The animals living in open plains exhibit cursorial adaptations, whereas animals living in burrows or under the surface of earth show fossorial adaptations.
(A) Cursorial Adaptations
The terrestrial animals living in open plains have to depend on speed for searching their food and for escaping from their enemies.
They exhibit following cursorial modifications-
- Body Contour - Body of cursorial forms is stream-lined with compressed thorax and flattened ribs. Such a body contour offers least resistance to air while running.
- Change in Foot Posture - The crsorial animals possess digitigrades or unguligrade foot posture. In digitigrades foot posture, the animals run on the digits of their feet. The bones of the writs, palm, ankle and sole are raised from the ground. The digitigrades foot posture is found in Birds, Bogs and Digitigrades mammals. The unguligrades walk on the hoof, the modified nail. The unguligrade foot posture is found in Horse, Camels and Cows, etc.
- Lengthening of Distal Segments of Limbs - Digitigrades and unguligrade foot posture the length of digits increases due to lengthening of carpals, metacarpals, tarsal's and metatarsal segments of the fore and hind limbs.
- Loss of Digits - The digitrade and unguligrade foot posture is derived from plantigrade posture. In plantigrade posture there are 5 toes in each foot but in digitigrades the number is reduced to four and in ungligrade to three, two or one.
- Reduction of Ulna and Fibula - The ulna in the forelimbs and fibula in the hindlimbs are reduced as an adaptation to fast running. The clavicle bones of the pectoral girdle also tend to reduce.
- Loss of Universal Movement - The free movement of limb segments at the joints is lost except at the ball and socket joint of hip and shoulder. The limbs can neither bend at the knee joint or elbow joint nor at the ankle and wrist joints. This produces a powerful impact on the ground and helps to get a better speed.
- Musculature of Limbs and Development of Spring Ligaments - In cursorial forms the muscles of limbs are powerful. These are restricted in the proximal part of the limbs. Because of this Their power and speed of contraction is markedly increased A set of spring ligaments is developed on the posterior surface of the foot.
- Tail as a Balancing Organ - The tail acts as an organ of counterpoise. It is short and heavy in bulky forms and long and slender in lightly built creatures.
- Bipedality - In bipedal forms (birds and marsupials, etc.) the cursorial adaptations include (i) reduction of forelimbs which are used as organs of pretension, and (ii) Shortening of neck.
- Mental Precocity - The young one of cursorial forms are as active as the adults and follow their fellows in the herd almost immediately after birth.
(B). Fossorial Adaptations
The animals residing either permanently of for most of the time inside the burrows or under the earth surface are known as burrowing or fossorial animals. These animals exhibit following modifications:
1. Body Form - The true burrowing animals are often cylindrical and worm-like or possess a fusiform or spindle-shaped body.
2. Head - Head is incipient and indistinct from the trunk. It is roughly conical or sub conical in form or is produced into a conical or tubular muzzle.
3. Neck - In burrowing mammals neack is often absent or it is very short, so that their conical head directly passes into the trunk.
4. Tall - In burrowing animals, the tail is either reduced or vestigial, because it has no significant function to perform in burrowing or otherwise.
5. Limbs - Limbs are either absent or are adapted for digging and burrowing. Caecilians, snakes and burrowing lizards are limbless forms. In burrowing mammals forelimb are modified for digging and burrowing. The forearm is palm is broad and spade-like. The digits bear powerful claws for digging. In moles each hand possesses a falciform bone of the wrist that gives great breadth to the palm.
6. Limb Bones and Girdle - To withstand stress while burrowing the bones of forelimbs are short and strong and are provided with additional tuberosities for muscle attachment.
The shoulder girdle is strong and forwardly placed. Scapula and sternum are lengthened and shift the forelimbs still forward. The clavicles are thick and articulate both with acromian process of scapula and the hummers. This enables them to withstand immense muscular strain during digging.
7. Vertebral Column - The vertebrae of the cervical, sacral and lumber region are fused to give firmness and strength in pushing the animals through earth. In moles and hedgehogs, intercentral ossicles are present between lumbar vertebrae to strengthen the vertebral column. In Armadillos an extra pair of zygapophyses are present in the lumbar vertebrae.
8. Skin - Skin of burrowing animals is protected with mucus (annelids), chitin (arthropods), scales (caecilians, and reptiles), fur or spines (mammals).
9. Loss of Eyes - The eyes are often reduced, vestigial or non-functional. In limbless lizards the eyes are seen as white dots through skin. In snakes the eyelids are immovable and fused together forming a transparent membrane over the eyes. In moles eyes are small, reduced and functionless and are buried in the fur.
10. Disappearance of Pinnae - The external ear which collects sound waves from air has no utility in burrowing animals. For this reason, the pinnae are either absent or very much reduced. Burrawing animals are sensitive to sound waves passing through the soil.
11. Tactile Organs - The tactile organs are specially developed to compensate for sight and hearing. In moles the snout is richly innervated and highly sensitive. The tail of wombat is also tactile.
12. Skull - The skull of subterranean animals is compact and roughly conical. The suture between the bones of the skull are obliterated. The jaw bones are reduced and zygomatic arch is absent. The dentition is also reduced and jaw muscles are poorly developed.
13. Digging Organs - The most important modification of fossorial life is the presence of some digging structures. The digging structures are either the snout, forelimbs or teeth.
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