For chimpanzees, salt and pepper hair is not a marker of old age

Silver locks and gray hair are a sign of aging in humans, but things are not that simple for our closest ape relatives: the chimpanzee. A new study published today in the journal. MORE ONE George Washington University researchers discovered that gray hair is not indicative of a chimpanzee’s age.

This research questions the importance of the gray phenotype in non-human wild species. While aging is one of the most prominent traits a chimpanzee has, the world’s most famous chimpanzee was named David Greybeard, there is significant variation in pigmentation among individuals. Aging occurs until a chimpanzee reaches middle age and then stabilizes as it continues to age, according to Elizabeth Tapanes, Ph.D. candidate in the GW Department of Anthropology and lead author of the study.

“With humans, the pattern is quite linear and progressive. You get grayer as you get older. With chimps that is not the pattern we found,” said Tapanes. “Chimpanzees reach this point where they are just a little bit of salt and pepper, but they are never completely gray, so you can’t use it as a marker to age them.”

The researchers gathered photos of two subspecies of wild and captive chimpanzees from their collaborators in the field to test this observation. They visually examined the photos of the primates, evaluated the amount of visible gray they had and rated them accordingly. The researchers then analyzed that data, comparing it to the age of the individual chimpanzees at the time the photos were taken.

The researchers hypothesize that there could be several reasons why chimpanzees did not develop gray hair patterns similar to humans. Its characteristic dark pigmentation could be critical to thermoregulation or help people identify with each other.

Dr. Brenda Bradley, associate professor of anthropology, is the lead author of the article. This research dates back to an observation made by Dr. Bradley while visiting a field site in Uganda five years ago. While learning the names of various wild chimpanzees, she found herself making assumptions about how old they were based on their pigmentation. Investigators at the scene told him that chimpanzees did not turn gray in the same way as humans. Dr. Bradley was curious to know if that observation could be quantified.

There has been little previous research on the loss of pigmentation in chimpanzees or any wild mammal, Dr. Bradley said. Most of the existing research on human aging is focused on the cosmetic industry and clinical dermatology.

“A lot of work has been done to try to understand physiology and perhaps how to override it,” said Dr. Bradley. “But very little work done in an evolutionary framework of why this is something that seems to be so prevalent in humans.”

The researchers plan to take advantage of their findings by looking at the pattern of gene expression in the hairs of individual chimpanzees. This will help determine if changes are occurring at the genetic level that coincide with the changes that the eye can see.

This study anticipates World Chimpanzee Day on July 14. GW faculty and student researchers contribute to our global understanding of chimpanzees and primates as part of the GW Center for the Advanced Study of Human Paleobiology. Through various laboratories, researchers study the evolution of social behavior in chimpanzees and bonobos, the evolution of the primate brain structure, and lead field projects at the Gombe Stream Research Center in Tanzania. Dr. Bradley’s lab is also currently working on research on color vision and hair variation in lemurs.