Animals climbing trees or other vertical surface or stones are known as arboreal or scansorial animals.
The arboreal animals show following adaptations:
1. Size - Arboreal forms are usually small and slender so that the twigs and tree branches can easily withstand their body weight.
2. Body Contour - The arboreal habit has resulted in strengthening of chest, ribs and limb girdles. The thorax is sub-circular and the ribs are more curved. The number of ribs is increased in sloths so as to give support to the viscera while the animals is in inverted position. The lumbar vertebrae are elongated and their number is often increased.
3. Girdles - The pectoral girdle is strongly is strongly built because the forelimbs support the body Wight while hanging from tree branches. The clavicle and scapula are prominent to withstand the strain of powerful breast muscles.
4. Lengthening of Proximal Segments of the Limbs - The proximal segments of forelimbs, i.e. the humerus bone is elongated in brachial or forms. In gibbon, hands are so long that they touch the ground when it stands erect.
5. Reduction in the Number of Digits - In arboreal forms there is reduction of digits. In tree sloths there are two digits in hand and three in foot, while in the number (Bradypus) has are three digits in each fore and hind limb. IN spider monkey the thumb is reduced. In Koala, the second and third toes are syndactylous. In almost all the birds the number of digits in hind limbs is reduced to three or four.
6. Syndactyly and Zygodactyly - In some arboreal forms the hindlimbs or both hindlimbs and forelimbs are amodifed for grasping opposable digits.
(a) The prehensile limbs with opposable digits found in birds, arboreal reptiles and mammals. In Chameleon, the digits are arranged into opposable bundles of 2 and 3 digits for holding the branches. In scansorial birds (Parrots, Woodpeckers, etc.) out of digits two toes are directed forward and two backward and the 5th digit is absent. Such a of foot is known as zygodactylous type.
(b) Arboreal Structures - The arboreal forms which lack grasping feet exhibit other modifications for climbing. Some of them are:
- Claws - In forms which do not have grasping feet the digits are provided with long and curved claws as in Bats, Squirrels, Cats and Lizards. In birds where feet are of grasping type the claws are also well developed. Claws are modified into powerful hooks in Sloths.
- Adhesive discs - In Tree frogs the digits adhesive discs or suction discs. In mammals Tree-porcupine (Erethizon), Tree coney (Dendrohyrax) and Japanese macaque (Innus) the sole of the feet are provided with adhesive pads. In Tarsiers the finger tips are dilated. Lizards possess adhesive organs having lamellae.
8. Development of Accessory Organs - Parachute mechanisms have been developed in the form of patagia in wide variety of arboreal animals such as flying lizards (Draco).
No comments:
Post a Comment