Linking plasma sex steroid hormone levels to the condition of external genitalia in European badgers (Meles meles): A critical evaluation of traditional field methodology

Field biologists often rely on examination of external genitalia as a proxy for mammalian reproductive condition. In seasonally breeding European badgers, scrotal testes are used as an indicator of spermatogenesis, while a pink and swollen vulva with mucosal secretion is assumed to indicate oestrus. Systematic validation of these assumptions is lacking. Here we analysed sex steroid hormones from plasma samples collected from sexually mature adults during the winter mating season, the spring post-weaning period, the summer minor mating peak, and autumn reproductive quiescence, to compare circulating testosterone levels in males (n = 216), and oestrone (n = 143) and oestradiol (n = 36) levels in females with concurrent external genitalia condition (males: descended, intermediate, ascended testes, as well as testes volume; females: swollen, intermediate, normal vulva). Sex-steroid levels exhibited seasonal patterns, broadly reflecting the seasonal variation in genital condition. Nevertheless, on an individual level, male testosterone levels correlated significantly with testes condition and testes volume only during the mating season in spring, and neither oestrone nor oestradiol differed between female EGM categories. We thus conclude that, in males, EGM is a reliable indicator of reproductive activity during the mating season, but can provide only a tentative indication of physiological reproductive status outside this period, while in females EGM is generally less precise for assessing female reproductive status.