Male sperm whales (Physeter macrocephalus) leave their natal units before sexual maturity and form the all-male groups. Although there are several reports about cohesive all-male group of sperm whales called “bachelor schools” during the modern whaling period, present-day studies revealed that males feed dispersing over a few kilometers and rarely form the cluster. On 16 September 2019, a large group of male sperm whales was observed in Nemuro Strait, Hokkaido, Japan. Approximately 30 individuals, within a range of about 1 nautical mile, were observed moving, diving and surfacing in a coordinated fashion. The sperm whales formed clusters of various sizes (from 2 to 18 individuals) when they came to the surface. The cluster of 18 animals is the largest cluster recorded in Nemuro Strait since the start of the survey in 2006. The body length of all of the individuals found near the whale-watching boat was about 10 m long, which is smaller than minimum individuals estimated the body size in this area, and there were no calves. During this encounter, males made a formation similar to the “Marguerite formation” twice, which female groups form against predators. The observation of a cohesive and large scale group of male sperm whales and their behavior is extremely rare, and these provided important information to understand the social structure of male sperm whales.
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