Sexually Dimorphic Heads of Eurycea bislineata

Abstract

Gladwyn Kingsley Noble was the first investigator to collectively examine courtship glands and correlate their function to the tail-straddling walk in plethodontid salamanders. While mental glands and caudal courtship glands have received the majority of attention since Noble's seminal work, Noble described other glands from Eurycea bislineata that were putatively involved in courtship that have received little or no attention (e.g., glands at the temporal regions of the heads of males). Previous studies demonstrated that heads of males enlarge during the mating season and some studies indicated that the enlargement was because of Noble's previously described temporal glands; however, current consensus of male head enlargement is that skeletal muscle hypertrophy is the cause of the head growth. In this study, we examined male and female E. bislineata throughout the year to test Noble's hypothesis that males possess courtship glands in the integument of the temporal regions of their heads and to assess what underlying tissues are involved with male head enlargement during mating. We found that the temporal regions of male heads change dramatically from non-mating to mating months, exemplified by two-dimensional geometric morphometrics. This variation is a result of M. levator mandibulae externus hypertrophy rather than glandular activity underlying the integument. Although no glandular masses isolated at the temporal regions of the heads result in this dynamic shape change, simple alveolar glands that resemble stereotypical courtship glands are found in the integument of only males from the mating season. These putative courtship glands are scattered amongst mucous and serous glands within the integument around the eyelids to at least the posterior termination of the cranium. The function of these glands is unknown but is probably involved with pheromone production, similar to more thoroughly examined courtship glands.