A study by Dr. Ronald P. Hall and his colleagues focuses on how bats become specialized to eat different foods and live in different environments; it shows us how the senses are a really important part of being well adapted to one’s environment.
This study is about bats, but first let’s start with a more familiar example of evolution, a classic from intro high school biology class: Darwin’s finches.
Darwin’s finches, or the Galapagos finches, are an example about how closely related species have different adaptations so they can take advantage of their environment. Some of these finches have big beaks that are good for eating big seeds, some finches have small beaks that are good for eating nectar and small insects in flowers, but all the finches are related. This happened because, over time, birds with big beaks did better at eating seeds than birds with medium beaks, and birds with small beaks did better at eating nectar than birds with medium beaks. So all the medium beak birds died off and the two separate groups of birds stuck around. This example shows us how different species separate because they adapt to take advantage of different resources.
Here is an illustration of the Galapagos finches drawn by Charles Darwin himself! You can see how the birds on the top have larger beaks, and the ones on the bottom have smaller ones.
The finch example is about the evolution of a physical change, but what the authors of this paper argue is that before a physical change, often there needs to be a change in what the animals can sense. After all, if you can’t see seeds or smell nectar, neither a big or small beak will do you any good.
The scientists tested this theory by studying a specific family of bats called Neotropical leaf-nosed bats (the scientific name is Phyllostomidae, and you can see a picture of these cuties below!). These bats are a good example to use to investigate this idea because, although the bats are closely related, they have a wide variety of diets. Some eat only plants, only nectar, only bugs, fish, or even other bats. These bats all have different bodies that are well adapted to their special diets.
Here are some leaf-nosed bats! These are in the genus Artibeus and they primarily eat fruit, but sometimes pollen and insects too.
The scientists looked at the size of the bat’s eye orbits (the place where their eyes sit in their head), their olfactory bulb (the part of the nose that does the smelling), and their cochlea (a part of their ears that does the hearing), which correspond to how well they can see, smell, and hear. They studied bats from the whole family tree of the leaf-nosed bats.
They found that bats that eat mostly fruit had bigger eyes and noses, and that bats that eat mostly insects tended to have bigger ears. This makes sense because bats that need to eat fruit have to find fruit, which are usually brightly colored and smell good. On the other hand, insects are hard to see and also don’t have a strong smell, but they do make noise. Some bats are known for their ability to echolocate, which means they can use sound to find insects that are hard to see.
They also found that bats that eat mostly nectar have smaller sense organs in general, which surprised the scientists. Based on all these measurements from this family of bats, the scientists estimated that the ancestor of this family was omnivorous, and had a bigger head, eyes, and nose than bats from other families.
The scientists concluded that because their ancestor had such good sense organs in general, these bats’ sense organs were able to evolve really quickly. This meant that the rest of their body could evolve quickly in response. One piece of evidence for this is that fruit bats evolved large eyes and noses three separate times, showing that the eyes and noses were an important prerequisite to adapting to eating fruit, before physical traits.
These results, and this kind of study in general, are a rather new way of studying the evolution of animals. Looking at senses, rather than just physical features, opens up more possibilities to understand how ancestral animals experienced the world. This sort of study provides an example of learning more about how the story of evolution unfolds by looking not only at the shape of the animals, but also their experience.
Ultimately, the authors’ conclusion supported their initial idea. These bats only became specialized once they could sense their new environment, and new studies like this might find that this is true for many animals. After all, it is pretty hard to eat something that you can’t see.
Here is a link to the original study if you would like to check it out:
https://onlinelibrary.wiley.com/doi/10.1111/evo.14270
Citation:
Hall, R.P., Mutumi, G.L., Hedrick, B.P., Yohe, L.R., Sadier, A., Davies, K.T.J., Rossiter, S.J., Sears, K., Dávalos, L.M. and Dumont, E.R. (2021), Find the food first: An omnivorous sensory morphotype predates biomechanical specialization for plant based diets in phyllostomid bats*. Evolution, 75: 2791-2801. https://doi.org/10.1111/evo.14270
