Why is an elephant so big

1. Why is an elephant so big

3. FORM, FUNCTION AND BODY SIZE IN VERTEBRATES Dr.S.N.Hegde Professor, Department of Zoology University of Mysore Mysore - 570006

4. Body size, form are adaptive traits • Largest vertebrate is one million times larger than the smallest • Relation between habits and size • Size is related to the requirement – eg giraffe needs long neck and deer does not although both are related in habits – herbivores - grazing • This is because of surface volume ratio – This relationship between size and volume is called scaling relationship

6. Simplest of this relation is isometric (equal+measure) growth. • One animal looks like photographic enlargement of the other (eg.cat-tiger).

7. The large and small will have geometric similarity. The mass-volume ratio will be constant. • Geometric similarity is found in the ontogeny of most animals

8. The second is elastic similarity • Two animals look similar but different in their structure and adaptations (eg.bat, bird) • They may or may not be closely relatedfig • The mass volume ratio will vary

9. Disproportionate growth • One way to adapt to increased size is to reduce the need for surface-related function. • Rate of respiration by lungs, absorption through gut, excretion through kidney depends on the rate of metabolism and not on size. • Bird has higher BMR than elephant.

10. Second method of adaptation to increase in size is by making special structural modifications that increase surface area disproportionately. • Eg. To absorb water the large mammals have nephrons which are much longer although the size of the kidney does not increase proportionally • Stomach of ruminants four chamber – in others single chambered

11. The gray matter of mammalian forebrain is much convoluted than other vertebrates • The large herbivores have the cheek teeth with several occlusions produced due to in-folding of enamel. • These folding provide large surface area for grinding although the actual size of teeth do not increase proportionately.

12. Geometric similarity of support system may not pertain over wide range of body size; the bones and muscles of large animals would fail to do their function under their heavy load. • There may be no proportionate growth of the skeleton. • Eg. Cattle and antelope have geometric similarity, but their bone diameter and strength are different

13. In certain animals one part may be under geometric similarity and another may exhibit elastic similarity. • Eg. In certain mammals, the width of the lumbar vertebra maintain geometric similarity, while the height has elastic similarity • This is called dynamic strain similarity

14. Movements of small animals may be similar to large animals. • The performance of these small animals have to be multiplied by number of times that of the proportion of the large animal. • This is called kinematic similarity

15. Allometry = Disproportionate growth (unequal+measurement) • It is the study of correlation between the size and form. • It has developmental (ontogenic) and phylogenetic (paleontological) relations • Eg. Colt has longer legs than adult horses. Modern horses have longer legs than the fossils

16. Allometry is important to interpret both size and form of animals and their adaptive significance. • In mammals, skeletal muscle, blood, heart and lungs scale approximately isometrically where as skeleton, fat, contribute to the mass. • Proportion of the limbs and associated bones depend on the mode of life

17. Vertebrate giants • Evolutionary trend toward large body size is common in vertebrates. • Terrestrial animals like elephants, Rhino weigh 4 to 7 tons. • Such animals are said to be graviportal (heavy + to carry) • Marine beasts like sharks, whales are larger than these!

18. Advantages of large size • Virtual freedom from predation (except by man) • Ability to move over larger area in search of food, water, shelter, breeding grounds • Capacity to produce and use energy more slowly than smaller • Smaller are more active and require more energy • Low surface volume ratio allows slow heat up and cool up of body

19. Gigantic fishes • Siberian fresh water sturgeon is 1360kg • Largest living fish, tropical whale shark grows 18 m • Arthodires were 30 m long

20. Amphibians and Reptiles • Largest labyrinthodont was heaviest and 4.5 m long • Jurassic reptiles are known for large size • Many of them were really giants • Extinct marine lizard monosaur was 10 m • Longest snake measures 12 m long • A Crocodile measures 13 m long

21. Diplodocus of Dinosaur order – Saurischia was probably longest land animal – 26.6 m • Brachisaurus weighed 86 tons / 10 times more than elephants • The elephant bird was 436 kg

22. Mammals • Bull elephant seal – 3500 kg • Hippotomus – 400 kg • Bull African elephant 4 m height and weighs 7.7 tons • Blue sperm whale grows 30 m long and weighs 150 tons

23. Aquatic/marine animals are larger than terrestrial • Weight is compensated by resistance of water • They can effortlessly swim • Terrestrial animals reduce their requirement by avoiding unnecessary oscillations