Evidence Matters

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

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

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

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

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

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