In part two of our conversation with evolutionary biologist Dr. Nancy Simmons, we discuss the astonishing diversity of bats and how they came to dominate Earth's nighttime skies.

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ERIC: From the Museum of Science in Boston, this is Pulsar, a podcast where we buzz for answers to the best questions we get from our visitors. I'm your host, Eric. And it's time for part two of my conversation with Dr. Nancy Simmons, the curator-in-charge of the Department of Mammalogy at the American Museum of Natural History in New York City. Last episode, we heard about why bats evolved the ability to fly. And today we're going to see how flight has resulted in an amazing variety of bat species living across nearly all of the Earth's skies. And yes, we'll find out if vampire bats really do drink blood. So flying has worked out really well for bats. And you can tell that's true because they are everywhere. Can you talk a little bit about how many species of bats there are and how diverse they are?

NANCY: It's an ever-changing number. The most recent number of for living species of bats is 1,456. In some parts of the world, there can be as many as 150 species, all living in the same piece of forest, all doing slightly different things. There's little teeny bats, great big bats, bats that fly close to the ground, bats that prefer flying up high in the forest canopy, bats that want to fly mostly in open space. And they're all anatomically different so that they're good at these various different lifestyles, so that they can survive and not compete too much with each other.

ERIC: Most people imagine that sweeping through the air to grab bugs, is that mostly the case?

NANCY: That is what the majority of species do. And the way that they separate out this niche space of being a flying nocturnal insect-eating animal is that they specialize on different kinds of insects. So one species may eat little tiny gnats and other species may specialize in hard beetles. Larger animals may specialize in really big moths that fly in certain places. Bats do not eat only one thing. So the moth eater will also sometimes take beetles, and the the one that eats little gnats will also eat somewhat larger things. But there are limits to this. And so it just depends on the species and where they are and how many other species are doing similar things in their environment.

ERIC: And based on what you said at first, it sounds like not all bats eat flying insects?

NANCY: Some bats eat other insect-related things, but some that might surprise you, like scorpions. There are bats that live in the desert that eat scorpions and great big centipedes. Some bats eat spiders, but those are all preying on things in the insect arthropod world. But there are other bats which are carnivorous. And they're meat-eating, and so they will eat lizards, frogs, birds, other bats. So some of these get reasonably large. There's a bat in South America and Central America that has a wingspan of about three and a half feet. And it's the largest carnivorous bat in the in the new world.

ERIC: Yeah, that sounds like an enormous bat, much bigger than you'd think bats could get.

NANCY: It's quite large. It has a skull that's an inch and a half long, and wit will take other pretty good-sized prey, mice, rats, birds, things like that. So we have the carnivorous bats like that, that eat vertebrate terrestrial forms. Some bats eat fish, so they will catch fish with enlarged hind feet and so they will fly over the surface of the water and scoop up small fish and eat them.

ERIC: Okay, that's crazy. I've never heard that.

NANCY: Yes, yes. Vampire bats. Vampire bats exist. There are three different species of them. They all live in the Americas, so South America, Central America, all the way on up into northern Mexico, and there's probably going to be vampire bats in Texas before long with climate change.

ERIC: And vampire bats...do they really feed on blood?

NANCY: They do. They feed exclusively on blood, they don't eat anything else. The common vampire bat feeds primarily on mammals. So it would, in a completely untouched tropical rainforest, probably feed on sleeping monkeys and deer and things like that. However, in most areas, there are people, and there are cattle, and so most living common vampire bats actually feed on cattle. But there's another species, it's a bird specialist. It prefers to feed on bird blood. And so it'll feed on domestic chickens when there's people around, but it'll also feed on sleeping birds up in the trees. None of these are very big, they can sit in the palm of your hand, they have a wingspan of, I don't know, eight or 10 inches. So we have blood-feeding bats. But then there's a whole set of bats which don't feed on animal tissue much at all. They specialize in eating plants primarily. So mostly fruit, or they feed from flowers like hummingbirds.

ERIC: Okay, so I think most people have heard of fruit bats, but there are nectar bats?

NANCY: It depends on the species. So there are bats that specialize almost entirely on feeding on nectar and pollen from inside flowers. And some of those have really long nose and a really long tongue so they can stick them down into flowers. The fruit-eating bats often have shorter faces, the because they have strong jaws for picking big fruit and flying with it. One of the things that fruit-eating bats do is they never stay in the same tree to eat the fruit that they pick, because predators hang out in that tree. So you'll see snakes hanging out in trees at night trying to catch the bats.

ERIC: So you'll see a bat flying around with like a kiwi or something?

NANCY: Yeah, usually not quite that big. But yes, exactly. Sometimes when we go out to catch bats in the rain forest, we put up nets are called mist nets that have a very fine mesh. So if you think of like a volleyball net, but you make each one of the little squares smaller, and you make the threads like thread, rather than stringy. And we catch bats, you know, sometimes we catch fruit bats with their fruit. So there'll be a bat in the net, and there'll be a fruit dangling underneath it because the bat was carrying the fruit when it flew into the net.

ERIC: So do you do a lot of field work going out and searching for new species of bats in remote areas?

NANCY: Sometimes yes, I do fieldwork. I go every year at least once or twice to Belize, I have a long term field project there. I've also worked in French Guiana, Peru, Thailand. Mostly we're not looking for new species necessarily. What we're interested in is learning about the bat fauna, the whole assembly of bat species living in one place and learning about bat ecology. And this has become particularly important in recent years as we understand that bats and human health have have a big intersection, because of viruses and bacteria. And so we're trying to understand the dynamics of bat communities and what keeps bats healthy, and what prevents spillover of diseases from bats to other species. And so there's a lot of work being done now, in that area. We also want to conserve bat populations because bats are really important in the roles they play in different ecosystems. Among other things, there are many economically important fruits that the pollination of the flowers is done by bats, seed dispersal is done by bats. Also, bats are really important in insect pest control. So many of the farm crops that are grown in the central United States, a lot of the pest control is done by bats that eat the pests that come to eat the corn.

ERIC: Can you talk about one of bats' coolest abilities besides flying, and that's echolocation - this kind of other sense that humans don't have that they can use to chirp and figure out what's in their surroundings.

NANCY: Not all bats, but most bats have a sensory system, which is very different from what humans can do in most circumstances. So this is called echolocation. And it's basically biological sonar. So what the bats do is they create high frequency sounds in the voice box. So it'd be like you making a sound like I am now, talking, except it's very high frequency and very intense and very loud. And then they can listen for the returning echo bouncing off things and they can use that information. They have special processing abilities in the brain, which are very well developed, that allow them to use all these returning echoes and avoid obstacles and also detect little tiny flying prey in three dimensions. So if you think about the bat flying and the bug is flying at the same time, it's three dimensions, thy are using, in essence, sonar pinging off that flying insect, listening to the echoes and by comparing the the frequency of the returning echo and the speed with which it got back to their ears they can pinpoint in space where that bug is and where it's going. So echolocation is a fabulous sense for a nocturnal animal that's going to be operating in the dark and especially if it's going to be flying at the same time.

ERIC: I was gonna say, is that why we see birds during the day and bats come out at night, because birds don't have this?

NANCY: Right, so birds use vision for orientation. And so when it gets dark, basically they go to sleep. Bats evolved from night-dwelling mammals which were primarily using their sense of smell for orienting. And also vision because it is possible to see at night, a lot of times not very well, but like cats can do. There are specializations in many mammals that allow them to see a lot better than we can and night. But echolocation really opened up a whole new world of things that bats could do in terms of flying rapidly through vegetation and things like this and also detecting prey. So finding those insects to eat. And so it opened up all sorts of what we would call ecological niches, which are basically lifestyles, that bats could successfully do that other animals can't. And there was no competition because at night, there weren't birds out there. They were already eating those insects in that way. And so it allowed bats to become very successful, very diverse. But not all bats use echolocation. That's one of the really interesting things. The flying foxes, so the fruit eating bats, live in what we call the Old World. So that's Asia, Australia, India, Africa. Those bats use vision, and their sense of smell, olfaction, to find their fruit food. And it looks like they probably evolved from animals that could echolocate. But as they began to emphasize using vision and smell to find fruit, probably just because the brain can't do everything at once, they ceased using echolocation as a major way of finding food.

ERIC: It's fascinating when evolution works like that, where you have an animal in an area with a diet that, you know, doesn't need this amazing ability that it has. And so it just, you know, loses it and develops great smell. It's not evolution going backwards. It's just -

NANCY: It's change. Yeah.

ERIC: It's change. It's adaptation. It's that environment. Yep.

NANCY: And in this case, it's possible that some of these trade-offs had to be made because of the weight of the brain. So when you're a flying animal, every ounce matters, every gram matters, because you have to use energy to fly. And so to be able to maintain a functional system, you can't just grow your head bigger and bigger and bigger and add on more sensory systems and end up with this huge head. And so there are trade-offs. So most animals are not good at everything in terms of sensory. So you know, humans, we have wonderful vision, we have a good sense of taste, our sense of smell is nothing like many animals, our hearing is good, but it's not like echolocating bat hearing. So every species has sensory systems that work for its lifestyle but you don't find anything that can do everything. You can either be a good at echolocating, or you can be good at fishing, but you can't do both.

ERIC: So to wrap up, bats are sort of something that we the museum call a misunderstood animal. There's lots of negative connotations, there's a lot of misunderstandings. What's one thing that you wish everybody knew about bats that would kind of reverse some of that?

NANCY: I think the most important thing that some people don't understand about bats is how important they are in ecosystems. We don't see bats because they're active at night. And so many people may go through their whole life, but never see a bat. And so they just don't think about, they don't realize that they're there, and that they're critically important to maintaining the balance of life no matter where you live on the planet.

ERIC: Dr. Simmons, thank you so much for joining me and telling us all about bats.

NANCY: It's been my pleasure.

ERIC: You can learn more about the delicate balance of ecosystems on your next visit to the museum in our Garden Walk & Insect Zoo. Until next time, keep asking questions.

If you liked this episode, be sure to check out:

How Did Bats Evolve Flight?

What Is the Fastest Animal?

Why Did Dinosaurs Have Feathers?