ADAPTIVE SEASONAL VARIATION IN GRASSHOPPER OFFSPRING SIZE

Offspring size has been reported to vary seasonally in a diverse group of organisms: for example, flowering plants, isopods, cladocerans, insects, fish, amphibians, and reptiles (Wellington, 1965; Leonard, 1970; Kerfoot, 1974; Harvey, 1977; Ware, 1977; Howard, 1978; Richards and Myers, 1980; Nussbaum, 1981; Ferguson et al., 1982; Brody and Lawlor, 1984; Cavers and Steel, 1984; Marsh, 1984; Wiklund and Karlsson, 1984; Perrin, 1988; DeMarco, 1989; McGinley, 1989; Dangerfield and Telford, 1990). Seasonal increases and decreases in offspring size are both common, and some populations show even more complex responses. These studies have shown population level changes in offspring size, as well as phenotypic plasticity by individual females, with varying offspring sizes produced among a female's successive clutches. Although there has been some discussion relating observed seasonal changes in offspring size with seasonal changes in biotic and abiotic conditions (e.g., Kerfoot, 1974; Brody and Lawlor, 1984; Perrin, 1988), more work is needed concerning whether these seasonal responses are adaptive or not and, if adaptive, what selective pressures favor such plasticity. Many theoretical models have been developed, beginning with Smith and Fretwell (1974), which predict optimal offspring size, based on the relationship between offspring size and offspring fitness (e.g., Smith and Fretwell, 1974; Brockelman, 1975; Pianka, 1976; Parker and Begon, 1986; Winkler and Wallin, 1987; McGinley et al., 1987). Investing fewer resources per offspring is assumed to allow parents to produce more offspring, increasing a parent's potential fecundity. However, fewer resources per offspring may decrease offspring fitness, with the result that each individual offspring contributes less to parental fitness. The optimal offspring size maximizes parental fitness: the product of offspring number and offspring fitness. One general prediction of these models is that offspring size should increase under conditions that decrease offspring survival and/or future reproduction (Sibly and Calow, 1983; Taylor and Williams, 1984; McGinley et al., 1987). These models have been explicitly applied to seasonal changes in offspring size (McGinley et al., 1987). Offspring fitness may change seasonally due to changing abiotic and biotic conditions (Dixon, 1976; Lacey, 1982; Ohgushi, 1986; Kalisz, 1986). Consistent withinyear variation in environmental conditions affecting