NEGLECT OF ECHO-LOCATION BY A NOCTULE BAT FEEDING IN CAPTIVITY T H E EARL OF CRANBROOK
Introduction NORMALLY insectivorous bats feed while Aying, finding and capturing their prey by echo-location using ultra-sonic impulses at frequencies far above the compass of the human ear. Of recent years Instruments have been designed to receive ultra-sonic sound waves and to give out corresponding emissions at audible frequencies. In October, 1965, a Holgate Ultrasonic Receiver was used to see if a noctule bat used or gave out ultra-sonic impulses when feeding in captivity and not in flight. The frequency ränge of this instrument is 10 to 150 kilocycles and it can be tuned to receive on any frequency within that ränge.
Echo-location by bats The technique of navigating or seizing an insect by echolocation differs from doing the same thing visually in so far as the information given by echo-location must of necessity be obtained at intervals, as opposed to the uninterrupted information given by the eyes. A bat sends out an echo-locating impulse—in effect a squeak at a very high frequency—and has to wait for the echo to come back and be heard before it gets any information about what there is in front of it. These impulses are heard as " ticks " through the ultra-sonic receiver. While a bat is flying through the air searching for an insect these " ticks " are given out at relatively long intervals, several times a second, but clearly distinct. For the purpose of this note that procedure will be called " giving out ticks " or " ticking ". As the bat approaches an insect to seize it, it must obtain more information and so must send out its echolocating impulses more frequently and with shorter intervals beUveen them. As a bat flies about hunting the listener with an ultra-sonic receiver hears a long series of ticks interspersed at intervals with noises which sound like " burr-r-r-p " and which for the purpose of this note will be called " burps ". These burps " are heard when a noctule is seen to turn quickly or dive down when in füll flight. They are also heard when a bat dives down in pursuit of a stone thrown into its path and are obviously given out by a bat when in closc pursuit and needing more information. A " b u r p " is in fact a series of " t i c k s " given out very rapidly. The "ticks " heard when a bat is flying correspond theretor with the visual scanning of its path by a hunting animal, wishing both to avoid obstacles and find its prey. There is of course no Visual analogy to a " burp ".
150 Transactions of the Suffolk Naturalists',
Vol. 13, Part 3
Though the human ear can only hear sound waves of frequencies below 12-15 kilocycles the air is fßll of sound waves of higher frequencies, indeed there are ultra-sonic components in human speech. Through the ultra-sonic receiver the ultra-sonic components of e.g., the pronouncement of the letters C or S, footsteps in long grass or a quick sniffing through the nose sound likealoud hiss. Tests on a captive noctule The bat was tested through the following stages :— 1. The bat was taken from its roosting place and held in the hand. If it was disturbed in this way while warm and awake it gave out a continuous series of ticks while hanging up, moving its head as if looking around visually. This continued when it was held loosely in the hand or if it was allowed to hang from a finger or to crawl about, when it behaved as if it was about to take flight. If held firmly in the palm of the hand, as if pressed into a crevice in in a rock or tree, it settled down immediately and ceased to " tick ". If the bat was disturbed in this way when cold and torpid no ticks were given out, either when hanging in its roost or from a finger. Sometimes when disturbed the bat would give an audible squeak with an ultra-sonic component which would be picked up by the detector, but no ticks preceded or followed the squeak. 2. The bat would gradually warm up, the breathing rate would gradually increase and throughout this period a meal worm was continually held and moved in front of its face. Finally the bat would be sufficiently wide awake to seize the meal worm in its mouth but still too cold to chew it up. Sometimes this action would be accompanied by a squeak with an ultra-sonic component but throughout this stage no ticks were given out. 3. Suddenly the bat would Start to eat the meal worm, the chewing being audible to the human ear but also having a strong ultra-sonic component up to frequency of 110 kilocycles. By this time the bat was fully awake and active. If allowed to hang freely or crawl about it gave out ticks more or less continuously : if held tightly it became relaxed and silent. In short it had reached the stage described in the first paragraph of 1. above. 4. The bat was tarne and accustomed to taking meal worms from the hand and from a dish. If a meal worm was held between thumb and forefinger 6" or 8" in front of the bat held in the other hand, it would Stretch itself out towards the proferring hand (or proferred food) opening its mouth and struggling, as it were, towards the meal worm. It looked much as it did when
NOCTULE BAT
151
excitedly preparing toflyand giving out a continuous series of ticks, save that the mouth was continuously opening and snapping. In fact it was not echo-locating at all and must therefore have been picking up the meal worm or hand holding the meal worm by sight. 5. A dish containing meal worms was then held in front of the bat which promptly climbed into the dish and started to feel around it for food. No ticks were given out, the meal worms being apparently sought for and found by touch. 6. When the bat after feeding in the dish for a short time was removed and allowed to hang from afingerit looked around giving out ticks continuously. If the dish fĂźll of meal worms was held underneath it, it would go into the dish and search for meal worms by touch, the emission of ticks ceasing as soon as the bat started to feel around the dish. 7. When most of the meal worms had been eaten they became much more difficult to find and the bat would search around the dish frenziedly but still ultra-sonically silent. Sometimes it would look over the side of the dish and give out a burst of ticks, following which there might be an occasional tick as it feit around the dish for food but it seemed still to be searching by touch and not ultra-sonically. 8. When the dish was empty the bat was heldfirmlyin the hand relaxed and ultra-sonically silent, the dish just in front of it. When the grasp was relaxed the bat would Start to search around the room with ticks : it was then encouraged to climb down into the dish which it searched apparently by touch though still giving out occasional ticks. When this proved fruitless it looked around the room ticking continuously and if directed back into the dish continued to tick while it made what seemed to be somewhat perfunctory searches by touch. 9. At no time was a burp heard. Summary
A noctule bat which had been trained to take food from the hand and from a dish was fed in front of the microphone of a Holgate Ultra-sonic Receiver. It gave out no ultra-sonic echo-locating impulses while cold and torpid. When taking food from the hand it apparently recognized the food or proferring hand by sight and did not use echo-location. When taking meal worms from a dish it apparently found them by touch and did not use echo-location.