The single species, H. monstrosus, is found from Gambia to southwestern Ethiopia and south to northeastern Angola and Zambia (Hayman and Hill, in Meester and Setzer 1977; Koopman 1975; Largen, Kock, and Yalden 1974).
Head and body length is about 193-304 mm, there is no tail, and forearm length is 118-37 mm. The wingspan in males is as much as 907 mm. This genus has the greatest sexual dimorphism in the Chiroptera; Bradbury (1977) found that males, which averaged 420 grams, were nearly twice as heavy as females, which averaged 234 grams. The coloration is grayish brown or slaty brown. The breast is paler, and the lighter color extends up around the neck, forming a sort of collar. A white patch is present at the base of the ear. Shoulder pouches and epauletlike hair tufts are lacking in both sexes.
Male Hypsignathus may be recognized in flight by the large, square, truncate head. The muzzle is thick and hammer-shaped, hence the common name. Other distinctive features are enormous and pendulous lips, ruffles around the nose, a warty snout, a hairless, split chin, and highly developed voice organs in adult males. Females have a foxlike muzzle similar to that of Epomophorus.
In referring to this genus, Lang and Chapin (1917) commented: "In no other mammal is everything so entirely subordinated to the organs of voice." The adult male has a pair of air sacs that open into the sides of the nasopharynx and can be inflated at will, as well as a great enlargement of the voice box (larynx) and vocal cords. The larynx "is nearly equal in length to one half of the vertebral column," actually filling most of the chest cavity, pushing the heart and lungs backward and sideward. The voice thus produced, a continuous croaking or quacking, is quite remarkable and probably attracts the females. The gregarious chorus reminded Lang and Chapin of "a pondful of noisy American wood-frogs, greatly magnified and transported to the treetops."
The hammer-headed bat inhabits forests, being most common in swamps, mangroves, and palms along rivers. It usually roosts in foliage but has been found in a cave. Bradbury (1977) stated that Hypsignathus roosted at a height of 20-30 meters during the day and would forage up to 10 km from the roost at night. With the ripening of certain fruits, this bat often seeks the high forest or native clearings to feed. It may take the juices of mangoes, soursops, and bananas. Van Deusen (1968) reported that Hypsignathus killed and ate tethered chickens.