Examining the Pheromonal Potential of Estradiol in the Big Brown Bat

Abstract

Historically, physiologists have believed steroid hormones act exclusively within the individual producing them. However, studies in mice have shown that bioactive 17β-estradiol (E2) is excreted in male urine and absorbed by female conspecifics where it binds to estrogen receptors in reproductive other tissues. This can lead to pregnancy disruption and/or cause precocious puberty in female conspecifics. In bats the transfer of tritium-labelled estradiol (3H-E2) from male to females has been shown during the mating season. I investigated the influence of season on 3H-E2 transfer and showed that females housed with 3H-E2 injected males had significantly higher levels of radioactivity in reproductive, neural, and peripheral tissues during reproductively relevant timepoints compared to the non-reproductive season. Because urine has been hypothesized as the vector of steroid hormone transmission in mice, I examined the natural patterns of sex steroids in the urine of male and female big brown bats (Eptesicus fuscus) across the annual reproductive cycle. I found that creatinine-adjusted E2, which corrects for animal hydration and activity, was significantly higher in male than female urine, and in adult compared to yearling urine. Seasonal differences in urinary E2 levels were observed within and between sexes. Finally, I designed a protocol to investigate the attractant properties of body odour and urine between bat sexes during the mating season. Using a two-alternative Y-maze arena, I found that female bats first approach the test arm containing urine of a male conspecific before exploring the arm with female urine. Females also tended to spend more time in the test-arm containing male urine and being in the male test-arm at the conclusion of the 5 min trial. My data supports the hypothesis that E2 has the potential to act as reproductive pheromone with urine as a likely vector. Given the close proximity of individual bats within a maternity roost, steroid hormone transfer between conspecifics quite likely occurs in nature and could have profound influences on female reproductive behaviour (e.g. receptivity) and physiology (e.g. estrus cycling). My research provides new evidence for the potential pheromonal actions of E2 in bats that is consistent with how sex steroids act as pheromones in other mammals.