Vision in echolocating bats - Fladdermus.net

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