Vision in echolocating bats - Fladdermus.net

the visual acuity varies considerably between species of bats (Suthers 1966,
Manske & Schmidt 1976, Bell & Fenton 1986).
The evidence thus suggests that visual acuity may be correlated with the food
searching technique among bats. In particular, gleaners seem to have better
visual acuity than those that catch insects in the air. The purpose of this study
was to test this hypothesis by examining the optomotor response in some
sympatric insectivorous vespertilionid bats that use different foraging techniques
(gleaning and aerial-hawking), in order to establish a behavioural visual acuity
threshold for these particular species. We also tested the assumption that visual
acuity is positively related to the size of the eyes among insectivorous bats.
Materials and methods
The experiments were performed at the old magnetite mine of Taberg, located
south of Jönköping (57ºN) in southern Sweden. The bats were caught in a mist
net set outside the mine entrance. They were tested for optomotor responses
immediately after capture or as soon they had come to rest. The tests were made
outdoors in the evening between August and November 2002, and between
March and April 2003. To achieve optomotor responses, we used a device
similar to that employed by Suthers (1966) and Bell & Fenton (1986). A bat was
placed in a 20 cm high, 10 cm diameter Plexiglas cylinder surrounded by a 30
cm high and 60 cm diameter, revolving drum (Fig. 1). The natural light was
insufficient for direct observation of the response in most cases, so the study area
was lit up by a 40 W light bulb placed ca. 2 m above and 5 m away from the setup.
This provided a light intensity of 0.1-0.7 lux inside the drum (Photometer
IL1400A, International Light Inc.). The drum could be rotated freely and
independently of the cylinder containing the bat. Sinusoidal grating patterns, i.e.
stripes with continuously changing luminance from black to white, of different
fineness was attached to the inside of the drum. The drum was then rotated
around the bat by hand at ca. 5 rpm randomly in both directions, and the
behaviour of the bat was observed from above. Using sinusoidal patterns instead
of black and white stripes reduces the risk of optical illusions, which could
otherwise elicit responses from the bats and thus make the results hard to
interpret (D. Nilsson & E. Warrant personal comm.). We used six gratings with
different stripe width (distance from white to white): 2.84 cm, 1.42 cm, 0.57 cm,
0.43 cm, 0.28 cm and 0.14 cm. From the bats´ point of view this is equivalent to
subtending angles of 5°, 2.5°, 1°, 0.75° (45’), 0.5° (30’) and 0.25° (15’) of arc.
When a response was achieved the grating was switched to a finer pattern until
no response could be recorded, indicating that the bats no longer could resolve
the pattern. At this point a wider pattern was reintroduced to make sure that the
bats still responded to moving stripes. This also served as a control for responses
to stimuli other than the stripes, such as noise originating from the drum.
After testing optomotor responses, we photographed the bats, using a Pentax
645 camera, on 50 ASA medium format slide film. We held the bats by hand so
that the face of the bat was perpendicular to the camera. A ruler was held next to
the bats, providing us with a cm-scale. The photos were scanned and magnified
17x – 33x, and the eye size of the individual bats were measured from the
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