Brain weight across the adult lifespan in great apes.

Reduction in brain weight is a hallmark feature of human aging, which is exacerbated in pathologies, such as Alzheimer's disease and other dementias. However, it remains unclear whether a decrease in brain weight with age reflects a vulnerability unique to the human lineage or represents a consequence of an extension of aging processes shared with other primates. To address this question, we examined age-related changes in brain weight across our closest living relatives, the great apes (chimpanzees, bonobos, gorillas, and orangutans). The sample spanned the full adult age range in each species and included individuals reaching ages comparable to those at which age-related brain changes become apparent in humans. Across species, brain weight did not show significant change throughout adulthood and into advanced age. The observed pattern held even when body weight was included in the analysis in a subsample of chimpanzees. Overall, these results indicate that marked brain weight loss is not a general feature of great ape aging and instead appears largely specific to humans, with implications for understanding susceptibility to late-life brain pathology.