The question, then, is why. Recent research on orcas, based on more than four decades of data, is providing new insight. It appears that menopause has less to do with the biology of individuals than the structure of their societies.
The fundamental unit of orca society is the matriline, which consists of a matriarch — a grandmother or great-grandmother — and her descendants. Matriarchs and their offspring remain together throughout their lives. Sometimes several matrilines, each consisting of about six to twelve individuals, travel together as a pod. Although adult males routinely disperse to mate with unrelated individuals in other pods, they always return to their native matriline. In the wild, males usually die around age 30, but female orcas can live to 90 or older. Yet they stop having children by about age 40. Female orcas have the longest post-reproductive life span of any nonhuman animal.
Since the 1970s, various teams of biologists have been studying two resident populations of about 380 orcas in a coastal swath of the Pacific Ocean ranging from Alaska to Vancouver Island. The scientists stay on the water for weeks at a time, photographing orcas from the decks of boats and sometimes filming them underwater. By cataloging photos of unique pigmentation patterns around the orcas’ fins, as well as accumulated nicks and scars, the researchers have identified nearly every member of the Northern and Southern Resident Killer Whales, as they are known, and documented their evolving relationships.
The resident orcas specialize in hunting Chinook salmon. Orca matriarchs use decades of experience and knowledge to guide their family to the best hunting grounds, especially in times of hardship, when salmon is less abundant than usual. Although pod members share the day’s catch, individuals are technically competing for a limited resource. There are only so many fish to go around, and different members of the pod have different loyalties.
In a recent study analyzing foraging behavior among orcas in the Pacific Northwest, scientists observed that young mothers primarily shared food with their offspring and their sisters, whereas post-reproductive females favored their eldest sons. On the whole, adult females were the most generous, sharing more than 90 percent of their catches, whereas adult males shared only a quarter of theirs. Male orcas remain dependent on their mothers and grandmothers throughout life, often hanging close to them on hunting expeditions. When an elderly female dies, her son’s risk of dying the following year increases by three to fourteen times, depending on their respective ages.
The immense caretaking burden placed on the backs of orca matriarchs makes it difficult for them to keep reproducing. If an elderly female were to bear calves, she would have to catch enough salmon to feed herself, her adult sons, and her newest children, which would put her in intense competition with younger mothers and their children. Apparently, such a level of family conflict is unsustainable — even lethal.
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In a study published earlier this year, behavioral ecologist Darren Croft of Exeter University and his colleagues performed a statistical analysis on 42 years of data concerning the social lives of the Northwest’s resident orcas. They concluded that calves born to older females are 1.67 times more likely to die before age fifteen than those with young mothers. It appears to be much more advantageous for older females to stop reproducing altogether and instead focus on keeping their existing children and grandchildren alive. In particular, by helping adult males thrive — so that they are healthy enough to venture off and father children in other pods — an orca matriarch can continue spreading her genes without adding new calves to an already overcrowded family.