Redefining the study of sexual dimorphism in bats: following the odour trail

Abstract

Sexual dimorphism is the condition in which males and females of the same species are easily distinguished by specific traits (secondary sex characteristics), often related to body size, colour patterns, weapons, and ornaments. Males of many mammal species tend to be larger or more ornamented than females, and these characteristics tend to be more pronounced in polygynous, diurnal, and open‐habitat species. Bats have long been considered a largely non‐sexually dimorphic group due to lack of conspicuous differences in body size and other cranial and skeletal characters. However, bats, like many mammals, exhibit a diverse array of soft‐tissue integumentary glands and non‐glandular odour‐producing structures with intense odorous substances that have not been thoroughly investigated, although postulated functions include facilitating mate selection in their generally polygynous associations. To date, there has been no systematic assessment of the occurrence or expression of sexually dimorphic traits in bats, many of which show intriguing sexual dimorphism in soft tissue, and most of which involve intense odours. In this study, we review evidence of integumentary glands and non‐glandular odour‐producing structures known in bats, as a first step towards identifying future research pathways to study sexual dimorphism in bats. Highly variable glands and non‐glandular odour‐producing structures have been noticed in ten different regions of the body, but are most frequently found on the head and the ventral region of the neck. They have been described in nearly 9% of bat species and in 70% of 21 extant bat families. Our review, based on extremely scattered and unevenly detailed literature, unveils the extraordinary sexual dimorphism that has been observed in Chiroptera to date, identifying not only target body parts where sexually dimorphic traits are likely to be found, but also critical avenues for future investigation and discoveries, and stressing the importance of the timing of secondary sexual trait observations, behavioural studies, and chemical analyses.

Microstructural characterization of the skin in the African straw‐coloured fruit bat, Eidolon helvum (Pteropodidae)

“…The reason for this histochemical characteristic of the sebaceous glands of the withers and ventral abdomen in this species is not known. However, it is probable that the sebaceous glands in these body regions may be associated with individual recognition, sexual signalling, sexual dimorphism and insecticidal functions as reported in some bats (Muñoz-Romo et al, 2012;Muñoz-romo & Page, 2021;Nassar et al, 2008). Mucins, glycoproteins, proteoglycans and hyaluronic acid are biochemical constituents of skin normally expressed in different parts of the skin.…”

Acoustic signal dominance in the multimodal communication of a nocturnal mammal

“…Indeed, olfactory signals play an essential role in agonistic interactions in many mammals, including territorial defense ( Jordan et al 2007 ) and the modulation of aggressive behavior ( Stritih and Kosi 2017 ). In bats, 15 species have been reported to have an odor-producing gland or structure ( Muñoz-Romo et al 2021 ). Most of the glands and odor-dispersing structures are sexually dimorphic (as in H. armiger ; Zhang et al 2020 ), indicating that olfactory signals play a vital role in mate choice and copulation ( Muñoz-Romo et al 2021 ).…”

“…In bats, 15 species have been reported to have an odor-producing gland or structure ( Muñoz-Romo et al 2021 ). Most of the glands and odor-dispersing structures are sexually dimorphic (as in H. armiger ; Zhang et al 2020 ), indicating that olfactory signals play a vital role in mate choice and copulation ( Muñoz-Romo et al 2021 ). Moreover, we previously showed that male H. armiger uses olfactory signals to discriminate among individuals and also to mitigate the costs of conflict ( Zhang et al 2020 ).…”

Do bats use scent cues from guano and urine to find roosts?

“…Fourth, roost-finding bats might use scent cues because in some species male bats use glands to scent mark roost entrances (e.g., Pacific flying fox (Pteropus tonganus, Grant & Bannack, 1999), the black flying foxes (P. gouldii, Moulton, 1967;Nelson, 1965), gray-headed flying fox (P. poliocephalus, Moulton, 1967;Nelson, 1965), greater sac-winged bat (Caspers et al, 2009), pale spear-nosed bat (Phyllostomus discolor, Höller & Schmidt, 1996), and lesser sac-winged bat (S. leptura, Caspers et al, 2009). Females can recognize quite a few individual characteristics from this scent marking, such as age, sex, social status, and identity, that can help them select a mate (Muñoz-Romo et al, 2021). While the function of scentmarking in males is to increase mating opportunities by signaling to females or rival males, scent marks at the entrance of a roost could also act as a cue to whether that roost was occupied by familiar or unfamiliar conspecifics.…”