THE IMPLICATIONS OF OVUM SIZE VARIABILITY FOR OFFSPRING FITNESS AND CLUTCH SIZE WITHIN SEVERAL POPULATIONS OF SALAMANDERS (AMBYSTOMA).

Abstract

A prevalent attitude concerning parentoffspring relationships maintains that either preor post-fertilization care by parents toward offspring results in increased offspring survival at the expense of additional present or future reproductive success. This has led to models that have as an assumption monotonically increasing functions which relate parental investment per offspring to offspring fitness (Smith and Fretwell, 1974; Brockelman, 1975; Wilbur, 1977). A second assumption of such models, that offspring number is sacrificed for increased offspring care, has recently been found valid in some real situations in salamanders (Kaplan and Salthe, 1979). A third assumption is that there is intrapopulational variability in the energy invested in offspring which allows for the possibility of change in the system. Reproductive patterns of populations can evolve (or be maintained) only if there is sufficient intrapopulational variability in at least some of their components. The present study will establish the amount of intrapopulational variability in ovum size (pre-fertilization parental care) in several salamander populations. Focusing on the first assumption, the physiological ramifications of intrapopulational ovum size variability that may be related to fitness will be assessed. Whether such ramifications are monotonically associated with ovum size variability over the range of variability found will be determined. This approach will allow for an evaluation of how parameters that may be related to offspring fitness are affected by variation in pre-ovipositional parental investment. For example, if the timing of a developmental event (e.g., hatching) is of critical importance in some environments and changes in ovum size affect such timing, then in those environments variability in ovum size will potentially affect offspring fitness. However, in an environment where the timing of such an event is unimportant, variance in investment per offspring will not affect the offspring's fitness through this route. This study also relates to the problem of the determination of clutch size, that is, how and why parental investment during any breeding period is partitioned differentially among offspring within a population. Factors that determine the degree of total parental investment during a single breeding period have been dealt with elsewhere (Kaplan and Salthe, 1979), and for the purposes intended here can be assumed to be constant among individuals.