Would they intervene just as harshly if the desired participation in the work did not occur? 

“The theory predicts that kinship between social partners ensures that their fitness interests largely coincide, so that performing cooperative activities is in the own interest of all participants,” explains Michael Taborsky, head of the study. “This makes social control less important, so temporary inaction doesn’t need to be punished as harshly.” And indeed, breeding pairs punished those helpers with whom they were related much more leniently.

“This was the first experimental demonstration of the interplay between social control and shared interests in a social system,” says Irene García-Ruiz. “This general principle most likely plays an important role in many animal societies, including humans.” Psychological research is already investigating such relationships. “In order to be able to elucidate the evolutionary basis of this interplay between social control and kinship, however, further studies are now required on groups of other highly social animal species,” summarizes Taborsky. 

This story was adapted from a news release by the University of Bern.