Individual Variation in the Chemical Profiles of Odorous Male Bat Crusts

Visually conspicuous sexually dimorphic traits such as body size, coloration, weapons, and ornaments are less effective in nocturnal mammals, favoring the evolution of signals in alternative sensory modalities. Acoustic and chemical signals, in contrast, function well in low-light environments, and chemical cues in particular may act as honest indicators of individual quality, as physiological condition and immune responses can alter odor profiles. Reproductively active male fringe-lipped bats, Trachops cirrhosus, exhibit a sexually dimorphic forearm crust characterized by a strong odor and seasonal enlargement coincident with female fertility. Crust size is positively correlated with testosterone levels, and males with larger crusts are more frequently found roosting with multiple females, suggesting a role in sexual signaling. Previous work identified six major chemical components of the crust. Here, we evaluated inter-individual variation in both chemical composition and compound concentrations of the crust. Gas chromatography - mass spectrometry (GC-MS) analyses revealed twelve volatile organic compounds detected in crust samples, with individual males containing an average of 7 ± 3 chemical compounds. Chemical composition and chemical concentrations varied substantially among individuals, indicating that male crusts are not composed by a determined set or standardized ratios of compounds. Instead, chemical profiles were highly individualized. Crust size was positively associated with both chemical diversity and compound abundance. Larger crusts (> 10 mg) contained more compounds and higher overall concentrations than smaller crusts. Cholesterol and Cholest-7-en-3-ol were the most consistently detected compounds across males. Five of six previously reported compounds were confirmed. Concentrations of individual compounds differed by several orders of magnitude among males, and some compounds were absent in certain individuals. These results indicate pronounced individual chemical variability in male forearm crusts, supporting the hypothesis that this trait functions as a complex and individualized chemical signal potentially relevant to mate choice and sexual selection.