Chemical antlers: sexual dimorphism in salivary and lacrimal glands of house mouse subspecies

Sexual dimorphism (SD), the systematic difference in phenotype between males and females of the same species, can arise through sexual and natural selection. Although SD is traditionally associated with conspicuous traits such as body size or colouration, it may also occur in cryptic characteristics such as chemical signalling. In mammals, where olfactory communication plays a central role, SD may be reflected in differences in the size or morphology of scent glands, as well as in the abundance and composition of their secretions. Here, we investigate sexual dimorphism in the size, histology, and protein content of the submandibular and lacrimal glands in two house mouse subspecies, Mus musculus musculus and M. m. domesticus . We showed remarkable dimorphism in both glands, with males of both subspecies exhibiting larger glands, including a higher proportion of granular convoluted tubules (GCTs) in the submandibular gland. Subspecies-specific differences in gland size were detected only in the submandibular gland, which was larger in M. m. musculus . In contrast, SD was more pronounced in the lacrimal gland in both subspecies and was strongest in M. m. domesticus . Furthermore, we found subspecies-specific differences in tear protein content and odour cue preference, suggesting mate recognition systems may be more divergent between these closely related taxa than previously assumed. By integrating data from wild animals and wild-derived strains, we provide a comprehensive assessment of sex-specific morphological and biochemical divergence in these exocrine glands. Our findings underscore the evolutionary significance of cryptic sexual dimorphism in mammalian olfactory signalling systems.