Though northern muriquis are critically endangered, the population in Strier’s study site, which is protected from further deforestation and hunting, has increased. There are now 335 individuals in four groups, a sixfold increase since Strier started her study.
That’s a development worth celebrating, but it’s not without consequences. The monkeys appear to be outgrowing the reserve and, in response to this population pressure, altering millennia of arboreal behavior. These tree-dwellers, these born aerialists, are spending more and more time on the ground. At first the behavior was surprising. Over time, though, Strier made some sense of it. “They’re on an island, with no place to go but up or down. When humans didn’t have enough food, they invented intensive agriculture. Monkeys come to the ground. It makes me think of how hominids had to eke out an existence in a hostile environment. Our ancestors would have brought to that challenge the plasticity we’re seeing here.”
Initially the muriquis descended only briefly and only for necessities, Strier says. Now they’re staying down for up to four hours—playing, resting and even mating. …
Strier wonders about the potential for other changes. What will peaceful, egalitarian primates do if crowding becomes more severe and resources run short? “I predict a cascade of effects and demographic changes,” she says. Will the monkeys become more aggressive and start to compete for food and other essentials the way chimps and baboons do? Will the clubby camaraderie between males fall apart? Will the social fabric tear, or will the muriquis find new ways to preserve it? Strier has learned that there is no fixed behavior; instead, it’s driven by circumstances and environmental conditions. Context matters.
For humans, the stagnation in income in the Malthusian world hid an underlying dynamism as people competed for scarce resources. Each innovation that increased resources allowed population density to increase. In a similar way, the muriquis are able to increase their density through a new innovation, moving to the ground.
In the human case, the innovation in the Malthusian state was ultimately the seed for the Industrial Revolution. The muriquis are some way from that, but it is possible to see it on the same spectrum of change that humans have undergone in the past.