Researchers have discovered that a form of oxytocin—the hormone responsible for making humans fall in love—has a similar effect on fish, suggesting it is a key regulator of social behaviour that has evolved and endured since ancient times.
The findings, published in the latest edition of the journal Animal Behaviour, help answer an important evolutionary question: why do some species develop complex social behaviours while others spend much of their lives alone?
"We know how this hormone affects humans," explains Adam Reddon, lead researcher and a graduate student in the Department of Psychology, Neuroscience & Behaviour at McMaster University. "It is related to love, monogamy, even risky behaviour, but much less is known about its effects on fish."
Specifically, researchers examined the cichlid fish Neolamprologus pulcher, a highly social species found in Lake Tanganyika in Africa.
These cichlids are unusual because they form permanent hierarchical social groups made up of a dominant breeding pair and many helpers that look after the young and defend their territory.
For the experiments, researchers injected the cichlids with either isotocin—a "fish version" of oxytocin—or a control saline solution.
When placed in a simulated territorial competition with a single perceived rival, the isotocin-treated fish were more aggressive towards large opponents, regardless of their own size.
When placed in a larger group situation, isotocin-treated fish became more submissive when faced with aggression from more dominant group members. Such signals are important in this species because they placate the dominant members of the group, say researchers.
"The hormone increases responsiveness to social information and may act as an important social glue," says Reddon. "It ensures the fish handle conflict well and remain a cohesive group because they will have shorter, less costly fights."
"We already knew that this class of neuropeptides are ancient and are found in nearly all vertebrate groups," says Sigal Balshine, a professor in the Department of Psychology, Neuroscience & Behaviour. "What is especially exciting about these findings, is that they bolster the idea that function of these hormones, as modulators of social behaviour, has also been conserved."
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