18 January 2023
Why do hammer-headed fruit bats honk?
The Hammer-headed Fruit Bat (Hypsignathus monstrosus) is the largest of the African bats. Named for its unusual appearance, this species is a classic example of sexual dimorphism at work, with males and females displaying significant differences in both size and appearance. While females are smaller and possess the familiar fruit bat face that usually generates a stream of ‘awwww!’ comments on YouTube, males elicit a completely different response. Their large mallet-like faces, flaring nostrils, flappy lips and bulging eyes, teamed with a huge wingspan of up to a metre, undoubtedly influenced the selection of ‘monstrosus’ when zoologists named the species in the 19th century.
The first scientific description of the Hammer-headed Fruit Bat was published in 1861 in volume 13 of the Proceedings of the Academy of Natural Sciences of Philadelphia. The article’s author, the American physician and anatomist Dr. Harrison Allen, provided a highly detailed breakdown of the bat’s anatomy, including everything from dental records to the stiffness of its fur. The specimen studied by Allen had been collected by the French-American explorer Paul Du Chaillu who had been sent on an expedition to Africa by the academy in 1855. A second description was published in 1862 in the Proceedings of the Zoological Society of London, written by the Scottish naturalist and lawyer Andrew Murray. Murray only received a copy of Allen’s description after his own paper had gone to print (the journal had taken 7 months to arrive in the UK). In the postscript, Murray noted that if both were describing and naming the same species, Allen’s name of Hypsignathus monstrosus must take precedence (the name the species carries today). Their descriptions varied slightly, however Murray assumed this was due to differences in the preserved specimens being examined; Murray believed Allen was working from a dried skin whereas he had access to a specimen preserved in spirits. This enabled Murray to include a detailed illustration of the species alongside his written description.
Being able to observe living individuals in their natural habitat was the next step. Field studies conducted in the 20th century revealed a surprising aspect of the bat's behaviour. During the breeding season, males were seen to congregate at dusk for an evening of intense vocal competition. A chorus of loud, monotonous honking would fill the night air as males used their calls as a way to prove their genetic fitness to nearby females looking for a mate. This recent recording of a Hammer-headed Fruit Bat lek includes the characteristic honking and was made by Michael Mills in Kumbira Forest, Angola on the 8th September 2013 (British Library reference WA 2014/001/001/431).
An extremely long larynx, measuring half the length of its body cavity, is what allows males to take part in such sustained sonic battling. This gathering of displaying males in an arena-like setting, known as lek behaviour, is more commonly seen in birds. Though the mating system is also seen in mammals, only a handful of bats are known to use this process. Scientists continue to uncover previously unknown aspects of bat behaviour and so more species could be added to this list in the future. Our journey to fully understanding this complex and diverse group of mammals is far from over.