Monkeys on a Japanese island experienced a severe loss of tooth enamel during the 1980s, which researchers believe was caused by the stress of culling programmes and the destruction of their habitat.
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| Japanese macaques on Yakushima Island, where monkeys in the 1980s experienced stress-related dental issues Hiroya Minakuchi/Minden/naturepl.com |
The condition would have been extremely painful and made it difficult for the macaques to eat, says Ian Towle at the Spanish National Research Center for Human Evolution in Burgos. “Their teeth will wear down to nothing and it will expose the living part of the tooth, the pulp chamber. Once that happens, abscesses will follow quite quickly,” he says.
During the 1960s and 1970s, the island of Yakushima was heavily logged, with the natural forest cleared to make way for orange and conifer plantations. The native population of Japanese macaques (Macaca fuscata) began raiding the orange plantations for food, prompting the widespread use of traps and culls in the 1980s. At least 3000 macaques are thought to have died this way over the course of the decade.
Skeletal remains of macaques whose teeth developed during this period reveal that severe loss of dental enamel was common in those from the worst-affected areas of the island.
Plane-form enamel hypoplasia (PFEH) is a condition in which large areas of a tooth’s crown are devoid of enamel. It can be caused by periods of severe stress. In humans, there was a surge in children with enamel hypoplasia during the second world war, for example. PFEH is rare in humans and even rarer in other primates.
Towle and his colleagues looked at dental specimens from 48 macaques from three neighbouring islands in Japan. Only individuals from Yakushima had PFEH, affecting 10 out of 21 monkeys studied from the island. All were from the southern part of the island where deforestation and culling were most rife.
Towle says it is “very likely” that stress caused by deforestation and culling on Yakushima was the primary driver of PFEH occurrence in the macaque population, given that the condition affected a large proportion of the population, and the known stress on the population at the time of their dental development.
However, more research is needed before other causes, such as a genetic predisposition or extreme weather events, can be completely ruled out, he says. “There’s little doubt there was some sort of severe stress,” he says.
Studying current primate populations on the island may provide answers. Macaques are now protected on Yakushima; if living populations don’t have PFEH, it would lend credibility to Towle’s suggestion that human activity was the cause in the 1980s.
Goro Hanya at Kyoto University in Japan studies wild macaques on Yakushima. Although enamel hypoplasia isn’t an area of focus for his research, he says he hasn’t observed PFEH among the macaques he studies.
In the future, studying the incidence of enamel hypoplasia in living primates could be a useful tool for measuring how stressed populations are by human-induced habitat changes, Towle says. “There’s a lot of primate groups out there that are either close to extinction or there’s documented evidence that they’re going through population decline,” he says. “[PFEH] theoretically should show up more often, and then it would be possible to link it to the health of the population if you have got a big enough sample.”
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