Vision in echolocating bats - Fladdermus.net

the ability of brown long eared bats to find prey by using vision alone; i.e. to find
prey on dark and dimly illuminated backgrounds and also behind a transparent
surface.
METHODS
The study was conducted in the School of Biological Sciences at Bristol
University, UK. Four female Plecotus auritus were captured at their roost
(Ilminster, Somerset) 23 April, and released at capture site 12 May 2002. They
were kept in a 2.2m x 3m x 3m flight room, where also the experiments took
place. The room had ventilation installed and the bats could move around freely
and they had several places to hide, including boxes and pieces of cloths on the
walls. The bats were fed on mealworms with vitamin supplements. They were
fed by hand the first day, presented with bowls containing mealworms the
second day, and could feed by themselves from the bowls from day three. Water
was given in the same kind of bowls and was available to the bats on the flight
room floor all the time (and changed twice every day). The temperature of the
room was 13-16° C, except during the experiments, when it was 20° C. The
daylight period was partly reversed (lights on at 03:00 and off at 16:00) with
experiments starting at ca. 18:00. All the bats lost a little weight (0.5 - 1g) during
the first two days, but on the day of release, they all had their original weight
except one that had gained 1g.
Experiment 1
Two halogen light sources (Schott) with two fibre-optic goose necks each were
placed on the floor in the flight room. At the end of each goose neck, we attached
plastic tubes, two of them with neutral density light filters (reducing light
intensity with 50%), and the other two with dark covers (letting no light
through). This set-up provided us with two circular 25-cm diameter areas of dim
light (4 lux; measured with a Gossen Mavolux digital light meter) and two
similar “areas of darkness” (0.2 lux), 30-40cm apart on the flight room floor. The
overall illumination in the room never exceeded 0.1 lux. The plastic tubes on the
fibre-optic goose necks could be switched and, hence, the arrangement of the lit
and dark areas could be changed. On the floor; under each goose neck we put a
petri dish (9 cm diameter, 1.8 cm deep), placed inside the lid belonging to it,
either with mealworms in the petri dish itself or in the lid (hence between lid and
dish). This provided four different combinations of sensory cues.
1: lit area with mealworms in the dish (visual and sonar cues).
2: lit area with mealworms placed in the lid (visual cues only)
3: dark area with mealworms in the dish (sonar cues only).
4: dark area with mealworms in the lid (no cues except from the dish itself).
We assume that acoustical cues arising from the mealworms moving in the
dishes or the lids were the same in all cases.
During the experiment the bats were flying individually in the
flight room for 30-40 minutes, feeding from the petri dishes. Each landing at a
52