AN EXPERIMENTAL TEST FOR FOOD EFFECTS ON HEAD SIZE ALLOMETRY IN JUVENILE SNAKES.

Abstract

Many animal species show extensive morphometric shape variation both within and among populations. Although this phenomenon has attracted considerable scientific attention, most studies have aimed at identifying its adaptive significance (e.g., Darwin 1859; Hespenheide 1973; Endler 1986), and it is still unclear to what extent morphometric shape variation is environmentally induced (e.g., Kemp and Bertness 1984; Vermeij 1987; Blouin and Loeb 1991). While shape is a simple concept, it is difficult to define and analyze precisely (Bookstein 1989). In the simplest bivariate case, however, shape may be defined as the size of a body part relative to some measure of overall body size. Such body shape variation among populations may reflect either local genetic modifications (possibly due to adaptation) or simply a direct phenotypic effect of prevailing environmental conditions, such as food and temperature, on overall growth rates (Cock 1966; Weaver and Ingram 1969; Sprent 1972; Vermeij 1980; Endler 1986; Smith and Palmer 1994). In fact, Gould (1977) suggested that variation in growth rates may be a major source of shape variation in animals. Growth rate may control shape in two different ways (Blouin and Loeb 1991). If growth rate variation results in variation in ageor stagespecific body size, and if shape scales nonisometrically with body size, then shape variation will be a simple allometric consequence of body size differences. In addition, growth rate may affect shape via modifications of the allometric relationship, because changes in overall rate of growth or development may not have the same effect on the growth rate of all body parts (Cock 1966; Weaver and Ingram 1969; Sprent 1972; Vermeij 1980, 1987; Kemp and Bertness 1984; Blackstone and Buss 1993). Only the latter of these two mechanisms results in size-independent variation in shape. Snakes offer unique opportunities to study the causes and consequences of size-independent shape variation. Relative head size (head size corrected for snout-vent length) varies considerably among and within snake species (e.g., Klauber 1938; Pough and Groves 1983; Shine and Crews 1988; Forsman 1991, 1994; Shine 199 la, 1993). For instance, in Vipera berus, individual differences in relative head size may be equivalent to those resulting from snout-vent length differences of up to 90 mm (Forsman 1994). Such variation may have pervasive effects on the performance of individuals because snakes swallow their prey whole, and head or gape size limits the size of prey that can be ingested (Pough and Groves 1983; Shine 1991b). Feeding experiments performed with captive V. berus have shown that variation in swallowing capacity among snakes of the same body size is related to individual variation in relative head size, and several fitnessrelated characters (body condition, growth rate, and survival) correlate positively with relative head size in natural populations (Forsman 1991, 1994; Forsman and Lindell 1993). Comparisons among species, populations, and sexes further suggest that different head sizes may have evolved to allow differences in foraging behavior and diet (Pough and Groves 1983; Forsman 1991; Shine 1991a,b 1993). However, the proximate sources of head size variation in snakes remain obscure (Shine 1993). Here I report on an experiment where I manipulated overall growth rates in juvenile V. berus by rearing animals under different feeding regimes. Using data from repeatedly measured animals, I assess both static allometry (among likeaged individuals) and average longitudinal allometry (within growing individuals; i.e., shape trajectories; sensu Cock 1966) and perform corresponding tests for treatment effects. This enables me to determine whether individual and geographic body size-independent head size variation in this species reflects environmentally induced (rather than genetically determined) modifications of the allometric relationship between head and body size. I am aware of only two previous experimental studies that have addressed the determinants of head size variation in snakes. Both of these studies were carried out on Thamnophis sirtalis, and one aimed at explaining sexual dimorphism rather than geographic variation. Thus, Shine and Crews (1988) experimentally demonstrated that the difference in relative head size between sexes in this species was caused by androgens inhibiting growth of the head in males early during embryonic development. In another other study, Arnold and Peterson (1989) raised neonatal T. sirtalis at different temperatures. These authors found that although snakes with less access to heat grew more slowly, static allometric slopes and head shape trajectories were unaffected by maintenance temperature. By contrast, at least to my knowledge, no study has tested for food effects on the ontogeny of relative head size in snakes. There are two reasons why such an experiment would be important. First, the effect of food on overall growth rates in snakes is well documented (e.g., Barnett and Schwaner 1985; Madsen and Shine 1993). This renders it possible that food levels affect relative head size in snakes by controlling