Seasonal Dynamics of Head Shape in Eurycea chamberlaini (Caudata, Plethodontidae, Spelerpini)

Males of some species of Eurycea exhibit gross morphological shape changes of their heads throughout the year because of hypertrophy of jaw musculature during the mating season, although glandular swelling of the integument has not been rejected for all taxa. We assessed head shape variation of a species from the Eurycea quadridigitata complex, E. chamberlaini, throughout the year to examine seasonal variation in head shape in a diminutive species where this has never been reported. We found that male specimens differed from females in terms of morphospace and, similar to other species of Eurycea, male heads were wider posterior to the orbital region relative to anterior. Interestingly, we also recovered the same result for females, demonstrating that head shape varies seasonally in both sexes of E. chamberlaini. Collectively, these results indicate that hypertrophy and atrophy of jaw muscles, or potentially integumentary glands, not only occurs in males and females of the Eurycea bislineata complex, but also may be more ubiquitous within the genus. The previous lack of this observation within the E. quadridigitata complex is most likely the result of difficulty in observing head shape variation in a diminutive species similar to E. chamberlaini without the use of large datasets and morphometric analysis.