STRATEGIES FOR EGG PRODUCTION

The view that egg production is adapted to counter, and thus provides a measure of, the relative hostility of the environment in which organsms must live has been expressed repeatedly (e.g., Stresemann, 1934; Rensch, 1938; Moreau, 1944; Schmalhausen, 1949; Skutch, 1949, 1967; Cole, 1954; and Smith, 1954). Cole (1954) states that the high fecundity frequently seen in parasites and marine organisms is considered commonly as an adaptation to ensure the maintenance of a population when the probability of survival of an individual is low. Dobzhansky (1950), Fretwell (1969), Pianka (1970) and Willson (1971) have added their support to this "balanced mortality" hypothesis. The rationale (following Smith, 1954) is compelling. Over long periods of time the growth rate of a population is very close to zero. Thus the difference between an average natural environment and the optimal environment can be measured by the maximal intrinsic rate of increase (rmax), and a comparative survey of these rates therefore permits a ranking of species in relation to the relative environmental harshness to which they are exposed (see also Hairston et al., 1970). In addition, the reciprocal of the net reproductive rate during optimal conditions, l/Romax, indicates the probability of survival to maturity in a natural environment, where Ro represents the average number of female progeny produced during the life of a female alive at the beginning of the generation. Thus, when comparing species with similar mean generation periods and adult mortality rates, the reciprocal of gross reproduction per female may be used as a predictor of probability of survival (for calculation of R, see Birch, 1948). Lack (1947, 1949, 1954, 1966) argued against this point of view and has gained support from Cody (1966, 1971) and Johnson and Cook (1968). Lack (1947) pointed out that population balance can be achieved only by the operation of density-dependent mechanisms, and since egg production in many bird populations remains constant with changes in density it cannot contribute to population stability. Thus Lack infers that proponents of the balanced mortality hypothesis suggest an evolution of clutch size to promote population balance, which is not the case. This would clearly invoke the action of group selection. The claims of the supporters of the hypothesis that adjustment of egg production results in population stability is vastly different from Lack's interpretation of their argument that adjustment is in order to achieve population stability. Skutch (1967) and Willson (1971) have made the necessary distinction between the broad limits set by long-term adjustments to mortality and the finer tuning of egg production to current events which frequently comes under density-dependent influences. Lack seems to have considered only the finer tuning in his critique. Lack (1954) also argues that genotypes that confer greater fecundity to an individual must become more abundant in the population and therefore the hypothesis does not permit us to conceive of ways in which reduced fecundity might be selected for. But one cannot equate fecundity with fitness. The cost of laying more eggs usually must be exacted from other energetic commitments that promote the welfare of