Social behavior in cetaceans runs the gamut from species that are largely solitary to those that are highly social. Baleen whales are rarely seen in groups of more
than two or three; however, gray whales and other rorquals may form transient groups from 5-50 animals during migration. Among toothed whales, river dolphins and narwhals (Monodon menoceros) are examples of species that appear to have mainly solitary habits.
The most highly social species are found among the toothed whales. In bottlenose dolphins and killer whales, for instance, individuals typically have social bonds with many others, which may last for life. In these species, females form the core of the society: long-term bonds between females and their adult daughters are important in many aspects of life, including foraging, fending off predators and aggressive dolphins, and delivering and raising the young. For example, sperm whale mothers often leave their infants in the company of a "babysitter," while they make the deep dives for squid where a baby could not follow. In bottlenose dolphins, social alliances between adult males are a prominent feature of society. Trios of adult male bottlenose dolphins perform together in synchronous aggressive displays, herding a sexually receptive female, or attempting to dominate rival males from similar alliances.
Courtship and mating remains largely undescribed for many species, owing to the difficulty of observing events under water; systematic study of most species is simply lacking. The mating systems of many that have been studied are classified as promiscuous, meaning that individuals select a new mating partner each year, and both males and females may mate more than once in a given season.
Some cetacean species—for example, narwhals, killer whales, and sperm whales—exhibit rather pronounced sexual dimorphism in size, meaning that males are noticeably larger than females. Such a size difference is believed to indicate a relatively high degree of competition among males for a chance to mate. These species are assumed to be polygynous, meaning that a few males mate with most of the available females, excluding the rest of the males for mating opportunities. Male-male fights in the presence of a receptive female may become fierce, and adult males often bear the physical scars of such contests. Adult male narwhals have a long, spiral tusk made of ivory growing out from their head, which they use in jousting and sparring with their competitors.
Science EncyclopediaScience & Philosophy: Categorical judgement to ChimaeraCetaceans - Mysticeti: Baleen Whales, Odontoceti: Toothed Whales, Anatomy And Physiology, Sensory Perception, Social Behavior