Blind as a bat: Finding your way home in the dark
4 October 2013 by Richard Holland
How do bats navigate by night? Richard Holland has been doing ingenious experiments to find out.
"Blind as a bat," the saying goes. Bats have small eyes and are nocturnal, so the idea is that they have little need for vision at night. Like many popular sayings, this is a misconception; bats actually see well in the dark compared to other mammals of a similar size. Yet it does highlight one question. How do bats find their way around at night when it is dark and visual cues are not available?
Of course, bats use echolocation, making sounds and then listening for the echoes. This system, essentially the same as the SONAR system used by ships, is sophisticated enough for them to detect and hunt small insects at night, out in the open air, and perhaps even navigate around a familiar place.
Echolocation has its limits though. Sound fades very quickly in air, and even though bat echolocation calls are some of the loudest sounds produced by an animal (130 decibels), the maximum range for detecting large landmarks is around 30m. However, we know that bats travel over much greater distances than this, finding their way home from as far as 700km away. Some bats even migrate, making journeys of over 1000km between their summer and winter roosts. Echolocation can't explain these feats of navigation.
Tracking bats after release
So how do they do it? When I started to get interested in this question in 2006, I discovered that the answer was that we don't know. So for reference, we had to look at bird navigation, the field from which I had moved. Birds can navigate using a two-step process that is effectively like our navigation with map and compass. They first work out where they are - the map step - and then they fly in the right direction to reach their destination - the compass. In an area they already know the 'map' is made up of familiar landmarks, but birds can return from unfamiliar places as well. Here it is less certain what provides the stimulus, but we think the birds may be able to sense changes in the Earth's magnetic field strength, or subtle differences in smell.
The compass cues birds rely on are much better known - we know they use the sun, the stars and the magnetic field's direction to take a direction bearing. Evidence suggests they use these different compasses to calibrate their direction finding-system. In particular it seems that birds calibrate the magnetic compass to the sunset. If the magnetic field shifts at sunset, they miscalibrate and fly off in the wrong direction.
In 2006 I did an experiment to test whether big brown bats also use this system, and found, quite surprisingly, that they did. We placed bats in an altered magnetic field at sunset, using a device called a Helmholz coil, which works by passing current through two parallel coils to generate a magnetic field. After the bats were driven 20km north of their roost and released, they would fly off at 90° to their unmanipulated peers. This suggested that they were using the Earth's magnetic field as a compass to take a bearing. Further experiments I later carried out in Bulgaria on greater mouse-eared bats confirmed this.
By fitting bats with a small radio transmitter, we could follow their direction after release. Sure enough, bats that observed sunset in an altered magnetic field were shifted by around 90° compared to a control group. But if they got the same treatment after sunset, there was no difference - both groups flew away from the release site in the same direction, towards home.
Bats awaiting release at sunset.
This tells us that bats use the sunset to calibrate the Earth's magnetic field for use as a compass. But the story may be still more complicated. In birds, it's not the sun itself that is responsible for this calibration, but the polarised light it creates in the atmosphere as it sets. (This is light made up of waves that are all oscillating in the same plane.) Experiments on birds show that if they view the sunset from behind a polarizing filter that rotated it by 90° making it vertical in the sunset direction (the opposite to the natural situation), then they would fly off at right angles to their normal direction.
With that in mind, I have returned with postdoctoral researcher Stefan Greif to the Tabatchka Bat Research Station in Bulgaria, aiming to test whether bats also set their compass with polarized light cues. As I write this 'live from Tabatchka', I am currently living a nocturnal lifestyle, having got out of bed at 2pm. Last night we were releasing bats until 4:30am, having started the experiment at sunset, when we exposed some bats to an altered magnetic field and others to a shifted pattern of polarization. I got to bed at around sunrise. Working on bats is a bit like being a student again.
As well as our experiment on the polarization, we are also collaborating with Yossi Yovel and Ivo Borissov of Tel Aviv University. They have developed small GPS tracking devices to track bats' homing routes. While GPS tracking is widely used on birds, it is much rarer on bats, partly because most bats are too small to carry even lightweight devices. However, Yossi and Ivo have arrived with GPS trackers weighing just 3 grams - the lightest ever built.
The trouble with these devices is we need to recapture the same bat in order to recover the data. But we have recovered 19 of 24 of these devices so far, and although the data are preliminary, they are also exciting. Soon we may know as much about bat navigation as we do about how birds find their way around. In essence we'll no longer be in the dark about the way these fascinating animals navigate.
Dr Richard Holland is a lecturer in Animal Cognition at Queen's University Belfast.