REJECTION OF NONADAPTIVE HYPOTHESES FOR INTRASPECIFIC VARIATION IN TROPHIC MORPHOLOGY IN GAPE-LIMITED PREDATORS

Recent studies have interpreted intraspecific divergence in relative h ead sizes in snakes as evidence for adaptation of the trophic apparatu s in gape-limited predators to local prey size However, such variation might also arise from non-adaptive processes (such as allometry, corr elated response, genetic drift, or non-adaptive phenotypic plasticity) . We test predictions from these alternative hypotheses using data on the allometric relationship between head size and body size in two wid e-ranging snake species: eight populations of adders (Vipera berus) an d 30 populations of common gartersnakes (Thamnophis sirtalis). Our dat a enable strong rejection of the alternative (non-adaptive) hypotheses , because the relationship between head and body size differed signifi cantly among populations, the geographic distance separating pairs of populations explained less than 1.5% of their divergence in allometric coefficients, and the within-population allometric coefficients were higher than the among-population coefficients in each species. In addi tion, the geographical variability of allometric coefficients in femal es did nor parallel that in males, suggesting that allometric coeffici ents have evolved independently in the two sexes. Phenotypic plasticit y also cannot explain the data, because laboratory studies show that t he allometric relationship between head size and body size is relative ly insensitive to differing growth rates. We conclude that the intrasp ecific head size divergence in these snakes is better explained by spa tially heterogeneous selection to optimize prey handling ability, than by non-adaptive processes. (C) 1997 The Linnean Society of London.