Imagine fighting with a sibling over who’s going to get the last slice of cake from the fridge. Now, imagine two monkeys fighting over affection from their mother. Similar, right? This shared feeling is jealousy. Monkeys tend to form relationships and attachments with their counterparts and feel distressed when separated from each other. So, do monkeys feel jealousy like humans do?

Many different studies have been conducted to see if primates feel jealousy. In 2017, scientist Karen Bales conducted an experiment in which she and colleagues separated male Titi monkeys from their female partners, putting a female Titi in a room with a random male Titi for 30 minutes. The separated male Titis’ behavioral and hormonal responses were recorded. The monkeys’ testosterone and cortisol levels increased, which indicated a rise in stress. These changes correlated directly with the amount of time that the male monkeys watched their partners with the stranger male. The rise in testosterone was expected given its association with mating-related aggression. In another study from 2021, a team of scientists in Namibia followed chacma baboon families in a nature park. Adolescent baboons would attempt to divert their mother's attention from their siblings, sometimes using tricks to lure their siblings away from their parents. Furthermore, much of this behavior occurred during grooming, which involves the removal of bugs and dirt from a monkey’s fur. Grooming is known to strengthen social bonds, as hugs do for humans. So, these observations demonstrate jealousy’s role in bonding and socialization.

These related studies convey how monkeys feel jealousy in similar ways to humans, which can be attributed to evolutionary factors. Jealousy motivates humans differently than other emotions, and if humans feel it, then it may have helped related ancestors form bonds and relationships in a similar way. Jealousy exerts a powerful influence on behavior, different from other emotions like anger or sadness. It motivates individuals to pay closer attention to their feelings and repair bonds.

Jealousy in monkeys often reflects sibling rivalry in human relationships. Children tend to get upset when a parent shows favoritism for another sibling over them, and might seek their attention by making a mess or screaming. Monkeys show similar behaviors when competing for parental affection. Additionally, jealousy might serve as a survival tactic for both species. When humans feel jealous, it is because they are trying to protect themselves from internal insecurity and fear. This is reflected in monkeys, because they may feel a sense of abandonment when their sibling is being nurtured over them. Maintaining relationships is important for food access, learning how to live on their own, and finding their own mate.

As demonstrated by analyzing primates, our closest evolutionary ancestors, jealousy is an age-old trait that lays the foundations for motivation, relationships, and survival.