Estimating Ovum Size in Amphibians: Egg Size Increases Differently among Species during Early Development

Abstract

The amphibian egg has long been of interest to evolutionary biologists. Classical studies on interspecific variation in egg size and correlated characters such as body size, clutch size, hatchling size, and post-ovipositional parental care (reviewed in Salthe, 1969; Salthe and Duellman, 1973; Kaplan and Salthe, 1979; Nussbaum, 1985) provided a solid foundation for more recent work which relates to the subset of life history strategy theory that addresses the issue of optimal parental investment in offspring (e.g., Smith and Fretwell, 1974; Brockelman, 1975; Wilbur, 1977). While this theoretical approach is still of on-going interest, empirical studies on intra-specific and intra-individual variation in egg size in amphibians (e.g., Howard, 1978; Kuramoto, 1978; Travis, 1983; Crump, 1984; Berven, 1988; Williamson and Bull, 1989, 1995; Tejedo and Reques, 1992; Beachy, 1993; see Kaplan and King, in press, for further review) have resulted in the realization that an optimality approach intended to explain the evolution of egg size may have some deficiencies. Currently there is an emphasis on the evolution of phenotypic response patterns of genotypes (i.e., the norm of reaction, Steams, 1989; Sultan, 1995) and the evolution of adaptive maternal effects (Bemardo, 1996a, b). Within this framework, the answers to several important questions require accurate estimates of ovum size. These include: (1) How much variation is there in ovum size and what are the sources of variation?