Cats do it. Dogs do it. Giraffes and lions and lemurs do it, too. Some humans have even been observed licking their newborn babies clean. But for the most part, human mothers don't respond in this way after giving birth. Ever wondered why?
Among mammals, the instinct to lick the newborn clean exists for very practical reasons.
For one, licking removes the amniotic fluid from the new baby, according to veterinary professors Donald M. Broom and Andrew F. Fraser in their book Domestic Animal Behavior and Welfare. This keeps the infant warmer because thick amniotic fluid conducts heat away from the body. But licking also stimulates the newborn and draws the baby's attention toward the mother, they said.
But if you were to ask the baby-licking mammals themselves, they might have a variety of other reasons right at the ends of their nonexistent fingertips — perhaps safety, cleanliness and nourishment.
It's possible that the licking cleans up smells that signal "a newborn was here," protecting the infant from predators, and that eating the placenta and amniotic fluid provides nourishment and replenishes energy lost in labor.
In fact, licking occurs "for a combination of reasons," says Lee Dugatkin, an evolutionary biologist at the University of Louisville and co-author with Lyudmila Trut of the book, "How to Tame a Fox and Build a Dog," published in 2017.
"One would be health-related — removing any nasty creatures on the surface of the skin, such as bacteria, viruses, that sort of thing," Dugatkin says. Another reason, Dugatkin explains, relates to the mother recognizing the baby. "It may be the start of a chemical recognition system between mothers and offspring. Licking is one way to get that sorted out. There are all sorts of bonding behaviors that go on between mother and offspring," and licking may be one of those "early chemo-tactile links" that the mother has to her baby.
So why don't human mothers do it?
A major reason may be that humans no longer need to. "There hasn't been [evolutionary] selection for licking your offspring. We can clean without licking." Dugatkin says. Humans, after all, do have functional hands.
"My guess is that we have so many other ways to get the benefits and information associated with licking," he says. "Visual and tactile senses in humans are very strong," and so selection might be weaker for a licking behavior. "We rely so heavily on ... more sophisticated cognitive behavior. My guess is that licking is not as useful," he says.