Optimal Egg Size Theory: Does Predation by Fish Affect Egg Size in Ambystoma maculatum?

Abstract

-Amphibian larvae hatching from larger eggs may be better equipped to avoid predation than larvae hatching from smaller eggs. We use the context of optimal egg size theory to investigate Ambystoma maculatum egg size in lakes with and without fish. Optimal egg size theory predicts that organisms produce an egg size which balances the fitness advantage of producing large eggs against the fitness cost of producing few eggs. Consequently if offspring fitness is relatively lower at a given egg size in one environment than another, the optimal egg size should be relatively larger in the harsher environment. We investigated this prediction at eight permanent lakes in Algonquin Park, Ontario. We analyzed differences in egg size with a three-level nested analysis of variance at the levels of treatment (lakes with and without fish), lake, and clutch. We found no significant difference in treatment means, and, therefore, the hypothesis was not supported. There was no significant variance component due to lake of origin, but there was a highly significant variance component attributed to the clutch, and therefore, female effect This variation is presumably caused primarily by differences in female body size, and its adaptive significance, if any, is unclear. We reasoned that the presence of fish has not caused females to adapted their egg size because (1) small differences in egg size may make little or no difference in larval survival, and (2) the populations are not isolated enough to evolve large differences in egg size. Egg size is an unusual trait because it is a phenotype shared by two individuals at once: an offspring and its mother (Bemardo, 1996). The egg size phenotype can have an important effect on an offspring's survival, especially at hatching and shortly after, when the offspring is at greatest risk of predation. For example, amphibian hatchlings from larger eggs have greater survivorship (Nussbaum, 1987; Semlitsch and Gibbons, 1990), begin to feed earlier (Kaplan, 1980a), develop with more energetic efficiency (Kaplan, 1980b), and grow faster (Kaplan, 1980a, 1985; but see Walls and Altig, 1986; Semlitsch and Gibbons, 1990) than hatchlings from smaller eggs. Since larval growth is multiplicative, small differences in initial size can theoretically result in large differences in size later in development (Kaplan and Cooper, 1984). Clearly for the individual offspring, fitness would be greatest if it received the entire allocation of resources the female puts into reproduction. The female, however, must balance the demands of individual offspring fitness with the conflicting advantage of producing many offspring. The optimal egg size (OES) model of Smith and Fretwell (1974; see also Brockelman, 46 This content downloaded from 157.55.39.153 on Mon, 19 Sep 2016 04:56:37 UTC All use subject to http://about.jstor.org/terms EGG SIZE OF AMBYSTOMA MACULATUM