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Reproductive effort is defined as that proportion of the total energy budget of an organism that is. devoted to reproductive processes. Reproductive effort at a given age within a species will be selected to maximize reproductive value at that age. Reproductive effort is not directly affected by changes in juvenile survivorship, nor necessarily reduced by an increase in adult survivorship. Selection for high levels of reproductive effort should occur when extrinsic adult mortality is high, in environments with constant juvenile survivorship, and in good years for juvenile survivorship in a variable environment, provided that the quality of the year is predictable by adults. Data necessary to measure reproductive effort and to understand how selection results in different levels of effort between individuals and species are discussed. We make several predictions about the effect of increased resource availability on reproductive effort. The empirical bases for testing these predictions are presently inadequate, and we consider data on energy budgets of organisms in nature to be essential for such tests. We also conclude that variance in life table parameters must be known in detail to understand the selective bases of levels of reproductive effort.Reproductive effort has become a central concept in theories of life history evolution. Fisher (1) evidently first called attention to the significance of determining how natural selection adjusts the partitioning of the energy budgets of organisms among reproduction, growth, and maintenance. Theories of how selection effects this adjustment have been developed by Williams (2, 3), Gadgil and Bossert (4), Schaffer (5), and others. Empirical evidence has been variously in accord with, or contradictory to, theoretical predictions (e.g., refs. 6-8).The difficulties in understanding the evolution of reproductive effort stem from the fact that predictions from theory are, in many cases, results of assumptions in the models which require careful examination before the predictions may be considered relevant to organisms in nature. Difficulties also arise because it is not clear what data constitute adequate measures of reproductive effort. It is impossible at present to decide whether failure of the data to be consistent with theory is due to inappropriate estimators or to inadequate theory.The purposes of this paper are: (i) to reexamine the theory of the evolution of reproductive effort, cost, and fitness; (ii) to predict those environmental conditions that result in selection for high and low levels of effort; and (iii) to suggest a methodological framework for the study of reproductive effort that will allow unequivocal tests of theoretical predictions.

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Population studies were conducted on the eastern fence lizard in South Carolina, Texas, Ohio, and Colorado. Hatching occurs in early to middle summer and well into the fall in southern populations, but is restricted to late summer and early fall in Colorado and Ohio. The hatchlings in Texas reach a mature size in 3 months and these lizards, as well as those in South Carolina, reproduce before they are a year old. In Colorado and Ohio the lizards do not mature until almost 2 years of age, but at a larger size than those in Texas or South Carolina. Lizards in the four populations differ significantly in average clutch size (7.4-11.8) and in clutch frequency (1-3) and in egg size (.23-.42 g per egg). In all populations there is a significant positive correlation between clutch size and body size and the regression lines for these variables differ in slope between populations. Survivorships of eggs, hatchlings, yearlings, and adults differ among the populations and these differences have been related indirectly to increased predation in the southern and western populations. The adult mortality rates are inversely related to population density. Life tables for the populations show large differences among the populations in the contribution made by each age class to the total population replacement rate. The life table characteristics of each population show that the measured parameters are consistent with maintenance of stable population numbers even though the life history strategies are clearly different. Evolutionary explanations are provided for these differences and the relevance of the data to the concept of r-and K-selection is discussed.

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