Walruses are extremely social animals. They lie close together and sometimes even on top of each other in larger or smaller groups when resting on land or ice. These groups generally fall into two different types: all-male groups, or herds of females, calves, and males. Walruses can dive to depths of 400 metres and hold their breath for 40 minutes. Most dives take place in shallow waters, where walruses find most of their food. Their diet consists mainly of bivalve molluscs which they locate with the help of their sensitive whiskers. The suck out the soft parts of the mollusc, leaving the shells behind. Walruses also eat sea snails, crabs, fish, and ringed seals. Female walruses have their first pup at about ten years of age. The pup stays with its mother for 2–3 years, and about three years pass between walrus pregnancies. Males attain sexual maturity at about the same age as females. However, because of strong competition for females, most young male walruses do not participate in mating until they are around 15 years old. Walruses can live up to 40 years.