The origins of jealousy and the evolutionary significance of the emotion are difficult to parse, especially in humans. But new analysis of jealousy among primates has offered scientists fresh insights into the neurobiology behind the powerful emotion.

The latest research suggests the emotion triggers an increase in neural activity among parts of the primate brain associated with social pain. But it also excites parts of the brain associated with social bonding.

"Understanding the neurobiology and evolution of emotions can help us understand our own emotions and their consequences," Karen Bales, a researcher at the University of California, Berkeley, said in a news release. "Jealousy is especially interesting given its role in romantic relationships -- and also in domestic violence."

To better understand the relationship between jealousy and long-term pair bonding, researchers analyzed the neural response of titi monkeys during possible jealousy-inducing scenarios. Mating pairs of titi monkeys form strong bonds, exhibit mate-guarding behavior and show signs of emotional pain when split from their partners.

"Male titi monkeys show jealousy much like humans and will even physically hold their partner back from interacting with a stranger male," Bales said.

Scientists recorded brain scans of male titi monkeys while observing their mate with a male stranger. As a control, scientist recorded brain scans of males observing a female and male stranger. Researchers also measured changes in the levels of hormone believed to be related to social boding and male aggression.

As expected, jealousy-inducing scenarios inspired heightened activity in the cingulate cortex, a neural region associated with social pain in humans. The so-called jealousy condition also triggered an uptick in activity in the lateral septum.

"Previous studies identified the lateral septum as being involved in the formation of pair bonds in primates," said Bales. "Our research indicates that in titi monkeys, this region of the brain also plays a role in maintaining the pair bond."

While jealousy doesn't appear vital to the formation of social bonds, the latest research -- detailed in the journal Frontiers in Ecology and Evolution -- suggests the emotion is important to the maintenance of social bonds. The fear of separation pain keeps pairs together.

"Increased activity in the cingulate cortex fits with the view of jealousy as social rejection," Bales said.

Previously efforts to understand pair bonding among mammals have mostly focused on prairie voles, the monogamous rodent. The latest research suggests the neural pathways involved in primate bonding differ from those involved in rodent bonding. But the role of positive and negative reinforcement in bond formation and maintenance remains the same.

"Monogamy probably evolved multiple times so it is not surprising that its neurobiology differs between different species," said Bales. "However it seems as though there has been convergent evolution when it comes to the neurochemistry of pair bonding and jealousy."

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