In this article we will discuss about:- 1. Definition of Scansorial Adaptation 2. Categories of Scansorial Animals 3. Adaptive Features 4. Some Vertebrate Examples.
Definition of Scansorial Adaptation:
In Latin, ‘scansorius’ means capable of climbing and ‘arboreus’ means ‘of a tree’. Modification of organisms for leading life on the walls, rocks, branches of the trees or arboreal habitat, are called scansorial adaptation.
For safety, retreat, abundant supply and easy procurement of food relatively feeble creatures have taken refuge on trees and have undergone scansorial adaptation. It is believed that flying vertebrates except fishes were derived from scansorial types.
Categories of Scansorial Animals:
Scansorial animals are broadly grouped into three sub divisions, described as follows:
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A. Wall and Rock Climbers:
Such animals are well suited for climbing on the walls or building and similar surfaces. Geckos, flying squirrels, house lizards, mabuia etc. are the example of this type. Geckos are very old and widely distributed group, exhibiting wide range of adaptation. Some volant adapted animals also show scansorial adaptation as rock climbers.
B. Terrestrio-Arboreal Forms:
Such animals may nest in the trees with more or less extensive terrestrial excursion during daytime or they may climb for food and live on the earth unless impelled by hunger and enemies. The members of this group are squirrels, some rodents, insectivores etc. Their climbing adaptations are not very remarkable.
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C. Arboreal Forms:
This group includes the creatures, which make the trees their home, and when occasionally they come down to the ground, their terrestrial progression become slow and laborious.
Types of Arboreal Form:
(i) Branch Runners:
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They live and progress on all four legs on the upper surface of the branches of trees, e.g., Squirrels, Marsupials, Lemur, and Chameleons etc.
(ii) Arboreal Forms Suspended Beneath Branches:
Generally they cannot walk upon the branches but rest and move suspended from branches by powerful curved claws of all four limbs. On the ground the animals progress with utmost difficulty, e.g., Sloth, an insectivore flying lemur (Galeopithecus), bats etc.
(iii) Arboreal Forms Swinging by the Fore Limbs or Brachiates:
These forms progress by means of the fore limbs swinging with great speed, e.g., primates like apes, gibbon, orangutans etc.
Adaptive Features of Scansorial Animals:
Climbing adaptation exhibits certain body modifications and modification of limbs. Various modifications may be described as follows.
A. Body Modifications:
Scansorial habitat does not impose any drastic change in the body form. Whatever change have occurred, is for supporting the visceral mass during resting in inverted position from tree. The section of the thorax is anteriorly sub-circular and ribs are much curved. In sloths, the ribs are numerous and are able to contain the viscera in their inverted position. The dorsolumbar series of vertebrae are often elongated, e.g., tree sloth.
B. Limb Girdle Modifications:
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(a) Pectoral Girdle:
It is strong enough and includes well-developed clavicles and scapula, so that they can support the pressure of breast muscles.
(b) Pelvic Girdle:
The ileum is broadened out as a support for the viscera during hanging, e.g. sloth.
C. Limb Modifications:
In scansorial forms proximal segments of the limb are usually long for easy movement on trees. The humerus is very long in sloth, apes and gibbons.
Feet:
Climbing forms are generally plantigrades. The feet of arboreal animal may be either prehensile or non-prehensile. In non-prehensile type the claws are well developed, e.g., squirrels. Adhesive pads are present either on the tip of the digits or on the soles of the feet, e.g., tree frogs, Geckos etc. A sticky secretion of their pads aids the tree frogs for attachment on leaf during arboreal life.
Small and numerous vacua on the digits, e.g., Geckos, support the adhesion on the walls. In prehensile type of feet, e.g., opossum, grasping type of feet are found. In prehensile foot more or less opposable digits are found. Syndactyly condition, i.e., digits are bound together, is evident in some scansorials, like sloth and marsupials (Fig. 4.26). True African Chameleons exhibit remarkable syndactyly.
Here in the hand, the first three fingers from the inner bundle are opposed to the outer bundle composed of two fingers. In parrots, woodpeckers and cuckoos, the rotation of the outer toe is permanent and the foot is called zygodactyl (Fig. 4.26). The foot of the koala (a marsupial) is with syndactylus second and third toes, which functions as four toed, exhibiting digital reduction. In lemur fourth digit is largest.
D. Tail Modifications:
Tails of the climbers are also of two types – prehensile and non-prehensile. Prehensile tails are found in chameleon, monkeys and others. In spider monkey, Ateles, the prehensile tail is highly developed and functions as a fifth hand. In non-prehensile tails there are ectodermal spines or scales on the under-side, which pre vent the animal from slipping down, e.g., Anomalurus (a flying squirrel).
E. Modifications of Other Organs:
In woodpeckers the stiff spiny feathers are braced against the tree trunk to which the creature clings. Parrot’s beak and spines of forearm of lemurs are also used as a clinging organ. In Lemur catta, there is a patch of hardened skin on the forearm, which is considered as a climbing organ.
Among the scansorial forms the wall and rock climbers- as well as terrestrioarboreal forms exhibit least modification. They are not perfectly adapted to the arboreal life, because off and on they lead a terrestrial life.
On the other hand, the arboreal forms are mostly restricted to scansorial life and only this category exhibits much more specializations from the standpoint of adaptive radiation. Among the arboreal forms, suspended and swinging creatures show tremendous adaptive features in scansorial life and they are truly arboreal.
Some Vertebrate Examples:
Class Osteichthyes:
Climbing perch (Anabas), Mud skipper (Periopthalmus).
Class Amphibia:
Tree frogs (Hyla, Rhacophorus, Polypedates).
Class Reptilia:
Geckos (Gekko, Hemidactylus), Chameleons (Chameleo), tree snakes (Ptyas, Dryophis, Chrysopalea).
Class Aves:
Parrots (Psittacula), woodpeckers (Dinopium, Dendrocopos, Picus), trumpeters, tree ducks (Aix), snakebird (Anhinga), cormorant (Phalacrocorax).
Class Mammalia:
Opossum (Didelphis), tree kangaroo (Dendrolagus), tree sloth (Bradypus)’, ant-eater (Myrmecophagus), Jaguar (Felis), civets (Viverra), panda (Ailums), squirrels (Sciurus), flying squirrel (Anomalurus), flying lemur (Galeopithecus), all bats (Pteropus, Vespertilio), rhesus monkey (Macaca), langur (Presbytes); gibbon (Hylobates).