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Grand Rounds

The ‘Grandmother Theory’ and Menopause: New Supporting Evidence Among Killer Whales

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BACKGROUND AND PURPOSE:

  • Evolutionary biologists have grappled with why menopause is not universal but rather exists in humans and toothed whales (e.g., belugas and killer whales)
  • The ‘grandmother effect’
    • Postreproductive females gain a genetic benefit by increasing their number of surviving grandoffspring
    • By taking care of their daughters’ young children, the mothers are able to obtain food following the period of breastfeeding
  • Evidence for the grandmother effect has been found in humans, but has not been studied in other species where menopause is feature of aging
  • Nattrass et al. (PNAS, 2019) tested the grandmother effect in a population of killer whales

METHODS:

  • Data sources
    • Annual demographic data from two resident killer whale populations in the Pacific Northwest
    • Genealogical relationships and mother-calf relationships were extracted
  • Offspring groups used to test grandmother effect
    • Those with living grandmothers, which were consider as the baseline scenario
    • Those with grandmothers who died within the previous 2 years
    • Those with grandmothers who died prior to the last 2 years
  • Study design and data analysis
    • Measure for annual region-wide salmon abundance within each year: Annual indices of Chinook salmon catch from test fisheries
    • Cox proportional hazards model used to examine the consequences of a grandmother’s death on grandoffspring survival
    • Additional model used to examine the consequences of a grandmother’s status on her daughters’ interbirth intervals

RESULTS:

  • 378 whales with known maternal grandmothers were included
    • 92 males | 76 females and 210 individuals of unknown sex who died before reproductive maturity

The authors found evidence for the grandmother effect in killer whales based on the following

  • The death of a grandmother reduces the survival of her male and female grandoffspring in the 2 years following her death
  • Individuals with a grandmother death in the last 2 years have a mortality hazard ratio 4.5 times higher than an individual with a living grandmother
  • Losing a postreproductive grandmother will have a mortality risk increase by a factor of 6.7
  • An increase of salmon reduces the grandmother effect indicating the connection to food supply

CONCLUSION:

  • The authors of this study found evidence of the grandmother effect for the first time in non-human species that undergo menopause
    • Grandmothers give their grandoffspring a survival benefit
    • Greater effect is seen when grandmothers are postreproductive and when salmon is less abundant

Learn More – Primary Sources:

Postreproductive killer whale grandmothers improve the survival of their grandoffspring

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Related ObG Topics:

North American Menopause Society: Hormone Therapy Statement
What is the Relationship Between Age at Natural Menopause and Incidence of Cardiovascular Disease?
Can Anti-Müllerian Hormone Levels Predict Menopause?
How Does Exercise Affect Health Postmenopause?

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