Next to the crucial trade route of the Panama canal sits Panama city, where an intriguing example of urban evolution has taken place. As Panama city has grown from a population of around 200,000 in the 1950’s to a population of around 2 million today, massive urbanization has taken place [1]. Urbanization is dramatically reshaping ecosystems, with notable impacts on the behavior and evolution of species. This can often cause species to adjust their sexual displays as the effectiveness of mating signals is influenced by environmental conditions [2].
One example is observed in the túngara frog (Engystomops pustulosus). Here we go more in depth into research that shows how urban environments influence the sexual signaling of the túngara frog [2]. This offers insights into adaptive changes happening in response to altered selection pressures which must have arisen since the rise of urbanization.
What changes are we talking about?
Urban túngara frogs have significantly adjusted their mating calls compared to their forest-dwelling counterparts. Male frogs from urban areas exhibit more complex and frequent calls. Furthermore, urban males are less vigilant and more consistent in calling even when approached by observers, highlighting a behavioral shift likely influenced by reduced predation pressures in urban environments.
If you would like to know what the different túngara frogs sound like:
The 22 forested and urban sites where the sampled frogs where located can be seen here:
The urban sites typically have significantly higher levels of noise and light pollution and less canopy cover compared to forest areas [2]. These environmental differences necessitate changes in communication. Urban frogs have adapted to these new environments by altering their call structures to stand out amidst the urban noise, thus increasing their chances of attracting mates.
Furthermore, although urban frogs had to adapt to a new environment they now also face less selection pressures compared to forest frogs. There are fewer predatory bats and parasitic midges in the urban environment, both of which are also attracted by the frog’s mating call. This reduced risk from natural predators allows urban frogs to produce more conspicuous calls without the significant survival costs faced by their forest counterparts.
Sadly for the urban frogs there also seem to be fewer female frogs in the urban environment. When the researchers played back the urban frog’s calls, they attracted fewer predators, but also fewer females. Despite this, urban males are still the more attractive to females. It does not matter whether they come from an urban or from a forest location. This was tested with a phototaxis experiment and to quote the researchers [2]: ‘Females were only tested once and strongly preferred the urban over the forest male call, irrespective of the female’s origin.’ ‘In total, 30 out of 40 females approached the urban call.’.
This difference in call attractiveness could be due to urban males also being able to display a broader ranges of signaling phenotypes. This allows them to flexibly alter their signaling depending on environmental conditions. This was tested with a translocation experiment, where frogs were moved between urban and forest environments. The urban males could quickly adjust their calling behavior to match their new environment. Urban males placed in forest settings reduced their call complexity, while forest males did not significantly change their calls when moved to urban areas. This flexibility suggests that urban frogs are better equipped to modulate their behavior according to environmental cues, offering a competitive edge in diverse habitats. This finding suggests that surviving in and adapting to in the urban environment has increased túngara frog fitness in both environments.
References
- [https://www.macrotrends.net/global-metrics/cities/22063/panama-city/population#:~:text=The%20metro%20area%20population%20of,a%202.1%25%20increase%20from%202020
- Halfwerk, W., Blaas, M., Kramer, L. et al. Adaptive changes in sexual signalling in response to urbanization. Nat Ecol Evol 3, 374–380 (2019). https://doi.org/10.1038/s41559-018-0751-8
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