5 Questions for Evolutionary Biologist Wouter Halfwerk

(March 28th, 2014) Born in the Netherlands, Wouter Halfwerk currently spends most of his time studying the predator-prey relationship between bats and frogs in the South American jungle. He recently discovered that bats have even keener senses than previously thought.

Tropical rainforests, with their huge biodiversity, are top research destinations for evolutionary biologists like Wouter Halfwerk, Research Fellow at the Smithsonian Tropical Research Institute in Gamboa, Panama, and Guest Researcher at Leiden University, the Netherlands. Already during his Masters, he was drawn to the Ecuadorian jungle to study how cicadas’ singing affected birds’ own tunes. After completing his studies he went back to Europe to a more urban setting. There, he wanted to find out how traffic noise affected bird songs, song perception and reproduction in the Great Tit, a classic species for the study of bird song vs noise. Now, Wouter is back in the jungle. This time he is in Panama’s rainforest, where he is looking at the evolution of the amazing communication skills of bats. His most recent finding shows that bats are quite sensitive to even the slightest movement. Even a ripple in a puddle of water doesn’t go unnoticed.

LT: You recently published a paper reporting that bats can detect subtle ripples made by frogs in water. What makes this finding so important?

Our main goal is to understand how animal communication systems evolve. We are in particular interested in communication signals that combine components from multiple modalities. Birds e.g. sing a complex song but at the same time show off with bright chest feathers. One important route, we think, is through co-option. Receivers of signals attend first only to a signal in one modality, e.g. the sound of the singing bird but by chance start paying attention to other cues generated to produce the sound (e.g. bill or body movements). Once these by-products of the main component are attended to by receivers (females or rivals) you can get rapid and unpredictable changes to the overall display.

In our system frogs produce ripples when calling. They do not do this intentionally, which makes these ripples a by-product of calling. This acoustic signal has already been shown to be under strong sexual (by females) versus natural selection (by bats). We are thus looking at the early stages of the evolution of a complex multimodal signal. Whether and how this signal will evolve depends on which animals attend to it and how they use it.... in other words whether there is positive or negative selection on ripples.

LT: How did you study this, in practice?

We designed an experiment, in which we showed that rival males attend to these ripples, but only when accompanied by sound. Furthermore, when we broadcast ripples at varying distances, frogs notice this. This suggests that male frogs use ripples to get more information about their rivals. At this stage we do not know whether this use by rival males has a positive or negative effect on the focal calling male.

Next, we tested whether the frog's main predator used the ripples in their assessment of prey. We gave frog-eating bats a choice to attack one of two frog models; one next to a control pool with a flat water surface, the other model was placed next to a pool, in which we artificially generated ripples. Bats had a clear preference to attack the ripple pool models. This shows that multiple animals can make use of the cues generated during sexual displays and, most importantly, that they do not need to use the same sensory system. Frogs probably use some sort of touch system, related to lateral line sensing in fish. Bats probably use their echolocation sense to detect the ripples.

Frogs can detect bats as well and stop calling. If the bat is far away, this increases localisation error of the bats tremendously. Unfortunately, the ripples travel tenfold slower than sound and are detectable by the bats for up to 3s, which deteriorates the frog's anti-predator behaviour.

LT: How did you come up with the idea of looking whether bats could detect the frog's ripples?

I was already working on a related project with bats, in which we tried to test whether they used the dynamic movement of the frogs' vocal sac as prey cue. During these experiments, I and one of my co-authors, Patricia Jones, were talking about fishing bats and other species and the possibility that they use disturbances of the water surface to locate fish coming to the surface. Earlier I had noted that frogs make clear ripples when calling and all of a sudden we thought that it would be pretty amazing if our species could incorporate ripples with sound in their prey assessment.

LT: What was it like to live in the jungle for a year with your family (including your 3-year old daughter)?

We lived in Gamboa, Panama, in a house right next to the forest. It was pretty amazing seeing new types of wildlife such as toucans and howler monkeys moving through our backyard every day. Coming from the Netherlands, with no dangerous animals, we were quite afraid of snakes in the beginning. This was especially true at night and for our little daughter, who wanted to wander into the forest all the time. However, after some time you get used to the fact that you hardly see snakes, so it is not a big threat anymore. Unfortunately, you soon get sloppy and so after a couple of months, I stepped into any puddle without giving it any thought and nearly got bitten by a ‘fer-de-lance’, a very poisonous pit viper. However, the scariest experience was a beetle that got attracted by my headlights and hit me in the eye with some sort of acid. That hurt like hell and I had to drive the truck all the way back on pipeline road for about 30 minutes to the lab to wash it out.

Our most funny experience was waking up at night by some noise, which sounded like someone trying to climb into our house. You should know that most houses are on poles, so you can walk underneath them and all the plumping goes underneath the house, too. When I went out and looked under the house I saw a sloth hanging from our plumbing system as if it were ordinary tree branches.

LT: What are your plans for the future?

Right now we want to know more about potential benefits of ripples. Do they help defend the territory? Do they also attract females? We also want to know more about the perceptual system. How are ripples processed? Can receivers differentiate between ripples produced by frogs and other sources, such as rain, wind or other animals?

Karl Gruber

Photo: Alex Tran

Last Changes: 04.07.2014