Data from: Guppies in large groups cooperate more frequently in an experimental test of the group size paradox

The volunteer’s dilemma, in which a single individual is required to
produce a public good, predicts that individuals in larger groups will
cooperate less frequently. Mechanistically, this could result from
trade-offs between costs associated with volunteering and costs incurred
if the public good is not produced (nobody volunteers). During predator
inspection, one major contributor to the cost of volunteering is likely
increased probability of predation; however, a predator also poses a risk
to all individuals if nobody inspects. We tested the prediction that
guppies in larger groups will inspect a predator less than those in
smaller groups. We also predicted that individuals in larger groups would
perceive less threat from the predator stimulus because of the protective
benefits of larger groups (e.g. dilution). Contrary to prediction, we
found that individuals in large groups inspected more frequently than
those in smaller groups but (as predicted) spent less time in refuges.
There was evidence that individuals in intermediate-sized groups made the
fewest inspections and spent the most time in refuges, suggesting that any
link between group size, risk and cooperation is not driven by simple
dilution. Extensions of theoretical models that capture these dynamics
will likely be broadly applicable to risky cooperative behaviour.