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 Dirk Petzold, Diplombiologe

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Verhaltensökologie der Australischen Schwimmratte
Eco-ethology of the Australian Water Rat

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IEC 97
Field Study

Note: The field study proposed here has been carried out in Sept-Nov, 1998, by me and Dr. Martina Raffel . The map shows the final survey route throughout the wet eastern part of Australia, visiting some dozend national parks and nature reserves. As long as the PhD thesis is not published, the results may not be published here. Some photos are shown here.
Proposal written 5-98


Eco-Ethology of the Australian water rat
(Hydromys chrysogaster)

Context

 Diving is an energetically expensive and time limited behaviour that requires special adaptations in the behavioural patterns of terrestrial mammals that hunt under water.

 For air-breathing animals, diving is considered analogous to the 'optimal foraging' approach describing animals that always return to a certain place ('central place forager'), i.e. in this case to the water surface. Behavioural models give predictions about durations and time allocations during the diving cycle depending on depth, and about decisions the animals make during foraging, e.g. about prey types, sizes or the food density at which to stop exploitation. An experimental evaluation of these predictions is very difficult in freely diving animals.

 Australian water rats (Hydromys chrysogaster) are ideal mammals for experiments on diving behaviour in the laboratory because they are easy to keep and handle. These rodents are well adapted to semiaquatic life by a streamlined body, webbed hindfeet and nostrils that can be closed. In Bielefeld, they are kept, bred and studied for several years.

Previous results and ensuing questions

 When diving, water rats totally depend on their vibrissae to detect food since their eyes are closed and no other sense organs are used as evidenced by tests. Therefore, the animals only get local information on food availability and are not able to estimate it from the surface. Sensorical abilities, especially of the vibrissae system, are studied intensively by behavioural and discrimination experiments. Additional histological examinations are done by Dr. Dehnhardt (University of Bonn), who already supervised a master thesis on water rats, and who examines the vibrissae of pinnipeds.

 Almost nothing is known about foraging and use of sense organs in natural habitats. Thus, for a better understanding of our results, we want to examine in which kind of habitats the animals occur, which food they prey on and under which conditions foraging takes place.

 Predictions derived from optimisation models were confirmed in experiments by computer aided measurements: Water rats dive with constant speed, and durations of all phases of the dive cycle changed systematically with increasing depth. For experiments in depth up to 5 m, we were able to use the "diving tank" at the "Konrad-Lorenz-Institut für Vergleichende Verhaltensforschung" (KLIVV), Vienna. A maximum dive duration of 36 s and a water depth of 5 m could be measured, more than 10 m should be reached according to calculations.

 We aim to study to what extent the natural habitat requires diving behaviour that approaches this values resp. the diving limits of the animals. This is important for the interpretations of the laboratory data as well as to estimate the importance for equivalent "optimal diving" in the wild. For this purpose, we want to assess water depths, use of habitat and diving durations.

 We examined foraging strategies of water rats with preference tests and experiments considering the influence of food quantity and prey type. The animals performed as 'single prey loaders', i.e. every food item was taken to the surface for eating. Only when small food items were very abundant, animals switched to feeding submerged. This also depended on diving depth. In medium depth, performance was maximal, and most was invested in big food pieces. Food gain per trial decreased with depth, for small food items faster than for bigger ones, which were exploited more intense. Experiments with food of different sizes offered simultaneously revealed that the animals became less choosy when depth increased, most likely due to time-dependent costs of foraging like increasing recovery time.

 Observations of diving and foraging behaviour in different habitats can specify under which conditions such or similar strategies occur in the wild. An important factor is the existing prey supply, thus we plan an examination of type, size and numbers of prey items.

Planned examinations

Habitat analysis: The waters animals hunt in will be mapped, depth, degree of turbidity, flowing speed, temperature etc. will be measured, and bank type and ground characteristics will be determined. In addition, it is planned to achieve basic data on animal density, social structure and age of the animals in the study area. This will be contributed to by an examination of tracks, feeding places, runways and burrows.

Food spectrum: The prey of the water rats will be analysed for species, size and frequency in the habitat, and the importance as food sources will be determined. Hence we will collect samples and make direct observations of foraging and analyses of faeces and feeding places.

Behaviour: Most important will be observations of foraging and diving behaviour. Thereon, diving durations and diving cycles will be registered as well as frequencies and depth, if possible. Use of space, daily activity patterns and food utilisation will also be investigated in different habitats. Therefore, several animals shall be identified (marked) individually.

Timetable and locations

 The field studies in Australia will be a part of PhD and a master thesis. Both studies are supervised in Germany by Prof. F. Trillmich. The research will take two to three months. Including journey, organisation and preparations in Australia, the project is scheduled for September till Dezember 1998.

 Part of the PhD thesis: In September a habitat and food spectrum analysis at Healesville will take place for about 2 weeks. Then a survey of different habitats will be carried out, reaching from coastal habitats in southern Australia to tropical rivers and mangroves in northern Queensland. This will take about 2 1/2 months, depending on the opportunities that are found at the different locations. A variaty of wetlands has been considered; but still suggestions for good water rat habitats are welcome!


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