Donald R. Griffin

The echolocation of flying insects by bats

Abstract 1. 1. Bats of the genus Myotis ( M. lucifugus , M. subulatus leibii and M. keenii septentrionalis ) have been studied while pursuing and capturing small insects under laboratory conditions. It is apparently important to provide fairly large numbers of such insects in order to elicit insect catching behaviour indoors. 2. 2. Insect catches are individually directed pursuit manoeuvres; each insect is detected, located, and intercepted in flight within about half a second. 3. 3. Certain individual bats caught mosquitos ( Culex quinquefaciatus ) and fruit flies ( Drosophila robusta and D. melanogaster ) at remarkably high rates which could be measured conservatively by the gain in weight of the bat. Sometimes a bat would average as many as 10 mosquitos or 14 fruit flies per minute during a period of several minutes. In four cases motion pictures showed two separate Drosophila catches within half a second. 4. 4. The orientation sounds of the hunting bat are adjusted in a manner that seems appropriate for the echolocation of single insects one at a time. There is a search phase before the occurrence of any apparent reaction to the insect. In this phase the frequency drops from about 100 to 50 kilocycles during each pulse of sound, and the pulses are emitted by M. lucifugus at intervals of 50 to 100 milliseconds. 5. 5. When an insect is detected the search phase gives way to an approach phase characterized by a progressive shortening of the pulse-to-pulse interval and, if necessary, a sharp turn towards the insect. In this phase the pulse duration may shorten somewhat, but the frequencies remain approximately the same as in the search phase or drop slightly. 6. 6. When the bat is within a few centimetres of the insect there is a terminal phase in which the pulse duration and interval between pulses shorten to about 0·5 millisecond and 5 or 6 milliseconds respectively. Contrary to a conclusion reached earlier on the basis of much less adequate data ( Griffin, 1953 ), the frequency drops in the terminal phase, sometimes to 25 or 30 kilocycles. This is the buzz, which also occurs in many cases when the bat is dodging wires or landing. 7. 7. The distance from the insect at which detection occurs can be judged by the shift from search to approach patterns. This distance of detection is commonly about 50 cm. for Drosophila , and it occasionally may be as much as a metre with fruit flies or mosquitos. 8. 8. Two M. lucifugus which had become adept at catching Drosophila in the laboratory were exposed to broad band thermal noise either at low frequencies (0·1–15 kilocycles) or high (20–100 kilocycles). The low frequency noise had an approximately uniform spectrum level of about 50 decibels per cycle band width (re 0·0002 dyne/cm 2 ) from 0·1 to 8 kilocycles. It was thus very loud compared to the flight sounds of Drosophila which have a fundamental frequency of a few hundred cycles/second and a maximum sound pressure level of 20–25 decibels at the distances of detection by these bats. The high frequency noise was of low and varying intensity, but it discouraged or prevented insect catching. The low frequency noise, on the other hand, had no effect on insect catching; the bats gained weight in this noise (and in the dark) just as rapidly as in the quiet. Although bats sometimes detect insect prey by passive listening to sounds emanating from the insects themselves, these experiments appear to us to establish conclusively that small and relatively silent insects are often detected by echolocation.

The change in refractive power of the human eye in dim and bright light.

It has been reported that the human eye behaves as though relatively short-sighted in dim light. Observers tend to compensate for this change by setting optical instruments more negatively in dim than in bright light. New measurements of telescope settings by 21 observers reveal an average increase in power of the eye in dim light of 0.59 diopter (range +1.4 diopters to −3.4 diopters). The dilation of the pupil in dim light does not contribute significantly to this phenomenon. The chromatic aberration of the eye was measured in 14 observers with a specially designed spectral stigmatoscope. The refractive power of the eye increases about 3.2 diopters between 750 and 365 mμ. For this reason the Purkinje shift of maximum visual sensitivity from 560 mμ in bright light to 505 mμ in dim light produces a relative myopia in dim light of 0.35 to 0.40 diopter. Persons who display changes larger or smaller than this do so because of involuntary changes in the accommodation in bright and dim light. In dim light the eye enters a state of relatively fixed focus, little different from its condition when the accommodation is paralyzed with homatropine. In this fixed state the accommodation may be relaxed, or it may add as much as 3 diopters to the refractive power of the eye. Experienced observers focus optical instruments in dim light close to the optimal settings determined objectively. Departures of more than 0.5 diopter in either direction from the optimal focus depress the visual sensitivity and acuity. It is concluded that setting optical instruments about 0.4 diopter more negatively in dim than in bright light is justified on the basis of the chromatic aberration of the eye. Many observers gain a further advantage from individual adjustments of focus in dim light, appropriate to their accommodative behavior.

Prospects for a Cognitive Ethology

What, if anything, do animals think about? Do they have anything at all comparable to mental experiences as we know them? Are any of them ever aware of themselves, their surroundings, or the results likely to follow from their behavior? It would of course be manifestly absurd to overlook or underestimate the enormous differences in complexity, versatility, and range of comprehension between human and animal thinking (if the latter occurs at all). However, our direct knowledge of our own mental experiences, imperfect as it may be, is apparently quite sufficient to establish the importance of such experiences, at least in one species. Mental experiences will not wither away merely to keep science simple and tidy. Hence it is appropriate to inquire whether or not the differences between men and animals in this respect are qualitative and absolute, and to explore the possibility that particular animals may experience thoughts and feelings of one sort or another under various conditions. Determining the nature and extent of animal thinking is crucially important in our attempt to understand human uniqueness and our place in the universe, as has been discussed by Popper and Eccles (1977) and many others.

New evidence of animal consciousness

This paper reviews evidence that increases the probability that many animals experience at least simple levels of consciousness. First, the search for neural correlates of consciousness has not found any consciousness-producing structure or process that is limited to human brains. Second, appropriate responses to novel challenges for which the animal has not been prepared by genetic programming or previous experience provide suggestive evidence of animal consciousness because such versatility is most effectively organized by conscious thinking. For example, certain types of classical conditioning require awareness of the learned contingency in human subjects, suggesting comparable awareness in similarly conditioned animals. Other significant examples of versatile behavior suggestive of conscious thinking are scrub jays that exhibit all the objective attributes of episodic memory, evidence that monkeys sometimes know what they know, creative tool-making by crows, and recent interpretation of goal-directed behavior of rats as requiring simple nonreflexive consciousness. Third, animal communication often reports subjective experiences. Apes have demonstrated increased ability to use gestures or keyboard symbols to make requests and answer questions; and parrots have refined their ability to use the imitation of human words to ask for things they want and answer moderately complex questions. New data have demonstrated increased flexibility in the gestural communication of swarming honey bees that leads to vitally important group decisions as to which cavity a swarm should select as its new home. Although no single piece of evidence provides absolute proof of consciousness, this accumulation of strongly suggestive evidence increases significantly the likelihood that some animals experience at least simple conscious thoughts and feelings. The next challenge for cognitive ethologists is to investigate for particular animals the content of their awareness and what life is actually like, for them.

The role of the flight membranes in insect capture by bats

Abstract 1. 1. A large series of electronic flash photographs of bats catching insects on the wing has demonstrated several common techniques employed. 2. 2. A small and slow-flying insect such as a fruit fly may be sometimes seized directly with the mouth. In most cases, however, the interfemoral membrane is formed into a pouch by forward flexion of the hind legs and tail just before an insect is intercepted. Immediately after contact with the insect, the head is enclosed within the pouched tail membrane while the insect is seized in the jaws. Examples of this technique occur when a Myotis lucifugus catches mealworms tossed into the air, and when Lasiurus borealis catches flying moths. 3. 3. When the insect is not directly in front of the approaching bat one wing is often extended so as to intercept it. Sometimes the terminal joints of the 3rd and 4th fingers are flexed to form a scoop in which the insect is rapidly conveyed to the mouth, usually by way of the pouched tail membrane. This technique has been photographed in Myotis lucifugus catching fruit flies and also mealworms that had been tossed into the air. The wing was also employed in this manner during a single case where a greater horseshoe bat, Rhinolphus ferrum-equinum , was photographed catching a flying moth. 4. 4. In a few cases the photographs show that the wing is used either deliberately or accidentally to flick an insect into a position where it is seized in the mouth or pouched tail membrane a fraction of a second later. This technique has been clearly photographed only with Myotis lucifugus catching tossed mealworms. 5. 5. While the use of tail and wing membranes greatly increases the potential area of contact with insect prey over the area of the opened mouth alone, the photographs almost invariably show that the bats head is pointed at the insect well before contact with it. Preparatory movements such as cupping the tail membrane, flexing the terminal joints of the fingers, and reaching the wing toward the moving insect, all show that the insect is located quite accurately before it touches any part of the bat. These photographs strongly indicate that each insect is individually located and intercepted. 6. 6. The wing of these bats thus retains some of the prehensile functions of the hand in non-flying mammals.

THE SENSITIVITY OF ECHOLOCATION IN BATS

1. The distance at which bats (Myotis lucifugus) react to the presence of a row of small wires has been measured by a photographic determination of the distance at which the pulse repetition rate first increases as the bats fly towards the wires. Distinct changes in this rate were measured in almost every flight towards wires spaced 30 cm. apart and ranging in diameter from 0.18 to 3 mm.2. The interval between successive pulses averaged 60 to 80 msec as the bats flew along the room towards the row of wires, and dropped to 20-40 msec just before the barrier. The intervals decreased less with the smaller sizes of wire.3. All but the largest of these wires are well below one wave-length of the emitted sounds of these bats (50-60 kc, or 6-7 mm., at the peak intensity and 120 kc, or about 3 mm., at the very beginning of some pulses).4. Clear evidence that the wires had been detected was furnished at the point where the interval between pulses first dropped significantly below the level that prevailed before an...

The Sensitivity of the Human Eye to Infra-Red Radiation

The spectral sensitivity of human vision has been measured in the near infra-red, in two areas of the dark adapted eye: the central fovea (cones) to 1000 mμ, and a peripheral area, in which the responses are primarily caused by rods, to 1050 mμ. In both cases the estimates of spectral sensitivity are based upon determinations of the visual thresholds for radiation passing through a series of infra-red filters. By successive approximation, sensitivity functions were chosen which were consistent with the observed thresholds.The spectral sensitivity of the fovea determined in this way is consistent with previous measurements of Goodeve on the unfixated eye. At wave-lengths beyond 800 mμ the periphery becomes appreciably more sensitive than the fovea. This tendency increases at longer wave-lengths, so that at the longest wave-lengths studied, the radiation appeared colorless at the threshold and stimulated only rods.Lengthening the exposure time increases the sensitivity of the peripheral retina relative to the fovea. Our measurements involved exposures of 1 second and fields subtending a visual angle of 1 degree. With shorter exposures or smaller fields the fovea is favored, so that under such circumstances the fovea may become more sensitive than the periphery well into the infra-red.At 1050 mμ the sensitivity of the peripheral retina is only 3×10−13 times its maximum value at 505 mμ. A computation shows that by 1150 or 1200 mμ radiation should be more readily felt as heat by the skin than seen as light by the eye.

The homing ability of the neotropical bat Phyllostomus hastatus, with evidence for visual orientation

Summary o 1. The neotropical bat Phyllostomus hastatus studied in Trinidad, West Indies, showed a superior homing performance to any previously reported for bats of temperate latitudes. In one night 94 per cent of seventy-four returned from 3 to 6 miles, 57 per cent of 257 from 7 to 21 miles, and 26 per cent of 112 at 25–33 miles. None of fifty returned the first night from 39 to 40 miles, but 12 per cent were observed over the next several days. 2. On one occasion two out of five bats returned from 6 miles within 1 hr and after some releases at 15–20 miles 40 per cent of the bats returned within 3–4 hr. The homing performance was not significantly affected by intervening mountains of 2000–3000 ft or by up to 10 miles of open water. Bats released 1 mile apart homed as well or better than those released in groups. 3. Blindfolded P. hastatus flew normally and appeared to live normal lives for several days. From distances of 5–8 miles a considerable number of blindfolded bats homed during the first night. The data do not suffice to demonstrate whether or not there was a significant difference in homing performance from these distances between blindfolded bats and controls equipped with goggles similar to the blindfolds except for clear plastic windows over the eyes. But at 15 to 20 miles none of thirty-six blindfolded bats returned in one night with their eyes effectively covered, even though half those equipped with goggles did so. 4. It thus seems likely that within familiar territory these bats can find their way without the need for vision, but that at 15–20 miles they are severely handicapped if deprived of visual orientation. One simple explanation would be use of the northern range of mountains for visual piloting.

High-Altitude Pursuit of Insects by Echolocating Bats

Bat detectors on helium-filled kite balloons revealed echolocating bats active at altitudes ≤ 600 m above the ground over Brachystegia woodland in the Sengwa Wildlife Research Area, Zimbabwe. Feeding buzzes indicated that bats were actively foraging to 600 m. At least seven species of bats were detected, including six molossids and one emballonurid.

Radar Observations of Bird Migration over the Western North Atlantic Ocean

Summary1.During two fall migration seasons, a large number of flying targets were tracked by a radar mounted on an oceanographic research vessel. Obervations were made in September and October in a region extending from the New England coast to the area south and east of Bermuda (Figs. 1 and 3). Accurate local wind measurements were made at short intervals during the observations.2.Evidence was obtained that many of the targets were birds engaging in migration between coastal North America and the neotropics. Such birds often appeared in discrete ‘waves’ associated with cold fronts passing the North American coast. Distant from land, long periods occurred between ‘waves’ when no targets were observed.3.Other targets, which were probably also migrating birds, flew in directions which were difficult to explain, or flew at speeds relative to the air which were lower than seem consistent with the energetic requirements of long-range flight. Extrapolations of the possible flight paths of such birds (Figs. 8–10) suggested that birds, even some of those flying at the higher speeds, may take several days in attempting to cross the Western North Atlantic.4.Birds appeared to fly at higher altitudes during the day than at night, perhaps to allow temperature regulation without evaporative water loss.5.The results are discussed in relation to previous laboratory measurements of caloric expenditures and flight speeds of birds in wind tunnels, possible use of structure in the atmosphere by small migrants, and the selection pressures which might play a role in the phenomenon of long migrations over water.