Animals may begin to live together for a variety of reasons—most
obviously, safety in numbers. In one of his most engaging
papers, Hamilton observed that a tight flock, herd, or shoal
will readily appear if every animal tries to make itself safer
by moving into the middle of the group—a phenomenon he termed
the “selfish herd.” But protection from predators isn’t the only
benefit of bunching together. A bird in a flock spends more time
eating and less time looking about for danger than it does when
on its own. Indeed, eating well is another common reason for
group living. Some predatory animals— chimpanzees, spotted
hyenas, and wild dogs, for example— have evolved to hunt
together. Many social animals thus live in huge flocks or herds,
and not in family groups—or even if the nexus of social life is
the family, the family group is itself part of a larger
community. In species such as these, social behavior must extend
beyond a simple “Be friendly and helpful to your family and
hostile to everybody else” approac