Hunnicutt, K. E.; Glenn, E. S.; Perry, J. C.; Tatro, M. Q.; Conrad, L. K.; Juergens, M. Y.; Wilsterman, K.; Skinner, B. M.; Larson, E. L.; Velotta, J. P.

Extreme environments like high elevation impose chronic challenges to endothermic animals who must sustain metabolic demand despite reduced oxygen availability. Male reproduction is a tightly regulated physiological system, and hypoxia exposure has negative but transient effects on male reproductive output in diverse mammal species not adapted to high-elevation. Little is known about whether high elevation populations adapt to the detrimental effects of hypoxia on male reproductive physiology. Here, we use high- and low-elevation populations of deer mice (Peromyscus maniculatus) and a lowland congener, P. leucopus, to test how male reproductive phenotypes have diverged across populations and in response to hypoxia exposure across acute, intermediate, and chronic timescales. We complement these phenotypic assays with bulk RNA sequencing of testes to assess the testis-specific response to hypoxia. Contrary to expectations, hypoxia exposure had a negligible effect on low-elevation strains, while highland mice had both lower baseline fertility and heightened sensitivity to hypoxic stress including complete absence of mature sperm. Gene expression patterns indicate that hypoxia exposure dramatically reorganizes testes architecture in highland mice, consistent with testicular regression similar to that observed in seasonally breeding mammals. We hypothesize that this increased sensitivity of male reproductive physiology in highlanders reflects a life history trade-off, allowing energy reallocation between reproduction, survival, and thermogenesis in response to stress in extreme environments.