The red-nosed cuxiu (Chiropotes albinasus) is a terrier-sized monkey whose diet is dominated by the unripe seeds of super-hard fruits. While you or I would need a hammer to access them, the cuxiu brings its own toolkit; enormous dagger-like canines, mounted on robust jaws and powered by massive jaw muscles. Combined, these give the glossy-black monkeys a bite-force that comes very close to a jaguar’s mighty 1500 pounds-per-square-inch.
With this much power, scientists had always thought that the pennant-tailed beasts would simply pick up any hard fruit they fancied and crack into it. But when you are eating hundreds of nuts a week, such a thug-like approach risks splitting a tooth or breaking off a tip. Dental disasters like this need to be avoided, since damaged teeth mean a lingering death by starvation, or predation by an alert predator.
So, how do they manage to avoid such fates?
That's exactly what a multi-national team of scientists wanted to find out during an extensive field study on the central Amazonian Brazil’s Tapajós River. The team have just published their results in the journal Biotropica.
The scientists found that, when feeding, the power-toothed primates don't bite just anywhere on the surface of fruits. Instead, they place their tooth tips very precisely, choosing natural lines of weakness (known as sutures) where the fruit would burst open once ripe (think of the lines on a walnut).
When eating fruits lacking such lines (imagine a miniscule melon), they simply chose the thinnest spots.
In both cases, the scientists discovered, the chosen sites take up to 60% less force to penetrate than other areas of the fruits’ surface.
“This highlights how these monkeys will selectively bite fruits in ways that not only reduces the risk of tooth breakage, but also means they spend much less energy when feeding," says study author Sarah Boyle from Canada’s Thompson Rivers University.
“This helps clarify the ecological and evolutionary pressures shaping their specialised dentition," points out study author Justin Ledogar from East Tennessee State University.
But does such finessed foraging work? To find out, the team spent time away from the field, looking at tooth damage in museum specimens of cuxius and their close relatives the uacaris.
Of the 148 canines of these monkeys that they studied, just two had damage linked to bite forces. So, clearly deploying such power-punches with pin-point precision allows these spectacularly chonk-canined primates to bite longer and prosper.
"The finding that these medium-sized monkeys regularly open some of the hardest fruits in the forest and do so with both speed and care suggests a successful feeding strategy to avoid competition in large Amazonian communities of fruit-eating monkeys," says Kent State University’s Marilyn Norconk.
“And the results are of more than academic interest,” emphasises study author Tadeu de Oliveira from Maranhao State University, "as they show which tree species are most important to the cuxius, and without those, they can’t survive."
Top image: Chiropotes albinasus. Credit: Getty
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