Tuesday, March 8, 2016

A Tale Of Monkey Tails, And Why Curious George Is Not A Monkey.

“George was a good little monkey and always very curious.”

This line opens nearly every one of the adventures that Curious George has set upon since he was first brought to life by HA Rey in the 1939 children’s book “Cecily G. and the Nine Monkeys.” These stories, which have sold over 25 million copies over the last 75 years, and are beloved by child and parent the world over, begin with a lie – a big fat lie because, in fact, Curious George is NOT a monkey.

We at THE ‘SCOPE thought it would be a good idea – in this Year of the Monkey – to clarify some things about our beloved primate pal, George. So what is a monkey, and why is George not one? Generally speaking, monkeys are haplorhine primates with external tails. Another way of saying this is that they have dry, furry noses (not wet ones like lemurs or dogs or cats) with more than just the vestigial tailbone that apes have, including humans. All monkeys have tails,* although some are longer than others. And because Curious George does not have a tail (along with other reasons explained elsewhere), he is not a monkey but an ape.

Frankly, and we’re just gonna throw this out there… Curious George would be way cooler if he were a monkey, precisely because he would have a tail. And monkey tails are fascinating structures because they come in so many varieties: long, short, skinny, fat, hairy, partially bare, prehensile, nonprehensile… wait, what does that mean?

We’ll tell ya!

Prehensile tails are ones that are capable of suspending the entire body weight of the animal. So, an animal with a prehensile tail can hang from its tail, which frees up its hands and feet for other activities like picking fruit or leaves to eat. Based on observations of living and fossil species, it is generally thought that prehensile tails evolved at least 14 times independently among 40 different genera of extant mammals. And this doesn’t include all the other living vertebrates with prehensile tails, like some snakes, lizards, salamanders, and seahorses, to name a few.

Not all monkeys have prehensile tails, which is a shame for those that don’t because prehensile tails are much more interesting than nonprehensile ones. In fact, the only monkeys that have prehensile tails are in Central and South America (platyrrhines or New World monkeys), but not even all those have prehensile tails. Prehensile tails likely evolved twice in the New World monkeys that have them: once in the ateline monkeys, which include spider monkeys, woolly monkeys, howling monkeys, and muriquis, and then a second time in the capuchin monkeys.

Left: Capuchin monkey (Cebus apella); Right: Spider monkey (Ateles geoffroyi)
In most respects, both instances of prehensile tail evolution in monkeys resulted in similar anatomical structures. Monkey prehensile tails are comprised of vertebrae that are structured to resist higher bending and torsional forces. This seems intuitive because animals that hang from their tail put more stress on the bones in their tail. So having bones that can resist higher forces would safeguard against accidental fracture. The ends of these tail vertebrae are more convexly rounded at the point where they articulate with one another, which allows for a greater range of motion than in nonprehensile tails. Also fascinating is that some of the muscles in prehensile tails are structured to produce higher contraction forces than those of nonprehensile tails, while other muscles have developed shorter extrinsic tendons that cross fewer joints along the tail, enabling the animal to have tighter control of the tail to wrap around substrates.

But the parallel evolution of prehensile tails in New World monkeys has also led to some pretty interesting anatomical differences that reflect functional differences in the way the tail is used. In addition to hanging from the tail during feeding bouts, ateline monkeys use their prehensile tails during locomotion to assist with moving through the forest canopy – much like a fifth limb. This is fundamentally different from the way that capuchin monkeys use their tails because capuchins tend to brace themselves with their feet and tail (like a tripod) for stability while feeding on fruits at the ends of thin tree branches, but they do not use their tails while moving. And this basic distinction in the way the tail is used can be seen in specific anatomical differences between ateline and capuchin prehensile tails.

Friction pad on tail of mantled howling
monkey (Alouatta palliata) at Hacienda La Pacifica,
Guanacaste, Costa Rica, 2004.
Capuchin monkeys, like most other primates, have tails that are completely covered in hair. The skin underlying the hair is replete with slow-adapting pressure and touch receptors (Ruffini endings) that allow the animal to detect the position of the tree branch substrates during postural behaviors like sitting or lying down. The ateline monkeys, however, uniquely possess a hairless friction pad on the ventral (anterior) and distal (tip) of the tail. This friction pad senses touch and pressure with slow-adapting sensors like the skin of the capuchin tail, but it is also full of rapid-adapting sensors (Meissner’s and Pacinian corpuscles) that are useful for detecting the substrate during locomotion. Additionally, the ateline tail friction pad contains dermatoglyphics, or “finger” prints, similar to those found on fingers and toes. This pad, with its dermal ridges, provides the source of friction so that the tail does not slip during suspension and locomotion.

So, as you can see, if Curious George was actually a monkey, there is no question that he would be infinitely more interesting** because monkey tails are interesting. And if he were a New World monkey with a prehensile tail, I have no doubt his curiosity would get him into so many more pickles because he would have the ability to hang from his tail and manipulate things with his hands and feet. Unfortunately, for over 75 years George has been given credit for being a monkey, when all signs point to him being an ape – the biggest clue being that he does not have a tail.

*Ok… so almost all monkeys have tails. There is an interesting case of near taillessness in Macaca sylvanus, also called the Barbary macaque. These monkeys, who live in Gibraltar and constitute Europe’s only wild population of monkeys, have a vestigial, stumpy, soft tail – a bulbous nubbin. And although they have tail vertebrae like every other monkey, these vertebrae do not actually extend into the external tail at all, so they are more like the tailbone (coccyx) that apes have than those of a typical monkey tail. For this reason, Barbary macaques are mistakenly called Barbary apes. But they are not apes. They are monkeys – unlike Curious George.

**Tamping down the hyperbole for a moment, the truly most interesting aspect of George, given that he is not a monkey, is the history of how he came to be so beloved by so many. You can read this fascinating history here.

Contributed by: Jason Organ, Ph.D.

Tail Jason on Twitter.

Organ JM, Teaford MF, & Taylor AB (2009). Functional correlates of fiber architecture of the lateral caudal musculature in prehensile and nonprehensile tails of the platyrrhini (primates) and procyonidae (carnivora). Anatomical record (Hoboken, N.J. : 2007), 292 (6), 827-41 PMID: 19402068

Organ JM (2010). Structure and function of platyrrhine caudal vertebrae. Anatomical record (Hoboken, N.J. : 2007), 293 (4), 730-45 PMID: 20235328

Organ JM, Muchlinski MN, & Deane AS (2011). Mechanoreceptivity of prehensile tail skin varies between ateline and cebine primates. Anatomical record (Hoboken, N.J. : 2007), 294 (12), 2064-72 PMID: 22042733

Deane AS, Russo GA, Muchlinski MN, & Organ JM (2014). Caudal vertebral body articular surface morphology correlates with functional tail use in anthropoid primates. Journal of morphology, 275 (11), 1300-11 PMID: 24916635

Russo GA (2015). Postsacral vertebral morphology in relation to tail length among primates and other mammals. Anatomical record (Hoboken, N.J. : 2007), 298 (2), 354-75 PMID: 25132483

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