Comparative Morphometrics in Ranid Frogs (Subgenus Nenirana): Are Apomorphic Elongation and a Blunt Snout Responses to Small-bore Burrow Dwelling in Crawfish Frogs (Lithobates areolatus)?

Abstract

The subgenus Nenirana of North American ranid frogs encompasses Pickerel Frogs (Lithobates palustris), Crawfish Frogs (L. areolatus), Gopher Frogs (L. capito), and Dusky Gopher Frogs (L. sevosus). All four species inhabit caves, crevices, stump holes, and/or burrows when not in breeding wetlands. Crawfish Frogs obligately inhabit crayfish burrows as their primary retreat sites, and in this study we examine whether the deep, small-bore crayfish burrows used by Crawfish Frogs have influenced Crawfish Frog morphology. Specimens of all four species of Nenirana were radiographed and snout–urostyle length, maximum headwidth, head length, femur length, and tibiofibula length were measured from films. Our results suggest that if Crawfish Frog morphology is a response to life in burrows, it is due in part to having the size characteristic of being the largest member of the clade and in part through the shape characteristic of generally exhibiting an intermediate morphology between Pickerel Frogs and the two Gopher Frog species. Not all shape metrics, however, are intermediate; among Nenirana, Crawfish Frogs have the longest hindlimbs and the relatively bluntest snouts. Further, Crawfish Frogs exhibit positive allometry in headwidth, a reversal of the ancestral pattern exhibited by Pickerel Frogs. None of the morphological features of Crawfish Frogs fit neatly into known or predicted functional/morphological cause-and-effect relationships associated with burrow occupancy. It may be that the ranid body plan is generalized enough to permit Crawfish Frogs to inhabit, despite being unable to dig, deep small-bore burrows without undergoing major morphological changes.