What's the News: The human brains, capable as it is of amazing mental feats, comes with a downside: it shrinks as we get older, contributing to memory loss, reduced inhibitions, and the other cognitive dysfunctions of age. But even chimpanzees, our closest living relatives, don't suffer this sort of brain loss, according to a study published online yesterday in Proceedings of the National Academy of Sciences. This unusual shrinkage of the human brain, the researchers say, may be a result of our long lifespan. How the Heck:

• Using an MRI machine, the researchers scanned and measured the brains of 87 humans and 69 chimpanzees. For both species, the participants' ages covered the full range of the adult lifespan: 22 to 88 years old for humans, 10 to 51 for chimps. (Chimps rarely survive past age 45 or so in the wild; in captivity, a few have lived into their 60s.)

• No chimp brain areas shrank significantly with age, the researchers found. On the other hand, every part of the human brain did. Some regions---such as the frontal lobe, an area important in making decisions and planning ahead---lost 25% of their earlier volume by age 80.

• Most of the age-related shrinkage in the human brain occurs beyond the lifespan of a chimpanzee, the researchers noted---some late in the fifth decade of life, but much of it in the seventh or eighth decade.

• A shrinking brain, the scientists suggest, may be the price we pay for those extra decades of life. Our brains use huge amounts of energy---25% of the energy our bodies have available, compared to 10% for a chimpanzee---which takes a toll on our neurons over time. "If you're going to take on another 40 years of life span and these neurons have to function, that ultimately seems like it's going to be hard to keep up with," anthropologist Chet Sherwood, the study's lead author, told LiveScience.com.

What's the Context:

• Earlierstudies have looked at age-related brain loss in rhesus monkeys, but none have looked at great apes. Since humans and rhesus monkeys parted ways, evolutionarily, more than 20 million years before humans and chimps did, looking at how chimps age can tell scientists more about when the human pattern of aging came about.

• This study showed that humans' brains shrink and chimpanzees' don't, but it didn't test possible hypotheses why---such as the exhaustion of added years that the researchers suggest. It will take further research to test out what, exactly, causes the brain loss that seems to be unique to humans.

The Future Holds:

• If human brains shrink due to our extra decades of life, it could be interesting to see if other long-lived animals---parrots, elephants, tortoises---also lose brain volume as they age, neuroscientist Bruce Yankner told ScienceNOW.

Reference: Chet C. Sherwood et al. "Aging of the cerebral cortex differs between humans and chimpanzees." Proceedings of the National Academy of Science, published online before print July 25, 2011. DOI:10.1073/pnas.1016709108

Image: Wikimedia Commons / Ranveig