Researchers have found that the greater mouse-eared bat uses polarised light patterns to navigate the skies — a technique that is not known to be used by any other mammal. A new study that was recently published in Nature Communications shows that these bats use patterns that form as sunlight is scattered within the atmosphere at sunset to calibrate and align their internal magnetic compass, aiding them in flight direction. Yet, despite this discovery, scientists are still baffled as to how they are able to detect the polarised light.
Scientists are aware that other animals make use of polarised light patterns in the sky, and for these creatures they do tend to understand how this is done. Bees for example are equipped with photoreceptors in their eyes that are specially-adapted for this purpose, while amphibians, reptiles, fish and birds all have cone cells in their eyes, which scientists believe may play a role in detecting polarisation. Yet, they are not sure what structure gives bats this ability.
Polarised light patterns are influenced by the position of the sun, and are clearer in the section of sky that extends across the skyline 90° from the sun’s position at sunrise or sunset. However, animals are still able to see the polarised light patterns even after the sun has set, which means that even when they are unable to see the sun, such as on overcast days, they are still able to orientate themselves.
‘Every night through the spring, summer and autumn, bats leave their roosts in caves, trees and buildings to search for insect prey. They might range hundreds of kilometres in a night, but return to their roosts before sunrise to avoid predators. But, until now, how they achieved such feats of navigation wasn’t clear,’ says Stefan Greif of Queen’s University Belfast, lead author of the study.
It is quite probable that bats utilise multiple senses for navigation, including sight, smell, echolocation, the magnetic field of the Earth, as well as the position of the stars of sun.
‘Most people are familiar with bats using echolocation to get around. But that only works up to about 50 metres, so we knew they had to be using another of their senses for longer range navigation,’ says Greif.
The research team set about trying to solve the riddle by showing a sample of 70 mouse-eared bats one of two different polarisation pattern types at sunset. The bats were then taken to one of two different sites to be released. Both sites were situated in Bulgaria and were located between 20-25 kilometres from the bats roosting sites. All the bats had tiny radio transmitters fitted onto their upper bodies before being released at 1am in the morning — when there is no visible polarised light in the sky.
The group of bats that were shown a shifted pattern of light took off in a direction which shifted at right angles to the direction bats in the control group (which were simultaneously released) took.
Populations of many species of bats are declining in both the United Kingdom and Europe, even though they are protected. For example, poorly positioned wind turbines can be a hazard for bats, and may be contributing to their decline.
According to Dr Richard Holland a co-author of the study from Queen’s University Belfast, scientists know that bats are ‘seeing’ wind turbines, but it appears that changes in air pressure surrounding active turbines results in barotrauma — similar to the bends that divers can experience with sudden changes in air pressure — which can be deadly.
Migratory species tend to be most affected, with an estimated 300,000 bats killed annually in Europe alone. Dead bats are found around the base of turbines, often with no visible external injuries. Holland suggests that reducing wind turbine activity during peak migration periods may be one option to reduce the number of bat fatalities.
Insectivorous bats provide a key service to humans — by consuming insects they in effect assist with natural pest control, reducing the need for costly and unhealthy chemical pesticides. It’s estimated that they save us millions of pounds in pesticides by eating insects.
‘Anything we can do to understand how they get about, how they move and navigate will be a step forward in helping to protect them,’ adds Holland.
Greif, S. et al. A functional role of the sky’s polarisation pattern for orientation in the greater mouse-eared bat. Nat. Commun. 5:4488 DOI: 10.1038/ncomms5488 (2014).