The Uses of Crying and Begging

Natural History, Oct, 2000 by Bryant Furlow

Throughout the animal kingdom, helpless offspring have ways of demanding attention that parents can't ignore. But all the yelling and screaming and peeping and bleating may communicate much more than mere hunger and thirst.

From a hatchling magpie's, first tentative squawk to a newborn baby's cry, the care-soliciting signals of dependent offspring are the first form of communication in many species. They are also among the most ostentatious forms of communication around. Writhing broods of songbirds, necks outstretched and beaks agape, confront parents at the nest. Chinstrap penguin siblings chase their parents through crowded rookeries for a meal. Robin chicks, jockeying for preferred feeding locations in the nest, get into shoving matches. Even before hatching, when heat and not food is the resource chicks crave, embryonic white pelicans vocalize in the egg. Nor are these displays limited to birds and mammals. Tadpoles of the poisonous frog Dendrobates pumilio perform vigorous swim displays when their mothers check on them, and mother frogs often respond by leaving behind unfertilized eggs as food. Even insect larvae try to get adult attention; immature burying beetles gesticulate at their parents to earn a helping of regurgitated, rotten meat.

Indeed, most offspring displays seem much more extravagant than one would think necessary to elicit attention from parents already eager to protect and feed their own. Surely natural selection designed maternal brains to be responsive. The excited squeals of hungry piglets and the bleats of insistent lambs seem better designed for pestering reluctant mothers than for conveying a simple message of need. Why hasn't evolution programmed human babies to seek parental favor with pleasant sounds and gestures--a nudge, for example, or a hushed whimper?

Back in 1974, Harvard biologist Robert Trivers (now at Rutgers University) supplied part of the answer. Trivers challenged the idea that the interests of a caretaking parent and its helpless young are essentially harmonious. He pointed out that because the offspring of almost all vertebrates share only half their genes with each parent, the stage is set for conflict. In Darwinian terms, he reasoned, each individual is trying to maximize the presence of his or her own genes in succeeding generations, so exaggerated assertions of need--each offspring trying to secure more than its fair share of parental resources--should be favored by natural selection. Parents, on the other hand, should be evolutionarily designed to favor a relatively equal partitioning of resources among offspring, each of which is equally related to the parent and thus carrying an equal number of parental genes into the next generation. The result of the differing interests of parents and young, Trivers predicted, would be a runaway process favoring the evolution of more and more ostentatious demands by offspring.

Others were quick to point out, however, that at some point it would benefit parents to cease responding altogether to begging displays. Extravagant displays entail costs. Noise and activity may attract a hungry rat, hawk, or snake. Indeed, birds that nest in the open or on the ground, such as ovenbirds and Louisiana waterthrushes, perform relatively inconspicuous displays, whereas those that nest in safer locales (tree nesters such as western bluebirds and American robins, for example) tend to produce louder, more easily located calls.

That this difference has a lot to do with predators is supported by a series of imaginative experiments by ecologist David Haskell, now at the University of the South in Sewanee, Tennessee. Haskell placed clay eggs and miniature walkie-talkies in artificial nests. He then broadcast recorded begging calls of a tree-nesting species (the black-throated blue warbler) from ground-nesting ovenbird abodes and vice versa. Later, he counted rodent tooth imprints on the clay eggs. Playing the louder calls on the ground increased visits by predators there, he found, but playing the relatively soft begging calls of ovenbirds from tree nests did not. Haskell got similar results with the begging calls of cardinals, bluebirds, and catbirds. (Some researchers have suggested that the attracting of predators could actually be the point of begging. Were this the case, solicitation displays--particularly vocalizations--should be designed to maximize the risk of attracting a predator, forcing parents to immediately feed the young in order to quiet them. Haskell's work effectively refutes this argument.)

Besides attracting predators, begging displays also take a toll on an animal's store of energy. John McCarty, at the University of Florida in Gainesville, measured oxygen consumption in chicks and found that when they begged, their metabolic rates increased--in starlings by 8 percent over resting rates and in tree swallows by 42 percent. According to Dutch zoologists Simon Verhulst and Popko Wiersma, only between a tenth and a third of the energy a chick successfully extracts from food is available for growth, so expending just 10 percent of its total energy on begging displays could reduce a chick's growth budget by as much as half.