This intraspecific variation in eye use resulting from the
hormone treatment may be related to the explanation for interspecific
variation in eye use that is based on interspecific variation
in the tendency to shoal (Bisazza et al. 2000), which may also vary
with predation pressure and other environmental variables (Brown
&Warburton 1997). In shoaling species, individuals must be able to
monitor both predators and shoalmates simultaneously. Selection
for such lateralization might be lacking in nonshoaling fish, but
present in shoaling fish. It is intriguing that lateralization at the
level of the population is seen as an adaptation for social behaviour
(Vallortigara & Rogers 2005), that many social behaviours are under
the influence of testosterone (Nelson 2005), and that we found an
effect of testosterone on lateralization. Perhaps within species,
testosterone induces a change in life history stage from being more
solitary to becoming more social, inducing fish to come together on
the breeding grounds, making synchronous escape movements
away from predators beneficial.