Abstract for IEC 1997 in Vienna
International Ethological Conference
Foraging and diving behaviour depending on depth and food types in Australian Water Rats (Hydromys chrysogaster)
D. Petzold, F. Trillmich and G. Dehnhardt
Dept. Anim. Behav., University of Bielefeld
For mammals, submerged feeding is an expensive, time limited behaviour. Australian water rats dive with eyes closed. These rodents totally depend on their fully protracted vibrissae for searching the ground. Foraging costs increase with depth, as rats usually surface to eat (`single prey loader'). Four animals were offered fish and mealworm pieces (25 to 500 mg) on a platform in depths of 1 to 5 m. Dive cycle durations and food found were recorded.
At great depth rats switched to multiple prey feeding. This became more frequent when food size declined; animals searched longer and less successful, which is expected if the rate of food exploitation is maximised. The same applies to stopping foraging when food density falls to a threshold value: Rats terminated food exploitation at higher remaining densities, this occurred earlier in greater depths. In shallow water submerged feeding was rare due to many but short dives. In medium depths, where food was exploited most intensely, submerged feeding occurred most frequently. It became less likely when food density declined or in the course of searching, most likely due to time dependent costs of foraging, e. g. increasing subsequent surface time. Multiple feeding was influenced by the food type through handling time and encounter rate. Therefore, frequency of submerged feeding and stopping criterion are measures for increased costs of diving.
Key words: Australian water rat, diving, optimal foraging, single prey loader, vibrissae
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