Animal Behaviour, 2004 67, 155-164.
Context-dependent group-size choice in fish
Hoare, D. J.1*, Couzin, I. D.2 Godin, J-G. J.3 and Krause,
J.1
1Department of Biology, University of Leeds, Leeds, LS2 9JT, UK
2. Department of Ecology and Evolutionary Biology, Princeton University,
Princeton 08544, USA.
2Department of Biology, Mount Allison University, Sackville, NB, E4L
1G7, Canada
*Author for correspondence (danieljhoare@yahoo.co.uk).
The costs and benefits of group membership vary with
the size of groups, and individuals are expected to modify their choice
of groups in response to ecological factors such as food availability
and predation risk. Here we experimentally examined context-dependent
group-size choice in a shoaling fish, the banded killifish (Fundulus
diaphanus), by using non-directional odour cues to simulate a food
source or a successful attack by a predator (food or alarm treatments)
in the laboratory. Group sizes were significantly smaller in the food
treatment and larger in the alarm treatment than in control trials.
When presented with food and alarm cues together, fish formed groups
of intermediate size that were larger than control groups but smaller
than those seen with alarm cues alone. The number of groups observed
reflected a similar pattern, being largest in the food treatment and
smallest in the alarm treatment. These results are consistent with theoretical
predictions based on the known benefits and costs of grouping and with
previous laboratory work examining the individual shoal choice behaviour
of single fish. To examine possible mechanisms of group formation, we
developed an individual-based model of shoaling behaviour in which simulated
fish were allowed to modify the area over which they interacted with
neighbouring individuals. Group size distributions produced by the model
were a good approximation of our experimental data. We suggest that
local behavioural interaction rules of this type are a potential mechanism
by which fish may individually adjust grouping behaviour without requiring
extensive information on the position and movement of all possible shoalmates.
