Mammalian odors are frequently sexually dimorphic, with males often exhibiting a stronger, or otherwise distinct, odor relative to females, which can be especially useful for nocturnal species with reduced use of vision. Some male bats exhibit intense odors to attract females and reproduce, presumably as a consequence of a high concentration of testosterone. This usually coincides with the fertile period of females, which can be difficult to recognize visually because female mammals rarely advertise their precise reproductive condition. Recently, a novel odorous crust was discovered on the forearms of adult male fringe-lipped bats, Trachops cirrhosus. To understand its reproductive significance, we explored potential relationships between the size of the crust and testosterone concentration, testes size, female reproductive state, ectoparasite load, and body condition. We sampled bats during the mating season and determined crust size, testosterone concentration (plasma), testes volume, precise reproductive status of females (vaginal smears), body condition (ratio of body mass and forearm length), and ectoparasite load. Crust size (0–38 mm2) was positively correlated both to testosterone concentration (0.7–36.4 ng mL−1) and to relative testes volume (6.3–349.1 mm3). During the sampling period, vaginal smears confirmed that all females were in estrus. Ectoparasite load in males was significantly lower than in females, although both sexes were in similar body condition. Our results suggest that testosterone concentration is a key factor in the exhibition of a larger crust and presumably a stronger odorous signal to estrus females during the reproductive season. Male secondary sexual characteristics are critical for attracting females for mating. Male odor, in particular, can be indispensable in nocturnal species, as visual signals may lose their effectiveness under dark conditions. The most important male hormone, testosterone, increases the development of male exclusive traits, including odors. In the Neotropics, reproductive male fringe-lipped bats exhibit large odorous crusts on their forearms. The largest male crusts correspond with the highest concentration of testosterone, a relationship we find in the mating season, when vaginal smears confirm females are fertile. We also show that males with crusts have low ectoparasite loads. Our results suggest that the male crust could potentially reveal critical information. Our research highlights the significance of testosterone in the expression of male exclusive traits.

中文翻译：

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男性的外壳：当女性生育时，大小重要吗？

哺乳动物的气味通常是两性异形的，与雌性相比，雄性通常表现出更强烈或不同的气味，这对于视力使用减少的夜间物种尤其有用。一些雄性蝙蝠表现出强烈的气味来吸引雌性和繁殖，这可能是高浓度睾丸激素的结果。这通常与雌性的生育期相吻合，这在视觉上很难识别，因为雌性哺乳动物很少宣传其精确的生殖状况。最近，在成年雄性流唇蝙蝠 Trachops cirrhosus 的前臂上发现了一种新的有气味的外壳。为了了解其生殖意义，我们探索了外壳大小与睾酮浓度、睾丸大小、女性生殖状态、体外寄生虫负荷和身体状况。我们在交配季节对蝙蝠进行了采样，并确定了外壳大小、睾丸激素浓度（血浆）、睾丸体积、雌性的精确生殖状态（阴道涂片）、身体状况（体重和前臂长度的比率）以及体外寄生虫负荷。外壳大小 (0-38 mm2) 与睾酮浓度 (0.7-36.4 ng mL-1) 和相对睾丸体积 (6.3-349.1 mm3) 呈正相关。在采样期间，阴道涂片证实所有雌性都处于发情期。尽管两性的身体状况相似，但雄性的体外寄生虫负荷明显低于雌性。我们的研究结果表明，睾酮浓度是显示较大外壳的关键因素，并且可能是生殖季节发情女性发出的更强气味信号。雄性第二性征对于吸引雌性交配至关重要。特别是雄性气味在夜间活动的物种中是必不可少的，因为视觉信号在黑暗条件下可能会失去效力。最重要的男性荷尔蒙，睾丸激素，会促进男性独有特征的发展，包括气味。在新热带地区，繁殖的雄性带唇蝙蝠的前臂上有大的有气味的外壳。最大的雄性结痂对应于最高浓度的睾酮，这是我们在交配季节发现的一种关系，当时阴道涂片证实雌性可以生育。我们还表明，有痂的雄性的外寄生虫负荷较低。我们的结果表明，雄性地壳可能会揭示关键信息。