Vision in echolocating bats - Fladdermus.net

computer screen. The bats were released outside the mine immediately after the
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 for orientation 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.
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