Sex and reproductive condition shape thermal acclimation strategy in a plethodontid salamander

Abstract

The question of whether males and females differ in their responses to temperature is essential to understanding adaptive capacities in a warming world. However, sex has traditionally been neglected in the field of thermal ecology, and our understanding of the factors that promote sex differences in thermal plasticity is underdeveloped. Here, we investigate the independent and interactive effects of sex and reproductive condition on thermal acclimation capacity in a plethodontid salamander (Plethodon cinereus). We carried out two stop-flow respirometry experiments, with salamanders acclimated to one of two thermal environments designed to simulate a Cool or Warm breeding season. In the first experiment, we compared the thermal acclimation responses of males and gravid females across a gradient of three ecologically relevant test temperatures and observed distinct patterns of sexual dimorphism of standardized metabolic rate (SMR) between thermal treatments that were not attributable to differences in body size. Specifically, gravid females in the warm treatment showed reduced SMR relative to gravid females in the cool treatment, particularly at higher test temperatures. In the second experiment, we expanded on our initial findings by directly testing the contribution of reproductive condition to observed sex differences in thermal acclimation capacity. By repeating the experiment of the early breeding season but including a third group of non-gravid females, we: (1) recapitulated our original finding - that gravid females exhibit a pronounced response to thermal acclimation relative to males; and (2) showed that female thermal acclimation responses are absent in nongravid females, and therefore we concluded that responses are contingent on reproductive condition. Taken together, our results provide a first glimpse into how sex and reproductive condition contribute to intraspecific variation in thermal acclimation capacity in an amphibian and underscore the need for more hypothesis-driven studies to directly test when, where, and how such patterns arise.