The diversity of bat faces

Last updated:

10/09/26, 14:25

Published:

24/09/26, 08:00

The New World leaf-nosed bats of the ‘Phyllostomidae’ family are known for their diversity, specifically their diverse skull shapes and therefore face shapes.

Whether it’s saving Gotham City or lurking in dark caves, the general public tends to have one standard idea of what a bat looks like. However, bats are a diverse group; approximately one in five mammalian species is a bat species. The New World leaf-nosed bats of the ‘Phyllostomidae’ family are known for their diversity, specifically their diverse skull shapes and therefore face shapes. This article will explain how and why the New World leaf-nosed bats have such varied skulls.

Diet variation leads to skull variation

Different bat species in the ‘Phyllostomidae’ family have different diets: either insects/arthropods, fish, land animals, nectar/pollen, fruit, blood, or a combination (i.e. omnivorous). A bat’s skull shape is adapted to eating a specific food group. For example, bats with long snouts are adapted to reach inside flowers for their nectar (see purple in Figure 1) while bats with short, wide skulls can easily bite into fruit (see blue in Figure 1). Although closely related bats tend to have the same diet, there are two separate groups of nectar-eaters that independently evolved the same long, thin snout (Figure 1). A 2019 paper by Camacho et al. found that all faces and skulls of ‘Phyllostomidae’ bats lie on a spectrum between the long, thin skulls of nectar-eaters and the short, fat skulls of fruit-eaters.

How bat skulls are analysed

To create their spectrum of bat face morphology, Camacho et al had to quantify the shape of a bat skull. They did this by taking computed tomography (CT) scans of bat skulls and picking “landmarks” that were common amongst all skulls (Figure 2A). These landmarks were treated like coordinates to quantify the distance between landmarks of the same skull (Figure 2B), and the distances between landmarks are fed to a geometric formula to quantify a skull’s shape. Alternatively, the same landmarks on different skulls can be compared to quantify the morphological differences between them (Figure 2B).

Not all studies of bat skulls use complex mathematics and computing. A 2014 study by Karen E. Sears took linear measurements on real skulls, finding that the simple ratio of palate length to palate width is a good proxy for a bat’s face shape. The researchers also found that palate width correlates with diet preference after standardising for body size - the palate of nectar-eating bats grows disproportionately long during embryonic development, while the palate of fruit-eating bats grows disproportionately wide. Thus, the elaborate computer-based measurements of the 2019 paper and the simple measurements of the 2014 paper came to the same conclusion about how diet is related to bat skull morphology.

What skull shape can tell us about evolution

By comparing skull shape measurements between bats, we can track the evolutionary trajectory of bat faces. In their 2019 paper, Camacho et al. compared ‘Phyllostomidae’ bat skulls of various species, ages, and embryonic stages using their CT scan landmark measurements. Using their singular skull shape measurement, they concluded that the adult skulls of ancestral bat species looked like the juvenile or embryonic skulls of species that evolved from that ancestor. Baby skulls from different bat species all looked similar, and their common baby skull morphology resembled the adult skull of their common ancestor. This phenomenon, called ‘peramorphosis’, would be like if humans evolved into a species whose foetuses and babies had adult human heads (which is weird to think about).

Peramorphosis can happen in two ways, both of which are found across the ‘Phyllostomidae’ family tree. If the skull develops faster in the later-evolved bat species than in the ancestral species, the later-evolved species will already have the adult skull while it is still a pup (Figure 3). This is called ‘acceleration’, and is thought to occur due to a faster rate of cell division in the embryo. Fruit-eating bats are an example of this acceleration - their faces are short because the palate cells growing outwards finish their growth and differentiation early on in the bat’s development.

The second way peramorphosis can occur is called ‘hypermorphosis’, which is when the skull of a later-evolved bat species continues to develop past where the ancestor’s skull would have stopped (Figure 3). Hypermorphosis occurs when the palate cell signal to stop dividing is delayed in the later-evolved species compared to the ancestor. An example is the nectar-eating bats with long faces - their palate cells divide and differentiate at the same rate as the ancestor, but they divide more times before dying, so the palate grows further out than in the ancestor. Therefore, evolution at the cellular level in a bat’s palate controls skull evolution by peramorphosis and ultimately species evolution.

Conclusion

A change in bat diet puts selective pressure on the bat, causing its cells to change how they divide, which causes a change in face shape, adapting to the change in diet. In peramorphosis, different bat species have a common skull shape as pups - resembling the adult skull of their ancestors - and unique skull shapes as adults that are adapted to their varied diets. This shows how ecology, mathematics, cell biology, developmental biology, and evolution all work together to explain why animals look the way they do.

Written by Simran Patel

Project Gallery

Figure 1- Diversity of New World leaf-nosed (a.k.a phyllostomid) bats and their diets.