Females live long past the birth of their last offspring
An old (~44 years old), post-reproductive, female mountain gorilla in Bwindi Impenetrable National Park, Uganda
© MPI-EVA/ Martha M Robbins
To the point:
- Length of post-reproductive life span: seven out of the 25 study females qualified as post-reproductive, exhibiting a post-reproductive life span of at least ten years
- Female ages: Six of the seven post-reproductive females were older than 35 years, which is the maximum age of observed reproduction
- Mating decline: Post-reproductive females were not observed mating for an average of 7.5 years before they exit the study
New research from the Max Planck Institute for Evolutionary Anthropology and the University of Turku reveals that female mountain gorillas, like humans and a handful of other mammals, can live long past the birth of their last offspring. Drawing on over three decades of life-history and behavioural data, the study shows that nearly one-third of adult female mountain gorillas from four wild groups in Bwindi Impenetrable National Park, Uganda stop reproducing yet survive for more than a decade afterwards, spending at least a quarter of their adult lives in a post-reproductive phase. This discovery represents a critical addition to our understanding of hominid life history evolution.
Most animals reproduce for as long as they live, so when females stop producing offspring long before death as humans do it poses an evolutionary puzzle. Why give up the chance to have more offspring? Until now, this so-called post-reproductive life span has been clearly documented only in humans, a few whale species and a population of chimpanzees. New research on wild mountain gorillas in Uganda reveals that some females also live for many years after their last birth. The finding likely offers an important clue to understanding how and why menopause evolved and whether living long after reproduction might have deep evolutionary roots shared with our closest primate relatives.
"We wanted to formally test the presence of long post-reproductive life span in mountain gorillas, as we had observed old females that had ceased reproduction for long time, yet still appeared in very good health. Two females that were mature when we started the study in 1998 are still alive, yet had their last offspring in 2010," says senior author Martha Robbins, director of the long-term Bwindi mountain gorilla research project, which provided the data for this study.
Long post-reproductive life-span
Based on behavioural observations spanning three decades and four social groups of wild mountain gorillas, this new study shows that female mountain gorillas can exhibit long post-reproductive life span of more than 10 years. Given that female mountain gorillas rarely reach 50 years of age in the wild, the observed 10-year post-reproductive life span represents at least 25 percent of adult female gorilla life span (adults: >10 years old).
"The evolutionary pressures which might have favoured the evolution of post-reproductive life span, or even menopause, in gorillas remain unclear - we are still far from deciphering the evolutionary roots of these traits in gorillas and beyond," says lead author Nikos Smit, a postdoctoral researcher at the two institutions involved.
Menopause?
Post-reproductive life span is difficult to document in the wild because of detailed information about the ages of the animals and collection of decades-long data needed in long-lived species. Additionally, the documentation of menopause (a physiological permanent cessation of menstruation due to loss of ovarian function) needs detailed hormonal analysis. An extensive duration of post-reproductive life span, a reduced or lack of mating activity, and previous endocrine analyses of old female gorillas suggest that menopause is a highly plausible cause for the reproductive patterns observed in this study. However, detailed hormonal analysis needs to verify this hypothesis.
Regardless, this study adds to observations of chimpanzees and suggests that the evolution of post-reproductive life span of humans might date back to the common ancestor of African hominids.
