Social animals interact with each other in friendly and not so friendly ways. The patterns of these interactions over time reveals the kinds of relationships individuals have; are they friends or rivals? In turn, the pattern of social relationships, taking into account sex and kinship, reveals the social structure of a population. Some toothed whales, like sperm whales, have a very complex matrilineal (led by the females) social structure as females remain with their mothers and males leave home. Some killer whale populations have a very unusual social structure in that both males and females remain with their mothers as long as she is alive! In bottlenose dolphins, the strongest associations are between adult males. Such complex social structures have not been described in mysticetes. For example, gray whale populations, have no long-lasting associations or social cohesion.