Frogs Living Near Loud Waterfalls Dance to Attract Attention

First evidence of visual communication among tropical glass frogs.

Posted Feb 11, 2021 | Reviewed by Matt Huston

Rebecca Brunner, used with permission.
The glass frog Sachatamia orejuela.
Source: Rebecca Brunner, used with permission.

Communication is vital for frogs. Most male frogs call during mating season, advertising their quality and location to females and their territorial boundaries to other males. But in certain environments, such as loud streams and waterfalls, vocalizations may not be enough to get noticed.

A handful of frog species have arrived upon the same solution to this problem, producing higher-pitched calls and adding dance moves to communicate in loud environments. These species are found around the world, in Brazil, India, and Borneo. While each of the species calls and visually displays near rushing water, they are not closely related to one another. These behaviors likely evolved independently in response to similar conditions, a concept called convergent evolution.

Now, another species can be added to the list of frogs that use visual communication in response to a loud environment. Scientists have documented the glass frog Sachatamia orejuela visually showing off with hand-flaps, foot-waves, and head-bobs when calling near a waterfall. This is the first record of a member of the glass frog family using long-distance visual communication. (Watch a video here).

S. orejuela are relatively large glass frogs native to the rainforests of Ecuador and Colombia. They live on rocks and boulders within the spray zones of waterfalls, where their green-gray, reflective skin makes them nearly impossible to spot. As a result, little is known about these frogs.

Rebecca Brunner, used with permission.
Rebecca Brunner conducting the fieldwork.
Source: Rebecca Brunner, used with permission.

Rebecca Brunner, a doctoral candidate at the University of California, Berkeley, wanted to change that — starting with documenting this species’ call. With co-author Juan M. Guayasamin of the Universidad San Francisco de Quito, Brunner surveyed waterfalls within Mashpi Reserve, part of the Tropical Andes hotspot of Ecuador.

Brunner was excited when, after months of spotting S. orejuela individuals near waterfalls but never hearing them calling, she finally heard a new call. She located the source — an S. orejuela male — and started recording. But she almost couldn’t believe what she saw next. As she filmed, the frog repeatedly raised its front and back legs, waved its hands, and bobbed its head. At the same time, Brunner also observed another male S. orejuela performing the same actions a few meters away, on the other side of the waterfall.

Even more surprisingly, all this vigorous visual signaling was taking place at night. Other frog species that similarly dance do so during the day. It is possible that S. orejuela relies on moonlight to be seen, although they may not even need much of that: Recent studies have shown that frogs, in general, have extremely good night vision, with many species even able to see color at night.

Brunner was not surprised to find that the call of S. orejuela is very high-pitched. High-pitched calls are more likely to be heard over the white noise generated by rushing water. Even though S. orejuela is larger than most other glass frogs, it calls at a much higher pitch, suggesting this is an adaptation to living near loud waterfalls. And if its calls still can’t be heard, S. orejuela has a backup: its dance moves.

Rebecca Brunner, used with permission.
S. orejuela visual signals.
Source: Rebecca Brunner, used with permission.

“Since hearing the call isn’t guaranteed, especially since high-frequency sounds cannot travel as far as low-frequency sounds, adding a visual component increases the chances of a receiver noticing you,” says Brunner. “At this point, we have no idea whether the intended receiver of the visual signal is a potential mate or a competing male.”

Though the COVID-19 pandemic has temporarily stalled Brunner’s fieldwork, she hopes to return to Ecuador soon to continue her research, which links bioacoustics and conservation. She says the rainforest is an exciting place to work because so much communication is happening, even though the animals are difficult to see.

“Animals will make sounds for a myriad of reasons and those sounds can help researchers conduct biodiversity surveys without ever needing to see the animal — provided we know what each animal sounds like,” she says.

“That’s one of the reasons why recording different species is so important for conservation. If we don’t know they are there, we can’t conserve them.”

References

Brunner, R., & Guayasamin, J. (2020). Nocturnal visual displays and call description of the cascade specialist glassfrog Sachatamia orejuela, Behaviour, 157(14-15), 1257-1268. doi:10.1163/1568539X-bja10048.