Post-reproductive lifespan, the period of life after females stop reproducing, seems to be a paradox as it is unclear whether and under what conditions this trait has evolutionary benefits on fitness. For females, it would be more beneficial to reproduce until the end of their lives. Besides humans, post-reproductive lifespan has only been documented in a few long-lived mammal species, particularly in toothed whales.

Researchers from the Max Planck Institute for Evolutionary Anthropology, Germany, and the University of Turku, Finland, examined the occurrence of post-reproductive lifespan in wild mountain gorillas. Using long-term data on 25 female mountain gorillas in Bwindi Impenetrable National Park, Uganda, the researchers analysed female reproductive patterns and life span.

Seven out of 25 females were classified post-reproductive. The post-reproductive phase represented a considerably large proportion of the females' adult life span. Females stopped reproducing on average at age 35 years but continued to live for more than 10 additional years.

Overall, the study suggests that the prolonged post-reproductive lifespan observed in mountain gorillas may reflect menopause and no other causes of sterility. According to the researchers, this is supported by the lack of mating activity observed in these females and previous endocrine analyses of old females at another location.

The researchers discuss different hypotheses that could explain the evolution of post-reproductive lifespan in mountain gorillas. For example, the reproductive conflict hypothesis posits that old females stop reproducing to avoid competition for limited reproductive opportunities with younger and possibly related individuals. Another hypothesis discussed in the publication is the grandmotheringhypothesis, which suggests that old females cease reproduction to increase their fitness by helping to raise descendants. However, gorilla social and ecological structure do not support these two hypotheses. The mother hypothesis seems to have more support. It states that old females rather invest into existing offspring than into reproduction and hence cease reproduction. However, further research is needed to clarify the evolutionary drivers of post-reproductive lifespan in mountain gorillas. According to the researchers, the results of this study add to the knowledge of post-reproductive lifespan in primates and humans and hence contribute to the understanding of the evolution of life history in humans and their ancestors.

Original publication
Smit, N. & Robbins, M. M. (2025): Post-reproductive lifespan in wild mountain gorillas. PNAS 122 (42), e2510998122