Many animals live in groups: herds of zebras, flocks of starlings, schools of sardines, packs of wolves. In Community Interactions you looked at how species affect each other. Here the question is why individuals of one species stay together, and the answer comes from natural selection.
Key idea
Group behavior persists because it helps the individuals who do it survive and leave offspring. Common benefits: many eyes spot predators sooner; the dilution effect spreads the risk (a predator that takes one animal from a group of 50 takes you with chance 1 in 50); predators can be confused by a swirling school; and group members can hunt together for prey too big to catch alone.
Why it works
Natural selection acts on the alleles that individuals pass on. An animal that stays in the middle of the herd is less likely to be eaten than one alone at the edge, so behaviors that keep an animal in a group are favored even though no animal is "trying" to help the herd. Groups have costs too: more competition for food and faster spread of disease.
Worked example: The dilution effect
A hawk catches one pigeon per attack. What is a pigeon's chance of being the one caught if it is alone? In a flock of 25?
- Alone: 1 in 1, or 100%.
- In a flock of 25: 1 in 25 = 4%.
Answer: 100% alone; 4% in the flock.
Worked example: Many eyes
Ostriches in larger groups each spend less time looking up for predators, and more time feeding, yet the group spots predators at least as fast. Why?
- With more birds, someone is almost always looking.
- So each bird can look up less often without the group being caught off guard.
Answer: Shared vigilance: more eyes let each individual feed more.
Common mistake
A common idea is that animals form groups for the good of the group or the species. Group behavior is kept by selection because it raises the survival of the individuals who do it. A pigeon in a flock is safer itself; that is enough for flocking to spread.