EVOLUTION OF OBLIGATE SIBLICIDE IN BOOBIES .1. A TEST OF THE INSURANCE-EGG HYPOTHESIS

Obligate siblicide 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 2nd egg as insurance against the 1st egg's failure. The insurance-egg hypothesis predicts that, if a 2nd egg sometimes produces a fledgling after a 1st 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. In the obligately siblicidal masked booby Sula dactylatra, 2nd eggs contribute a surviving hatchling after the 1st egg's failure in 19.2% of 2-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 51-61%, whereas that in single-egg species is at least 85%. -from Author