| Summary: | 1. The sperm whale (Physeter macrocephalus) is one of the eight species of cetaceans routinely encountered in the Mediterranean Sea; however, information on the social organization of sperm whales living in the basin remains scarce. 2. The social behaviour of sperm whales within female units, and groups of males are reported, made over an 11-year period (2002–2012) in waters around the islands of Ischia and Ventotene (Tyrrhenian Sea, Italy), an area characterized by the presence of a submarine canyon system and a coastal marine protected area (‘Regno di Nettuno’ MPA). 3. Different types of aggregations were identified, consisting of social units and two arrangements of males (bachelor groups and clusters). Close clustering at the surface was recorded both for social units and bachelor groups, with evidence for long-term relationships between females (as expected from other studies) and, surprisingly, also among some immature males. 4. Such long-term associations between individuals in bachelor groups may allow immature males to benefit in several ways, including optimizing feeding efficiency.
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