In the Guadarrama Mountains the larvae of the Iberian frog (Rana iberica) coexist with many predators, including the Brown trout (Salmo trutta). To avoid predation, the Iberian frog larvae have developed the fascinating ability to detect chemical signals produced by them (their 'smell'). Thus, when the larvae detect the presence of the fish through its odour, which is dissolved in water, they reduce their movements to avoid detection, as the trout locate prey visually. However, in the Guadarrama Mountains there are also Iberian frog populations that have never lived with trout, since the terrain has prevented trout colonizing the small and steep streams where these populations are found. The larvae of these Iberian frog populations therefore have never been in contact with the chemical signals of the trout, and we wondered how they would react when they came into contact with them.
To answer this question, we conducted an experiment in the laboratory with Iberian frog larvae from six local populations of the Guadarrama Mountains, three of which coexist with trout, and three of which have never come into contact with trout. We used PVC drainpipe shaped like a "U" divided into sections by lines to test the individual behaviour of the larvae. At one end of the drainpipe we placed a larva of the frog, while at the other end we placed an ice cube containing chemical signals of the trout (experimental group) or an ice cube with clean water (control group). Then we measured the activity of the larvae counting the number of times they crossed each of the marked lines within the drainpipe over a period of five minutes.
The results of this experiment showed that both larval populations living with trout, and those who have never lived with this predator, significantly reduced their movements when the ice cube contained chemical signals of the trout. Therefore, it appears that this behaviour is innate and not learned, as the larvae of all populations showed the same predator-avoidance behaviour.
How fascinating to be born knowing instinctively how to avoid a predator without even seeing one?