Quantification of Urinary Sex Steroids in the Big Brown Bat (Eptesicus fuscus).

Bats (order Chiroptera) are the second largest group of mammals, diverging ~52.5 million years ago. Many species exhibit an unusual reproductive cycle and extreme longevity without reproductive senescence, yet steroid profiles exist for few bats. Big brown bats (Eptesicus fuscus) are temperate insectivores found throughout North America. They mate promiscuously in fall, store sperm during winter hibernation, and have delayed ovulation and fertilization in spring. Here, we report the first urinary steroid profile in bats by quantifying 17β-estradiol (E₂) in captive male and female E. fuscus across their reproductive cycle. Male bats had higher urinary E₂ levels than females, and adults had higher levels than yearlings following creatinine adjustment for hydration. In nonpregnant females, several seasonal differences in creatinine-adjusted and unadjusted urinary E₂ levels were observed. Urinary E₂ was higher in males than females in winter for both conditions and in autumn for creatinine-adjusted levels. We quantified progesterone (P₄) in a subset of females. In nonpregnant females, urinary P₄ was constant across seasons except for unadjusted levels, which were highest in the summer. In pregnant females, urinary E₂ and P₄ levels peaked beginning ~20 d before parturition, with both steroids returning to baseline in the following weeks. Knowing how urinary steroid levels fluctuate with age and sex and across the annual season is key to understanding reproductive cycling in bats. Our research furthers the potential for bats as a model for medical reproductive research. Moreover, it complements previous studies on the potential role of steroids in primer pheromonal effects in bats.