Environmental Drivers of Variation in Labial Teeth Number in Ascaphus Tadpoles

Morphological variation is often maintained by complex and interrelated factors, complicating the identification of underlying drivers. Tadpole oral morphology is one such trait that may be driven by the independent and interacting effects of the environment and variation in developmental processes. Although many studies have investigated tadpole oral morphological diversity among species, few have sought to understand the drivers that underlie intraspecific variation. In this study, we investigated potential drivers of labial tooth number variation among populations of two species of tailed frogs, the Rocky Mountain tailed frog (Ascaphus montanus) and the Coastal tailed frog (A. truei). We counted labial teeth from 240 tadpoles collected across elevation (both species) and latitudinal (A. truei) gradients, providing a natural temperature gradient. We tested the effects of developmental stage and local temperature conditions on labial tooth number. We found that labial tooth number variation was independently affected by both developmental stage and local temperature, as well as the interacting effects of these two variables (pseudo-R2 = 67-77%). Our results also uncovered consistent patterns in labial tooth row formula across the ranges of both species; however, A. truei tadpoles from northern British Columbia had a unique bifurcation of a posterior tooth row. This study highlights the diversity in intraspecific tadpole oral morphology and the interacting processes that drive it.