Unable to even hold their heads up on their own, they rely on nearby humans for food, water, and the rest of their needs. That makes figuring out who can be counted on to provide those things pretty important for the little guys. According to a new study published in the journal Evolution and Human Behavior, they might accomplish this important goal by watching who swaps spit.
To find out, Woo and fellow psychologists from Harvard and MIT got their hands on some infants and treated them to a series of experimental puppet shows. In the first, an adult placed an orange slice in their mouth and then in the mouth of a puppet—a clear representation of saliva sharing. The team then had another adult pass a ball to the puppet to represent sharing without the spit.
In the next act of the production, the puppet expressed distress. The researchers monitored the infants to see which person (orange-sharer or ball-passer) they looked to as a signifier of which relationship they deemed closer, which is common in other primates. In studies of vervet monkeys, for example, juveniles will look to the mother of another juvenile if it emits a distress call.
The cast of the next puppet show included two puppet actors and a single human. Instead of oranges, a human put their finger in their mouth, placed it in one puppet’s mouth, and then back in their own. They subsequently touched their forehead, touched the second puppet’s forehead, and then their own.
After that bit of character development, the human protagonist acted like they were crying while the researchers watched which puppet the babies looked to. Put simply, it seems the little tots inferred a certain closeness in the relationship of those who swapped spit. It’s not the first time researchers have discovered babies making this inference, but it’s the first time in infants this young.
The team performed the same experiment on babies between the ages of 8 and 10 months as part of another study. By that age, 90 percent of babies are being fed solid food and are probably more familiar with witnessing the intimate behavior of saliva sharing, which means it may be learned. But between the age of 4 and 6 months, only around half of babies have started eating solid food.
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