constrict in cold water, the walrus can appear almost white when swimming. As a secondary sexual characteristic, males also acquire significant nodules, called "bosses", particularly around the neck and shoulders. The walrus has an air sac under its throat which acts like a flotation bubble and allows it to bob vertically in the water and sleep. The males possess a large baculum (penis bone), up to in length, the largest of any land mammal, both in absolute size and relative to body size. Life history Reproduction Walruses live to about 20–30 years old in the wild. The males reach sexual maturity as early as seven years, but do not typically mate until fully developed at around 15 years of age. They rut from January through April, decreasing their food intake dramatically. The females begin ovulating as soon as four to six years old. The females are diestrous, coming into heat in late summer and around February, yet the males are fertile only around February; the potential fertility of this second period is unknown. Breeding occurs from January to March, peaking in February. Males aggregate in the water around ice-bound groups of estrous females and engage in competitive vocal displays. The females join them and copulate in the water. Gestation lasts 15 to 16 months. The first three to four months are spent with the blastula in suspended development before it implants itself in the uterus. This strategy of delayed implantation, common among pinnipeds, presumably evolved to optimize both the mating season and the birthing season, determined by ecological conditions that promote newborn survival. Calves are born during the spring migration, from April to June. They weigh at birth and are able to swim. The mothers nurse for over a year before weaning, but the young can spend up to five years with the mothers. Walrus milk contains higher amounts of fats and protein compared to land animals but lower compared to phocid seals. This lower fat content in turn causes a slower growth rate among calves and a longer nursing investment for their mothers. Because ovulation is suppressed until the calf is weaned, females give birth at most every two years, leaving the walrus with the lowest reproductive rate of any pinniped. Migration The rest of the year (late summer and fall), walruses tend to form massive aggregations of tens of thousands of individuals on rocky beaches or outcrops. The