Vision in echolocating bats - Fladdermus.net

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Introduction Functions normally served by vision in most vertebrates have been taken over by ultrasonic echolocation in insectivorous bats. In particular, the detection and tracking of flying insects is usually believed to be entirely acoustic (Kalko & Schnitzler 1993). Ultrasonic echolocation allows detection of very small targets, but its practical range is normally limited to a few metres, which is due to severe atmospheric attenuation and spreading loss of high-frequency sound and the poor reflective power of targets as small as insects (Lawrence & Simmons 1982, Kick 1982). On the other hand, although the eyes of insectivorous bats are small, they generally have good light-gathering capacity and good focal depth (Suthers 1970, Suthers & Wallis 1970). Vision can therefore be assumed to provide important cues particularly at ranges beyond that of echolocation, and is presumably useful for orientation and navigation at night. However, adaptation of the visual system for nocturnal conditions occurs partly at the expense of acuity, the ability to resolve details, and this presumably limits the use of vision for some short-range purposes such as finding prey (Suthers 1970). Nevertheless, at least some bat species, particularly those that glean prey from surfaces and for which acoustic clutter (background echoes) makes echolocation less useful (Arlettaz et al. 2001), have a visual acuity that is good enough for detection of insects and other objects at close range (Bell 1985, Joermann et al. 1988). We recently discovered that vision also plays a role in prey detection by the northern bat Eptesicus nilssonii (Family Vespertilionidae), an aerial-hawking species (Eklöf et al. 2002). However, because aerial-hawking bats generally seem to have poor visual acuity (Suthers 1970), this can be expected to set a lower limit to the size of insects that can be detected by vision. Experimental setting and methods To determine the visual acuity for E. nilssonii foraging under practical conditions in the field, we took advantage of a natural situation where bats regularly exploit groups of male ghost swift moths Hepialus humuli (Hepialidae) displaying over hayfields during midsummer evenings in southern Sweden (57°N). These moths are silvery white and highly reflective on the dorsal side and display visually in hovering flight among the grass panicles to attract females (Andersson et al. 1998). Hepialids are unusual among larger moths in that they are earless and do not show any evasive response to bat echolocation calls, whether these are natural or synthetic (Rydell 1998). At two different moth display sites, each regularly patrolled simultaneously by up to 10 northern bats (which were not marked), we added dead and spread individuals to the naturally displaying moth population. The dead moths were glued on top of steel wires and presented pair wise about 2 m apart and 0.5-0.7 m above the grass in various parts of the fields. One moth in a pair had its white 73
(dorsal) side up and the other had its dark grey (ventral) side up towards the patrolling bats. We assumed that the two were equally detectable by echolocation but that the white moth was more detectable by vision. This assumption was based on a previous experiment, showing that the moths´ silvery white coloration, which also contains a UV-component, is particularly contrasting during their natural display time just after sunset and against a background of green grass (Andersson et al. 1998). We thus expected the white and the dark moth to be attacked with equal frequency if bats use echolocation alone but with unequal frequency if they also use visual cues. To determine the minimum size of moths detectable by vision, we presented pairs of moths (one white and one dark) which were either intact (ca. 6 cm wingspan; Eklöf et al. 2002) or where both had the wingtips cut to give a total wingspan of either 5, 4 or 3 cm. Hence, size differed between the pairs of moths but the white and the dark moth that formed a pair were always of the same size. The moths were replaced when destroyed by the bats, but otherwise reused as long as possible. To prevent the bats from learning the exact positions of the moths, the pairs were moved at least a few metres following each attack by a bat. Hence each pair of moths was attacked only once while in each position. We deliberately presented the moths at a height where the bats´ echolocation would be complicated by clutter from the grass, overlapping with echoes from the moths, so that the bats were encouraged to use visual cues to find the moths. The extent of the ”clutter overlap zone”, which depends on the duration of the echolocation calls (7-8 ms), was 0.6-1.2 m above the grass (Jensen et al. 2001). Moths and bats were observed visually and also acoustically with a Pettersson D-940 bat detector from a distance of 2-10 m. The visual observations were facilitated by the relatively good light conditions prevailing at 57°N around midsummer (June 2002), which always made it possible to see what happened in sufficient detail. The experiments were performed only as long as moths were displaying naturally nearby, which occurred for about 30 minutes each evening (Andersson et al. 1998). Results Neither bats nor moths showed any obvious response to our presence. The bats seemed to forage normally, perhaps because they had become habituated to our presence over several seasons. The bats typically patrolled in large circles over the field at a height of 3-4 m (mean 3.5 m). The height was determined by using a measured and marked lamppost at the edge of the field as a reference. The bats always emitted echolocation calls during the search as well as throughout the attacks on the moths. An attacking bat typically performed a rapid and more or less vertical dive towards the grass while switching from search phase echolocation calls to a typical “feeding-buzz”, i.e. short pulses and high pulse repetition rate (Jensen et al. 2001). This behaviour strongly suggests that the attacks consistently were guided by echolocation. We counted the number of attacks on white and dark moths and compared the results for each moth size using one-tailed chi square statistics. Attacks on white moths were more frequent than on dark moths when the moths were 5 cm or larger (Fig. 1), suggesting that the detection was facilitated by vision in these 74
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Vision in echolocating bats Johan E
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A doctoral thesis at a university i
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to be precedence of vision over son
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och för att undvika hinder på vä
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B ats (Order: Chiroptera) are among
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VISION IN ECHOLOCATING BATS The mic
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The microchiropteran eyes are shape
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structures, and the dorsal lateral
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10 lux, which is equivalent to the
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The device used for the optomotor r
Page 23 and 24: Tab 2 - cont Light Visual Species (
Page 25 and 26: jamaicensis, Desmodus rotundus and
Page 27 and 28: (Davis & Barbour 1965) or when they
Page 29 and 30: Insectivores and carnivores For bat
Page 31 and 32: Moreover, the eyes of Macrotus cali
Page 33 and 34: combined with broadband echolocatio
Page 35 and 36: Multimodality - vision and echoloca
Page 37 and 38: superior colliculus. It has been sh
Page 39 and 40: Acknowledgements First of all I wou
Page 41 and 42: Buchler, E. R. & Wasilewski, P. J.
Page 43 and 44: Heffner, R. S. 1997. Comparative st
Page 45 and 46: Masterson, F. A. & Ellins, S. R. 19
Page 47: Suthers, R. A. & Chase, J. 1966. Vi
Page 51 and 52: Animal Behaviour - In Press Use of
Page 53 and 54: the ability of brown long eared bat
Page 55 and 56: General observations RESULTS When r
Page 57 and 58: are influenced to some extent by th
Page 59 and 60: Eklöf, J., Tranefors, T. & Vazquez
Page 61 and 62: Table 1. - The number of feeding at
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Page 73: Submitted manuscript Vision complem
Page 77 and 78: (Anderson et al. 1998). Hence, the
Page 79: Figure 1. - Frequency of attacks by
Page 83 and 84: Manuscript Visual acuity and eye si
Page 85 and 86: computer screen. The bats were rele
Page 87 and 88: study. On overcast nights the amoun
Page 89 and 90: Koay, G., Kearns, D., Heffner, H. E
Page 91 and 92: Tab. 1. - Eye diameter and optomoto
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