The Complex Calls Of City-Living Frogs
City mouse and country mouse aren’t just characters from stories—cities are unique ecosystems built by humans, and animals adapt when they move into urban areas. Researchers recently compared the calls of male túngara frogs in Panama that lived in the forest with those in the city. They found that the city frogs had more complex calls and that female frogs preferred these calls—but the less complex calls of country frogs made them easier to hide from predators. Biologist Alex Trillo, an author on the study, talks about the costs and benefits of changing calls for the túngara frog.
Listen to the difference in the calls below.
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Paula Alex Trillo is an assistant professor of Biology at Gettysburg College in Gettysburg, Pennsylvania.
IRA FLATOW: And now, it’s time to play “Good Thing, Bad Thing.”
Because every story has a flip side. You’ve heard about the country mouse and the city mouse, of course. This story is about the country frog and the city frog. Turns out that male frogs that lived in the city were more attractive to female frogs than frogs that lived in the forest. The city frogs’ mating calls– they’re called chucks– were more complicated and actually attracted more females.
But what are the costs and the benefits of this change? This study was published this week in the journal Nature Ecology and Evolution. My guest is an author on that study. Alex Trillo is an assistant professor of biology at Gettysburg College in Pennsylvania. Welcome to “Science Friday.”
ALEX TRILLO: Hi, hello. How are you?
IRA FLATOW: Hi, nice to have you. How different were the calls?
ALEX TRILLO: From the city to the forest, the calls are actually quite different. So in urban environments, males have the ability to increase their call rate. So they basically have a few more calls every minute. And males can also produce calls that are what we call the more complex. And let me just take a second to explain that.
So tungara males have two parts to their call. First, they have a main syllable called the “whine” and then a second syllable called a “chuck.” And males can either call just with a whine, or they can also add a number of chucks to their call. And so urban males have the ability to add more chucks to their call and make them more complex and, therefore, change their calls when they are attracting females.
IRA FLATOW: And actually, we have an example of that. We have a clip of these different chucks. First, we’re going to play the forest frog– just small.
OK, you heard that. Let’s play the urban frog. Oh, Dr. Trillo, you can hear the difference there.
ALEX TRILLO: Yes, there’s at least two or three chucks to add to that call and that is something that females are basically really excited about. So that makes females much more attractive to those males.
IRA FLATOW: I’m going to ask you to tell me why the females are excited by the one with the chucks. Do you know?
ALEX TRILLO: Yes, it appears that it’s basically a quirk of their nervous system, of their neurological system. So females are generally just more excited when males add these extra notes at the ends of their call– the extra chucks. And you can actually go to a closely related species of tungaras where males do not make chucks, and then you can take that tumor– and put it at the end of the call. And then the females of the other species will still love it– love it more than their own calls.
IRA FLATOW: Do the forest frogs– if they go into the city, can they develop the chuck themselves? Or how did the city frogs get that chuck?
ALEX TRILLO: So forest frogs can also add chucks to their calls. They just don’t add them at the same numbers that the urban ones do. And in fact, part of this study shows that when we transport urban male frogs from the forest to urban areas, those males do not have the ability to add their game to basically add more chucks to the calls. And so on the other hand, urban males can actually decrease the number of chucks in their calls when they’re in the forest. So urban males are more able to basically move back and forth between these two places and rearrange the number of chucks.
IRA FLATOW: Too bad for those guys.
ALEX TRILLO: Yeah.
IRA FLATOW: So, OK, what’s the bad news here? What did this tell us about how urban environments might affect animals?
ALEX TRILLO: Yeah, so basically, moving to the city can come with a change in the male frogs’ environment. There are definitely benefits, and parts of those benefits are the fact that they don’t have predators or many predators and parasites because also these predators and parasites also use these chucks to eavesdrops on the males and then basically get them– get them as froggy meals. So that’s a benefit to the urban male.
But now a cost to the urban male is that it’s a lot harder for those males to find the females, so they are actually less likely to find a female. And so what they do is they have to up their game. They have to change their calls, make them more complex in order to deal with the females, but that is an adaptive situation because there’s also less predators and parasites.
IRA FLATOW: So what do you want to know next?
ALEX TRILLO: Well, one of the things that basically comes right after the information that we got in these studies, whether these populations are still intermixing quite a bit or not? Are these populations quite separated? And could this start a strong divergence between the urban populations and the forest populations? Or can we still see a lot of mixing between these individuals in which case potentially the urban males are going to– the urban phenotype– is going to take over?
IRA FLATOW: All right, well, we’ll watch for the kids’ book, Urban Frog, City Frog, Country Frog.
ALEX TRILLO: All right, thank you.
IRA FLATOW: Thank you, Dr. Trillo. Alex Trillo is an assistant professor of biology at Gettysburg College in Pennsylvania.