Head morphology predicts prey traits and drives individual dietary specialization in generalist anurans

Functional morphology often constrains the diversity of resources an organism can exploit, but the extent to which morphological variation drives differences in the magnitude of individual specialization across populations remains unclear. We investigated whether predator functional morphology (i.e. head shape) predicts individual variation in prey traits (size and mobility) and, ultimately, explains differences in the degree of dietary individual specialization across populations of two co-occurring species of tropical generalist frogs. Specifically, we tested three morphology-function predictions: (i) individual frogs with laterally short jaws relative to head height primarily consume slow and large prey; (ii) individuals with laterally long jaws relative to head height primarily consume fast prey; and (iii) individuals with relatively wide heads primarily consume large prey. We found support for all three morphology-function predictions, although patterns were species-specific and depended on predator and prey traits. Also, we found that populations with stronger head shape-prey size relationships showed higher degrees of individual specialization in prey size. Our findings provide the first empirical evidence that functional morphology is a key driver of individual dietary specialization in populations of generalist anurans and highlight the potential role of intraspecific niche variation in maintaining phenotypic variation in natural populations.