The New York Times, March 10, 2005
NEW YORK — Parents of wailing babies, take comfort: You are not alone. Chimpanzee babies fuss. Sea gull chicks squawk. Burying beetle larvae tap their parents’ legs. Throughout the animal kingdom, babies know how to get their parents’ attention. Exactly why evolution has produced all this fussing, squawking and tapping is a question many biologists are trying to answer.
Someday, that answer may shed some light on the mystery of crying in human babies.
“It may point researchers in the right direction to find the causes of excessive crying,” said Joseph Soltis, a bioacoustics expert at Disney’s Animal Kingdom in Lake Buena Vista, Florida. Soltis published an article on the evolution of crying in the current issue of Behavioral and Brain Sciences.
Young animals vary in how much they cry, squawk or otherwise communicate with their parents, and studies with mice, beetles and monkeys show that this variation is partly based on genes.
Some level of crying in humans, of course, is based on gas pains and messy diapers. But as for the genetic contribution, you might expect that natural selection would favor genes for noisier children, since they would get more attention.
Before long, however, this sort of deception may be ruinous. If the signals of offspring became totally unreliable, parents would no longer benefit from paying attention. Some evolutionary biologists have proposed that natural selection should therefore favor so-called honest advertisements.
Some biologists have speculated that these honest advertisements may not just tell a parent which offspring are hungry. They might also show their parent that they are healthy and vigorous and therefore worth some extra investment.
The babies of rhesus macaque monkeys cry out to their mothers and tend to cry even more around the time their mothers wean them. The mothers, in response, begin to ignore most of their babies’ distress calls, since most turn out to be false alarms.
“Initially, mothers respond any time an infant cries,” said Dario Maestripieri, a primatologist at the University of Chicago. “But as the cries increase, they respond less and less. They become more skeptical. So infants start crying less. So they go through these cycles, adjusting their responses.”
Kim Bard, a primatologist at the University of Plymouth in England, has spent more than a decade observing chimpanzee babies. “Chimps can cry for a long time if something terrible is happening to them, but when you pick them up, they stop,” Bard said. “I’ve never seen any chimpanzees in the first three months of life be inconsolable.”
Maestripieri and other researchers say these evolutionary forces may have also shaped the cries of human babies. “All primate infants cry,” Maestripieri said. “It’s a very conserved behavior. It’s not something humans have evolved on their own.”
Copyright 2005 The New York Times Company. Reprinted with permission.