The Evolution of Helping. I. An Ecological Constraints Model

Abstract

The ecological factors underlying the evolution of helping behavior in birds and mammals are examined. I argue that a necessary first step for the evolution of cooperative breeding is a substructuring of the population into small, stable, social units; in most known cases these are extended-family units. The ecological conditions leading to the development of such units are explored, and a general model is presented that emphasizes ecological constraints that limit the possibility of personal, independent breeding. When severe constraints occur, selection will favor delayed dispersal and continued retention of grown offspring within their natal units. Differing proximate factors can be responsible for limiting the option of personal reproduction. In stable, predictable environments where marginal habitat is scarce, high population density and resulting habitat saturation can lead to a severe shortage of territory openings (Brown 1974; Koenig and Pitelka 1981). This decreases the chance for independent establishment by new breeders. In variable and unpredictable environments, erratic changes in the carrying capacity can create the functional equivalents of breeding openings and closures. During harsh years, the cost of successfully reproducing can be magnified to prohibitive levels. When the chance of successful reproduction is sufficiently restricted, for either reason, selection will favor individuals remaining as nonbreeders within their natal groups. In essence, grown offspring remain at home only when the cost of doing otherwise is prohibitive. This ecological constraints model predicts that the frequency of occurrence of nonbreeders will vary directly with (1) the degree of difficulty in becoming established as a breeder (for species in stable, saturated, habitats), and (2) the level of environmental harshness (for species in erratic, unpredictable habitats). Available evidence is presented for the acorn woodpecker, Melanerpes formicivorus (representing 1), and the white-fronted bee-eater, Merops bullockoides (representing 2), in a preliminary test of the predictions.