Understanding the evolutionary origins and purposes of menopause presents a longstanding challenge spanning various disciplines. Unlike most female mammals, human women cease reproduction well before their natural lifespan concludes. While strides have been made in understanding the adaptive value of menopause in humans, its broader implications remain unclear.
Toothed whales offer a unique opportunity for comparative analysis, as they are the only mammalian group where menopause has evolved independently multiple times. Studying these whales can help elucidate theories surrounding the emergence and function of menopause in mammals.
(Photo : Pixabay/nikkikeldsen)
Five Whale Species Experience Menopause Like Women
Menopause, a common phenomenon among women, is notably rare in other species. While a single population of chimpanzees exhibited prolonged post-reproductive lifespans, menopause is unequivocally observed only in five whale species.
The evolutionary rationale behind menopause has been a subject of debate among scientists. Some propose it as an adaptive advantage for females, while others suggest it may stem from unrelated beneficial aspects of their lives.
In a recent study, titled "The evolution of menopause in toothed whales" published in the journal Nature, researchers focused on five whale species and argue that menopause offers these animals an evolutionary edge. For instance, it may prevent older females from competing with their daughters for resources, thereby benefiting both generations.
Samuel Ellis, lead biologist at the University of Exeter, suggests that whales may have evolved menopause akin to humans due to its uncommon nature.
In most species, females continuously produce eggs throughout their lives, a pattern aligned with natural selection. However, the five menopausal whale species, including killer whales, false killer whales, beluga whales, short-finned pilot whales and narwhals, deviate from this norm.
While killer whales are relatively easier to study due to their coastal presence, other menopausal species present challenges with their deep-sea habitats.
Rather than direct observation, researchers utilize existing data, such as mass strandings, to glean insights into whale biology. Statistical analyses of age estimates and reproductive patterns reveal distinct patterns among menopausal and non-menopausal whale species.
READ ALSO: Wild Chimpanzees Could Also Experience Menopause, Hormonal Changes Like Humans, Evidence Reveals
Whale Menopause Supports 'Grandmother Hypothesis' in Humans
Their findings combined different ideas, especially about how long animals live. This suggests that menopause in whales probably didn't come from changes that made females stop having babies earlier, but instead came from changes that made them live longer after they stopped having babies.
Lead researcher Samuel Ellis from the University of Exeter noted that female killer whales often outlive males, with females frequently living into their 60s and 70s, while males typically die by 40. This supports the "grandmother hypothesis," indicating that older females contribute to species survival by caring for their grandchildren.
The study's co-author, Darren Croft, highlighted the significance of competition within killer whale families. When both mothers and daughters attempt to breed simultaneously, calves born to older females face lower survival rates due to resource competition.
Thus, the extended lifespan of older females is coupled with a shorter reproductive window. Croft underscored parallels between human and whale social structures, emphasizing the importance of older matriarchs in both societies. Older female whales contribute valuable experience that aids families during challenges like environmental stress or food shortages.
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