I like bats. They’re flying mammals that can echolocate; how cool is that? And I have exciting bat news today: I now have a favorite bat. I present to you the glorious, beautiful, and majestic hammer-headed bat (Hypsignathus monstrosus).
Hammer-headed bats are a type of megabat – bats that belong to the family Pteropodidae. Megabats are also called fruit bats, Old World fruit bats, or flying foxes. They are the only family of bats who don’t echolocate with their larynx (although a few species primitively echolocate with their tongues or wings). Instead, megabats typically rely on their sense of sight and smell to navigate and find their typical source of food: fruit.
The hammer-headed bat is found throughout West and Central Africa and is the largest bat in continental Africa. It is, as the youth say, an absolute unit. Hammer-headed bats also have some cool adaptations and behaviors that are unique among bats.
- Like many bats, hammer-headed bats are nocturnal and sleep during the day. They often roost alone, but small groups of around four individuals are also found. Larger groups of up to twenty-five individuals have also been documented, but these are much rarer. In most bat species, roosts are segregated by sex: males roost together, and female roost together. In contrast, the hammer-headed bat will roost in mixed sex groups. However, even though the sexes will sleep together, they don’t really pay attention to each other. Individuals of the same group will just sleep during the day, wake up at night to go forage without interacting with their group mates.
- Sexual dimorphism
- Male and female hammer-headed bats look super different from each other; this species is the most sexually dimorphic bat species in the world. First off, males are much larger than females. Male hammer-headed bats are almost twice as heavy as the females: 420 grams vs 234 grams on average. Males also have a very striking face with large lips. In contrast, females have a narrower snout with a foxlike face. You can definitely tell males and females apart!
- One interesting but non-visual difference between male and female hammer-headed bats is in their sex chromosomes: they are one of the few mammals that have an XO sex-determination system. While females have the classic XX chromosomes that you are probably familiar with, males have just a single X chromosome and no Y chromosome. If we count up their chromosomes, this means that female hammer-headed bats have 36 chromosomes while males have only 35.
- Male face
- Why do male hammer-headed bats have such a different face than the females? It has to do with their mating system. Males attract females through the sounds that they make: loud, honking vocalizations. In order to make these sounds, male hammer-headed bats have larynges, or voice boxes, in their throats that are almost three times bigger than the ones in females. This larynx is half the length of the spine and is so large that organs like the heart, lungs, and digestive tract are displaced in males. In addition, male hammer-headed bats have large resonating chambers to amplify the sounds made from the larynx. These chambers are air sacs in the pharynx (the area between the back of the nasal cavity and start of the esophagus) that are connected to a large sinus in the snout.
- As I alluded to, males rely on the sounds they make in order to find mates. Hammer-headed bats are particularly cool because they have a lek mating system and are thought to be the only lekking bat species. In lek mating, males will gather together in a particular region (called a lek) where they establish display territories. These territories are literally just areas where the male can display for females. Then, the males just sit around displaying at the lek. Females look at all the possible options, decide who they want to mate with, mate with their chosen male, and then leave to go raise the offspring on their own.
- Here’s how lekking works specifically for hammer-headed bats. Males will form leks along streams or riverbeds during the mating season. Each lek contains between 20 and 135 males in a strip of land 130 feet wide and between 1,300 and 5,200 feet long. Males will compete with each other in the lek to get the best display territory; once established, each male will have a display area that is about 33 feet in diameter. Then, the male just hangs from his branch, honking and flapping his wings until a female chooses to land next to him. After a quick mating session (30 – 60 seconds), the female flies off and the male resumes honking until another female appears. Pretty straightforward! You can hear the honking of a hammer-headed bat lek here.
- One other fun sex difference in the hammer-headed bat is how males and females search for food. Both sexes are frugivores and eat fruit. They mostly eat figs, but other fruits like mangos, guavas, and bananas are also on the menu. Hammer-headed bats even have a fun tongue with backwards-facing papillae (taste buds) that help extract juice from fruit! Now, female hammer-headed bats forage using what is called trap-lining: in this foraging strategy, females travel along an established route that has dependable sources of food. In other words, they go to places where they know food will be, even if that food may be of lower quality. In contrast, males actively search for patches of high quality food and may even travel up to 6 miles to reach exceptional patches. This difference in foraging strategies may reflect the size difference between males and females. For males, they may have higher metabolic needs and require high food quality, while the smaller females can survive on moderate quality food patches.
Hammer-headed bats, although large, are mostly harmless. They do, of course, eat a lot of fruit crops (which is annoying), and their honking can keep people up at night (also annoying). But they (like many bats!) don’t actually harm people.
The only potential harm of hammer-headed bats is that they are possibly a reservoir for the Ebola virus. Some individuals that have been tested have Ebola anti-bodies, but these individuals didn’t have the virus itself. Also, as of 2015, no Ebola outbreak has been traced back to a hammer-headed bat hunter or researcher. Still, you should be careful if interacting with these large bats or if interacting with wild animals in general. It’s best to always be respectful of wild animals: even though they probably won’t hurt you, it doesn’t mean they can’t.