Insurance, Developmental Accidents, and the Risks of Putting all Your Eggs in One Basket

Parents may hedge against developmental accidents, the premature and unforeseen failure of dependent offspring, by adding supernumeraries to the clutch or brood. The success of such an insurance strategy depends upon backup or marginal offspring being available to replace failed core offspring. Marginal offspring that fail at the same time as core offspring have no insurance value. Here we build models to examine two sources of non-random offspring failure: (i) density-dependence and (ii) correlated offspring failure. The efficiency of an insurance strategy is diminished when the very presence of marginal offspring elevates the rate of offspring mortality. Elevated mortality, per se, does not, however preclude an insurance strategy. Somewhat paradoxically, an insurance strategy can pay when the very presence of insurance offspring elevates the need insurance. Under an ideal insurance strategy, the failure of core and marginal offspring is inversely related; conversely, an insurance strategy is undermined if marginal offspring are likely to fail at the same time as core offspring. An insurance strategy can be enhanced, therefore, if offspring are reared in environments that are at least partially uncorrelated—parental strategies to avoid simultaneous offspring failure include spatial separation of individual progeny, temporal synchrony of offspring development, and/or phenotypic diversification of the brood.