For most frogs there are only two categories in life: to eat or to be eaten. Accordingly, they perceive their surroundings to a limited extent. Objects that approach them parallel to the body axis – a worm crawling on the ground, for example – are food. Objects approaching you vertically from above or below are not worms. So in all likelihood enemies to jump away from.
The escape reflex of frogs and, incidentally, also of toads in the face of objects coming from above or below, can also be triggered artificially by means of dummies. Biologists therefore speak of the “anti-worm stimulus”. A study that is currently in the science journal Proceedings of the Royal Society B. appeared, but now suggests that frog life may not be that easy after all.
According to this, waving frogs of the species Staurois parvus shamelessly exploit the primal fear of their conspecifics and trigger their flight reflexes in order to drive away competitors for females who are ready to mate. According to the scientists led by biologist Nigel Anderson from Brown University in Providence, America, that would be a good explanation for the strange behavior of winking frogs, which gave the amphibians their name. The animals kick their hind legs high in the air, stretch them out completely with spread toes and then pull them back again. You are waving your legs, so to speak.
Anderson and his colleagues suspect that the amphibians with their outstretched leg imitate an enemy who attacks from above and thus scares their male competition to death. Driven by the anti-worm reflex, the competitors put the third most important thing in the life of a frog – reproduction – on the back burner and first of all get to safety.
Basically, the waving of the winking frogs, of which there are six different types, probably originated because the animals live in a noisy environment in which the type of communication (croaking) that is otherwise common among frogs does not work. The little waving frog (Staurois parvus), for example, the research object of Nigel Anderson, lives by rushing brooks in the tropical rainforest of Borneo. The animals, which have only recently been recognized as a separate species, often sit on rocks in the middle of the water, preferably near rapids. In this environment, croaking would be drowned out by the rush of the water. Leg communication, on the other hand, is visible to everyone from afar – especially since the skins between the animals’ splayed toes are colored white and therefore stand out well against the black rocks.
The thesis that the wave should deter male competition, the biologists support with their observation that the male sex hormone testosterone intensifies this behavior. Little waver frogs in the zoo in Vienna, to which they had given testosterone for their examination, waved more violently and threw their leg higher in the air than frogs from the control group, which they had only injected with a saline solution.
The researchers consider it rather unlikely that the males want to attract females with their waving. On the contrary, some males keep waving during mating. And also that female frogs no longer pay attention to the acrobatic exercises of the males: they usually mate with whoever is closest to them.