The stress of scramble: sex differences in behavior and physiological stress response in a time-constrained mating system
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Iteroparous species maximize lifetime reproductive fitness by balancing current and future reproductive investments. In order to maximize fitness in the face of social or environmental heterogeneity, individuals of the same species may vary in whether they prioritize current reproductive opportunity or sacrifice immediate reproduction in order to prioritize survival and future reproductive potential. Glucocorticoid (GC) secretion plays an important role in mediating this trade-off by promoting behavioral and physiological responses associated with survival, often at the expense of nonessential (e.g., reproductive) functions. We used wood frogs (Lithobates sylvaticus [Rana sylvatica]) to test whether males and females differed in their (a) physiological response (plasma corticosterone [CORT] concentration) to standardized handling stress—a proxy for predation threat—and (b) performance of reproductive behaviors that may enhance their conspicuousness to predators. We also tested whether levels of male competition influenced sex differences in these factors, as more intense competition may require males to devote more time to risky reproductive behaviors. We found that females had lower baseline CORT but exhibited a significantly greater CORT response to a stressor and spent less time performing potentially risky behavior (surface floating) than did males. These sex differences were consistent across different levels of male mating competition. Our results reveal that during breeding, males and females may differentially respond to stressors and perform risk-prone behaviors, despite facing the same extreme breeding constraints, providing new insight into the survival-reproduction trade-off of explosively breeding species.
KeywordsAmphibian Corticosterone Life-history trade-off Reproduction Male competition Sex differences
We thank the Cavener Lab for use of their plate reader. We are grateful to Jennifer Tennessen, Brad Carlson, Vincent Faguliani, Jill Newman, and John Swierk for the field assistance; Courtney Norjen and Tyler Jacobs for the laboratory assistance; Crystal Kelehear Graham for the statistical advice and support; and the Hildebrand family for their support and enthusiasm. We thank B. Chitterlings for the comments on an early draft of this article. This research was funded by the National Science Foundation (DGE-1255832 to LS and IOS-1051367 to TL); any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.
This work adhered to the national and international standards on animal welfare, the legal requirements of the USA, and the Institutional Guidelines of Penn State University (IACUC #33346). This work was approved by the Pennsylvania Fish and Boat Commission (Permit #483, Type 1) and the Pennsylvania Game Commission (Permit #NC-028-2012, NC-027-2011, NC-019-2010).
Conflict of interest
The authors declare that they have no conflict of interest.
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