Short notes on Different types of Structural Adaptations

Structurally adaptations may be morphological i.e. confined to the shape, size and external features or anatomical in which visceral organs are adapted according to need. Lull (1952) classified adaptation into the following types.

1. Cursorily or terrestrial adaptations.

2. Fostoria adaptations.

3. Aquatic adaptations.

4. Censorial adaptations.

5. Volant adaptations.

6. Cave adaptations.

However, Mathew added one more adaptation to the above i.e. Deep sea adaptations.

Animals living on land need bodily structures enabling them to run away quickly. Thus these adaptations are also called speed adaptations. These adaptations can be seen in reptiles like snakes, lizards, birds like Emu, Ostrich, Rhea, Moa (all are flightless birds) etc. and more perfectly in mammals. They are characterized by following changes.

1. Body contour

The bodies of all the speedy animals are molded externally in such a way as to offer least resistance to the medium through which they pass. The perfectly adapted mechanism is evident in a race-horse with head and neck extended, ears thrown backwards, every tense muscle of the body working with machine-like precision exhibits the beautiful body contour.

2. Less Development of Fore-limbs

Among the propelling organs, the forelimbs are less developed as they may be concerned in food gathering. The hind-limbs are highly modified and are main divers.

3. Changes in food poiture

The primitive terrestrial foot is plantigrade while walking, the entire palm and sole rests on the ground. Examples are: bear, racoon, and baboon. Next stage is digitigrade i.e., walking or running upon the digits themselves. Examples: dogs, birds, dinosaurs and mammals except ungulates. The third stage is unguligrade i.e., walking on modified nail or hoofs. Examples: horse, and other ungulates.

4. Loss of digits

Plantigrade animals are those who’s entire palm or sole rests on ground. The plantigrade animals are generally five-toed: The digitigrade animals are four-toed. Plantigrade four-, three-, two-or even one-toed. Arteodactyles have two toes while Parissodactyles have only one. Hence speed adaptation has its influence in determining digital reduction.

5. Reduction of Fibula and ulna

These bones tend to be reduced and in exclusively locomotors limbs these bones are mere vestiges. These are well developed in slow moving forms.

6. Loss of universal movement

The entire movement of limbs is restricted to movement is only one plane. However, where universal movement is retained through the movement of ball and socket joint.

7. Lengthening of distalia of limbs

The lengthening of limbs is accomplished by the growth of the distal elements only. The powerful muscles are attached at the proximal end of the limbs. Moreover, the muscles are short and stout which increase their power and speed of contraction.

8. Bipedality

In bipedal forms (birds, marsupials and man) the cursorial adaptations include reduction of fore-limbs. Fore-limbs are also often used as efficient organs of prehension and food capturing.

9. Importance of tail

Tail takes up different functions. In some, the tail may be short or heavier or it may be extremely long and slender. In kangaroos, tail is a powerful organ and serves as a third limb, when animal rests. Tail is not found in birds but feathers serve collectively as a balancing organ.

10. Shortening of neck

In bipedal mammals, there is a tendency of reduction in the neck size (e.g. Jerboa) though these are seven cervical vertebrae.

11. Mental alertness

Animals, which depend upon speed for safety, have alert young ones which keep up with the head almost at once. In carnivores and rodents, young ones are feeble and are, therefore, kept hidden until able to move independently. The newly born camel, deer, or horse with long limbs, have more mental alertness, therefore, there is little or no period of helpless infancy.