1.1 Diameter 1.2 Incline 1.3 Balance 1.4 Crossing gaps 1.5 Obstructions
2 Anatomical specializations
2.1 Limb length 2.2 Prehensile tails 2.3 Claws 2.4 Adhesion 2.5 Gripping 2.6 Reversible feet 2.7 Low center of mass 2.8 Small size 2.9 Hanging under perches
3 Behavioral specializations 4 Ecological consequences 5 Climbing without trees 6 Brachiation 7 Gliding and parachuting 8 Limbless climbing 9 Arboreal animals 10 References 11 Sources
Biomechanics Arboreal habitats pose numerous mechanical challenges to animals moving in them, which have been solved in diverse ways. These challenges include moving on narrow branches, moving up and down inclines, balancing, crossing gaps, and dealing with obstructions. Diameter Moving along a narrow surface poses special difficulties to animals. During locomotion on the ground, the location of the center of mass may swing from side to side, but during arboreal locomotion, this would result in the center of mass moving beyond the edge of the branch, resulting in a tendency to topple over. Additionally, foot placement is constrained by the need to make contact with the narrow branch. This narrowness severely restricts the range of movements and postures an animal can use to move. Incline Branches are frequently oriented at an angle to gravity in arboreal habitats, including being vertical, which poses special problems. As an animal moves up an inclined branch, they must fight the force of gravity to raise their body, making movement more difficult. Conversely, as the animal descends, it must also fight gravity to control its descent and prevent falling. Descent can be particularly problematic for many animals, and highly arboreal species often have specialized methods for controlling their descent. Balance Due to the height of many branches and the potentially disastrous consequences of a fall, balance is of primary importance to arboreal animals. On horizontal and gently sloped branches, the primary problem is tipping to the side due to the narrow base of support. The narrower the branch, the greater the difficulty in balancing a given animal faces. On steep and vertical branches, tipping becomes less of an issue, and pitching backwards or slipping downwards becomes the most likely failure. In this case, large-diameter branches pose a greater challenge, since the animal cannot place its forelimbs closer to the center of the branch than its hindlimbs.
Gibbons are very good brachiators because their elongated limbs enable them to easily swing and grasp on to branches.
Crossing gaps Branches are not continuous, and any arboreal animal must be able to move between gaps in the branches, or even between trees. This can be accomplished by reaching across gaps, by leaping across them or gliding between them. Obstructions Arboreal habitats often contain many obstructions, both in the form of branches emerging from the one being moved on and other branches impinging on the space the animal needs to move through. These obstructions may impede locomotion, or may be used as additional contact points to enhance it. While obstructions tend to impede limbed animals, they benefit snakes by providing anchor points. Anatomical specializations Arboreal organisms display many specializations for dealing with the mechanical challenges of moving through their habitats. Limb length See also: Intermembral index Arboreal animals frequently have elongated limbs that help them cross gaps, reach fruit or other resources, test the firmness of support ahead, and in some cases, to brachiate. However, some species of lizard have reduced limb size that helps them avoid limb movement being obstructed by impinging branches. Prehensile tails Many arboreal species, such as tree porcupines, chameleons, silky anteaters, spider monkeys, and possums, use prehensile tails to grasp branches. In the spider monkey and crested gecko, the tip of the tail has either a bare patch or adhesive pad, which provide increased friction. Claws Claws can be used to interact with rough substrates and re-orient the direction of forces the animal applies. This is what allows squirrels to climb tree trunks that are so large as to be essentially flat, from the perspective of such a small animal. However, claws can interfere with an animal's ability to grasp very small branches, as they may wrap too far around and prick the animal's own paw.
The silky anteater uses its prehensile tail as a third arm for stabilization and balance, while its claws help better grasp and climb onto branches.
Adhesion Adhesion is an alternative to claws, which works best on smooth surfaces. Wet adhesion is common in tree frogs and arboreal salamanders, and functions either by suction or by capillary adhesion. Dry adhesion is best typified by the specialized toes of geckos, which use van der Waals forces to adhere to many substrates, even glass. Gripping Frictional gripping is used by primates, relying upon hairless fingertips. Squeezing the branch between the fingertips generates frictional force that holds the animal's hand to the branch. However, this type of grip depends upon the angle of the frictional force, thus upon the diameter of the branch, with larger branches resulting in reduced gripping ability. Animals other than primates that use gripping in climbing include the chameleon, which has mitten-like grasping feet, and many birds that grip branches in perching or moving about. Reversible feet To control descent, especially down large diameter branches, some arboreal animals such as squirrels have evolved highly mobile ankle joints that permit rotating the foot into a 'reversed' posture. This allows the claws to hook into the rough surface of the bark, opposing the force of gravity. Low center of mass Many arboreal species lower their center of mass to reduce pitching and toppling movement when climbing. This may be accomplished by postural changes, altered body proportions, or smaller size. Small size Small size provides many advantages to arboreal species: such as increasing the relative size of branches to the animal, lower center of mass, increased stability, lower mass (allowing movement on smaller branches), and the ability to move through more cluttered habitat. Size relating to weight affects gliding animals such as the reduced weight per snout-vent length for 'flying' frogs. Hanging under perches Some species of primate, bat, and all species of sloth achieve passive stability by hanging beneath the branch. Both pitching and tipping become irrelevant, as the only method of failure would be losing their grip.
The gecko's toes adhere to surfaces via dry adhesion, to allow them to stay firmly attached to a branch or even a flat wall.
Arboreal species have behaviors specialized for moving in their
habitats, most prominently in terms of posture and gait. Specifically,
arboreal mammals take longer steps, extend their limbs further
forwards and backwards during a step, adopt a more 'crouched' posture
to lower their center of mass, and use a diagonal sequence gait.
Arboreal snails use their sticky slime to help in climbing up trees, since they lack limbs to do so.
Primates Cats brushtail possums opossums Sloths Anteaters Treeshrews Goats Colugos Kinkajous Viverrids Tree squirrels and many other rodents Parrots Geckos Chameleons Many other lizards Mambas Brown Tree Snakes Many other snakes Stick insects Many other arthropods Tree snails Koalas
^ a b c d e f g h i Cartmill, M. (1985). Climbing. In Functional
Vertebrate Morphology, eds. M. Hildebrand D. M. Bramble K. F. Liem and
D. B. Wake, pp. 73–88. Cambridge: Belknap Press.
^ Fröbisch, Jörg; Reisz, Robert R. (2009). "The Late Permian
Bertram, J. E. A.; Ruina, A.; Cannon, C. E.; Chang, Y. H.; Coleman, M.
J. (1999). "A point-mass model of gibbon locomotion" (PDF). J. Exp.
Biol. 202: 2609–2617.
Cartmill, M. (1974). Pads and claws in arboreal locomotion. In Primate
Locomotion, (ed. F. A. J. Jenkins), pp. 45–83. New York:
Jayne, B. C.; Riley, M. A. (2007). "Scaling of the axial morphology
and gap-bridging ability of the brown tree snake (Boiga irregularis)"
(PDF). J. Exp. Biol. 210: 1148–1160. doi:10.1242/jeb.002493.
Lammers, A.; Biknevicius, A. R. (2004). "The biodynamics of arboreal
locomotion: the effects of substrate diameter on locomotor kinetics in
the gray short-tailed opossum (Monodelphis domestica)" (PDF). J. Exp.
Biol. 207: 4325–4336. doi:10.1242/jeb.01231.
Lammers, A. R. (2000). "The effects of incline and branch diameter on
the kinematics of arboreal locomotion". Am. Zool. 40: 1094.
Socha, J. J. (2002). "
v t e
Concertina movement Undulatory locomotion Rectilinear locomotion Rolling Sidewinding Other modes
Comparative foot morphology
Canine gait Horse gait Human gait