Variation in Propagule Size as a Function of Environmental Uncertainty for Tree Frogs

Abstract

Predictions concerning egg size and the associated within-clutch variability component were tested with tropical treefrogs, genus Hyla. It was expected that species breeding in temporary versus permanent aquatic breeding sites would exhibit significant differences in clutch parameters as a result of differing selective forces in these contrasting environments. Although the intraclutch variability in egg size, as measured by CV, was not significantly different between the two groups, there was a significant difference in the shape of the distribution of values around the mean, as measured by kurtosis values (g2). The clutches from species breeding in temporary ponds tended towards platykurtosis; this distribution is interpreted as "bet-hedging" as a response to environmental uncertainty. The clutches from species breeding in permanent ponds tended towards leptokurtosis; this distribution is interpreted to be the result of stabilizing selection around an optimal egg size in the more predictable environment. I wish to stress that although these two groups are significantly different from each other in their distributional trends, most of the distributions are not significantly different from normal. In other words, there is a range of egg size, rather than one optimal egg size, in both environments. Numerous factors, including stochastic environmental effects, probably influence egg size in ways that are not necessarily synergistic and unidirectional. The central limit theorem (relative to all the selective and proximate factors affecting egg size) is invoked as an explanation for the fact that nearly all clutches exhibit a normal distribution of egg sizes.