Evolution of Obligate Siblicide in Boobies. 1. A Test of the Insurance-Egg Hypothesis

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Abstract

Obligate siblicide, the unconditional killing of an individual by its sibling, occurs in a small number of bird species. Although clutches of two eggs are frequently laid, obligate siblicide eliminates the younger hatchling before it reaches the age of independence. The phenomenon presents a challenge to adaptationist evolutionary theory because parents produce chicks that are virtually certain to be killed and because surviving offspring actively sacrifice the inclusive-fitness increment represented by the victim. Dorward's (1962) insurance-egg hypothesis explains the production of a second egg as insurance against the first egg's failure. The insurance-egg hypothesis predicts that, if a second egg sometimes produces a fledgling after a first egg fails, then the evolution of insurance-egg production in single-chick species is governed by the ratio of the benefit of the increased probability of producing a hatchling to the cost of egg production. A field study of the obligately siblicidal masked booby (Sula dactylatra) demonstrated that second eggs contribute a surviving hatchling after the first egg's failure in 19.2% of two-egg clutches. The primary source of hatching failure was exceptionally high infertility or early embryonic death. Not all single-chick booby species produce insurance eggs, however, and the prediction of the insurance-egg hypothesis that the occurrence of insurance eggs is associated with the high benefit-to-cost ratio was supported by a compilation of booby breeding parameters. Because females make similar relative investments in each egg, the cost of insurance eggs is assumed to be constant across species, but benefits differ: hatching success in obligately siblicidal species ranges from 51% to 61%, whereas that in single-egg species is at least 85%.

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