A single calf is born in late December to early February of the following year, after a gestation period of about 13 months. Most females bear calves once every two years. Growth slows as the whales reach sexual maturity at about age 8 but continues until the age of 40. They apparently feed by lying on their sides and sucking up sediment from the sea floor. Small invertebrates are captured on the baleen while sediment and other particles are expelled through the baleen fringes.
Gray whales eat primarily amphipod crustaceans.
Concentrations of 12,000 to 20,000 amphipods per square yard have been found in the southern Chukchi Sea and northern Bering Sea where the whales feed. In general, gray whales feed little along their migration route and on the calving grounds. Animals taken by early whalers before the northward migration weighed 11 to 29 percent less than whales of similar length taken on the northern feeding grounds. The only major predators of gray whales are killer whales and humans. Many gray whales have healed scars and killer whale teeth marks on their flukes and flippers.