The Wall Street Journal reported on Chet Sherwood's work late last month: "Brain Shrinkage: It's Only Human".
The human brain normally can shrink up to 15% as it ages, a change linked to dementia, poor memory and depression. Until now, researchers had assumed this gradual brain loss in later years was universal among primates.
But in the first direct comparison of humans to chimpanzees, a brain-scanning team led by George Washington University anthropologist Chet Sherwood found that chimpanzees don't experience such brain loss. From that, researchers concluded that only people are afflicted by this oddity of longevity.
The paper is in PNAS . The press article doesn't really explain the findings of the paper very well. Sherwood and colleagues found that the age effect in their sample of humans was limited to ages older than any chimpanzee in their samples. So there's no evidence that humans and chimpanzees differ across the same ages. Now, whether we expect chimpanzees to shrink their brains at a younger age (because they develop and senesce faster) is an open question; I can see arguments both ways. Anyway, I think the study goes as far as gross morphological comparisons can take this question, and more detail will have to wait for us to understand the cellular mechanisms that influence brain size senescence.
- . Aging of the cerebral cortex differs between humans and chimpanzees. Proceedings of the National Academy of Sciences. 2011;108(32):13029 - 13034.