Shades of Green: Earth's Forests -- Arboreal Adaptation

A howler monkey's tail weighs as much as one of its legs. What advantage could there possibly be to lugging around all that extra weight? It is a "prehensile" tail, or one which can be used as a fifth limb: for grasping, holding, and supporting the entire weight of the animal's body. With prehensile tails, animals can dangle upside-down to reach thin branches, and feed high in the crowns of the tropical forest without danger of falling.

Animals with prehensile tails are found in the tropical forests of South and Central America. Photo courtesy Naomi Woods.

Prehensile tails can be useful for more than feeding. Mouse-opossums have been observed collecting nesting material and wrapping their tails around it to transport the material back to their nesting sites.

Animals which mainly eat fruit, leaves, and flowers are the most common group with prehensile tails. Fruit and new leaves grow at the ends of branches which might not be able to support an animal's weight. With a prehensile tail, however, animals can hang from more solid branches while feeding, or use their tails for added support. Large spider monkeys can be seen feeding over 50 meters from the ground, hanging on only by the tips of their tails.

In forests with thick vegetation and lots of vines, fewer animals have evolved prehensile tails because it is easy enough to move through the forest already. Photo by Maya Walters.

But eating a diet of fruit isn't the only reason to have a prehensile tail. Some snakes and lizards also have this adaptation, including Asian pythons, and some boas and vipers. These reptiles are highly arboreal, and make use of prehensile tails to hold tight to a branch while handling large prey such as birds and lizards.

Whatever a particular animal's use for the tail, prehensile tails are only found in arboreal species, and are sometimes considered the epitome of arboreal adaptation. Ground-dwelling Bothrops vipers are not prehensile, while those Bothrops species which inhabit trees do make use of prehensile tails. Terrestrial anteaters lack prehensile tails, yet the arboreal silky anteater uses its prehensile tail for support along with its hind feet, and can keep its forelimbs available for foraging.

Porcupines are one species not generally thought of as arboreal, as those which inhabit the temperate zone are terrestrial. But the tropical American porcupines spend much of their time in trees. The prehensile-tailed porcupine is one of the most arboreal of all, foraging for seeds at the very tips of branches.

This is one case where the evolution of the prehensile tail may have a slight drawback. This porcupine's tail is un-protected, compared to that of its temperate counterpart which is well armed with quills. The tropical porcupine has evolved into a much more agile and acrobatic animal, but is not as well defended as the temperate species.

An alternative to the prehensile tail is another arboreal adaptation: gliding. Gliding mammals generally have flaps of skin stretching between their front and back legs, which they can spread to make "wings". They move through the forest by climbing fairly high and sailing to another tree, then climbing and sailing again.

Some lizards have ribs that are actually extended from the sides of the body and joined with a flap of skin to form "wings" that function similarly to those of the gliding mammals. However, instead of having "wings", most reptiles flatten out their bodies and leap through the trees. Frogs, lizards, and some snakes use this behavior to escape predators.

There are no gliding animals in the American tropics, the very place where prehensile tails are so common, and this distribution is even stranger when you consider that there are gliding squirrels in North America. The fact that prehensile tails are almost absent from the forests of Asia and Africa adds to the biogeographical mystery of these adaptations.

Some people think different forest structure on different continents accounts for this odd distribution of gliding animals and animals with prehensile tails. In general, African forests are full of liana vines, and Asian forests lack liana vines. Tropical American forests have a moderate number of vines, and trees with relatively fragile branches. Because of all the vines, animals can get around in African forests without special adaptations. But gliding makes sense in the more open Asian forests, and prehensile tails are the way to go for the particular characteristics of American forests.

Orang utans are one of the many Asian primates which do not have prehensile tails, while a large percentage of American mammals do. Photo credit Corel Photo Clipart CD.

This idea certainly doesn't explain everything, and some people disagree with this theory. Why are there no prehensile tails in Asia? Primates such as gibbons and orang utans are unable to suspend themselves from a tail like howler monkeys, leaving both arms free to forage. Why don't some tropical American species glide? So far, nobody has a definite explanation.

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