Vision in echolocating bats - Fladdermus.net

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Visual acuity and eye size in four species of insectivorous bats
Johan Eklöf
Zoology Department, Göteborg University, Box 463, SE-405 30 Göteborg,
Sweden, E-mail: johan.eklof@zool.gu.se
Abstract
Behavioural tests on optomotor responses establish a visual acuity threshold in
four species of bats of the family Vespertilionidae. Three species of Myotis spp.,
which are aerial-hawking bats, responded only to a stripe pattern equivalent to 5
degrees of arc, whereas Plecotus auritus, which is a gleaner, responded down to
0.5 degrees of arc. Eye diameter was positively correlated with visual acuity, and
varied from 0.9 mm in Myotis mystacinus to 1.8 mm in Plecotus auritus. These
results are consistent with earlier findings. The variation in eye size and visual
acuity presumably reflects differences in foraging technique (aerial-hawking vs.
gleaning) and, in particular, how vision is used as a complement to sonar.
Key words: Chiroptera, grating, optomotor response, resolving power, spatial
resolution, vision
Introduction
The microchiropteran eyes are generally adapted for nocturnal conditions in that
they have large corneal surfaces and lenses relative to the size of the eye, and
generally large receptor fields, which give them good light gathering power at
the expense of acuity, i.e. the ability to resolve fine spatial details (Suthers 1970;
Suthers & Wallis 1970). Bat eyes are generally better suited for long- than short
distance operation, and due to the short effective range of sonar, vision is
probably of major importance in guidance over longer distances (Griffin 1958,
1970). Loss of vision drastically reduces the homing performance in many bats
(Williams et al. 1966, Hassell 1963, 1966, Davis & Barbour 1970).
At least some bats are able to use vision over short distances as well, for
example during escape and obstacle avoidance (Chase 1981, 1983, Chase &
Suthers 1969, Bradbury & Nottebohm 1969). There is also evidence that some
species of bats use visual cues to find prey (Bell 1985, Grant 1991, Vaughan &
Vaughan 1996, Eklöf et al. 2002), a task which presumably requires relatively
fine detail discrimination.
Visual acuity has been estimated theoretically, based on counts of the number
of retinal ganglion cells, in several species of bats (Marks 1980, Pettigrew et al.
1988, Heffner et al. 2001), and has shown a large range of variation; from 16’ of
arc in the gleaning carnivorous species Macroderma gigas (Megadermatidae) to
1.4° of arc in Rhinolophus rouxi (Rhinolophidae), an insectivorous flutterdetector
(Pettigrew et al. 1988). Optomotor response tests have also shown that
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