Oddity and the confusion effect in predation

Let largemouth bass hunt shoals of silvery minnows at a range of shoal sizes, and separately introduced a small number of visually distinct individuals — dyed blue — into otherwise uniform shoals.

Studied in

Micropterus salmoides, Notemigonus crysoleucas

Sample size

bass hunting shoals of varying size and composition

Bass capture success fell sharply as shoal size increased, and rose again when odd-looking individuals were present. The odd individuals were taken far out of proportion to their numbers, and their presence improved the predator’s success against the shoal as a whole.

• One predator and one prey species in a tank, where escape options are limited.
• Dyeing individuals changes appearance in a way that may exaggerate natural variation.

• Dyeing a few fish blue

  Predators do worse against larger shoals. Is that because there are more prey to share the risk, or because many similar targets are genuinely harder to catch?

• Group defence

  A group helps in three separate ways: your share of the risk falls, targeting gets harder for the predator, and you can put somebody between yourself and it. Only the second needs the group to look alike.

• Group defence

  A predator’s success drops sharply against a large shoal — until a few individuals look different. Odd ones are taken far out of proportion, and their presence restores the predator’s success against the whole group.