What is it Like to be a Bat?

Prof Gareth Jones

Bristol University

25 February 2005

Professor Gareth Jones is from the School of Biological Sciences at Bristol University with an interest in evolution and echo-location.

Prof. Jones introduced his talk with a reference to Thomas Nagel's famous 1974 paper[1] with the same title, in which he asks whether it is possible to put oneself in the position of an alien creature sufficiently well to understand how it thinks and feels. Anyone who has been in a room with a bat flying around will realise how differently it must see the world.

 

Bats have an ancient history, evolving probably about 60 million years ago, and are now highly specialised. They are the only mammals with the ability for powered flight, and in fact the only other vertebrates with this facility are birds. Although we often refer to their blindness, they have better vision than we usually credit. But their primary adaptation is to be able to use pulses of sound to give them clues as to their whereabouts and that of prey; and to that end, some have enormous ears relative to their body size (like horseshoe bats) or convoluted nasal features to direct the outgoing pulses (like noseleaf bats).

Due to their nocturnal habits and flight patterns they are not easy to examine, but we are now building up some solid information on their social interactions and feeding ranges as well as the mechanics of how they navigate, avoiding obstacles and finding prey. By using a rapid sequence of flash still photographs at 1/15 sec. intervals and then correlating with recordings of their calls, as they pick up mealworms off a water surface, we are learning some of the principles of their bio-sonar abilities.

Analysis of their flight through clouds of helium filled bubbles have shown that their wings generate lift from vortex rings on their downstroke only in slower flight, but at higher speeds they can generate more lift on the upstroke as well.

Bats have traditionally been divided into two classes suborders. Those with large wings, Tthe Megachiropterans, like or Old World fruit bats, which have very little if any echo-locating abilityn , and the smaller ones, the Microchiropterans, which use the facility extensively. But recent DNA comparisons show that one group of the latter, the horseshoe bats, are more closely related to the fruit bats than the other echo-locating species, which may imply that the ability to use echo-location evolved very early, and that later the fruit bats lost it.

To get an idea of how sound can be used to find objects, a visit to the website at [2] is worthwhile, where the principles are illustrated by analogy of using light. In daylight, we can see from reflected light in the environment, but in the dark, we must provide our own source of light, like a torch. In the same way, a bat provides its own source of sound by issuing short pulses and listening for the response from the surroundings. The general principles of their use of sound after 60 million years closely resembles what radar and sonar engineers have discovered much more recently.

As the sound returns from other objects, the further away they are, the weaker the echo. In the case of bats, this is particularly apparent as the high frequencies they use attenuate rapidly with distance. To counter this, the pressure levels in the calls of bats have been measured higher than a jet taking off 60 metres away. To detect objects, narrow pulses are used to provide acoustic glints from insects and other prey. But to localise it, longer frequency modulated broadband signals are used. They are clearly trading localisation against detection. It is thought theyat horseshoe bats can prevent themselves from deafening themselves by relying on the Doppler shift of the return signal from their own movement through the air. Other bats using shorter calls shut down their middle ear muscles when calling.

An examination of the calls of bats as they flew along an irregular hedge, showed that they adapt the timing between calls, and the type of call, so that they can follow the hedge very closely. But they are not perfect. From work with whiskered bats, it appears that they tend to overestimate distance as they increase their speed, but that they underestimate it because they are actually moving. These effects largely cancel out at a distance called the "focus" distance. As a bat approaches an object, the pulses get shorter in order to s horten the focus distance.

There are places where echo-location does not work, especially where there is a lot of clutter, like leaf litter on a forest floor. In these circumstances, bats merely listen and rely on being able to determine the direction and location of a moving target from the nosies it makes. If prey is alive, they can find it quickly anywhere. But if it is dead, then they have difficulty locating unless it is in the open.

Bats are remarkably long-lived for such a small mammal. Horseshoe bats are known to live to 30 years of age, and keep long-term links with their kin. Within a single roost, their are a number of separate family groups, following the female line of descent. The mothers do all the nurturing of the infants, and share their feeding ranges with them and their siblings. They produce at most one baby per year, and normally with the same male, which creates the kin preferences that are evident in their behaviour. Although the father has no part in the upbringing of the children offspring, theire must be some form of information transfer which prevents mating with the father, but it is not yet known how this is accomplished.

Recently it has become apparent that their may be among a single area a number of species, called cryptoic-species, which look identical, but which have different patterns of behaviour. For instance, among pipistrelles, there is one subgroup that uses sounds at 45kHz frequency, and another group at 55kHz. Other such divisions are now being found elsewhere in the world.

Bats have been declining in numbers, but a recent countryside stewardship scheme to help farmers adapt their methods has allowed some populations to increase again. In particular, it has been noted that organic farms have more insects around them, which in turn has attracted more bats to those areas.

During the questions from the audience, a number of other points were brought to light.

Bats often feed at up to about 3km range from their roosts. But to get there and back is not eased by echo-location, that cannot find landscape featureswhich is only effective over short distances . We do not know how they find their way over longer distances.

The brains of bats show adaptations to using sound too. Their auditory cortex is very well developed and ha strong connections to their visual cortex, indicating that the processing of the signals is of vital importance to their abilities - not just the ability to hear well. But exactly which model they use to process the sounds is not yet clear.

The frequency response of their ears well matches that of the signals they emit in general. Some bats though also have good low frequency hearing, and they search for ground-dwelling prey where hearing the movement of prey is important to locate them. The calls that we can hear at dusk are social calls, which are believed to be a means of telling other members of the colony same species to stay away, perhaps to allow room for feeding in the area defend feeding areas.

To allow for some genetic development, Ssome males are known to move between colonies, and in fact a male may have progeny from 6 to 8 females.

Some prey of bats can hear them too, in particular many moths will immediately dive for cover when they hear an approaching bat.

Further reading

[3] Griffin, D.R. Listening in the Dark. (Cornell University Press. 1986)

References

[1] Nagel, T. "What is it Like to be a Bat?", The Philosophical Review LXXXIII, 4 (October 1974): 435-50

[2] http://www.biosonar.bris.ac.uk, "Biosonar - seeing with sound"