Menopause Averted a Midlife Energetic Crisis With Help From Older Dependent Children and Parents: A Simulation Study

Area of Science:

• Human evolutionary biology
• Anthropology
• Demography

Background:

• The grandmother hypothesis suggests ancestral women stopped reproducing midlife to provision grandchildren.
• Alternative theories include biparental investment due to offspring energetic costs and father absence impacting resource availability.
• Juvenile production's role in these hypotheses has been underestimated.

Purpose of the Study:

• To investigate the energetic and reproductive dynamics underlying menopause in ancestral human populations.
• To evaluate the validity of existing hypotheses (grandmother, two-sex, father absent) through simulation.
• To determine the role of juvenile production and intergenerational resource transfer in human reproductive strategies.

Main Methods:

• Simulations of hunter-gatherer lifespans, incorporating age- and sex-specific survival rates.
• Modeling of interbirth intervals and foraging skill acquisition.
• Analysis of energy consumption and production across the lifespan.

Main Results:

• A significant midlife energy deficit was identified in simulations.
• Cessation of reproduction at midlife helped mitigate this deficit by reducing the number of dependents.
• Energy transfers from older parents and younger couples were crucial for averting deficits.

Conclusions:

• Menopause is integral to the human life history strategy, characterized by short interbirth intervals and prolonged juvenile dependency.
• Findings support and extend the "two-sex" and grandmother hypotheses.
• Extensive food sharing and intergenerational resource transfers are key components of human reproductive success.