Jens Krause *, Dirk Bumann, and Dietmar Todt. Institut ffir Verhaltensbiologie, Freie Universitfit Berlin, Haderslebenerstrasse 9, W-1000 Berlin 41, Federal ...
Behav Ecol Sociobiol (1992) 30:177-180
Behavioral Ecology and Sociobiology © Springer-Verlag 1992
Relationship between the position preference and nutritional state of individuals in schools of juvenile roach (Rutilus rutilus) Jens Krause *, Dirk Bumann, and Dietmar Todt Institut ffir Verhaltensbiologie, Freie Universitfit Berlin, Haderslebenerstrasse 9, W-1000 Berlin 41, Federal Republic of Germany Received October 29, 1990 / Accepted November 24, 1991
Summary. Position preferences o f well-fed and food-deprived juvenile r o a c h were investigated in schools o f 2 and 4 fish in the laboratory. F o o d - d e p r i v e d fish appeared significantly m o r e often in the front position than their well-fed conspecifics. F o r fish at the same h u n g e r level, individuals at the f r o n t o f the school h a d the highest feeding rate. These results represent the first evidence for a relationship between the nutritional state o f individual fish and their positions in a school and suggest a functional a d v a n t a g e o f the preference.
Introduction Schools o f fish were viewed as egalitarian and leaderless groups for a long time (Breder 1954; Shaw 1962). The specific position o f an individual fish within a school was n o t considered important. This idea has c h a n g e d during the past 2 decades as position preferences were f o u n d in a n u m b e r o f different species o f fish (Healey and Prieston 1973; Partridge 1978; Pitcher 1979; M c K a y e and Oliver 1980; Pitcher et al. 1982; Musikalwitz and Cullen, unpublished, f r o m Pitcher 1986). Two factors which seem to have a great influence on the positioning b e h a v i o u r o f animals in social groups are predation risk ( H a m i l t o n 1971; M o r g a n and G o d i n 1985; J a k o b s e n and J o h n s e n 1988; Parrish 1989; Parrish et al. 1989) and foraging success ( O k a m u r a 1986; R a y o r and Uetz 1990). There are theoretical expectations (Eggers 1976) as well as experimental ( O ' C o n n e l l 1972) evidence o f a higher f o o d intake for p l a n k t i v o r o u s fish in the f r o n t o f schools c o m p a r e d with positions at the back. Based on the observations by O ' C o n n e l l and Eggers, we tested in a first experiment whether h u n g r y fish showed position preferences for the f r o n t position, which was predicted to provide the highest possible f o o d intake _
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* Present address: Department of Zoology, University of Cam-
bridge, Downing Street, Cambridge CB2 3EJ, UK Offprint requests to: J. Krause at the present address
rate. In a second experiment, we investigated the relationship between f o o d intake rate and spatial position to c o n f i r m that the front position provided the highest f o o d intake for the particular species and school sizes used in the first experiment.
Methods Some 100 juvenile roach (2+_SD=5.5_+0.3 cm) were caught in lakes near Berlin and kept in captivity for 4 months before the experiment was started. The fish were fed twice daily on dry food (ca. 40 mg TetraMin per fish per day). Relationship between food deprivation and positioning behaviour.
One group of 50 fish was not fed over a period of 2, 4 and 6 days, respectively, and the other 50 fish continued to be well-fed twice daily. For this purpose they were kept in two separate tanks, each 60 × 60 cm and 30 cm deep. For the experiments food-deprived and well-fed fish were chosen at random from these groups, transferred to the test tank and then released at the same time. The test tank used in all trials was 2.50 m long and 1.60 m wide, with a water depth of 30 cm and a water temperature of 20°+ 1° C. In trials with schools of 2 fish, I food-deprived fish (2 or 4 days of food deprivation, respectively) and 1 well-fed fish were put together. Ten trials were run for each period of food deprivation. With schools of 4 fish, I food-deprived fish (4 or 6 days of fooddeprivation, respectively) and 3 well-fed fish were put together. Five and 4 trials were run for each period of food deprivation, respectively. For the experiments with schools of 4 fish, position preferences were more difficult to measure than with just 2 fish because the larger school took up more space, and individuals were more often constrained in their movements by the walls of the tank. As a result, periods in which the whole school swam in one direction (where we could measure positions) occurred less often. We therefore used a large period of food deprivation, namely 6 days, in the hope that stronger differences in position preference would be induced, which should be easier to measure. There was no indication that this larger period of food deprivation caused distress to the fish. Each fish was used in only one trial. A video recording lasting 30 min was started with the introduction of the fish into the test tank. The food-deprived fish was released on the opposite side of the tank, so that it could be distinguished from the well-fed fish on the video monitor and then followed throughout the 30-rain period. In a few cases, fish could also be distinguished by natural markings such as unusual scale
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a) 2 fish
b) 4 fish p < 0.01
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