To protect themselves against predators, less resilient animals usually use one of two strategies: they flee or rely on camouflage. A study using frogs as an example explains why some species have developed one strategy while others have developed another. Accordingly, the selection pressure exerted by predators plays a decisive role in determining whether the energetically more expensive escape strategy is worthwhile.
In nature, there has always been an eternal struggle to adapt between predator and prey, which has produced countless hunting and camouflage strategies in the course of evolution. When it comes to prey that is not very strong, there are usually two options: flee or hide. The escape behavior is cognitively more demanding because the animals have to recognize potential enemies in good time and react to them in a targeted manner. This requires a larger brain and the appropriate physical abilities to escape, such as strong hind legs.
However, this complex strategy for fleeing predators consumes a lot of brain capacity and energy and therefore also has disadvantages. Species that have adapted to their environment in the course of evolution in such a way that ideally they are simply overlooked by their predators rely on a different method. But the camouflage can also be too good and, for example, make it difficult to find a partner if the animals are not even discovered by their own species. In addition, such species are not optimally camouflaged on every surface and can only feel safe on brown tree bark or green leaves, for example.
Frogs as a test case
The two different strategies of the prey animals are well described and understood. But why some species have developed one strategy and others the other is largely unclear. Therefore, Wen Bo Liao from the China West Normal University in Nanchong, China and his colleagues investigated this question using over one hundred Chinese frog species as an example. Using a spectrophotometer, they measured the reflectance of the frogs' skin and their immediate surroundings, such as the leaves, rocks or tree stumps they were sitting on. This allowed them to determine how well each species is visually adapted to its environment. This, in turn, told them whether the frog species used flight or camouflage to escape predators.
In addition, the researchers determined the respective hunting pressure by determining the frequency and number with which snakes, which are considered to be the primary predators of frogs in their study area, were found at the frogs' breeding sites. "Since most snakes are visual hunters with well-developed eyesight, the evolution of frog camouflage is most likely adapted to snakes as predators," Liao and his colleagues explain.
It depends on the predators
The study results show that frogs that have few predators are more likely to use flight as a survival strategy. Because they rarely have to flee from a predator, they have enough time in between to find food and get the energy they need to flee. This allows them to offset the higher cost of a larger brain and the energy-consuming flight. In addition, these frog species are mostly
more conspicuously colored, which can, among other things, support visual communication when finding a partner.
However, if the frogs have many enemies, so that the predation pressure increases, the escape strategy is less efficient. They simply don't have the time - for example to eat, to gather energy to escape or to reproduce. "Under these difficult conditions, the disadvantages of the necessary cognitive performance for the escape outweigh the disadvantages," says senior author Stefan Lüpold from Wageningen University in the Netherlands. "Evolutionally, this has led to a shift towards less mobility and better camouflage -- and a smaller brain in return."
Better camouflage if necessary
For the first time, the study establishes a direct connection between brain evolution and different strategies for avoiding predators. She shows that camouflage is not necessarily an ideal primary strategy, as is often assumed, but rather a secondary adaptation triggered by the high cost of flexible, cognitive escape behavior. However, whether such a connection also exists with other strategies in frogs, such as the poison glands or in other animal groups, still needs to be researched further.
Source: University of Zurich; Specialist article: Science Advances, doi: 10.1126/sciadv.abq1878