Vision in echolocating bats - Fladdermus.net

The brown long-eared bat Plecotus auritus responded to patterns equivalent to
30’ of arc, which means that this species should be able to detect objects as small
as 0.9 cm at a distance of 1 m. Among the Vespertilionidae, only Antrozous
pallidus has been shown to have a better resolving power (15’; Bell & Fenton
1986). These results and the fact that P. auritus has larger eyes than most
Vespertilionids (Cranbrook 1963; Tab 1.) suggest that it should be possible for
long-eared bats to detect prey sized objects visually. It typically feeds on
relatively large insects including many moths and beetles (Swift & Racey 1983,
Rydell 1989). As P. auritus is a gleaner and sometimes takes insects from leaves
(Swift 1998), it faces potential problems with clutter and therefore use other
sensory cues in addition to sonar. In fact, passive listening plays a major role in
prey detection by P. auritus (Anderson & Racey 1991). These bats are
exceptionally sensitive to sounds around 15 kHz, which is close to the
frequencies emitted by insects moving in clutter (Coles et al. 1989). However,
the long-eared bats may also use visual information when searching for prey. In
a recent study on feeding behaviour, it was shown that P. auritus preferred to use
visual cues to sonar when possible, and that they could detect ca. 2 cm long
mealworms visually (Eklöf & Jones, in press).
Visual acuity has previously been tested in a number of species (Table 2),
both behaviourally by optomotor response tests (Suthers 1966, Bell & Fenton
1986), and theoretically by counting the number of retinal ganglion cells (Koay
et al. 1998, Heffner et al. 2001). Both methods give indications of the minimum
separable angles, i.e. the minimum distance between two points that an animal
need in order to be able to separate them. The acuity values estimated by
counting retinal ganglion cells tend to be higher than those estimated from
behavioural studies, suggesting that the anatomical method gives a theoretical
threshold, rather than what the bats actually respond to. Nevertheless, although
the acuity values obtained from the different methods are roughly in the same
order, comparisons across the two methods should be made with care.
As shown by the literature data presented in Table 2, frugivorous and
nectarivorous bats seem to have better spatial resolution than most insectivorous
species. Nevertheless, the finest spatial resolution in any bat (3’38’’) is found in
the gleaning insectivore Macrotus californicus (Phyllostomidae), and this
happens to be the only bat known to find prey, using vision alone (Bell 1985,
Bell & Fenton 1986). Indeed, gleaning insectivores may have better visual acuity
than aerial-insectivores in general, and this suggests that the aerial-hawking
insectivores rely mostly on echolocation rather than vision for detection of small
targets, while the opposite may be true in gleaners. At the same time it seems as
if, among aerial-hawking insectivores, Emballonuridae have better resolution
than Vespertilionidae.
The visual resolving power may depend on ambient light intensity. In the
common vampire bat Desmodus rotundus, for example, the acuity drops from
48’ at a light intensity of 31 mL (ca. 310 lux) to over 2° in 4*10 -4 mL (ca. 0.004
lux) (Manske & Schmidt 1976). Other bats, such as Macrotus californicus and
Antrozous pallidus retain their visual acuity down to light levels as low as 2*10 -4
mL (ca. 0.002 lux) (Bell & Fenton 1986). As a comparison, a light level of 0.1
lux is equivalent to light levels at full moon, and similar to the conditions in this
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