The long-fingered bat (Myotis capaccinii) is, as far as we know, the only bat in Europe that feeds on fish as well as on insects. But this behaviour is more than a mere anecdote, since the level of subtlety it has achieved in the development of its senses and in its way of flying has been a significant factor in the evolutionary history of the long-fingered bat, as well as in the survival of this species. This is what emerges in the paper that a research group of the UPV/EHU-University of the Basque Country has published in the scientific journal Scientific Reports and entitled "Insight on how fishing bats discern prey and adjust their mechanic and sensorial features during the attack sequence".

"The use of one technique or another to catch insects and fish shows that these bats are capable of distinguishing between the two preys, and the first step in this research was to understand how this distinction was made: in other words, to find out what stimulus they use to identify that the prey is a fish," said Ostaizka Aizpurua, a member of the UPV/EHU's research group Ecology and Evolution of Behaviour and lead author of the paper.

The dragging and buzzes

The long-fingered bat uses one technique to hunt insects and another to catch fish, and each one is adapted to each type of prey. It catches insects located on the surface of the water by performing a dragging movement with one of its feet or with its uropatagium (the membrane attached to its hind limbs). Then, to fish, it inserts its feet more deeply and the dragging action takes longer. Yet the differences are not limited to flight, because the echolocation pattern is also different. Echolocation is the mechanism bats use to orientate themselves and to hunt, and it consists of using their echo capability to locate objects around them. Specifically, the acceleration in the echolocation pulse that occurs at the end of the hunting phase is known as a buzz, and in the case of these fishing bats, two parts with differentiated features can be distinguished: the first buzz and the second buzz. Long-fingered bats are capable of adjusting their two buzzes; in other words, depending on what they are interested in, they can extend one and shorten the other. To hunt insects, the importance of both buzzes is similar, but when it comes to fishing, the second buzz is considerably shortened to the point where it even disappears in some cases.

The first step to find out the specific stimulus in this case has been to discover what type of stimuli bats respond to. And to do this the UPV/EHU's research group analysed the behaviour of the bats in three different situations. As the researcher Aizpurua explained, "in one of the stimuli we caused waves to form in the water without any apparent prey. In another we placed a fish that was stationary in the water with its upper lip out of the water. And the last stimulus was a fish that came up to the surface and then went down, appearing and disappearing and creating waves in the water". As a result of this observation, the research team noticed that the bats only responded to the stimuli in which the prey was visible, without paying any attention to the waves in the water.

Once this knowledge had been obtained, they went on to study the differences between the fish that was stationary and the one that appeared and disappeared and they had a clear aim in mind: to see whether the bats identified the fish on the basis of its morphology or its movement. And the correct answer was the second one, because they observed that the reaction towards the two stimuli was similar to the reaction displayed towards insects and fish. "When a fish is stationary, they attack it as if it were an insect, with short dragging movements across the surface and using the two buzzes in a similar way. But when the fish disappeared, they made deeper, longer dragging movements and relegated the buzz to the pulses of the first type, the same as what happens when they fish," said the researcher.

When the prey vanishes

The technique used by bats to fish is more tiring than the one they use to catch insects, since the friction produced when they insert their feet under the water causes them to lose a lot of kinetic energy. So using the fishing technique continually is not to their advantage if the fish are not caught in most of the attempts. The research group designed one final experiment to see how bats react when the prey disappears. They submerged the fish at various moments during the bat's hunting action to analyse the change in their flight patterns and their echolocation.

The bat's response was not of the yes/no type but gradual. In other words, there is a link between the disappearance of the fish and the change in the hunting pattern. They adjust the intensity of the dragging action to the uncertainty of the location of the prey, and they adapt the echolocation pattern to receive the information they are interested in. The regulating behaviour therefore could be the important element that makes fishing advantageous.

Bibliographical reference

Ostaizka Aizpurua, Antton Alberdi, Joxerra Aihartza & Inazio Garin. Insight on how fishing bats discern prey and adjust their mechanic and sensorial features during the attack sequence. Scientific Reports (2015). http://dx.doi.org/10.1038/srep12392