The niche variation hypothesis (NVH) states that
populations with wider niches are more phenotypically variable. The NVH has important
ecological and evolutionary implications but has been controversial since its
inception. Recent interpretations have supported the NVH by directly comparing
among-individual diet variation with population dietary niche breadth.
Traditional studies of the NVH focused on morphological traits as proxies of
niche variation, with contradictory results. Gape-limited predators may be
relatively likely to show effects of morphological variation on diet breadth
because gape size can strongly limit diet. We used five anurans to test NVH
predictions, including three true frogs, <i>Rana
catesbeiana</i>, <i>R. clamitans</i>, and <i>R.
sphenocephala</i>, and two toads, <i>Anaxyrus americanus </i>and <i>A. fowleri</i>.
We combined recent and traditional approaches by comparing both individual
variation in diet and variation in gape width with dietary niche breadth. We
found support for the NVH within two species of the three true frogs but not
for either toad species, a difference likely driven by greater strength of the
feeding limitation caused by gape width in the frogs. Toads had higher gape
width to snout-vent length ratios, reducing the strength of the feeding
limitation imposed by gape width. We found strong support for the NVH among
species; species with more among-individual variation in diet and species with
more variation in gape width had broader niches. Our results highlight the
circumstances under which the NVH is applicable and demonstrate an example in
which the NVH is supported through both traditional and recent interpretations
