They apparently have a kind of compass in their body: some animal species can orientate themselves amazingly precisely on their long journeys. But where is this sixth sense? The results of an experimental study on wandering bats now suggests: In mammals with a magnetic sense, the mysterious orientation system is in the cornea of the eyes.
Seeing, hearing, smelling, tasting and feeling are standard – but some animals have a further ability to perceive, as many studies have shown: some species of birds, fish, turtles and also mammals such as dolphins, whales or bats have an orientation ability when moving over a large area, which is apparently not based on the classic five senses. It is now assumed that they are able to perceive magnetic fields and use them as navigation aids. Experiments already suggest that this ability is based on iron oxide particles in certain body cells that function as “microscopic compass needles”. Especially in the case of mammals with the sixth sense, however, it is unclear where these directional cells could be located.
Where is the compass?
So far there have only been indications that gray mulls can perceive magnetic fields through sensory receptors in their stunted eyes in order to find their way around their branching tunnel systems. As part of their study, the researchers led by Oliver Lindecke from the Leibniz Institute for Zoo and Wildlife Research in Berlin have now investigated the question of whether the magnetic sense could also be in the eye in mammals with large-scale migration behavior. The focus was on the European native bats (Pipistrellus nathusii), which, like some bird species, migrate from their summer quarters in Northern and Eastern Europe to areas with a mild climate for the winter.
For the study, the scientists caught some rough-skin bats at night on the Baltic Sea coast, which were on their late summer migration to the south. Some of the animals were given a drop of oxybuprocaine in the eyes. It is a short-acting anesthetic that is used in human ophthalmology for diagnostic and surgical procedures. It numbs nerves in the cornea. However, the eyesight is not impaired, as the scientists also demonstrated in the course of their study in the case of bats through experiments. This enabled them to rule out that the observed effects are due to impaired vision, which bats sometimes use for orientation in addition to their echolocation system.
In one group of bats, the scientists treated both eyes, while in a second group, only the cornea of one eye was anesthetized. Bats served as the control group and were given only an ineffective saline solution as eye drops. The researchers then released the animals one by one in an open field eleven kilometers away from the trapping site. In doing so, they recorded the directions in which the animals flew away into the darkness.
Brought off course with eye drops
It was found: Individuals from the control group and the group with unilateral corneal anesthesia immediately orientated themselves south according to their migration route. But that was not the case with bats with corneas anesthetized on both sides: “These animals flew away in random directions,” reports Lindecke. “This indicates that the corneal anesthesia permanently disrupted the sense of direction – and that this apparently also works well with one eye.” continue to the south, emphasize the scientists.
“We were able to observe for the first time in an experiment how a migrating mammal was literally taken off course – a milestone in behavioral and sensory biology that allows us to research the biological navigation system of mammals in a more targeted manner,” says Lindecke Study a. Specifically, the results have now provided clear indications that the sense of direction in migrating mammals is typically in the eye. However, the sixth sense still remains mysterious, the scientists finally emphasize: How and where exactly it is in the bats’ cornea, how it works and whether it is actually the suspected magnetic sense must now be shown by future studies.
Source: Leibniz Institute for Zoo and Wildlife Research, specialist article: Communications Biology. doi: 10.1038 / s42003-021-02053-w