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Tölch, Ulf (2006): Bat Time Stories: Decision-making in spatio-temporally predictable environments. Dissertation, LMU München: Faculty of Biology
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Abstract

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.