Scansorial or Arboreal Adaptations (complete information)

Only a few animals take to trees for safety, retreat and for abundant and easily procured food.

Climbers can be of three types (a) Wall and rock climbers (b) Terrestrio-arboreal forms (c) Arboreal forms.

(a) Wall and Rock Climbers

These are well-suited for climbing on the walls of buildings as well as on similar surfaces in nature. Examples are: gecko-lizards. Among mammals, the example is that of flying squirrels.

(b) Terrestrio-arboreal forms

These are capable of climbing but they feel at home beneath the grounds. Examples are: a number of carnivores, rodents and insectivores.

(c) Arboreal forms

These make the trees their home and some occasionally descend to the ground. The forms include some primates and on the basis of locomotion they can be divided into following three subdivisions:

(i) Branch runners:

Various squirrels, lemurs, chameleons etc. are the examples of this subdivision. They live and move on the upper surface of branches by their four legs.

(ii) Suspended forms:

Bats, sloths and flying lemurs (Galeopithecus) etc. are the examples. These animals rest and moves

(iii) Branchiator forms:

These forms swing by their fore-limbs. Examples are monkeys, man-like apes etc. The scansorial adaptations acquired are as follows:

1. Body contour:

Chest and ribs are strengthened and in more arboreal forms the ribs are highly curved. Usually dorso-lumbar vertebrae are elongated and become increased in number from 19-25 or 27.

2. Girdles:

Pectoral girdle is strong and well developed clavicles and scapulae. Ilium in pelvic girdle is broad to support the viscera in forms having inverted posture on trees.

3. Limbs:

In fore-limbs the proximal segment or humerus becomes elongated in suspending form. In sloths the humerus is very long. In gibbons, arms are very long so that the hands touch the ground when it stands erect.

4. Syndactyly and zygodactyly in limbs:

The feet are prehensile. If the feet are non-prehensile, the digits possess claws eg. Squirrels, cats etc. Adhesive pads on the tip of digits or on the soles of feed occur in Hyla, geckoes (lizard) and Dendrohyrax (mammal) etc. opposed to outer bundle formed by the fusion of 4th and 5th fingers. In foot the condition is reverse, outer bundle is of three fingers and inner bundle.

If the feet are prehensile, syndactyly (union of digits) is common. The well known example is that of chameleons. Here syndactyly is marked in both the fore-limbs and hind-limbs.

On the hand first three fingers are of two fingers. In parrots, woodpeckers etc. the outermost toe becomes rotated backward in such a way that 4th toe and halluxes become opposed to the 2nd and 3rd toes. This type of foot where the outer toe rotation is permanent is called zygodactylous.

5. Tail:

Tail may be prehensile (e.g. chameleon, oppossums and some monkeys). If tail is not prehensile it possess spines or scales as in flying squirrels (Anomalurus).

6. Other Accessory Organs:

Other accessory climbing organs are beaks (as in parrot), spines and tubercles found on the fore-arms in some lemurs (Hapalemur grisous). In Lemur cetta there is a climbing organ which is a piece of hard skin on the fore-arm as a projection.