Usually, these family-oriented whales spend their entire lives within the same group, with population gains and losses coming only from births and deaths. This social structure is the only one of its kind among the world's mammals. But another type of killer whale exists, called transients. These whales roam in and out of resident whales' boundaries but don't mix with resident populations. Both types of orcas live in pods ranging from 1 to 55 individuals, but transient pods tend to be smaller than resident pods.
Transient orcas are different from their resident cousins in other ways too.
Also, it is the transients that sometimes eat marine mammals, a practice that gave the whales their common name, killer. In spite of this malevolent name, documented attacks on humans are rare. In those attacks that have occurred, the whale was provoked either by harpooning or by an attempt to capture it or a member of its family. Collisions between whales and boats are usually accidents. This doesn't mean that orcas can't sometimes be pests.