Sperm whales live in either nursery or bachelor groups. Nursery groups consist of a number of adult females and immature males and females (2) (6). Males leave these groups when they become mature and join bachelor groups, which consist of males of 7 to 27 years of age (6). Older males live in small groups or singly, and visit nursery groups to mate with females during the breeding season (6). Most groups of sperm whales tend to number between 10 and 15 individuals (4). Sperm whales use echolocation to find their prey in the dark ocean depths (6). When foraging, powerful sound waves are emitted from the large head; these can stun and even kill the squid, octopuses and fish on which they feed (4). These whales make deep dives to depths of up to 3,000 meters (almost 2 miles) that can last as long as two hours (2) (6). This is the deepest dive made by any species of mammal (6). Males reach maturity at 10 years of age, but they do not begin to mate until they are around 19 years old and a length of 13 metres. Females become mature at between 7 and 11 years, when they are around nine metres in length. A single calf is born between July and November after a gestation period of around 16 months. The calf is suckled for up to two years (4). Groups of females protect their young by adopting a defensive 'marguerite formation' in which the calves are placed in the centre of the group surrounded by a circle of females, facing tail outwards (4).