Frog »

Last Updated on Tuesday, 12 July 2011 04:24 Written by Sandesh Monday, 24 May 2010 04:47

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FROG
The study of amphibians and reptiles is known as Herpetology. Amphibians include all living tetrapods i.e. four-legged vertebrates. They are among the oldest creatures on earth. The earliest amphibian evolved from a prehistoric fish about 350 million years ago and became the first vertebrate (an animal with a backbone) to walk on land. There are around 6,200 described, living species of amphibians.

EXTINCT AMPHIBIANS: Eryops, Seymouria, Lysorophus and Diplocaulus
LIVING AMPHIBIANS: Frogs, Toads, Salamander, Newts and Caecilians

The frog is an Amphibian in the order Anura which means “tail-less”, from Greek (an-, without + oura, tail). Frogs are an ancient group. The earliest known fossils of frogs date from the Jurassic Period about 208 million to 144 million years ago. They probably evolved from salamander-like animals that had long bodies, tails, and short hind and forelegs.

The smallest frog: 8.5 mm (0.34 in) long and less than 30 g., found in Brazil.
The largest frog: West African goliath frog, which is 30 cm (12 in) long and weighs 3.2 kg

1. SYSTEMATIC POSITION
Kingdom: Animalia
Phylum: Chordata
Subphylum: Vertebrata
Division: Gnathostomata
Superclass: Tetrapoda
Class: Amphibia
Order: Anura
Family: Ranidae
Genus: Rana

Species: tigrina

KEY CHARACTERISTICS OF FAMILY RANIDAE: Slim waist, long legs with pointed toes and extensive webbing, horizontal pupils, dorsolateral folds, smooth skin, excellent jumpers.

FROG
2. HABITAT:
Frogs live on all landmasses except Antarctica and some oceanic islands. They can be found in all but the most extreme habitats, such as the Polar Regions and the highest mountain peaks. Like many groups of animals, frogs reach their greatest diversity in the tropical rainforests. Frogs are among the most diverse groups of vertebrates, consisting of more than 5000 species. About 47 species of frogs occur in Nepal (IUCN, 2004). However, populations of certain frog species are significantly declining.
The common Indian bull frog, Rana tigrina, lives like other species of frogs in or near pond water. It is in water most of the time. It lives near water mainly for two reasons:
1) To keep skin moist to carry on cutaneous respiration, and
2) To immediately jump or slip into water to escape from enemies.

3. HABITS:
Frogs are non-poisonous, harmless and normally silent animals. The presence of frog is difficult to detect unless it is disturbed. It is very lively and an excellent jumper.

i. Locomotion: Frog moves in two ways, by leaping on land and by swimming in water. In both cases long hind limbs plays the chief part.

a. Leaping: A frogs rests on land with its short forelimbs upright and very long and powerful hind limbs or legs flexed or folded in the manner of “Z”. It jumps or leaps by suddenly extending the hind limbs which act like springs throwing the body into air. On landing back the forelimbs act like shock absorber.

b. Swimming: The frog swims in water by powerful backwards thrusts of its hind limbs which act like propellers. During their backwards strokes, the toes are spread apart and the broad webs push against water, moving the body forward. Fore limbs usually do not take part in swimming. When frog comes to the surface to breathe, or simply to float, only the tip of the snout carrying nostrils is visible, with both fore and hind limbs extend in water. When disturbed in this position, it immediately dives under water.

ii. Feeding: Frogs are carnivorous and food consists mainly of living insects, worms, mollusks and tadpoles, which are caught by sudden flip of its tongue. The tongue is attached at the front end and free behind. The food is not chewed but simply swallowed.

iii. Croaking: The characteristic noise or sound made by frogs is known as croaking or ‘rain-music’. It is commonly heard in breeding season during rains and is a mating call. It is particularly produced in the evenings and at night. The croaking is done by male frogs; female frogs do not croak. It is produced by forcing air from lungs over vocal chords into mouth cavity and back again. Frogs can croak under water as well as on land. It is louder in males due to the presence of a pair of distensible balloon-like loose skin folds on throat, called vocal sacs. These act as resonator (sound producer).

iv. Hibernation and Aestivation: Being poikilothermous animals, the body temperature of frog fluctuates with that of environment. During adverse environmental conditions in clod climates or hot dry seasons, frogs burry in soft damp bottom mud to keep their body for protection. They become metabolically inactive and stop feeding, living only on glycogen stored in their bodies. Lung respiration is suspended and cutaneous respire through damp skin is carried on. This state of dormancy is known as hibernation or winter sleep during winter or aestivation or summer sleep during summer.

v. Camouflage: Frogs are not easily preyed upon by their predators as they can change the colour of their skin to match with that of surroundings. This type of protective colouration is known as camouflage. Changes in colour are made possible by depression or concentration of special amoeboid pigment cells in their skin.

vi. Breeding: As the rainy season begins, frogs emerge out of aestivation and immediately start breeding which lasts from about July to September. During breeding seasons males gather in appropriate shallow water and start croaking to attract females for mating or copulation. During copulation the male mounts upon the back of the female and grasps firmly around her thorax by his forelegs. The nuptial pads which are fully developed during the breeding season helps in holding the slippery females. This sexual embrace is called amplexus which may last for several days until the female deposits several hundred ova or eggs into water. Mass of eggs which are embedded in gelatinous material swells into a protective transparent jelly when comes in contact with water. The male discharges seminal fluid or milt containing spermatozoa over eggs to fertilize them. Thus fertilization is external. After fertilization, male releases his grip and leaves the female. The fertilized eggs are hatched into tadpoles which undergo metamorphosis to become adult terrestrial frogs.

4. ECONOMIC IMPORTANCE:
i. Importance to farmers.
Most frogs live on a diet of insects, and in many areas they help control populations of mosquitoes and crop-damaging insects.

ii. Frogs as food:
Frogs may be a food source for humans—the legs of one type of frog are considered a delicacy in many parts of Europe.

iii. In teaching and scientific research:
Adult frogs are often used to teach students about the anatomy and physiology of vertebrates, or animals that have a backbone. Frog eggs, meanwhile, help scientists learn about embryonic development.

iv. As bio-indicator:
Ecologists, scientists who study the Earth’s living ecosystems, are interested in frogs and other amphibians because these animals are considered bioindicators. This means that the health of frog populations is thought to reflect the health of the ecosystem as a whole.

In some lower vertebrates the Epiphysis Cerebri – Pineal Gland – has a well-developed eye-like structure; in others though not organized as an eye, it functions as a light receptor. In lower vertebrates, the pineal gland has an eye like structure and it functions as a light receptor and is considered by some to be the evolutionary forerunner of the modern eye.

B. EXTERNAL FEATURES
1. Shape and size:
The body of frog is bilaterally-symmetrical, somewhat spindle-shaped, pointed anteriorly and rounded posteriorly. It is slightly flattened dorso-ventrally and streamlined to swim through water. It can be divided into head, trunk and limbs. Neck is absent.
Size of adult Rana tigrina varies from 12 to 18cm in length and 5 to 8 cm in width.
2. Skin and colour:
The skin is moist, slimy and loosely fitted to the body. There are several dorso-lateral folds or thickening, present on the dorsal surface known as dermal plicae. A mid-dorsal line extends from the tip of snout to the cloacal region.
The colour of skin is green with black patches dorsally but lighter pale-yellow ventrally. Rana tigrina got its name due to mottled colouration like that of tiger.
3. Head
Head of frog is roughly triangular and somewhat flattened. It bears a short, blunt anterior snout, terminating into transverse mouth. There are two external nares or external nostrils lying dorsally above the mouth, at the tip of the snout. They serve in respiration.
• Eyes- Two very large, protruding eyes are situated dorsally on the top of the head.Each eye bears three eye lids, one upper eye-lid and two lower eye-lids. The upper eye-lid is pigmented, thick and almost immovable where as the lower eye-lid is semi-transparent and freely movable. The third transparent eye-lid or nictitating membrane covers and protects eye during swimming and keeps it moist in air. Frogs, like other vertebrates have monocular vision.
There is also a presence of median light coloured patch or brown spot just in front of eyes which represents vestigial pineal eye.
• Tymphanum- Below and behind each eye are flat and deeply pigmented circular patches known as ear drum or tymphanum which receives sound waves. External ears are lacking.
• Vocal Sacs- In male frogs, throat bears vocal sacs which act as resonator to intensify sound of croaking during breeding.
4. Trunk:
Head is joined behind to somewhat flattened trunk. Its back is raised in the middle in a characteristic hump or scaral prominence which becomes distinct especially when the frog is squatting. At the posterior end of the trunk is a small, circular cloacal aperture through which fecal matters, urine as well as reproductive products are discharged.
5. Limbs:
Laterally the the trunk bears two pairs of unequal limbs, anterior forelimb and posterior hind limb.
i. Forelimb:
The forelimbs are shorter and consist of three distinct parts, namely:
o the upper arm ( brachium),
o the fore arm (antebrachium),
o wrist( carpus )and
o hand (manus) bearing four digits without webs
In male frogs, the base of first inner finger is thickened, especially in breeding season, forming nuptial pad for clasping the female during amplexus.
ii. Hindlimb:
The hind limbs are much elongated and powerful than the forelimbs. Each hind limb consists of -
o thigh
o shank (crus),
o ankle(tarsus),
o large foot (pes) having 5 slender toes connected by webs which assists in swimming.

SEXUAL DIMORPHISM
Male – Female
1. Males are generally smaller and darker in colour. 1. Females are larger and lighter in colour.
2. Males are slimmer. 2. Females are stouter(fat)
3. Males croak loudly due to the presence of vocal sacs 3. Females do not croak as vocal sacs are absent in them.
4. Presence of swollen, copulatory or nuptial pad 4. Nuptial pads are lacking in females