Life in the (Urban) Jungle

The sun sets quickly in Panama, and the advancing darkness signals it is time to go. The car packed up with a variety of equipment, we head out onto the road. The stops tonight include hiking along an eerie powerline, skulking down the famous Pipeline Road (known internationally for its bird diversity), and visiting a variety of areas surrounding Panama City, city of about 2 million inhabitants. The work around the city differs quite significantly from that in the forests. Where we encountered only the high pitch calling of insect and frog species in the forest, we find the lower frequency noise of cars and trains passing by in the city. The more cavalier strolling in the forest turns to more surreptitiously poking around people’s homes. These urban sites also have some surprises—occasionally being chased away by dogs and security guards, or coming across the beautiful and formidable fer-de-lance next to the street. At this point you might be wondering ‘what might inspire someone to do these things?’ For frogs, of course! Specifically, the túngara frog, or Engystomops pustulosus. Though perhaps not quite as charismatic as other tropical frogs, such as the impressive Red eyed treefrog (Agalychnis callydrias), these frogs are magnificent in their own right. 

The túngara frog has a rich history of research, predominantly with the Smithsonian Tropical Research Institute. Starting in the 1970s, primarily through the research of Michael Ryan and Stanley Rand, this frog has become one of the more recognizable species in the field of sexual selection and mate choice. Like many frog species, male túngara frogs will gather at night to try and attract females to mate with. The call of these frogs has two distinct parts to it; it starts with a sound that decreases in pitch, called the whine, and then males can add a nice loud “chuck” at the end of the whine. The name túngara is onomatopoeic for a frog producing a whine with two chucks! The addition of the chuck makes a male more appealing to females, but it is also more appealing to predators, including the fringe lipped bats (Trachops cirrhosus). Therefore, males are in quite a bind, and have to balance the risk of attracting females with the risk of attracting bats as well. This balance of different selection pressures (via the predators and mates) is one of the many reasons this frog is so captivating to work with.  

Our late-night adventures to find and study these frogs around Panama City are due to our interest in how urbanization might alter this fascinating communication system. Having just reached 8 billion people on the planet, with increasing percentages living in densely populated areas, these urban ecosystems have become an important place to study. Urban environments have been radically altered over short time spans, and many animals or plants that manage to live in these new urban environments must adapt to their changed surroundings. This means that these urban organisms might actually evolve different ways of coping with living in cities, allowing researchers to see evolution in action. Additionally, as more species face urbanization worldwide, understanding the impacts of urbanization on species is vital to properly assess how our behaviors as humans are influencing and altering ecosystems and species.  

Previous work in our lab, led by Dr. Wouter Halfwerk, has shown that there are in fact differences in calling behavior between urban and forest frogs, with males in urban environments producing more attractive calls on average. However, we don’t fully understand what is driving these differences, or what the full implications of these changes are. To find out, we have conducted a variety of lab and field experiments to understand how sexual communication is changed in response to different urban sensory pollutants, focusing on light and noise pollution. By introducing light and noise pollution to frogs in the forest, and by bringing frogs to the lab, we have begun to examine how these pollutants can affect how males call, how females respond to these calls, and how the behavior of predators hunting the frogs changes. We have found that light and noise pollution alter both when and how males produce their calls. We also found that exposure to multiple sensory pollutants at the same time can have unexpected effects. For example, the effects of noise pollution on male calling were changed when light pollution was also added. Additionally, in an experiment led by fellow PhD candidate Judith Smit, we have carried out a large-scale breeding experiment using both urban and forest frogs to understand whether there are any genetic bases for differences between these frogs. These experiments involved many long nights driving around in forests and around Panama City, looking for frogs and taking a variety of measurements when we found them. 

During these nighttime excursions, it is quite common for people to come up and ask, ‘What on earth are you doing?!’ Many people are shocked to hear that we are working with frogs, and many also admit having fears of these small creatures. But regardless, they are always curious, and are excited to learn about the research happening right outside their houses. And this curiosity represents one of the fascinating aspects about urban ecology. Evolution, often thought of as an intangible concept, might in fact be occurring right in front of your doorstep. 

Andrew Cronin | READ MORE

Andrew Cronin is a PhD candidate at Vrije Universiteit Amsterdam in the Netherlands. A native of Maryland, he is interested in sexual selection and mate choice, including in the context of urbanization. He is also interested in how urbanization might influence development, and his current work in this area is part of his STRI PhD Fellowship project entitled ‘Developmental responses to urban conditions’.