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Frogs – How Amazing Are They?
All of us are familiar with them. They are easily available in the ponds, streams, lakes etc. during the rainy season their croaking sounds can be easily heard which disturbs our peace. We are talking about the frogs. Frogs belong to the phylum Chordata and class Amphibia. They have the potential to live on water as well as on land. They are anuran which means that they are tailless and belong to the order Anura. Majority of the frogs possess long hind limbs, webbed digits, short body, protruding eyes and no tail. Frogs have great powers of jumping from here and there which is governed by their short body and long hind legs which help in performing jumping. They have a permeable skin and are semi-aquatic, prefer to live in the humid areas but can easily walk on land. They are oviparous and lay their eggs in ponds and lakes. The larvae that hatch out of the eggs are called tadpoles which are characterized by the presence of gills and they complete their development in water. Adult frogs are carnivorous and feed on gastropods, annelids and arthropods. They are easily noticed during their mating season by their croaking sound.
Frogs are easily available in the tropic and subarctic regions but majority of the species are found in the tropical rain forests. They form the diverse group of vertebrates and about 5,000 species are known till present. Some species are however on the verge of extinction. Frogs are different from the toads as the toads prefer to live in dry habitats and this distinction is made on the basis of convergent evolution. The word frog has been derived from an Old English word frogga which means ‘to jump’.
The order Anura encompasses about 4,810 species under 33 families out of which Leptodactylidae includes 1100 species, Hylidae includes 800 species and Ranidae includes 750 species. 88% of the amphibian species are the frogs. All the members of Anura are frogs but those belonging to the family Bufonidae are considered as true toads. The skin of the frogs is moist and smooth while that of the toads is dry and warty except the fire bellied toad Bombina bombina whose skin is slightly warty. Frogs and toads are broadly classified under three suborders. The first one is Archaeobatrachia, which comprises 4 families of primitive frogs. Second is Mesobatrachia, which includes 5 families of more evolutionary intermediate frogs and the third one is Neobatrachia, which is the largest group and contains the remaining 24 families of “modern” frogs,found throughout the world. Neobatrachia is further divided into the Hyloidea and Ranoidea. Some species of frogs can be hybridized easily for example the common edible frog Rana esculenta is the hybrid of Pool frog R.lessonae and the Marsh frog R.ridibunda.
Morphology and Physiology
Frogs are quite peculiar among the amphibians. Among the two groups of amphibians, the salamanders and the caecilians they are unusual as the adults lack tail and their hind legs are adapted more to jumping as compared to walking. Their physiology is similar to other amphibians as oxygen can easily penetrate through their moist skin. This feature enables them to breathe through their skins. The oxygen is easily dissolved in the moist layer of the skin in the form of a film and is passed to the blood from where it is transported later on. The frogs always keep their skin moist. This mode of respiration also makes frogs susceptible to the toxins dissolved in water and is responsible for decline of population of certain frog species. Frogs have long hind legs with elongated ankle bones. Their vertebral column is also short comprising of about ten vertebrae followed by a fused tail bone called as coccyx.
The body size of frogs range from 10 mm to 300 mm. the skin loosely covers the body as there is no connective tissue. The texture of the skin may be smooth, warty or folded. They have three eyelids. One is transparent which protects the eyes underwater and the two are translucent to opaque. They have tympanum on either side of the head which helps in hearing. They have pedicellate teeth. Most frogs have teeth only on the upper jaw and vomerine teeth on the roof of their mouth. The teeth are basically for holding the prey in place from where it is swallowed.
Feet and Legs
The legs and feet vary in structure depending upon the habitat of the frogs whether they live on trees, ground, water and in burrows. The frogs are supposed to be very quick to capture their prey and escape from their predators and must be suitably adapted to their environment. Frogs those living in water have webbed toes but the degree of toes to be webbed is directly proportional to the time spent in water by the frog. The African dwarf frog has completely webbed toes as it is completely aquatic whereas White’s tree frog has half webbed toes. The frogs adapted to arboreal mode of life have toe pads located on the ends of the toes for gripping on the vertical surfaces. The pads are made up of interlocking cells that have very small gap between the adjacent surfaces. When the frog applies pressure on the pads the interlocking cells help in gripping the irregular surface. The small gaps between the cells help in gripping by capillary action and this also allows the frog for maintain grip on the smooth surface also. The ground dwelling frogs lack all the adaptations that are present in the aquatic ad arboreal frogs. They have very small toe pads and very little or no webbing at all. Some burrowing frogs have a toe extension called the metatarsal tubercle which helps in burrowing.
Frogs are generally considered as exceptional jumper but perhaps the best jumpers of all the vertebrates. The Australian rocket frog can leap about 50 times higher than its body length resulting in jumps of about 2 meters. The acceleration of the jump may be twice as that of gravity. The capability of jump, distance of jump varies with the species. The musculo-skeletal morphology is highly modified for performing the jumping activity. The bones tibia, fibula and the tarsals are fused into a single strong bone just like the radius and ulna in the forelimbs. The metatarsals are also elongated and help in jumping. The ilium is fused with the sacrum and aids in jumping especially in the Ranids and Hylids and functions as the power provider for jumping. The elongation of limbs is the basic factor which governs the height as well as the distance of the jump.
The muscular system is also highly modified according to the habitat of frogs. In modern frogs almost all muscles are involved in performing the jumping movements and only few are involved in bringing back the body to its original posture. The muscles are also greatly enlarged in size and the muscles that are involved in jumping form about 17% of the body mass of the frog. Presently it is hypothesized that the frogs store their energy by stretching their tendons and use it like spring and release the store energy in one stroke only.
Many frogs are reported to store water and oxygen through their skin near the pelvic area. However, the permeable skin is also responsible for water loss. Some tree frogs have a waterproof layer over their skins which help in reducing the water loss. Other frogs also minimize water loss by becoming nocturnal, living under the shades during the daytime and coming out in night for capturing the prey. Some frogs rest in such a way that their skins are in direct contact with each other and this helps in reducing water loss.
Majority of the frog species use camouflage as their prime defensive weapon. Most camouflaged frogs are nocturnal in habit which adds a good source of their survival from the predators. Some frogs have the ability to change colour but they are capable of acquiring shades of only one or two colours like the White’s tree frogs vary in shades of green and brown. Warts and skin folds are generally found in the ground dwelling frogs and the arboreal frogs have smooth skin as they can easily cover themselves under the leaves. Some forgs change their colour during the day and the night time as the light and moisture content helps in contraction and expansion of the pigment cells.
Many frogs bear mild poisons that make them unpalatable by their predators. All the toads have poison gland called the parotid gland located behind the eyes on top of the heads. Some poison dart frogs are completely toxic. The chemicals present the toxins of the frogs may vary from irritants to hallucinogens, convulsants, nerve poisons and vasoconstrictors. Many predators of frogs are capable of protecting themselves from the poisons of frogs. Others including human beings may be severely affected by the toxins of frog. Some species of frogs obtain poison from the ants and the arthropods they eat and produce an alkaloid that is toxic and this alkaloid is not derived from the food they eat. Some native people of South America extract poison from the poison dart frogs also. The alkaloid epibatidine, a painkiller which is derived from the poison dart frogs is 200 times more potent than morphine. Other chemicals that have been isolated from the frogs are thought to be useful against the treatment of HIV infection. The dart and arrow poisons are under investigation for use in therapeutic drugs. The skin secretions of some toads contain bufotoxins especially the bufotenin is psychoactive.
Respiration and Circulation
The skin of the frog allows entry of oxygen as well as carbon dioxide. The skin houses large number of blood vessels. When the frog is underwater the oxygen is absorbed through the skin and is transported directly to the bloodstream. On land lungs are used for respiration. The lungs resemble human lungs but the chest muscles are not involved in respiration an also the frogs lack ribs and diaphragm. While breathing through lungs the frogs take air through the nostrils and compress it at the floor of the mouth and then the compressed air is sent to the lungs.
The heart of frogs is three chambered. In the three chambered heart the oxygenated blood from the lungs and the deoxygenated blood from the tissues enters the atria which are separate and which later on enters the aorta by spiral valve. The blood finally enters the pulmonary artery. The spiral valve prevents the mixing of two types of blood and enables the frog to maintain higher metabolic rates and remain active. Some frogs are even able to survive in oxygen deficient water also.
Digestion and Excretion
The digestive system of frogs begins with the mouth. The upper jaw bears teeth called the maxillary teeth which are used for grinding the food before swallowing. These teeth are very weak and cannot be used for catching the prey. The frogs use their sticky tongues for catching the prey. The food after grinding enters the stomach through esophagus. The food then enters the small intestine where most of the digestion takes place. The pancreas secretes pancreatic juice and the liver produces bile which enters the intestine and mixes with the food and thus the frog extracts nutrients. The food after digestion enters the large intestine where water is absorbed and then the waste products are passed towards the cloaca. All the waste products leave the body through cloaca and cloacal vent.
The fogs bear a highly developed nervous system consisting of brain, spinal cords and nerves. Many parts of the brain resemble with that of the humans. The medulla oblongata is responsible for controlling the activities of digestion, respiration and other automatic functions. Cerebellum controls the muscle coordination and body posture. The relative size of cerebrum is smaller in comparison to that of the humans. There are ten cranial nerves and ten pairs of spinal nerves. The ear has semicircular canals that are responsible for balancing and orientation.
The life stages of frogs are similar to that of other amphibians including egg, tadpole, metamorphosis and adult. The breeding behaviour of frogs is very interesting as they produce a mating call in order to attract the female. After mating the female lays thousands of eggs wrapped in a gelatinous covering in water. The eggs are highly vulnerable to predation so frogs use a number of diverse techniques in order to save their progeny from predators. In the colder climate the embryo is black in colour in order to absorb more heat from the sun which fastens the development of the frog. Some frogs lay their eggs in small packets of leaves which they make in order to save them from the predators and provide necessary moisture to them. In some frogs the tadpoles after hatching directly jump into water. The eggs hatch into tiny tadpoles which have oval bodies and vertically flattened tails. Only one species have semi-terrestrial tadpoles. The tadpoles are completely aquatic and they lack eyelids, lungs, front and hind legs and possess a cartilaginous skeleton. Like fishes the tadpoles also have lateral line system, gills for respiration and tails with dorsal and ventral folds of skin foe swimming. Some species which hatch from the eggs as tiny frogs do not have gills. Tadpoles also lack true teeth but they have keratinized structures in their upper jaws called keradonts and the lower jaw also bears three keradonts surrounded by a horny beak.
The tadpoles feed on algae and diatoms which are filtered by the gills indicating that they are herbivorous. Some species are carnivorous and also show cannibalism. Tadpoles are at the risk of being easily caught by the fishes, birds especially the kingfishers, newts and predatory diving beetles. Tadpoles of some species are also toxic to their predators. Some tadpoles have the capability of overwintering and then metamorphosing in the next year. The tail of the tadpole also contains some vertebral structures that give rise to the urostyle in later part of the life and the tail also lacks solid segmental structures but it bears a notochord. When fully developed the tadpole undergoes metamorphosis which includes transition from larval stage to the adult form. Metamorphosis includes many drastic changes that help the tadpole to develop into an adult. These changes include development of fore and hind limbs, loss of gills and development of lungs and loss of tails. The length of the intestine also shortens as they transform their feeding habit from herbivorous to carnivorous. The eyes also migrate rostrally and dorsally and are adapted for the binocular vision as found in the adult frog. The final stage of transformation of a tadpole to the adult frog involves apoptosis which means programmed cell death and loss of tail.
After completing metamorphosis the frogs leave water and start their terrestrial lives and if they are aquatic then they may live in the water as well. Almost all species of frogs especially the adults are carnivorous feeding on gastropods, arthropods and annelids. Frogs are also consumed by many mammals and birds. Frogs are also consumed by humans. The average life span of frogs is about 40 years and some species especially those of the temperate region are also known to hibernate during the winters.
When the frogs reach maturity they assemble near a water source which may be a pond of stream. Some also reach back to the places where they were born. During the breeding season the male frog calls a female by a special sound called the mating call or croaking and it becomes a chorus used by the males to attract the females towards themselves. The mating call is unique for every species. The females do not croak at all. After that the male and female undergo amplexus where the male tightly holds the female with his forelegs. The fertilization is external the female releases the eggs outside her body and the male soon releases a sperm solution over them resulting in the swelling of the eggs which attain a protective covering. The eggs are brown or black with a clear gelatinous covering. Most temperate species reproduce between the late autumn and early spring. Reproducing in the early season ensures better availability of food as well as faster development of the tadpoles.
20% species of frogs show parental care and this topic needs further concern. Some species of poison dart frogs lay their eggs on the forest floor and protect them by continually guarding them and also urinate on them to keep them moist. After hatching the parent carries the eggs to a water holding bromeliad and feeds them on unfertilized eggs. Some frogs protect their eggs by keeping them inside their bodies. Some frogs keep their eggs inside their vocal sacs and release the tadpoles after hatching of the eggs.
The sound of some species is so loud that they can be heard from miles away. The call of a frog is unique for a species. Frogs call by passing air through the larynx in the throat which is then amplified by the vocal sacs. Some species lack vocal sacs but can produce loud calls as they have greatly enlarged buccal cavity acting as the resonance chamber that amplifies their call. The species of frog that lack vocal sacs and are not able to produce large calls gather near the flowing water and communicate by other means. The main reason of producing a call is to attract a female by the male. Males either call singly or in groups. All calls are produced by the males by keeping their mouths closed. When frogs are in danger they also produce a distress call by keeping their mouths open resulting in high pitch sound.
Frogs are distributed all over the world except Antarctica and many oceanic islands. The diverse population of frogs is present in the tropical areas as plenty of food is available and the temperature range is suitable according to their need and also they are capable of defending themselves. The population of frogs has declined since 1950s resulting in complete extinction of 120 species and one third has joined the category of threatened species by 1980s. Habitat loss, change in climate conditions, pollution, infectious diseases including chytridiomycosis. Many environmental scientists believe that frogs are good biological indicators as they participate actively in the food chain as well as in the food web. Many programmes including the conservation of frogs have been started since 2007. They have also been shifted to zoos and aquarium in some countries and the year 2008 has been declared as the Year of the Frog. The earliest frogs evolved in the Jurassic (188-213 million years ago). The major evolutionary changes include shortening of the length of body and loss of tail. The fossils of frogs have been recorded all over the world except Antarctica.
Frogs play a very important role from commercial point of view. They are a source of food as frog legs are a source of delicacy in China, Philippines and France. Dead frogs are also used for dissection in the schools and colleges for studying the anatomy and physiology but this practice has been banned as the population of frogs is declining. Frogs are also used as model organisms since the history of science. Galvani discovered the relationship between electricity and nervous system by studying the physiology of frogs. The human chorionic gonadotropin found in substantial quantities in urine of the pregnant females when injected into the frog Xenopus laevis induce her to lay eggs. Robert Briggs and Thomas King cloned a frog by the technique of somatic cell nuclear transfer in 1952 and later on through this technique the sheep Dolly was cloned. Frogs are used in the research of embryology as they resemble very closely with that of the development of human embryo. Xenopus laevis is a very efficient experimental frog used in the concepts of human pregnancy. The genome sequence of X. leavis is under study and will be completed latest by 2015.
Frogs have also been a part of fairy tales from a long time as they are ugly, clumsy and are considered very talent. The Moche people of Peru are famous for worshiping frogs. We can conclude that frogs are very important in various fields of our life.
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