Hen’s Teeth and Horse’s Toes by Stephen Jay Gould Read Free Book Online Page A

blue-foot lays her eggs several days apart, and they hatch in the same order. The firstborn sibling is thus larger and considerably stronger than its one or two ring mates. When food is abundant, parents feed all chicks adequately and the firstborn does not molest its younger siblings. But when food is scarce and only one chick can survive, the actions of younger sibs evoke (how, we do not know) a different behavior by big sister or brother. The oldest simply pushes its younger siblings outside the guano ring. As human mammals, our first reaction might be: so what? The younger sibs are not physically hurt and they end up but a few inches from the ring, where parents will surely notice their plaintive sounds and struggling motions and gather them quickly back.
    But a parental booby does no such thing, for it operates like our proverbial sailor who made an either-or judgment by invoking the single criterion of movement. Parental boobies work by the rule: if a chick is inside the ring, care for it; if it is outside, ignore it. Even if the chick should flop, by happenstance, upon the ring, it will be rejected with all the vehemence applied to my transgressing toe.
    We saw a chick on Hood Island struggling just a foot outside the ring in plain sight of the parent within, sitting (in an attitude that we tend to read as maternal affection) upon the triumphant older sibling (which did not, however, seem to be smirking). Every mother’s son and daughter among us longed to replace the small chick, but a belief in noninterference must be respected even when it hurts. For if we understand this system aright, such a slaughter of the innocents is a hecatomb for success of the lineages practicing it. Older chicks only expel their siblings when food cannot be secured to raise them all. A parental struggle to raise three on food for one would probably lead to the death of all.
    The rule of “nurture within, ignore or reject outside” cannot represent all the complexity of social behavior in nesting boobies. After all, most birds are noted “egalitarians” in their division of labor between sexes, and male boobies are almost as attentive as females in incubating eggs and chicks. Since each brooding stint lasts about a day, boobies must permit their mates to transgress the sanctity of the guano ring when exchanging roles of care and provision. Still, the basic rule remains in force; it is not flouted but rather overridden by specific and recognized signals that act as a ticket of admission. K. E. L. Simmons, working on Ascension Island with the related brown booby, described the extensive series of calls and landing rituals that returning mates use to gain admission to their territory. But when adults trespass upon the unattended territory of an unrelated bird (as they often do to scrounge nest material on the cheap), they enter as “silently and as inconspicuously as possible.”
    If chicks could perform the overriding behavior, they too could win readmission to the ring. Indeed, they learn these signals as they age, as well they must, for older chicks begin to wander from the ring as they gain sufficient mobility for such travels at about four to five weeks of age. (Nelson argues that they wander primairly to seek shade when both parents are foraging; overheating is a primary cause of death in booby chicks.) Yet hatchling boobies display only a few behaviors—little more than food begging and bill hiding (appeasement) gestures, as Nelson demonstrates—and the overriding signals for entrance into the ring are not among them.
    The third species of the Galápagos, the white, or masked, booby, works on a more rigid system, but follows the same rules as its blue-footed cousin. Masked boobies are distant foragers, feeding primarily on flying fish. By Nelson’s maxim, they should be able to raise but one chick. Sometimes, masked boobies lay only one egg, but usually they provision each nesting site with two. In this case, “brood