White and clear wings in bats (Chiroptera)

Abstract

White or clear (“whitish”) wings are a distinct feature in about 30 species of tropical insectivorous bats (Mammalia: Chiroptera) belonging to three families (Emballonuridae, Molossidae, and Vespertilionidae). Such wings may provide camouflage against the sky at dusk and dawn, when bats commute to and from the roost and are vulnerable to aerial predation from birds. We tested this hypothesis by comparing the contrast of black, white, and transparent plastic models against the evening sky. Compared with normally dark wings, white and particularly transparent wings indeed reduce the contrast against the sky and may also prevent overheating in bats flying in daylight. Whitish wings could facilitate earlier evening emergence and later morning return, increasing access to crepuscular or diurnal insects as food. But whitish wings become maladaptive near artificial lights, where they are highly visible when illuminated against the dark sky. Pale but colored (not whitish) wings and reticulated patterns on translucent wings in some African and south Asian bats may be variations on the same theme, functional as camouflage against a lit background of vegetation and shades.