photographs had been taken.
Results
Sixteen individual bats belonging to four different species were caught and tested: Plecotus auritus, Myotis mystacinus, M. brandtii and M daubentonii. When tested, the bats typically moved about in the Plexiglas cylinder for a while before coming to rest, and they sometimes continued to move around during the tests. However, most bats unambiguously responded to the rotating stripes by moving their heads in a snappy, stereotyped manner, either following the rotational direction or the opposite direction, as described earlier by other authors (Suthers 1966; Bell & Fenton 1986).
The results were relatively consistent within a species and genus but differed considerably between the two genera. The species of Myotis responded only to the largest pattern (5° of arc), while all the Plecotus auritus individuals except one responded down to the pattern equivalent to 1-0.5° of arc (Table 1). The eye size varied with visual acuity as expected (Table 1). The Myotis species had smaller eyes (ca. 1 mm diameter) than Plecotus auritus (ca. 1.7 mm). Discussion
Visual acuity is highly variable among vespertilionid bats, which presumably reflects the extent to which bats of the different genera make use of vision and what they do with it. As might have been expected, the relatively big-eyed gleaner Plecotus auritus did much better than the aerial-hawking and trawling Myotis spp., which also had much smaller eyes.
The reaction to the 5° but not to the 2.5° pattern by the Myotis species used in this study is consistent with an earlier investigation of another Myotis species, the little brown bat (M. lucifugus), which responded down to 3-6° (Suthers 1966). A visual acuity in this range suggests that these bats can only detect 5-9 cm objects at a distance of 1 m, and hence it seems unlikely that they can use vision to detect the insects that they eat. Prey items captured by any of these species are much smaller than this and they are presumably detected using sonar cues alone (Swift & Racey 1983, Kalko & Schnitzler 1989). However, vision could well be used to detect large objects at distances beyond the range of echolocation, i.e. objects important fororientation and navigation. Indeed, it has been shown that loss of vision drastically reduces the homing performance by other Myotis species, such as M. sodalis (Hassell 1963, 1966, Davis & Barbour 1970) and M. austroriparius (Layne 1967).
Nevertheless, Bradbury and Nottebohm (1969) showed that hearing impaired M. lucifugus could avoid 2 mm wide strings in dim light, when the strings contrasted sharply against the background. Considering their visual acuity, it is unlikely that the bats could have seen the strings more than 5 cm away. Nevertheless, the results from this and other studies (Mueller 1966, 1968) suggest that vision may be important for normal flight behaviour in these bats, although contrast sensitivity might perhaps be more important that visual acuity in some cases.
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