. Current Herpetology. 66 Current Herpetol. 20(2) 2001 o. (0 Fig. 2. Proportion in wet mass (mean + SE) of diet composition in large- (â ; n=21) and small-headed larvae ( â¡ ; n=23) of Hynobiits retcirdatiis collected from a pond in Tomakomai Experimental Forest. Terres=terres- trial invertebrates; Diptera=Diptera larvae; Epheme=Ephemeroptera nymphs; Tricho=Trichoptera larvae; Cope=Copepoda; Salamander=salamander larvae (H. retardcitus)\ Detritus=fine litter. absolute value of HW did not differ between the two groups of larvae. It is therefore likely that the variation in diet was caus
. Current Herpetology. 66 Current Herpetol. 20(2) 2001 o. (0 Fig. 2. Proportion in wet mass (mean + SE) of diet composition in large- (â ; n=21) and small-headed larvae ( â¡ ; n=23) of Hynobiits retcirdatiis collected from a pond in Tomakomai Experimental Forest. Terres=terres- trial invertebrates; Diptera=Diptera larvae; Epheme=Ephemeroptera nymphs; Tricho=Trichoptera larvae; Cope=Copepoda; Salamander=salamander larvae (H. retardcitus)\ Detritus=fine litter. absolute value of HW did not differ between the two groups of larvae. It is therefore likely that the variation in diet was caused not by the difference in gape size between the large- and small-headed larvae, but by other factors. Considering the difference in shape of the snout tip between the two groups (, relatively pointed in the small-headed larvae, and more rounded in the large-headed larvae: Fig. 1), it is probable that the dietary varia- tion actually reflects difference in foraging environments: the small-headed larvae may forage using their snout for digging around the pond bottom where their main food items, detritus and Diptera larvae, chiefly occur, whereas the large-headed larvae may chiefly feed on fallen terrestrial inverte- brates near the water surface. For larvae of some salamanders, varia- tion in diet is a cause of variation in growth in body mass (Brunkow and Collins, 1996). The present results suggest that H. retardatus larvae also shift prey items under the influence of flexible devel- opment of large head width (Fig. 2). Such an intra- populational variation in diet could influence the population dynamics. For instance, the divergence of diet selec- tion within a population may reduce intraspecific competition, thereby allowing a greater population density and higher averages of individual fitness (Polls, 1984). Because individual variation in head shape depends on the initial density (Kohmatsu et al., 2001), the prediction of population regulation for H. retardatus shou
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