Bornean orangutans, the largest tree-dwellers on the planet, are vanishing. The population of these great apes was halved between 1999 and 2015, per an estimate published Thursday in the journal Current Biology.

A survey of orangutan nests, coupled with a statistical analysis of habitat changes, indicates that more than 100,000 animals were lost in the last 16 years. It is a dramatic drop for the animals who, because their genomes and unique physical characteristics so resemble ours, are among the closest living relatives to humans.

Orangutans’ exact numbers are uncertain. They are intelligent and shy and prefer thick forests. You could walk by an orangutan hiding in the canopy and never know the 4-foot-tall animal was there, said Maria Voigt, an expert in sustainability and ape habitat at the Max Planck Institute for Evolutionary Anthropology in Germany. Counting the shaggy orange creatures by sight would be a very difficult task.

Instead, surveyors tally orangutan nests. Orangutans, before they sleep, bend long branches into structures that look like leafy baskets. The nests are so large that researchers can use helicopters to spot them. Since 1999, surveyors have covered a total of 500 square miles in Borneo looking for their nests.

One nest does not equal one ape. Some orangutans, especially infants and mothers, may crowd together into the same nest. Researchers must also account for abandoned nests, too. But extrapolating population counts from the nests is possible.

When the study began, surveyors found 22 nests per every kilometer traveled. By 2015, they found 10 nests in the same distance. Voigt and her co-authors, an international team of ecologists, biodiversity experts, conservationists and others, created a mathematical model to track the ape population. They bundled the decreasing number of nests together with human population density, deforestation and rates at which orangutans are hunted and killed.

By the researchers’ best estimate, there were 148,500 more orangutans in 1999 than in 2015. Experts were shocked to see such a precipitous decline. Others doubted that there were so many orangutans in 1999. Voigt summarized their responses: “That can’t be, that’s too much, we don’t think there are so many left.” But the findings are on par with other declines in great ape populations, she said. Grauer’s gorilla populations have dropped by 80 percent in 20 years. The western chimpanzee population dropped by 80 percent in 25 years.

The researchers estimate there are 17,000 to 100,000 Bornean orangutans left, Voigt said. Looking to 2050, a business-as-usual model suggested a less dramatic decline – a loss of 45,300 animals from habitat destruction.