Coppery titi monkey
A coppery titi monkey looking glum. Is its partner off with someone else? Wikimedia Commons

Male coppery titi monkeys are jealous types, showing distress and aggression when their mate hangs out with another monkey.

The first study to look at the neuroscience of jealousy in a monogamous primate has found that male coppery titi monkeys experience anxiety, stress and aggression when they see their mate spending time with a male rival. The results are published in the journal Frontiers in Ecology and Evolution.

Coppery titis form strong bonds with a single mating partner. Behavioural studies have shown that when a mating pair is separated, they become distressed. When another male approaches, the male partner tries to stop him coming anywhere near the female.

Scientists at the University of California, Davis, have looked at what is going on in the monkey's brains and hormonal systems when they are jealous. Brain scans revealed that the monkeys had increased activity in a part of the brain activated by social rejection, known as the singulate cortex, when their mate was hanging out with another monkey.

The monkeys' testosterone and cortisol levels also rose when they saw the stranger male with their mate. Cortisol is involved in the stress response, and testosterone is associated with aggression and competition.

Jealousy may play a role in maintaining social bonding in monogamous pairs, the researchers suggest. But in coppery titis and humans, it also leads to stress, anxiety and – in extreme cases– violence.

"Understanding the neurobiology and evolution of emotions can help us understand our own emotions and their consequences," said study author Karen Bales from the University of California, Davis. "Jealousy is especially interesting given its role in romantic relationships – and also in domestic violence."

"The neurobiology of pair bonding is critical for understanding how monogamy evolved and how it is maintained as a social system," said Bales. "A better understanding of this neurobiology may also provide important clues on how to approach health and welfare problems such as addiction and partner violence, as well as autism."

The researchers now plan to monitor female monkeys watching their partners spend time with other females, to see if they react with jealousy in the same way.