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

reduction” (to use the somewhat euphemistic jargon) is obligatory. The older chick always pushes its younger sibling outside the nest (or occasionally stomps it to death within). This system seems, at first, to make no sense. The blue-foots, whatever our negative, if inappropriate, emotional reactions, at least use sibling murder as a device to match the number of chicks to a fluctuating supply of available food.
    By what perverse logic should masked boobies produce two eggs, yet never rear more than one chick invariably branded with the mark of Cain? Nelson argues forcefully that clutches of two eggs represent an adaptation for greatly increased success in raising one chick. The causes of death in eggs and young hatchlings are numerous—siblings intent upon murder being only one of many dangers to which booby flesh is heir. Eggs crack or roll from the nest; tiny hatchlings easily overheat. The second egg may represent insurance against death of the first chick. A healthy first chick cancels the policy directly, but the added investment may benefit parents as a hedge worth the expense of producing another egg (they will, after all, never need to expend much energy in feeding an unnecessary second chick). At Kure Atoll in the Hawaiian Archipelago, for example, clutches of two eggs successfully fledged one chick in 68 percent of nests examined during three years. But clutches of one egg fledged their single chick only 32 percent of the time.
    Evolutionary biologists, by long training and ingrained habit, tend to discuss such phenomena as the siblicide of boobies in the language of adaptation: how does a behavior that seems, at first sight, harmful and irrational really represent an adaptation finely honed by natural selection for the benefit of struggling individuals? Indeed, I have (and somewhat uncharacteristically for me) used the conventional language in this essay, for Nelson’s work persuades me that siblicide is a Darwinian adaptation for maximizing the success of parents in rearing the largest number of chicks permitted by prevailing abundances of food.
    But I am most uncomfortable in attributing the basic behavioral style, which permits siblicide as a specific manifestation, only to adaptation, although this too is usually done. I speak here of the basic mode of intelligence that permits siblicide to work: the sailor’s system (of my opening paragraph) based on yes-no decisions triggered by definite signals. John Alcock, for example, in a leading contemporary text ( Animal Behavior: An Evolutionary Approach , 1975) argues over and over again that this common intellectual style is, in itself and in general, an adaptation directly fashioned by natural selection for optimal responses in prevailing environments: “Programmed responses are widespread,” he writes, “because animals that base their behavior on relatively simple signals provided by important objects in their environment are likely to do the biologically proper thing.”
    (On the overwhelming power of natural selection, no less a personage than H.R.H. Prince Philip, duke of Edinburgh, has written in the preface to Nelson’s popular book on birds of the Galápagos: “The process of natural selection has controlled the very minutest detail of every feature of the whole individual and the group to which it belongs.” I do not cite this passage facetiously to win an argument by saddling a position I do not accept with a mock seal of royal approval, but rather to indicate how widely the language of strict adaptation has moved beyond professional circles into the writing of well-informed amateurs.)
    As I argued for siblicide and guano rings, I am prepared to view any specific manifestation of my sailor’s intellectual style as an adaptation. But I cannot, as Alcock claims, view the style itself as no more than the optimized product of unconstrained natural selection. The smaller brain and more limited neural circuitry of nonhuman animals must impose, or