A GENERAL THEORY OF CLUTCH SIZE

Abstract

It is possible to think of organisms as having a certain limited amount of time or energy available for expenditure, and of natural selection as that force which operates in the allocation of this time or energy in a way which maximizes the contribution of a genotype to following generations. This manner of treatment of problems concerning the adaptation of phenotypes is called the "Principle of Allocation" (Levins and MacArthur, unpublished), and one of its applications might be the formulation of a general theory to account for clutch size in birds. At this stage we will assume that clutch size is a hereditary phenotypic characteristic which can be affected to a greater or lesser extent by the prevailing environmental conditions and which exhibits the normal variability of such characteristics. Lack (1954) discusses the validity of several hypotheses which' attempt to account for clutch size and its variation under different circumstances and conditions, all of which were rejected in favor of his now widely accepted theory that clutch size is adapted to a limited food supply. This paper is an attempt to show that this and other existing hypotheses when taken singly are inadequate in some respect to account for all the data, that each holds for some particular set of conditions , and that each is but a part of the complete explanation. The theories will be dealt with individually and it will be shown that as environment varies so will the factors which determine clutch size. PRESENTATION OF THE THEORY It is known that in temperate regions, because periodic local catastrophes reduce and maintain populations below the carrying capacity, K, of the habitat, natural selection is proceeding to maximize r, the reproductive rate (Fisher, 1929, whose fundamental theorem is density independent). In these regions any phenotypic variation which enables parents to leave more offspring will be selected for. Any increase in clutch size, up to a limit determined by natural resources, would suffice to increase the reproductive rate. In the tropics, however, with a more climatically stable environment where the advent of such catastrophes is rare, populations will be at saturation densities, and any adaptive variations which will increase the carrying capacity K will usually be favored by natural selection (MacArthur, 1962). Increasing K is equivalent to increasing the population density with the same resources, or maintaining the population density with decreased resources. Whereas in unstable regions all energy was perforce utilized to increase r, an individual living in a stable environment well suited to the needs of the species would need to spend much less energy on maximizing r than individuals living elsewhere. By the "Principle of Allocation ," maximum contribution to future generations will be achieved by those individuals which utilize, to increase K, some of the energy conserved by reducing r. Such individuals will be favored by natural selection over others which do not allocate surplus energy to this end. We can guess suitable recipients for the conserved energy, possible candidates for its use being predator avoidance, more successful intraspecific competition (i.e., reduction in resource density necessary for maintained existence), or perhaps devoting more energy per individual to the raising of the young. All these considerations could serve to increase the contribution to future generations. Such energy requirements or drains, together with the number of eggs laid, may EVOLUTION 20: 174-184. June, 1966 174