Scansoriality refers to the ability or propensity to climb. An animal that spends at least some of its time climbing is therefore scansorial. Although the term is not exclusive to climbing on plants—it can also refer to climbing on cliffs, buildings, or other three-dimensional surfaces—it is most often used in the context of climbing on vegetation like trees or vines. Scansoriality differs from arboreality, though they are frequently used interchangeably. An arboreal animal spends most of its life in trees and conducts many of the important aspects of its life history above ground, including feeding, nesting, reproduction, etc., while a scansorial animal may nest on the ground but acquire some of its resources from trees. While the distinction may be blurred in species such as the red squirrel (Tamiasciurus hudsonicus), which nests in either trees or on the ground and feeds on both pine and spruce nuts as well as terrestrial fungus, there is no doubt that this species is scansorial. (Cartmill, 1985; Steele, 1998)
The two primary reasons mammals climb trees are to gain access to resources that are above the ground and to protect themselves against predators. Additional reasons for scansoriality may include availability of arboreal nesting sites, or cache sites. Trees can also be launching sites for gliding mammals.
Scansoriality plays a major role in resource acquisition and has likely evolved in many orders of mammals for this reason. Trees and shrubs contain many unique and valuable resources that may not be exploited as frequently as terrestrial resources. Examples of resources (and the mammals that consume them) include nectar (noolbenger or honey possum (Tarsipes rostratus)), tree bark (North American porcupine (Erethizon dorsatum)), leaves (koala (Phascolarctos cinereus)), and fruits (spider monkey (Ateles geoffroyi)). Some predatory mammals are also highly adapted climbers that prey on scansorial or arboreal animals, such as the American marten (Martes americana) that is known to chase and capture red squirrels in trees, and margays (Leopardus wiedii) that prey on climbing mammals and birds. Scansoriality is not specific to locomotion in trees; mountain goats (Oreamnos americanus) are a good example of mammals that have a unique climbing ability that allows them to access unexploited grazing areas while positioning themselves in areas that are difficult to reach for predators.
In addition to food resource acquisition, climbing also provides escape from predators or danger. Female black bears (Ursus americanus) will often encourage their cubs to climb trees when danger is imminent. Squirrels, raccoons (Procyon lotor), and domestic cats (Felis catus) are all known to rapidly climb trees to escape dogs or other predators.
Scansoriality is a highly convergent behavior found in a wide variety of mammals. Below is a list of mammalian orders. Bolded text denotes orders that contain scansorial species, with an example provided.
Scansorial mammals are found in a variety of habitats. Tropical forests probably host the largest number of climbing mammals and often contain primates, one of the groups most adapted for life in the trees. Scansoriality is not limited to the tropics, however. In the boreal forests of the Northern Hemisphere, many mammals are adept climbers, including many species in the family Mustelidae (e.g., marten and sable) and rodents such as squirrels. North American porcupines regularly ascend trees, and if good terrestrial den space is not available, they often rest in trees for much of the winter. Desert ecosystems also support climbing mammals. In the Great Basin of the United States, where few trees are found, the chisel-toothed kangaroo rat (Dipodomys microps) climbs to the tops of spiny saltbush shrubs and uses its specialized incisors to access water in the inner layers of the leaves. This is an excellent example of how climbing can allow access to a highly valuable and untapped resource (Kenagy, 1972).
When an animal climbs a sloped or vertical surface or crosses a branch with a rounded surface, it needs some way to make secure contact with the substrate to prevent falling. This can be accomplished by gripping the surface using friction or some other force (adhesion or suction are common methods in non-mammalian climbers) or by interlocking the feet to the surface as is done with claws. As scansoriality has evolved independently in many different mammals, there is no single body plan specific to climbing. Nevertheless, many physical similarities can be found among climbing mammals. (Cartmill, 1985)
Grasping hands or feet: Climbing on small branches, trunks, or limbs require a way to secure contact with the surface to prevent a fall. One evolutionary strategy that is common in mammals is modification of the hands to allow grasping or gripping of the substrate, which adds a rotational force that counteracts the downward force of the animal’s weight. This is often accomplished by having an opposable digit, usually digit 1 (the thumb, or pollex) that can flex horizontal to the direction of the other digits. In some marsupials, both the first and second digits are opposable, creating a powerful grip for round surfaces; this adaptation is known as schizodactly. (Cartmill, 1985)
Hand proportions: Climbing mammals often possess different hand proportions than their terrestrial counterparts. The palm (metacarpals) is often shortened and the fingers (proximal and/or intermediate phalanges) are often elongated in scansorial and arboreal species. Such a pattern has been found in carnivores, rodents, marsupials, and primates. It is thought that having longer fingers and shorter palms allows the fingers to wrap farther around the branches, providing a better grip. (Kirk et al., 2008)
Claws: Claws can be used to lock an animal into position on a branch or tree trunk and are common in many scansorial mammals. Having claws that dig into the substrate helps prevent roll while crossing small limbs and distributes body weight across many digits when ascending a tree or other surface. Claws used for climbing are often shorter, taller at the base (dorsoventrally), and have a stronger curvature than claws used for other purposes. Shorter, hook-like claws can dig in to firmly grip substrates, holding the animal’s weight without breaking off or bending. Alternatively, burrowing mammals tend to have narrower, longer claws for digging in soil. Some scansorial mammals lack claws, including many primates and marsupials, and instead have nails that do not extend beyond the tips of the digits. Many clawless species are more modified for manual or pedal grasping and therefore do not requiring claws to stay above the ground. It is thought that claws may have been lost in these species since they interfere with the gripping of small branches, but marmosets (Callithrix spp.) retain claws and are equally dexterous at grasping, climbing, and foraging as non-clawed primates. (Tulli et al, 2009; Ji et al., 2002)
Hind foot reversal: One challenge for mammals is climbing downwards on a tree trunk or other surface in a way that does not leave them vulnerable. Descending head first is important since it allows the animal to see where it is going and avoid obstacles or any other potential hazards. This is made difficult by the fact that the claws face the wrong direction to grip the substrate. This problem can be overcome by a specialized ankle morphology that allows reversal, or inversion, of the hind feet. When an animal descends, its rear feet can be reversed, allowing the claws to face in the appropriate direction for supporting its weight. This adaptation has been reported in some procyonids, viverrids, tupaids, sciurids, and one felid (margay (Leopardus wiedii)). (Jenkins & McClearn, 1984)
Prehensile tail: A grasping tail is a highly specialized adaptation to climbing and is usually indicative of arboreality. Using a tail to grasp branches and limbs provides an additional force to offset the weight of the animal, allowing it to loosen its manual or pedal grip, thereby freeing its hands to cross gaps in branches or conduct other activities such as feeding or grooming. This trait is well known in the Virginia opossum, which is commonly portrayed fully suspended by its tail, but is also found in a variety of other mammals, including other marsupials (e.g., ground cuscus (Phalanger gymnotis)), primates (e.g., Central American spider monkeys (Ateles geoffroyi)), and rodents (e.g., prehensile-tailed hutias (Mysateles prehensilis) and Brazilian porcupines (Coendou prehensilis)) (Cartmill, 1985).
One of the challenges of climbing trees is navigating small branches. Morphological adaptations such as grasping hands can facilitate movement across branches, but another way to deal with this is by lowering the center of gravity. Crouching and walking close to the branch reduces the possibility of toppling to one side or another. Shortened limbs are a modification that can keep the center of gravity close to the branch, but mammals with longer limbs or a large body mass, such as leopards (Panthera pardus), can crouch low to trees when climbing.
Another method to prevent toppling entirely is to move suspended below the branch instead of balanced on top of it. Three-toed sloths [Bradypodidae|(Bradypus spp.)] are an excellent example of this behavior, as their hook-shaped claws are ideal for suspensory locomotion. Many primates move through the trees by brachiation, a familiar form of hanging and swinging from branch to branch commonly seen in primates like gibbons (Hylobates spp.). This movement requires grasping hands and a pectoral (shoulder) girdle that permits a wide range of fore-aft movements but can be a very quick and efficient way to travel in the canopy.
For an animal that spends much of its life in the trees, it becomes inefficient to climb down to the ground, move to another tree, and ascend. It is much easier to cross from tree to tree, but this often involves navigating the smaller, thinner, terminal branches. Some of the aforementioned adaptations help to deal with this issue: a prehensile tail allows hands to be free to grasp branches from neighboring trees, and brachiation builds momentum that allows the animal to swing one arm out and grab a branch while retaining a grip on a different branch. However, these features are not found in all climbing mammals. One alternative to brachiation is jumping. Many climbing mammals, such as squirrels, are skilled jumpers and can leap great distances from tree to tree (or branch to branch). Gliding, found in colugos, squirrels, and gliding marsupials (sugar gliders (Petarus breviceps)), is an adaptation that makes leaping from tree to tree much more energetically efficient while allowing the animal to cover much larger distances. Not all climbing mammals need to be able to move quickly over a large area, as some are content staying in one tree for long periods of time. For scansorial mammals such as tarsiers (Tarsius spp.) that are ambush hunters of insects and small vertebrates, slower, more deliberate movements are more advantageous. Tarsiers’ long digits provide a strong grip for their small mass, allowing them to slowly move through small branches in search of prey without relying on momentum. (Cartmill, 1985; Jenkins & McClearn, 1984)
Body size is an important determinant of how and where mammals climb. One way to think of this is to imagine walking along a downed tree like it was a tight-rope. If the tree is large, the circular shape of the trunk is less of a challenge. A very large tree may be as easy to walk across as a flat surface. If the tree is smaller, it becomes more difficult to cross. At some point the tree becomes too small for anyone but the best acrobat to walk along. The same principal is true for any animal moving through trees. A small mouse would not notice the curvature of the surface of a large downed tree trunk while running along it. A 20 mm-diameter branch may be relatively simple for a small climbing mouse to cross, but might require some careful movements from a squirrel, and may be impossible for a black howler monkey (Alouatta caraya). Mammals with a smaller mass may move freely on smaller branches, while larger climbing mammals require more sturdy limbs for support.
For a small mammal living on the forest floor, the world is a very three-dimensional place. Rocks, twigs, and downed logs all present obstacles that must be climbed over or around. This has led to the hypothesis that terrestrial and arboreal habitats are not distinct for small mammals, but that both habitats involve near constant climbing. While this hypothesis provides an interesting perspective on how scansoriality evolved, it is not appropriate to think of every small forest dwelling mammal as scansorial. One reason to differentiate the two is a scansorial mammal’s ability (and inclination) to ascend and descend larger trees. Vertical climbing can be more difficult for a small mammal as a large tree trunk becomes a flat vertical wall to scale. Climbing over fallen branches is not the same as ascending a tree for food. (Cartmill, 1985; Jenkins, 1974)
Many extinct mammals in the fossil record have been described as scansorial. Often it is easy to determine that the fossils belong to climbers. For example, many primate fossils look very similar to extant arboreal primates. Often, however, a fossil mammal may not closely resemble a living species and inferences about its locomotor adaptations must be made based on comparisons to one or more extant taxa that may or may not be appropriate for comparison. If a fossil is well preserved, many of the aforementioned morphological characters can be seen, such as elongated fingers for grasping and thickened claws for climbing. These particular features were observed in the oldest known fossil placental mammal, †Eomaia scansoria, and led researchers to describe it as scansorial (as its species name “scansoria” implies). Additionally, several other small mammal fossils from Cretaceous deposits are considered to have been climbers. Some caution should be taken in inferring scansoriality in fossil mammals. Often climbing is described based on a loose diagnostic character, such as the shape of the distal phalanx (the last finger bone) where the claw is attached. While climbing mammals often have thick, sharply curved claws, they are not the only mammals that do, as many terrestrial carnivores have similarly shaped claws. Another difficulty in identifying climbing in the fossil record is the number of scansorial taxa that may go unnoticed due to a generalist body plan. For example, some cats are scansorial but may not have any obvious morphological features that indicate that. A smaller-sized example of this is the red tree-vole (Arborimus longicaudus), a fully arboreal vole that, if fossilized, would likely be inferred to have been an unspecialized terrestrial small mammal. Additionally, some mammals that are actually scansorial lack obvious adaptations to scansoriality (see below), and it is almost certain that these taxa would be interpreted as terrestrial. (Ji et al., 2002; Luo, 2007)
As is often the case in biology, there are exceptions to the rule. Tree kangaroos (Dendrolagus spp.) are secondarily arboreal marsupials that share many physical features with their terrestrial kangaroo cousins. Although they are reported to be quite agile in trees, their outward appearance does not give the impression of a climbing mammal. Nevertheless, their rear feet are shorter and wider than those of terrrestrial kangaroos and have a sponge-like padding on the soles to improve grip on branches. One study reports that they are the only known mammals outside of primates to have a complete rotator cuff in their shoulders, allowing for increased flexibility to reach their arms up above their heads to assist in climbing.
Tree-hyraxes (Dendrohyrax spp.) are small (<5 kg) relatives of elephants that inhabit forested areas in sub-Saharan Africa. They live in crevices in trees and feed on leaves and other vegetation. They have small fore-feet with very short stubby digits and nails that have been described as hoof-like. When looking at a tree-hyrax there is no immediate indication of scansoriality. However, they are almost strictly arboreal and widespread in Africa from lowlands to high altitude cloud forest.
One of the most surprising scansorial mammals is the domestic goat (Capra hircus). Goats are known to climb on various structures when kept in barnyards, but some herds in Morocco readily climb argan trees to access fruit. (Sonnabend & Young, 2009; Jones, 1978; Lybbert et al., 2011)
Jonathan A. Nations (author); Link Olson (instructor)
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