Locomotor adaptations in the disc-winged bat Thyroptera tricolor. I. Functional organization of the adhesive discs

Locomotor capability on smooth vertical surfaces is rare in mammals. The adhesive discs on thumbs and soles of bats of the family Thyropteridae are uniquely specialized for this purpose, but their functional organization remains virtually unexplored. The face of the discs is concave, has a characteristic dermatoglyphic pattern, and is permanently moist. Each disc is supported by a biconcaval fibrocartilagenous framework; its circular "rim" is divided into 60–80 chambers, each accommodating a sudoriparous gland which discharges on the disc face. Glandular form and cytology are unique, reflecting special functional adaptations. Other significant adaptations involve the epidermis, distribution of adipose tissue, extrinsic muscle insertions, disc relations to digital joints, and distribution of certain sensory endings. The discs have no intrinsic musculature and are manipulated from without. Experiments with living Thyroptera and analysis of the internal organization of the discs verify that their functional mode is suctorial rather than some other adhesive mechanisms; special relationships within the thumb discs suggest that the adhesive force may be to some extent adjustable under normal use.