A Darwinian Look at a Wailing Baby
By CARL ZIMMER
Published: March 8, 2005
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 now trying to answer. Someday, that answer may shed some light on the mystery of colic in human babies.
''It may point researchers in the right direction to find the causes of excessive crying,'' said Dr. Joseph Soltis, a bioacoustics expert at Disney's Animal Kingdom in Lake Buena Vista, Fla. Dr. Soltis published an article on the evolution of crying in the current issue of Behavioral and Brain Sciences.
When a young animal relies on its parents for survival, they need a constant stream of information to decide how to care for it. If a rat pup tumbles out of its nest, for example, it may die of hypothermia unless it gets help from its mother.
A young animal sends its parents signals to let them know how it is faring. But in the 1970's, Dr, Robert Trivers, an evolutionary biologist at Rutgers University, pointed out that parents and their offspring wanted different things from these signals.
Dr. Trivers's argument ran like this: An infant's odds of surviving to adulthood go up if it gets a lot of food, protection and care. But the infant's parent may not benefit, in an evolutionary sense, from lavishing so much attention on one offspring. The parent's odds of passing down its genes to future generations depend on the total number of offspring it raises.
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.
These advertisements demand energy, meaning that a young animal cannot make them without paying a real cost. ''You pay a price if you lie,'' said Dr. Jonathan C.K. Wells of the Institute of Child Health in London.
Dr. Rebecca Kilner of the University of Cambridge in England was able to show that these costs exist by studying canaries. She removed 60 canary chicks from their nests and fed them by hand for six hours. She fed some birds quickly but let others beg for up to a minute. Dr. Kilner found that the longer a canary had to beg, the less weight it put on, despite the fact that all the birds were eating the same number of calories.
Some biologists have speculated that these 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. In the coot, an American bird, extravagantly long feathers are a sign of vigor in chicks. Coot parents give long-feathered chicks more food than short-feathered ones.
On the other hand, there may also be some circumstances in which it pays for babies to fake it. The babies of rhesus macaque monkeys cry out a lot to their mothers, and tend to cry even more around the time their mothers wean them --making it possible for the mothers to have more children. 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 Dr. Dario Maestripieri, a primatologists 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.''
Dr. Maestripieri and other researchers say these evolutionary forces may have also shaped the cries of human babies. ''All primate infants cry,'' Dr. Maestripieri said. ''It's a very conserved behavior. It's not something humans have evolved on their own.''
Dr. Wells points out that human babies are under particularly intense pressure to get food and care from their parents compared with other primates. Human brains are very large and demand a lot of energy. Making matters worse, they grow rapidly in the first year of life.
Although it may benefit babies to cry a lot, they do have to pay a price. ''Crying may be 20 times costlier than resting for a baby,'' Dr. Wells said.
Why then do babies cry even when they are healthy and full? Dr. Maestripieri thinks one possibility worth exploring is that colic is a form of deception. By crying more than other babies, colicky babies get more attention. ''If every infant had these genes for colic, it would not work,'' Dr. Maestripieri said. ''But if it's rare enough so that parents don't know if they're being honest or not, it works.''
Some scientists are not convinced by such arguments. Dr. 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,'' Dr. Bard said. ''I've never seen any chimpanzees in the first three months of life be inconsolable.''
Dr. Bard thinks colic is unique to humans and is the result of their big brains. As an infant primate matures, its brain reaches a point where it can regulate its emotions and its body. Because the human brain is growing so quickly, it is harder for human babies to reach the point where they can keep their crying in check. ''That's more delayed in humans than in other primates,'' Dr. Bard said.
But Dr. Bard and other researchers all agree that their ideas about crying still wait to be tested.
Dr. Soltis added, ''We are definitely in the stage of theorizing and speculation.''
Drawing (Drawing by Michael Witte)