Data from: 'Selfish herds' of guppies follow complex rather than simple rules when information is not limited

Under the threat of predation, animals can decrease their level of risk by
moving towards other individuals to form compact groups. A significant
body of theoretical work has proposed multiple movement rules, varying in
complexity, which might underlie this process of aggregation. However, if
and how animals use these rules to form compact groups is still not well
understood, and how environmental factors affect the use of these rules
even less so. Here, we evaluate the success of different movement rules,
by comparing their predictions with the movement seen when shoals of
guppies (Poecilia reticulata) form under the threat of predation. We
repeated the experiment in a turbid environment to assess how the use of
the movement rules changed when visual information is reduced. During a
simulated predator attack, guppies in clear water used complex rules that
took multiple neighbours into account, forming compact groups. In turbid
water, the difference between all rule predictions and fish movement paths
increased, particularly for complex rules, and the resulting shoals were
more fragmented than in clear water. We conclude that guppies are able to
use complex rules to form dense aggregations, but that environmental
factors can limit their ability to do so.