Adaptive Versus Constraint Explanations for Egg-to-Body Size Relationships in Two Butterfly Families

Abstract

We have used a comparative approach in order to understand what determines egg size by analyzing relationships of egg to female body size in 11 pierid and 17 satyrid butterflies. Adult female weight varies between 30 mg and 300 mg, and since scaling of morphological character traits is usually observed, we would expect a positive correlation between egg and female size as a result of nonadaptive allometry (in the sense of Huxley 1932) in the absence of selective pressures on egg size. However, two such pressures can readily be recognized, both of which should cause egg size to depart from the baseline allometry. First, a positive correlation between egg size and offspring fitness may select for bigger eggs. Second, since there is a negative relationship between egg size and fecundity, selection on females to maximize fecundity confers a selective pressure to decrease egg size. Since these selective pressures signify a parent-offspring conflict, we follow Parker and Begon's assumption that "selection acts to maximize the reproductive success of the female parent" (1986, p. 574). Then, assuming that the critical minimum value for egg viability is independent of female body size, we believe that the strong selection pressure on females to maximize their fecundity will reduce egg size to this minimum viable size, irrespective of female body size. Only if female ability to achieve high fecundity is somehow constrained would we expect scaling of egg to body size as a result of either nonadaptive allometry (in the sense of Huxley 1932) or adaptive adjustment of egg size. Among the 11 pierids tested, egg weight does not increase with female weight. Therefore, we must conclude that pierids are selected to maximize fecundity and that the strength of this selection pressure overrides the effects of any positive correlation between egg size and offspring fitness. By contrast, egg size does increase with body size among the 14 satyrids that are adapted to low-temperature environments but not among the 3 species from sunny habitats. We tentatively conclude that female ability to achieve high fecundity is somehow constrained among the 14 species and that the constraints are related to their adaptation to low temperatures (by butterfly standards). This would be true if there is an upper limit to the rate at which eggs are laid each day and if egg laying can be extended over a limited number of days. Thus, the observed scaling of egg to body size among these satyrids can be due to an unraveling either of a positive correlation between egg size and offspring fitness or of a nonadaptive "baseline" scaling expected from some common law of growth (see Huxley 1932). Two lines of evidence suggest that the latter explanation agrees better with the data. First, the adaptive explanation rests on the assumption that egg size and offspring fitness correlate positively. Empirical tests for a correlation between egg weight and seven offspring-fitness parameters in two of the satyrid butterflies, Pararge aegeria and Lasiommata megera, have failed to reveal any such positive relationship (Wiklund and Persson 1983; Karlsson and Wiklund 1984), suggesting a low correlation between egg size and offspring fitness. If so, the adaptive explanation becomes more or less identical with the nonadaptive-allometry explanation. Second, the slope of the log-log correlation between egg and body weight for the 14 satyrids adapted to low-temperature habitats was 0.677, which is in general agreement with the correlation for the 17 orders of birds that have been tested, all of which appear to have the same slope, 0.675 (Rahn et al. 1975). This may suggest that some common kind of nonadaptive (in the sense of Williams 1966) growth mechanism is responsible for the observed scaling. The inference that temperature constrains the realization of high fecundity in the 14 satyrids above is corroborated by the demonstration that the 3 satyrids adapted to sun-exposed habitats effectively behave like pierids; that is, they lay a higher number of eggs per day compared with the species adapted to low temperatures, and they also lay eggs smaller than expected for their body size.