Frogs are sexy in the city

When looking for a partner, animals have to adapt to their surroundings. And the Tungara frog, from Central America, is no exception. New research from Vrije University has found that frogs living in cities have evolved to call more often, and make a more complex sound than their forest-dwelling counterparts. Adam Murphy heard all about it from Wouter Halfwerk...

Wouter - They're much smaller than you would expect. They're about two centimetres big. Different brownish colours and normally during the day they're very camouflaged on the forest floor. But, when they start to call, it's a very impressive sound. They call very loud. They call in big groups, so they're hard to miss. If you compare their call to us, they have the same fundamental frequency as our voice and more or less the same amplitude as when we are yelling at each other. So they have a very impressive mating signal, or mating display you could say.

Adam - So they are tiny frogs screaming at our volume looking for a mate?

Wouter - Yeah they're tiny frogs and they really scream at the maximum capacity. They also have a very big larynx, a voice box, very similar to what we have. But in their case, their voice box is much bigger than their brain. And in humans it's the other way around. It just shows you that sexual selection has operated on their voice and not on their brain.

Adam - What did you find then when you look at these frogs?

Wouter - We compared frogs in 22 urban sites and forest sites. We recorded up to 100 males. On average we see that frogs in the city call at about 25 percent higher call rates and about 40 percent higher complexity. So what you should know is that in this species males always make this first part of their call, that we called a ‘tun’, that sounds like “boom”. And that's how they start. We call that a simple call. But then when they get really excited they start to add elements known as ‘chucks’ and that sounds like “ah” or “ah, ah”. And if you get “tungara” - that's why they're called tungara frogs. And when they do that they make themselves more attractive to females. But we also know from previous studies that predatory bats and parasitic midge are also paying attention to these ‘chucks’. And if you give them a choice they prefer to attack males that make more of these so-called ‘chucks’. That was the first thing we found - we found that these urban males make more of these ‘chucks’ and then we thought, okay, that must be related to on the one hand attractiveness to females but also on the other hand related to predation risk.

Adam - Can the city frogs, could you take them into the forest and they'd revert back to being rural frogs and vice versa? How would that work?

Wouter - Yes so that's also what we tried. So when we found out that there are these differences, we wanted to know: how flexible are urban or forest males in changing these calls? We put an urban frog in a forest environment and then immediately saw that it would change back its call to match that of a forest frog. And that way it adjusted its calling behavior to match the predation risk in these forest environment. But when we did the other experiment, where we moved a forest frog to the urban environment, we saw that they could only partly change their calls and they could not make as much of these ‘chuks’. They had less complex calls than the urban frogs in the same environment. And this suggest that we are looking at an evolutionary response and that the urban males have been selected by the city for their increased complexity.

Adam - So this is witnessing the start of natural selection onto evolution then?

Wouter - Yeah, this could potentially be to start off of a new species as a result of changes in sexual and natural selection pressure. You're right. So the key question here what we really want to find out is whether these differences are heritable - whether this trait this calling behavior is passed on from one generation to the other. That's what we are planning to do next summer. We are planning this large-scale breeding experiment where we keep forest frogs together and urban frogs in the same kind of lab environment and then see if their offspring have the same differences in calling behavior.