Mothers carry their babies in utero for a 17-month gestation period, and they give birth every three to 10 years until becoming menopausal around age 40. They are exceptionally protective of their offspring, and often adolescent females are recruited to assist in caring for calves. Mothers will nurse their babies for up to two years, forming a bond so strong that if a mother dies, the mortality rate of her sons increases. Orcas are some of the affected mammals in the ocean due to pollution, and the contaminants make their way into breastmilk, increasing the mortality rate of all young to 50% within the first year.