Oikos, 2003 100, 429-438.
Mechanisms Underlying the Social Organisation
of the Trinidadian Guppy (Poecilia reticulata)
Croft DP 1, Arrowsmith BJ 1, Bielby J 1, Skinner K 1, White
E 1, Couzin ID 1, Magurran AE 2, Randamarine I 3 & Krause J 1
1 School of Biology, University of Leeds, Leeds LS2 9JT, UK.
2 School of Biological and Medical Sciences, University of St Andrews,
Fife KY16 9TS, UK.
3 Department of Life Sciences, The University of the West Indies. St
Augustine, Trinidad.
Free-ranging groups are frequently assorted by phenotypic
characters. However, very little is known about the underlying processes
that determine this structuring. In this study, we investigate the mechanisms
underlying the phenotypic composition of shoals of guppies (Poecilia
reticulata) in a stream in the northern mountain ranges of Trinidad.
We collected 57 entire wild shoals, which were strongly assorted by
body length. Shoal encounters staged within an experimental arena showed
shoal fission events to be an important mechanism in generating phenotypic
assortment. In the wild, fission and fusion between guppy shoals occurred
extremely frequently and thus are unlikely to constrain the opportunities
for shoal assortment. However, fission and fusion processes occur under
the restrictions imposed by the distribution of individuals within the
environment. We observed size specific segregation within the habitat
in three dimensions, providing a passive mechanism that contributes
to the maintenance of the observed homogeneity of group composition.
Furthermore sex differences were found in social behaviour. Male guppies
switched between shoals, and left shoals as an individual, more frequently
than did females. We suggest that fish shoal composition is determined
by habitat segregation on a medium spatial scale and by fission/fusion
processes on a small spatial scale (with sex-specific shoal dynamics
adding a additional layer of complexity).
