The monkey had a full morning, but now it was time to head home.
Small, about the size of a rabbit, with dusky red fur and a long prehensile tail, he had been taken from his cage earlier that day for a shot. Now that the task was complete, it was finally time to rest and spend time with his mate.
But the scientists weren't quite done with him. In fact, they were setting him up for an incredible betrayal.
As the copper titi monkey settled in, he saw his mate—not at home, but in the cage of a rival. Instantly, his brain fired up. He arched his back, smacked his lips, and his tail lashed violently.
If he could, he’d have stormed over to that cage, pulled his mate away, and likely confronted the rival.
If this reaction seems similar to human jealousy, you're not far off. This entire setup was part of an experiment by Nicole Maninger and Karen Bales from the University of California, aiming to uncover where jealousy is processed in the brain and how it functions.
When they examined the monkey's brain, two areas lit up. An MRI and blood sample revealed elevated testosterone and stress hormones, and activated two deep brain regions: the cingulate cortex, linked to social rejection, and the lateral septum, associated with bonding.
These findings, as well as cultural depictions of jealousy in romantic comedies and novels, suggest that jealousy is closely tied to monogamy. Interestingly, monogamy is rare in the animal kingdom, with fewer than 1 in 10 mammal species forming lasting bonds with a single partner. Even humans aren’t strictly monogamous, but we do form uniquely strong, lasting relationships.
According to Bales, the pain of jealousy might play a key role in why monogamous animals form such strong bonds. This may provide an evolutionary advantage, as monogamous male monkeys help care for and provide for their offspring.
So, that tail-lashing, lip-smacking monkey could actually help us understand ourselves. “Understanding the neurobiology and evolution of emotions can help us understand our own emotions and their consequences,” says Bales.
This research could even shed light on how we form romantic relationships and what happens when they go wrong. Studies suggest jealousy may play a significant role in intimate partner violence, which affects about 1 in 3 women and 1 in 4 men in their lifetimes.
Of course, human emotions are complex, and we shouldn't assume monkeys feel exactly what we do. Titi monkeys have different societies and evolutionary histories. However, Bales notes similar brain activity has been observed in human studies.
Jealousy can be an intense, dark emotion, but next time you feel it, you might take comfort in knowing what’s happening in your brain.
Maninger and Bales' research was published in Frontiers in Ecology and Evolution.