Vision in echolocating bats - Fladdermus.net

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DISCUSSION We found that long-eared bats are capable of visual detection of prey, at least under the light intensity of 4 lux, and that they prefer visual cues to sonar cues if given a choice. The results also indicate that the bats were unable to detect the prey items using sonar alone. There is both experimental and observational evidence that echolocating bats make use of vision and even give precedence to visual stimuli in some situations, including long distance orientation (Griffin 1970), detection of landmarks (Davis 1966), obstacle avoidance (Bradbury & Nottebohm 1969) and prey detection (Bell 1985). When selecting an escape route in an experimental set-up, the phyllostomid Anoura geoffroyi used visual cues alone and disregarded acoustical cues that also were provided (Chase 1981, 1983). The observation that bats have a tendency to crash into windows of buildings when released indoors (Fenton 1975) or during natural migration (Timm 1989) also suggests that they predominantly rely on vision rather than on echolocation in situations when both acoustic and visual cues are available. The performance is greatly improved (i.e. fewer crashes) when the bats are blinded (Davis & Barbour 1965) or when flying under natural dark conditions, and hence are “forced” to rely on echolocation (Eklöf et al. 2002). Suthers and Wallis (1970) studied the eyes of two species of Vespertilionidae (Myotis sodalis and Pipistrellus subflavus) and four different phyllostomids (Desmodus rotundus, Carollia perspicillata, Anoura geoffroyi and Phyllostomus hastatus), and concluded that the visual capabilities of all the species tested would allow the bats to see well beyond the range of echolocation. Due to the more or less spherical lenses, it also follows that Microchiroptera has a short focal distance and hence a great depth of focus (Suthers & Wallis 1970). In fact, microchiropteran bats seem to be farsighted, indicating that vision is used preferably over longer distances, where it may not overlap with echolocation, which is a short-range operation (Kick 1982; Fenton et al. 1995). Nevertheless there are some bats, such as Phyllostomus hastatus, which also use vision within the range covered by their echolocation system, i.e. when approaching a landing spot (Joermann et al. 1988). We cannot exclude the possibility that the bats used passive listening to detect the mealworms, as some other gleaning bats do (Arlettaz et al. 2001). Sounds coming from the prey could have been detected from any of the dishes, including the dishes with visual cues only and the no cue situation, as live mealworms were crawling under the petri dishes (in the lids). Also olfactory cues from the mealworms may have been available to the bats in all four feeding situations. Hence, olfactory cues and prey-generated sounds may explain some of the feeding attempts at petri dishes with prey in the lids. During the experiments, bats were sometimes seen hovering at positions where no petri dish was available, but where dishes had been available previously, presumably relying on spatial memory (Neuweiler & Möhres 1966; Grant 1991; Höller & Schmidt 1996). Hence, the bats tended to revisit positions where they previously had been rewarded and it seems possible that the results 55
are influenced to some extent by this. However, we believe that our experimental design, where the position of the dishes were changed after every feeding attempt, would minimise these effects, and that the results show preference for feeding dishes, rather than spatial memory. The results of Experiment 2 allowed rejection of the hypothesis that bats associated structural features of the food dish with reward, and were therefore attracted to features of the dishes by associative learning (Siemers 2001). However, as all the bats flew simultaneously in this experiment, we cannot exclude the possibility that some individuals sometimes explored the dishes without getting a reliable indication of reward. In the first experiment there were equally many feeding attempts at the petri dishes providing sonar cues as at the no cue dishes, which means that we found no indication that the bats could detect the prey items by using sonar alone. One can therefore hypothesize that bats having to rely exclusively on sonar may learn to recognize structures rather than prey. There were more feeding attempts at the petri dishes providing visual cues only compared to those that provided sonar cues only. This suggests that the long-eared bats were capable of finding prey visually and that they even preferred using vision when possible. Long-eared bats emerge from their roosts late in the evening (15-55 minutes after sunset depending on the latitude; Swift 1998), which means that they normally operate in very low light levels (
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Vision in echolocating bats Johan E
Page 3:
A doctoral thesis at a university i
Page 6 and 7: to be precedence of vision over son
Page 8 and 9: och för att undvika hinder på vä
Page 11 and 12: B ats (Order: Chiroptera) are among
Page 13 and 14: VISION IN ECHOLOCATING BATS The mic
Page 15 and 16: The microchiropteran eyes are shape
Page 17 and 18: structures, and the dorsal lateral
Page 19 and 20: 10 lux, which is equivalent to the
Page 21 and 22: 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: General observations RESULTS When r
Page 59 and 60: Eklöf, J., Tranefors, T. & Vazquez
Page 61 and 62: Table 1. - The number of feeding at
Page 63: “The Eocene brought mammals mean
Page 73 and 74: Submitted manuscript Vision complem
Page 75 and 76: (dorsal) side up and the other had
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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