"To comb gray hairs" is a Spanish expression used to indicate that someone has reached a certain age, and they are not young anymore. It is an accurate idiom since, as we humans get old, our head hair generally becomes more gray. While little gray hair appears in the first half of our lives, once we reach midlife the color of our hair may totally disappear. But, is this the case in our closest evolutionary relatives? Can you tell a chimpanzee's age looking at the grey hair on their heads? 

To find out, a group of anthropologists took pictures of the faces of chimpanzees of different ages. They recruited 152 human observers, who were unfamiliar with these chimpanzees, and asked them to score the number of gray hairs they saw. 

The researchers discovered that hair graying in chimpanzees occurs at different times than it does in humans. Chimpanzees' hair becomes more gray from the time they are young up until the middle of their lives, but then the graying tends to stop. The opposite is true for humans, where graying is more striking from midlife on. They also found that a lot of variation between individuals, meaning that a 5-year-old chimp might have grayer hair than a 50-year-old ape. So, you cannot tell a chimp's age by the gray hairs on their heads!

It is unclear what the purpose of this difference might be, but it does indicate that going gray is not a reliable measure of age in mammals.