There has been some confusion concerning the animal group-size: an
exponential distribution was deduced by maximizing the entropy; lognormal
distributions were practically used; a power-law decay with exponent {3/2} was
proposed in physical analogy to aerosol condensation. Here I show that the
animal group-size distribution follows a power-law decay with exponent 1, and
is truncated at a cut-off size which is the expected size of the groups an
arbitrary individual engages in. An elementary model of animal aggregation
based on binary splitting and coalescing on contingent encounter is presented.
The model predicted size distribution holds for various data from pelagic
fishes and mammalian herbivores in the wild.Comment: 19 pages,9 figures, to appear in J. Theor. Bio
