Ask a Zookeeper: Clever Capuchin Monkeys

Photo Courtesy of Wiki Commons
Thanks for tuning in to my third installment of my "Ask a Zookeeper" series! Today's question comes from the lovely Mary Vettel (aka, Zooks), who writes:

"Can Capuchin monkeys see in the dark?
And do they see colors?"

Before we get started, let's chat a little bit about what a Capuchin monkey actually is. Probably best know for their mischievous roles in movies like Raiders of the Lost Ark, Night at the Museum, Pirates of the Caribbean, and The Hangover 2, Capuchin monkeys are considered the most intelligent of the New World monkeys. They are often kept as pets (like Ross's "Marcel" on Friends), and they are also the quintessential street performers.

"Where's my peanut?"
Photo Courtesy of Wiki Commons
In recent years, Capuchin monkeys have become quite popular and successful as service animals, and specially-trained Capuchins are often placed with individuals who are paralyzed or who suffer other severe mobility impairments. (Learn more about service Capuchins at Helping Hands, a non-profit organization that provides Capuchin placements.)

Native to Central and South America, Capuchin monkeys are tree dwellers, and they usually live in large, polygamous family groups of up to 35 individuals, typically led by one alpha male. They are omnivores, and they feed primarily on fruits, nuts, seeds, buds, insects, spiders, birds' eggs, and small vertebrates. They will also eat crabs and shellfish by cracking their shells with stones. (An excellent example of tool use as a measure of intelligence, which we discussed in my literary crow post!)

But getting back to your fabulous question about vision, Mary. What's interesting about Capuchins is that they are diurnal--or most active during the day--just like we are. At night, they sleep wedged between branches in an effort to evade an array of potential predators--including jaguars, cougars, jaguarundis, coyotes, tayras, snakes, crocodiles, and raptors.

Because they typically aren't active at night, nature hasn't blessed Capuchin monkeys with particularly powerful night vision. Most scientists agree that in many respects, Capuchin eyesight is similar to human eyesight. We have about the same ability to distinguish fine details, and our eyes react similarly to the presence of light after long periods of darkness. (Probably less swearing and grumbling from the monkeys, though.)

On to the color vision! Research has recently shown that Capuchin monkeys--just like humans--have a huge range of variation in their color vision.
Normal human vision is considered "trichromatic," which means that our retinas contain three types of color receptors (called cone cells) for conveying color information. (Basically, we can see in colors. Lots of colors.)

Some humans, however, have "dichromatic" vision, which means one of our three basic color mechanisms is absent or not functioning. According to Wikipedia, there are various kinds of color blindness found in humans:
  • Protanopia is a severe form of red-green color-blindness, in which there is impairment in perception of very long wavelengths, such as reds. To these individuals, reds are "perceived" as beige or grey and greens tend to "look" beige or grey like reds. 
Protanopia Test:
Can you see the very vague 37 in this picture?
(Photo Courtesy of Wiki Commons)
  • Deuteranopia consists of an impairment in perceiving medium wavelengths, such as greens.
  • Deuteranomaly is a less severe form of deuteranopia. Those with deuteranomaly cannot see reds and greens like those without this condition; however, they can still distinguish them in most cases.
Deuteranopia Test:
Can you see the very vague 49 in this picture?
(Photo Courtesy of Wiki Commons)
  • A more rare form of color blindness is tritanopia, where there exists an inability to perceive short wavelengths, such as blues. Sufferers have trouble distinguishing between yellow and blue. They tend to confuse greens and blues, and yellow can "appear" pink.
Tritanopia Test: 
Can you see the very vague 56 in this picture?
(Photo Courtesy of Wiki Commons)
Most mammals are dichromatic in some way, but many New World monkeys are a notable exception. Scientists now believe male Capuchins are dichromatic, but up to 60% of female Capuchins may be trichromatic! Their peak sensitivities lie in the blues, greens and yellow-greens--which makes sense, considering where they live--but their color vision may not be all that much different than ours.

"Your tie clearly doesn't match your shirt."
(Photo Courtesy of Wiki Commons)
Why, you may ask? According to The Evolution of Color Vision in Primates, "Diurnal primates generally eat fruits and young leaves, and it has been argued that trichromatic color vision is an adaptation for discriminating the most nutritive, colorful items.

"However, in dim light, trichromats have exhibited a slight disadvantage for discriminating fruit from foliage. In many situations, dichromats have a foraging advantage when food is camouflaged or similar in color to the background. Since almost all New World monkeys are known to search for food cooperatively, the entire group can benefit from the advantages of trichromacy and dichromacy."

Boom! So, there you have it. Discriminating, intelligent and cooperative. And pretty darn cute, too.

"With our powers combined..."
(Photo Courtesy of Wiki Commons)
Thanks for tuning in to my bi-weekly "Ask a Zookeeper" series, and thanks again to Mary Vettel for such a fantastic--and difficult!--question. (Not gonna lie, I had to call in some help from my primate keeper friends for this one. ;)) Please let me know if you have any questions for a future post, and please join me in two weeks for my next post. Have a wonderful Wednesday!