Groovy toothpicks are a mystery for using tools

Teeth have a story to tell, and sometimes that story has a surprising twist. Researchers have long studied the arrangement and condition of teeth to determine an animal’s age, diet, health, and even technical abilities – including the use of tools in early human species. Now a study published in American Journal of Biological Anthropology offers new insight into the diet and behavior of a world-famous macaque troupe.

When Ian Towle, a researcher at the University of Otago in New Zealand, and his colleagues inspected the teeth of a deceased macaque on Koshima Island in Japan, they were surprised to discover long, uniform scratches that ran along the front of the monkey’s front teeth. They also found distinct cone-shaped grooves on the posterior teeth. First, Towle assumed that they were looking at the result of an individual’s strange behavior. “And then I came to the next one, and it was exactly the same thing,” he says. The brands proved to be widespread in the famous well-studied Koshima squad.

“Teeth are the most resilient part of our skeleton,” said Almudena Estalrrich Albo, a paleoanthropologist at the University of Cantabria in Spain, who was not involved in the study. Unlike other bones, which can heal, the teeth do not reshape after being chipped or scratched. “Because of that, they gather information about an individual’s life,” adds Estalrrich Albo.

Towle noticed that the markings his team found were similar to wear patterns on fossilized human teeth. But in humans, similar properties are usually associated with the use of tools such as cleaning teeth with a long, sharp tool or processing animal skins – behaviors that Koshima macaques do not exhibit.

“This population has been studied since 1952,” said study co-author Andrew MacIntosh, a behavioral primatologist at the Center for International Collaboration and Advanced Studies in Primatology at Kyoto University in Japan. “No one has ever mentioned tool use.”

So where did these odd brands come from? In the Koshima macaques, they probably have a surprising dietary origin: eating seafood. The researchers postulate that when the monkeys pick molluscs from the island’s rocky shores, they ingest sand with their meal. This gravel scratches the enamel on the front of the monkeys’ teeth when they chew, and it gets stuck in grooves, where it must be removed with saliva jets. Such marine foraging is extremely rare in monkeys, according to University of California Davis primatologist Lynne Isbell, who was not involved in the study. “[Nonhuman] primates usually do not live close to marine habitats, she says.

If scientists “found these traces on the back teeth and these large scratches on the front teeth in a fossil human sample, I really think most scientists would identify them as tool use,” says Towle.

However, there are remarkable differences. Paleoanthropologist David Frayer of the University of Kansas points out that the macaque’s tooth scratches are vertical, while most fossilized human tooth markings are more angled. In fact, the angle of scratches on Neanderthal teeth has long been used to determine whether fossilized individuals were left-handed or right-handed. The macaque markings “are impressive, but they do not resemble any of the types of scratches we documented for Neanderthals” or other ancient human species, says Frayer, who was not involved in the study.

Estalrrich Albo agrees with Frayer’s assessment, although she emphasizes the value of examining tooth wear in non-human primates. “I think it’s a contribution with potential,” she says. “Looking at other species is a necessity to make sure we define the right subjects for study.”

Towle notes that the presence of tools not used by tools in macaques does not exclude tool use indicators in other primates, and he does not believe that most cases of fossil human tooth scratches came from eating shellfish. But he believes that a closer study of macaque teeth can help refine the analysis in the future. “I do not suggest that in the future it will not be possible to distinguish between the two,” he says.

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