Researchers studying sperm whale social networks in the Caribbean reveal that individual females form long-term relationships with others, and sometimes different groups stay bonded for decades. The findings, published in Animal Behaviour last week, suggests that units of these toothy cetaceans mirror the long-term bond groups among distantly related female African elephants.
Long-lived animals like sperm whales (Physeter macrocephalus), who can live up to 70 years, often have social systems with hierarchies and a substantial amount of complexity. Females and their calves form groups numbering a dozen or more, and solitary males will join them for the length of the breeding season. But because these animals spend the majority of their lives hunting squid in the deep, researchers only get to observe their interactions when they come up to breathe for a few minutes at a time, until now.
Dalhousie University’s Shane Gero and colleagues collected data from nine sperm whale social units over the course of six years. The team wanted to study social relationships across two levels of their hierarchical structure: how individuals interact with each other, as well how social groups relate to one another. They measured the diversity of relationships by calculating the social differentiation—marking it a 0 when all the relationships between units are more or less the same, and a 1 when there’s diversity. Then they compared patterns between individuals within these units.
Relationships within units are diverse, they found, with an average social differentiation of 0.80 among adult females and 0.91 when calves were included. As expected, the whales preferred to relax with family members, Science reports, but within families they played favorites, frequently swimming with the same ladies.
Preferences like these create complexity and diversity in the types of relationships formed at multiple levels of the sperm whale social structure and across various timescales, they write. While individuals show preferences for each other over hours, days, and years, some units form strong long-term bonds across decades. The team identified two groups that were consistently friendly since the 1990s (using additional data from other research groups). They also found that vocal dialects were used by the animals to mark social segregations between sperm whale cultures across generations.
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