Bat Time Stories

since environments underlie a constant change, animals need to keep
track of these changes by gathering information and by using this
information to make decisions. During the course of evolution,
cognitive abilities, information processing skills, have evolved in
many species to cope with the requirements of diverse habitats. In
this study I investigated the cognitive abilities involved in the
foraging on renewable resources. Examples for such resources
include nectar, fruits, or foliage. Renewable resources possess two
qualities that can be used by an animal to optimise its foraging
behaviour; first, once an animal discovers a location where such a
resource can be found, it is profitable to return to this location
later since most renewable resources are not mobile. Second, there
is often a temporal pattern underlying the renewal process so that
such a resource renews itself with a more or less constant
production rate. Thus, it would be a clear advantage if an animal
were able to remember the location and to estimate the production
rate of a resource. To remember the location of a resource can save
time and energy for searching, and the ability to assess the
production rate would allow an animal to time its return so that
the difference between the energy that is needed to travel to the
resource and the energy gained at the resource is positive. I
explored these possibilities in a flower-visiting bat, Glossophaga
soricina, which forages mainly on floral nectar. This species will
thus allow for the study of cognitive specialisations in the
domains of spatial memory and interval timing. This study aimed at
the following questions: 1.What spatial information will these bats
use to relocate already visited flowers and how is this information
encoded? 2.Can bats use temporal and qualitative information that
can be obtained when visiting a flower to time their revisits?
3.What implications arise from these results for the dynamics on a
population level? When relocating flowers, bats have several
spatial stimuli available. However, some of these stimuli are
spatially dissociated from the flower like conspicuous branches or
leaves. When the spatial contiguity between a stimulus and a
response location is not given, it is difficult or even impossible
to form associations for some species. However, in the case of
flower-visiting bats, it could be of advantage to use these stimuli
in the relocation process. In chapter 2, I explored this
possibility by providing the bats with additional cues in a task
where they had to exploit an array of 64 flowers with 16 randomly
distributed rewarding feeders. The additional cues were spatially
separated from the rewarding feeders. Even though bats employed
information from these spatially dissociated cues in the relocation
process by forming single stimulus response location associations.
However, the information obtained from additional cues seems, at
least in this experiment, of subordinate importance since bats were
even without cues able to achieve a good performance with respect
to their spatial accuracy. Bats encounter in their environments
different species of flowers that provide them with nectar. The
quality with respect to nectar content of these flowers can differ
considerably between as well as within species. In chapter 3, I
investigated, whether flower-visiting bats can discriminate between
different sugar water volumes. This was done in a two alternative
forced choice task in which two sugar water volumes were presented
to the bats, which differed. Bats discriminated well between the
different sugar water volumes. An analysis on basis of a
psychometric function that we obtained from the empirical data
showed that the discrimination threshold seems to be even lower
than the threshold for honeybees. The production rates of floral
nectar underlie temporal patterns, and the ability to estimate the
time interval since the last visit to a flower might help
flower-visiting bats to time their revisits according to such
patterns. In chapter 4, I examined, whether bats possess the
ability to estimate small time intervals. For this purpose, I
tested bats in a modified version of a fixed interval schedule, the
peak procedure. Here, bats were rewarded after a fixed time after
the onset of a signal. We analysed only empty trials, trials where
no reward was given, that were interspersed with ordinary trials.
Bats showed increasing response rates after the signal onset with
maximum response rates at the fixed interval time. After the fixed
interval time had elapsed, the response decreased again. This
reaction has been already found in several other species. It shows
that flower-visiting bats are able to estimate small time
intervals, which might help them optimise their foraging bouts. In
the previous two chapters, I looked at the perception of nectar
volumes and time intervals separately. However, only when bats were
able to integrate these two information, it could result in an
optimisation of their foraging behaviour. Therefore, I confronted
bats with six feeders with differing nectar secretion rates
(chapter 5). Results showed that bats adopted their visitation
pattern according to the underlying rates. Moreover, a
computational model could provide evidence that bats possess
reference memories for the two types of information. Thus, bats are
able to estimate nectar production rates and direct their foraging
decisions by this information. In all paradigms described above,
bats foraged alone. However, under natural conditions this is
seldom the case. In chapter 6, I explored the possible ecological
implications from chapter 2 through 5 and speculated on the impact
of the found cognitive abilities on foraging dynamics on a
population level. I tested several bats in the rainforest in Costa
Rica in a semi natural paradigm for their reaction to variable
resources while foraging in a group with other individuals. And
even though the amount of empirical data is not convincing yet, I
cannot rule out the possibility that the cognitive abilities we
found might also constitute the basis for the estimation of
competitional pressure at certain resource locations, which could
lead to an optimised exploitation of the standing crop. In this
study I could provide evidence for the existence of several high
level cognitive abilities in a flower-visiting bat. Through these
cognitive abilities, bats can plan into the future and direct their
foraging decision by the information they processed. It is probable
that these cognitive abilities represent unique adaptations to the
demands of the ecological niche of a flower-visiting bat.