The Zoologist/4th series, vol 3 (1899)/Issue 700/The Mode in which Bats secure their Prey, Oldham
THE MODE IN WHICH BATS SECURE
By Charles Oldham.
Observations made during the past few months have to a great extent confirmed my suggestion (ante, p. 51) that the method adopted by the Whiskered Bat (Myotis mystacinus) and the Long-eared Bat (Plecotus auritus) to secure their prey was common to other species. This curious habit seems to be little known, or, if noticed at all, to have been misunderstood, and is so remarkable that a further description of it, even at the expense of repetition, will, I trust, be forgiven.
When walking, most of our British Bats carry the tail curved downward and forward beneath the body, the interfemoral membrane forming a pouch or bag. If a moth or other large insect be encountered, the Bat seizes it with a rapid snatch, slightly spreading its fore limbs with the wings still folded, and, pressing them firmly on the ground at the carpus in order to steady itself, brings its feet forward in order to increase the capacity of the pouch, into which, by bending its neck and thrusting its head beneath its body, it pushes its prey. If the moth be a large one the Bat often struggles convulsively for a few seconds before it can adjust its grip to its satisfaction; but once in the pouch the insect rarely escapes, and, when effectually secured, is brought out and eaten openly. If the Bat can be induced to feed whilst hanging head downwards, suspended by its toes, its actions can be observed much more easily. Its tactics are then more efficacious, as the tail is not pressed close to the belly, and the pouch is in consequence held open, as it would be, of course, during flight.
This habit, practised readily and frequently in captivity, is so perfect an adaptation of means to an end that it must obtain with equal frequency among Bats in a free state. These creatures, when at large, capture most, if not all, of their food during flight—I have known a captive Long-eared Bat to remain on the wing for over an hour at one time—and it seems in the highest degree probable that they habitually use this method to secure insects which are large and vigorous, and therefore difficult to manage, without being compelled to alight.
One species at any rate has actually been observed to use the interfemoral membrane as a pouch when on the wing. My friend Mr. J.R.B. Masefield writes, under date March 1st, 1899:—"I have no doubt whatever that the Long-eared Bat makes use of the interfemoral pouch in the way you mention. I have been close to them when picking moths off sallows, and the Bat always hovers when taking off the moth, and bends up the tail so as to form a receptacle for the insect as it drops. As you know, the sallow-feeding Noctuæ (Tæniocampa gothica, stabilis, instabilis, cruda, &c.) all drop immediately the flower or bush is touched or shaken, and thus the head of the Bat and the interfemoral pouch form a trap from which the moth cannot escape. When feeding in captivity I have often seen this Bat, as soon as it had seized a moth, sit, as it were, on its tail and double up its head in the way you describe. The Long-eared Bat does not always succeed in holding a large moth at the first snap, and this is an additional argument in favour of your theory." A Long-eared Bat which I found in the old copper-mines on Alderley Edge, and kept for some days in February last, used always to thrust moths (Scotosia dubitata and Gonoptera libatrix) into its pouch, but only treated mealworms in this manner when they struggled violently, seizing and eating them at other times quite openly.
In July and August I caught several examples of Daubenton's Bat (Myotis daubentoni) as they emerged from a hole beneath the eaves of a house near Redes Mere, Cheshire. They seized and ate mealworms quite openly, but always thrust moths into the interfemoral pouch. Small thin-bodied moths (Cidaria populata) were thrust in and withdrawn again almost immediately; a larger species (Urapteryx sambucata) was obviously more difficult to manage, whilst vigorous thick-bodied species (Xylophasia polyodon, Triphæna pronuba, and Mamestra brassiccæ) occasioned many struggles, and were not firmly secured until they had been held in the pouch for some seconds. Once, one of the Bats, having seized a large and powerful T. pronuba, brought its feet so far forward that it fell over on to its back, but pluckily held the moth in its pouch until it was secured. Owing to the late hour at which it appears in the evening, it is not easy to distinguish the actions of this Bat as it skims over the shady pools which are its favourite haunts. It probably subsists to a large extent on gnats and other insects which fly just above the surface of the water and are too small to necessitate the use of the interfemoral pouch. Its behaviour in captivity shows, however, that, when occasion requires, this method of securing its prey is readily and effectually adopted. Daubenton's Bat has the tail only slightly curved during flight, to about the same extent as the Pipistrelle, less than the Whiskered and Long-eared Bats, and more than the Noctule, which holds its tail almost straight behind it. In his account of Daubenton's Bat, Tomes says (Bell, 'British Quadrupeds,' 2nd edit. p. 64):—"When a fly or other food was taken which was rather large, the carpus was always brought into use to do the office of a hand, and the food was pushed into the mouth with it." This is entirely opposed to my experience, for neither this Bat nor any of the other species I have kept has ever made use of either carpus or foot in feeding.
The Pipistrelle (Pipistrellus pipistrellus) is abundant in the neighbourhood of Alderley Edge, and I have kept several for a few days at different times during the past summer. The habit under consideration is much less pronounced in this species than in those already mentioned. My captives used to seize and eat mealworms quite openly, but on one occasion a particularly large and vigorous worm was thrust into the pouch. Thin-bodied moths (Larentia fluctuata and Cidaria populata) were also seized and eaten openly, as was a male Hepialus sylvinus; but larger moths (T. pronuba, Polia chi, and other Noctuæ) were pouched before being eaten.
The Noctule (Pipistrellus noctula) occurs commonly at Alderley Edge, but my efforts to obtain one alive have so far been unsuccessful. This Bat comes abroad early, and during the long midsummer evenings is silhouetted so clearly against the sky that the contour of its ears may be seen distinctly. Under such favourable conditions I have spent hours watching Noctules, both with the naked eye and with a strong glass, but have never seen them use the interfemoral membrane as a pouch, nor have I been able to detect them using the thumb to rend asunder their prey, as Mr. O. Grabham (ante, p. 131) states they do. It is certain that the oblique downward plunge, so noticeable in the flight of the Noctule, is not always due to the loss of balance which would be involved in bringing the thumb to the mouth, for I have often seen Noctules plunge when the light was sufficiently good to show that both wings were fully extended. Mr. T.A. Coward, who has constantly watched Noctules in Dunham Park, suggests that a loss of balance would involve a vertical fall such as occurs when one wing is broken by shot, and not an oblique dive with extended wings. It must be remembered, however, that the diet of the Noctule is not restricted to large beetles (Melolontha and Geotrupes), and neither the pouch nor the thumb would be required to secure or dismember small insects; but whether this species uses the interfemoral membrane as a pouch, as its congener the Pipistrelle undoubtedly does, could be definitely settled by observing individuals in captivity.
A number of Lesser Horseshoe Bats (Rhinolophus hipposiderus), obtained at Cefn, Denbighshire, on March 4th, died before the end of the third day of their captivity. I could not induce them to feed, and they were so loath to take wing that I was unable to ascertain definitely the position of the tail during flight. In repose this organ is reflexed over the back (cf. B. Newstead, Zool. 1897, p. 538), and when on the ground the Bat carries it erect, i.e. at right angles to the long axis of its body. The legs showed very distinctly against a white ceiling when viewed from below, but this was possibly due to the shortness of the tail, and not to its being erect or recurved. Even if the tail were curved beneath the body during flight, its shortness and the small extent of the interfemoral membrane would constitute only an inefficient pouch, and it seems improbable that in the genus Rhinolophus these parts subserve the same purpose as in Plecotus, Myotis, and Pipistrellus.
I have put together these notes in the hope that others interested in the British Bats, who may be able to obtain the Barbastelle, Natterer's Bat, the Noctule, and more especially the Horseshoe Bats, will make observations on the methods adopted by them to secure their prey.
- In Bell's 'British Quadrupeds,' 2nd edit. p. 64, Daubenton's Bat is described as thrusting its nose more or less downwards under its breast in feeding; and in 'The Zoologist,' 1890, p. 99, a captive Pipistrelle is said to have beaten moths against its breast to stun them.