How Do Amphibians Reproduce?

Sexual Reproduction

Amphibian Reproduction
At this stage, it breathes through gills and has a tail. Frog populations have declined significantly since the s. True toads completely lack teeth, but most frogs have them, specifically pedicellate teeth in which the crown is separated from the root by fibrous tissue. This results in the appearance, toward the lumen of each tubule, of clusters of mature spermatozoa. Three Cartoons — ".

Salamanders and Newts

Book - The Frog Its Reproduction and Development 3

From late summer until the spring breeding season the paired ovaries will fill the body cavity and will often distend the body wall. The mature eggs are highly pigmented on the surface of the animal pole, so that the ovary has a speckled appearance of black pigment and white yolk, representing the animal and the vegetal hemispheres of the eggs. There is no appreciable change in the size of the ovary during hibernation, nor is there any observable cytological change in the ova. However, if a female is forced to retain her eggs beyond the normal breeding period by isolating her from males or by keeping her in a warm environment and without food, the ova will begin to deteriorate cytolize within the ovary.

Immediately after the spring breeding season, when the female discharges thousands of mature ova, the remaining ovary with its oogonia to be developed for the following year is so small that it is sometimes difficult to locate.

There is no pigment in the tissue of the ovary in the stroma or in the immature ova , and each growing oocyte appears as a small white sphere of protoplasm contained within its individual follicle sac.

The histology of the ovary shows that within its outer peritoneal covering, the theca externa, are suspended thousands of individual sacs, each made up of another membrane, the theca interna or cyst wall, which contains smooth muscle fibers.

This theca interna is derived from the retro-peritoneal tissue. The smooth muscle fibers can be seen histologically and can be demonstrated physiologically. The theca interna surrounds each egg except for the limited area bulging toward the body cavity, where it is covered by only the theca externa. This is the region which will be ruptured during ovulation to allow the egg to escape its follicle into the body cavity.

The theca interna, plus the limited covering of the theca externa, and the follicle cells together comprise the ovarian follicle. These two membranes make up the rather limited ovarian stroma of the frog ovary, and they contain both blood vessels and nerves.

Within each follicle are found follicle cells, with their oval and granular nuclei, derived originally from oogonia.

These follicle cells surround the developing oocyte and are found in close association with it throughout those processes of maturation which occur within the follicle. Enclosed within the follicle cells, and closely applied to each mature egg, is the non-cellular and transparent vitelline membrane, probably derived from both the ovum and the follicle cells. This membrane is developed and applied to the egg during the maturation process so that it is not seen around the earlier or younger oogonia.

Since the bulk of the egg is yolk vitellus , this membrane is appropriately called the vitelline membrane. It is sometimes designated as the primary of several egg membranes. After the egg is fertilized this membrane becomes separated from the egg and the space between is then known as the perivitelline space, filled with a fluid. The fluid may be derived from the egg which would show compensatory shrinkage.

As the oocyte matures and enlarges, the follicle cells and membranes are so stretched and flattened that they are not easily distinguished. It is therefore best to study these structures in the immature ovary.

The egg will mature in any of a variety of positions within its follicle, the exact position probably depending upon the maximum blood supply.

As one examines an ovary the eggs will be seen in all possible positions, some with the animal hemisphere and others with the vegetal hemisphere toward the theca externa and body cavity.

It is believed that the most vascular side of the follicle wall will tend to produce the animal hemisphere of the egg, and hence give it its fundamental symmetry and polarity.

The frog's egg is essentially a large sac of yolk, the heavier and larger granules of which are concentrated at the vegetal pole. There is a thin outer layer of cytoplasm, more concentrated toward the animal hemisphere and in the vicinity of the germinal vesicle or immature nucleus. Surrounding the entire egg is a non-living surface coat, also containing pigment.

This pigment is presumably a metabolic byproduct. This coat is necessary for retaining the shape of the egg and in aiding in the morphogenetic processes of cleavage and gastrulation Holtfreter. Lateral to each ovary is a much-coiled oviduct suspended from the dorsal body wall by a double fold of peritoneum. Its anterior end is found between the heart and the lateral peritoneum, at the apex of the liver lobe.

At this anterior end is a slit-like infundibulum or ostium tuba with ciliated and highly elastic walls. The body cavity of the female is almost entirely lined with cilia, each cilium having its effective beat or stroke in the general direction of one of the ostia. These cilia are produced in response to an ovarian hormone and therefore are regarded as secondary sex characters.

They are found on the peritoneum covering the entire body cavity, on the liver, and on the pericardial membrane. There are no cilia on the lungs, the intestines, or the surface of the kidneys except in the ciliated peristomial peritoneal funnels which lead into the blood sinuses of the kidneys. The abundant supply of cilia of the female means that eggs ovulated from any surface of the ovary will be carried by constant ciliary currents anteriorly toward and into one or another of the ostia.

This can be demonstrated easily by opening the body cavity of an actively ovulating frog or by excising a strip of ventral abdominal wall of the adult female, inverting it in amphibian Ringer's solution, and placing on it some of the body cavity eggs. Any object of similar size or weight, such as pellets of paraffin, will be carried along by the ciliary currents in the original direction of the ostium.

These cilia function the year around, and will carry to the ostia any objects of approximately the size and weight of frog eggs that may be placed in the body cavity. One might suggest, therefore, that the oviducts may act as accessory excretory ducts, for certainly body cavity fluids must be similarly eliminated. As the egg leaves the ovary it is nude except for the non-living, transparent, and closely applied vitelline membrane.

Thus far it has been impossible to fertilize these body cavity eggs and have them develop. When they are placed in a sperm suspension some will show surface markings which resemble very closely the normal cleavage spindles and the cleavage furrows but none have developed as embryos as yet. These body cavity eggs are often quite distorted, due to the fact that the ovulation process involves a rupture of the follicle and forcing out of the egg from a very muscular follicle.

The egg is literally squeezed from the follicle, through a small aperture. The process looks like an Amoeba crawling through an inadequate hole. Ovulation rupture and emergence of the egg takes several minutes at laboratory temperatures, and is not accompanied by hemorrhage.

By the time the egg reaches the ostium within 2 hours , as the result of ciliary propulsion, it is again spherical. Ciliary currents alone force the egg into the ostium and oviduct. The ostial opening is very elastic and does not respond to the respiratory or heart activity, as some have described.

The eggs are simply forced into the ostium, from all angles, stretching its mouth open to accept the egg. As soon as the egg enters the oviduct and begins to acquire an albuminous mucin-jelly covering, it becomes fertilizable.

One can remove such an egg from the oviduct by pipette or by cutting the oviduct 1 inch or more from the ostium, and can fertilize such an egg in a normal sperm suspension. The physical or chemical changes which occur between the time the egg is in the body cavity and the time it is removed from the oviduct, which make it fertilizable, are not yet understood.

As the egg is propelled through the oviduct by ciliary currents, it receives coatings of albumen jelly. The initial coat is thin but of heavy consistency, and is applied closely to the egg. The egg is spiraled down the oviduct by its ciliated lining so that the application of the jelly covering is quite uniform.

There are, in all, three distinct layers of jelly, the outermost one being much the greater in thickness but the less viscous. The intermediate layer is of a thin and more fluid consistency.

There is hyperactivity of the glandular elements of the oviduct just before the normal breeding season, or after anterior pituitary hormone stimulation, so that the duct is enlarged several times over that of the oviduct of the hibernating female.

The presence of the jelly layers on the oviducal or the uterine egg is not readily apparent because it requires water before it reaches its maximum thickness. Eggs sectioned within the oviduct show the jelly as a transparent coating just outside the vitelline membrane. As soon as the egg reaches the water, however, imbibition swells the jelly until its thickness becomes greater than the diameter of the egg.

The function of the jelly is to protect the egg against injury, against ingestion by larger organisms, and from fungus and other infections. Equally important, however, is the evidence that this jelly helps the egg to retain its metabolically derived heat so that the jelly can be said to act as an insulator against heat loss. Bernard and Batuschek showed that the greater the wave length of light the less heat passed through the jelly around the frog's egg, in comparison with an equivalent amount of water and under similar conditions.

Passage of eggs through the oviduct. The eggs of the frog are greatly distorted as they pass down the oviduct toward the uterus. They accumulate albumen around them, but, since they spiral down the duct, the albumen jelly is evenly deposited and the eggs become spherical as the jelly swells when the eggs pass from the uterus into the water.

Oviducts of the frog under various states of sexual activity. A Post-ovulation condition, collapsed and dehydrated. B Actively ovulating condition, oviduct full of eggs, edematous. C Oviduct of non-ovulating, hibernating female. Originally, and erroneously, the jelly was thought to act as a lens which would concentrate the heat rays of the sun onto the egg, but since the jelly is largely water, which is a non-conductor of heat rays, this theory is untenable.

One can demonstrate that the temperature of the egg is higher than the temperature of the immediate environment, even in a totally darkened environment. So, the jelly has certain physical functions in addition to those as yet undetermined functions which aid in rendering the egg fertilizable. The egg takes about 2 to 4 hours, at ordinary temperatures, to reach the highly elastic uterus, at the posterior end of the oviduct and adjacent to the cloaca.

Each uterus has a separate opening into the cloaca, and the ovulated eggs are retained within this sac until, during amplexus sexual embrace by the male , they are expelled into the water and are fertilized by the male. Generally the eggs are not retained within the uterus for more than a day or so. There may be quite a few hours between the time of appearance of the first and the last eggs in the uteri.

The maturation process can best be described as it begins, immediately after the normal breeding season in the spring. At this time the ovary has been freed of its several thousand mature eggs and contains only oogonia with no pigment and little, if any, yolk. Even at this early stage each cluster of oogonia represents a future ovarian unit, consisting of many follicle cells and one ovum. There has been no way to determine which oogonium is to be selected for maturation into an ovum and which will give rise to the numerous follicle cells that act as nurse cells for the growing ovum.

It is clear, however, that both follicle cells and the ovum come from original oogonia. All ova develop from oogonia which divide repeatedly. These pre-maturation germ cells divide by mitosis many times and then come to rest, during which process there is growth of some of them without nuclear division.

These become ova while those that fail to grow become follicle cells. However, there are pre-prophase changes of the nucleus of the prospective ovum comparable to the pre-prophase changes in spermatogenesis. The majority of oogonia, therefore, never mature into ova, but become follicle cells. The process of maturation involves contributions from the nucleus and the cytoplasm. First, chromatin nucleoli aid in the synthesis of yolk, and second, the breakdown of the germinal vesicle allows an intermingling of the nuclear and the cytoplasmic components.

Only a small portion of the germinal vesicle is involved in the maturation spindle so that it may be at this time that the nucleus exerts its initial influence on the cytoplasm. All cytoplasmic differentiations must be initiated at a time when the hereditary influences of the nucleus are so intermingled with it. Growth Period to Primary Oocyte Stage.

Growth is achieved largely by the accumulation of yolk. As soon as growth begins the cell no longer divides by mitosis and is known as an oocyte rather than an oogonium. The growth process is aided by the centrosome, which is found to one side of the nucleus, and around which gather the granules or yolk platelets. The chromatin filaments become achromatic and the nucleoli increase in number, by fragmentation, and become more chromatic. Many of the nucleoli, which are concentrations of nucleo-protein, pass through the nuclear membrane into the surrounding cytoplasm during this period.

Such an ovary greatly enlarges. It attains black color with light yellow spots. Each oviduct is a long narrow and highly coiled tube. It is divided into three parts in accordance with its structure and functions. The anterior end of the oviduct forms a wide and fringed oviducal funnel. The ovoiducal funnel is located on the dorsal side of the lung. The margin and inner surface of the oviducal funnel is lined by ciliated epithelium.

The oviducal funnel leads into the oviduct. This oviduct is straight and thin-walled for a short distance. Thereafter it becomes highly coiled and thick-walled. This coiled oviduct runs posteriorly along the outer side of the kidney. The hinder portion of the oviduct becomes very thin walled.

It is sac-like and is called ovisac. The ovisac opens of the posterior end in the dorsal wall of the cloaca by its individual apertures lying anteriorly to the openings of ureters. The cloaca opens to the exterior by a cloacal aperture at the posterior end of the body. Females of many frog species, such as the common tree frog Polypedates leucomystax , reply to the male calls, which acts to reinforce reproductive activity in a breeding colony. The rationale for this is thought to be that by demonstrating his prowess, the male shows his fitness to produce superior offspring.

A different call is emitted by a male frog or unreceptive female when mounted by another male. This is a distinct chirruping sound and is accompanied by a vibration of the body.

All of these calls are emitted with the mouth of the frog closed. It is typically used when the frog has been grabbed by a predator and may serve to distract or disorientate the attacker so that it releases the frog. Many species of frog have deep calls. The croak of the American bullfrog Rana catesbiana is sometimes written as "jug o' rum". During extreme conditions, some frogs enter a state of torpor and remain inactive for months.

In colder regions, many species of frog hibernate in winter. Those that live on land such as the American toad Bufo americanus dig a burrow and make a hibernaculum in which to lie dormant.

Others, less proficient at digging, find a crevice or bury themselves in dead leaves. Aquatic species such as the American bullfrog Rana catesbeiana normally sink to the bottom of the pond where they lie, semi-immersed in mud but still able to access the oxygen dissolved in the water.

Their metabolism slows down and they live on their energy reserves. Some frogs can even survive being frozen. Ice crystals form under the skin and in the body cavity but the essential organs are protected from freezing by a high concentration of glucose. An apparently lifeless, frozen frog can resume respiration and the heart beat can restart when conditions warm up. At the other extreme, the striped burrowing frog Cyclorana alboguttata regularly aestivates during the hot, dry season in Australia, surviving in a dormant state without access to food and water for nine or ten months of the year.

It burrows underground and curls up inside a protective cocoon formed by its shed skin. Researchers at the University of Queensland have found that during aestivation, the metabolism of the frog is altered and the operational efficiency of the mitochondria is increased.

This means that the limited amount of energy available to the comatose frog is used in a more efficient manner. This survival mechanism is only useful to animals that remain completely unconscious for an extended period of time and whose energy requirements are low because they are cold-blooded and have no need to generate heat.

Different species of frog use a number of methods of moving around including jumping , running , walking , swimming , burrowing , climbing and gliding. Frogs are generally recognized as exceptional jumpers and, relative to their size, the best jumpers of all vertebrates. Within a species, jump distance increases with increasing size, but relative jumping distance body-lengths jumped decreases.

The Indian skipper frog Euphlyctis cyanophlyctis has the ability to leap out of the water from a position floating on the surface. Slow-motion photography shows that the muscles have passive flexibility. They are first stretched while the frog is still in the crouched position, then they are contracted before being stretched again to launch the frog into the air.

The fore legs are folded against the chest and the hind legs remain in the extended, streamlined position for the duration of the jump. When the muscles contract, the energy is first transferred into the stretched tendon which is wrapped around the ankle bone. Then the muscles stretch again at the same time as the tendon releases its energy like a catapult to produce a powerful acceleration beyond the limits of muscle-powered acceleration.

Frogs in the families Bufonidae, Rhinophrynidae , and Microhylidae have short back legs and tend to walk rather than jump. The Great Plains narrow-mouthed toad Gastrophryne olivacea has been described as having a gait that is "a combination of running and short hops that are usually only an inch or two in length".

By measuring the toad's uptake of oxygen it was found that hopping was an inefficient use of resources during sustained locomotion but was a useful strategy during short bursts of high-intensity activity. The red-legged running frog Kassina maculata has short, slim hind limbs unsuited to jumping. It can move fast by using a running gait in which the two hind legs are used alternately.

Slow-motion photography shows, unlike a horse that can trot or gallop, the frog's gait remained similar at slow, medium, and fast speeds. Frogs that live in or visit water have adaptations that improve their swimming abilities. The hind limbs are heavily muscled and strong. The webbing between the toes of the hind feet increases the area of the foot and helps propel the frog powerfully through the water.

Members of the family Pipidae are wholly aquatic and show the most marked specialization. They have inflexible vertebral columns, flattened, streamlined bodies, lateral line systems, and powerful hind limbs with large webbed feet.

Some frogs have become adapted for burrowing and a life underground. They tend to have rounded bodies, short limbs, small heads with bulging eyes, and hind feet adapted for excavation. An extreme example of this is the purple frog Nasikabatrachus sahyadrensis from southern India which feeds on termites and spends almost its whole life underground. It emerges briefly during the monsoon to mate and breed in temporary pools.

It has a tiny head with a pointed snout and a plump, rounded body. Because of this fossorial existence, it was first described in , being new to the scientific community at that time, although previously known to local people.

The spadefoot toads of North America are also adapted to underground life. The Plains spadefoot toad Spea bombifrons is typical and has a flap of keratinised bone attached to one of the metatarsals of the hind feet which it uses to dig itself backwards into the ground. As it digs, the toad wriggles its hips from side to side to sink into the loose soil.

It has a shallow burrow in the summer from which it emerges at night to forage. In winter, it digs much deeper and has been recorded at a depth of 4. During this time, urea accumulates in its tissues and water is drawn in from the surrounding damp soil by osmosis to supply the toad's needs. The burrowing frogs of Australia have a rather different lifestyle. The western spotted frog Heleioporus albopunctatus digs a burrow beside a river or in the bed of an ephemeral stream and regularly emerges to forage.

Mating takes place and eggs are laid in a foam nest inside the burrow. The eggs partially develop there, but do not hatch until they are submerged following heavy rainfall. The tadpoles then swim out into the open water and rapidly complete their development. One of these, the green burrowing frog Scaphiophryne marmorata , has a flattened head with a short snout and well-developed metatarsal tubercles on its hind feet to help with excavation.

It also has greatly enlarged terminal discs on its fore feet that help it to clamber around in bushes. Tree frogs live high in the canopy , where they scramble around on the branches, twigs, and leaves, sometimes never coming down to earth.

The "true" tree frogs belong to the family Hylidae, but members of other frog families have independently adopted an arboreal habit, a case of convergent evolution. These include the glass frogs Centrolenidae , the bush frogs Hyperoliidae , some of the narrow-mouthed frogs Microhylidae , and the shrub frogs Rhacophoridae.

The surface of the toe pads is formed from a closely packed layer of flat-topped, hexagonal epidermal cells separated by grooves into which glands secrete mucus. These toe pads, moistened by the mucus, provide the grip on any wet or dry surface, including glass.

The forces involved include boundary friction of the toe pad epidermis on the surface and also surface tension and viscosity. The reticulated leaf frog Phyllomedusa ayeaye has a single opposed digit on each fore foot and two opposed digits on its hind feet.

This allows it to grasp the stems of bushes as it clambers around in its riverside habitat. During the evolutionary history of frogs, several different groups have independently taken to the air. Typical of them is Wallace's flying frog Rhacophorus nigropalmatus from Malaysia and Borneo. It has large feet with the fingertips expanded into flat adhesive discs and the digits fully webbed.

Flaps of skin occur on the lateral margins of the limbs and across the tail region. With the digits splayed, the limbs outstretched, and these flaps spread, it can glide considerable distances, but is unable to undertake powered flight. Like other amphibians, the life cycle of a frog normally starts in water with an egg that hatches into a limbless larva with gills, commonly known as a tadpole.

After further growth, during which it develops limbs and lungs, the tadpole undergoes metamorphosis in which its appearance and internal organs are rearranged. After this it is able to leave the water as a miniature, air-breathing frog. Two main types of reproduction occur in frogs, prolonged breeding and explosive breeding.

In the former, adopted by the majority of species, adult frogs at certain times of year assemble at a pond, lake or stream to breed. Many frogs return to the bodies of water in which they developed as larvae.

This often results in annual migrations involving thousands of individuals. In explosive breeders, mature adult frogs arrive at breeding sites in response to certain trigger factors such as rainfall occurring in an arid area.

In these frogs, mating and spawning take place promptly and the speed of larval growth is rapid in order to make use of the ephemeral pools before they dry up. Among prolonged breeders, males usually arrive at the breeding site first and remain there for some time whereas females tend to arrive later and depart soon after they have spawned. This means that males outnumber females at the water's edge and defend territories from which they expel other males.

They advertise their presence by calling, often alternating their croaks with neighbouring frogs. Larger, stronger males tend to have deeper calls and maintain higher quality territories.

Females select their mates at least partly on the basis of the depth of their voice. They may intercept females that are approaching a calling male or take over a vacated territory. Calling is an energy-sapping activity. Sometimes the two roles are reversed and a calling male gives up its territory and becomes a satellite. In explosive breeders, the first male that finds a suitable breeding location, such as a temporary pool, calls loudly and other frogs of both sexes converge on the pool.

Explosive breeders tend to call in unison creating a chorus that can be heard from far away. The spadefoot toads Scaphiopus spp. Mate selection and courtship is not as important as speed in reproduction. In some years, suitable conditions may not occur and the frogs may go for two or more years without breeding. At the breeding site, the male mounts the female and grips her tightly round the body. Typically, amplexus takes place in the water, the female releases her eggs and the male covers them with sperm; fertilization is external.

In many species such as the Great Plains toad Bufo cognatus , the male restrains the eggs with his back feet, holding them in place for about three minutes. In these species, fertilization is internal and females give birth to fully developed juvenile frogs, except L.

Frogs' embryos are typically surrounded by several layers of gelatinous material. When several eggs are clumped together, they are collectively known as frogspawn. The jelly provides support and protection while allowing the passage of oxygen, carbon dioxide and ammonia. It absorbs moisture and swells on contact with water. After fertilization, the innermost portion liquifies to allow free movement of the developing embryo.

In certain species, such as the Northern red-legged frog Rana aurora and the wood frog Rana sylvatica , symbiotic unicellular green algae are present in the gelatinous material.

It is thought that these may benefit the developing larvae by providing them with extra oxygen through photosynthesis. The shape and size of the egg mass is characteristic of the species. Ranids tend to produce globular clusters containing large numbers of eggs whereas bufonids produce long, cylindrical strings. The tiny yellow-striped pygmy eleuth Eleutherodactylus limbatus lays eggs singly, burying them in moist soil.

The eggs hatch when the nest is flooded, or the tadpoles may complete their development in the foam if flooding does not occur. Aquatic eggs normally hatch within one week when the capsule splits as a result of enzymes released by the developing larvae. The larvae that emerge from the eggs, known as tadpoles or occasionally polliwogs , typically have oval bodies and long, vertically flattened tails. As a general rule, free-living larvae are fully aquatic, but at least one species Nannophrys ceylonensis has semiterrestrial tadpoles which live among wet rocks.

From early in its development, a gill pouch covers the tadpole's gills and front legs. The lungs soon start to develop and are used as an accessory breathing organ. Some species go through metamorphosis while still inside the egg and hatch directly into small frogs. Tadpoles lack true teeth, but the jaws in most species have two elongated, parallel rows of small, keratinized structures called keradonts in their upper jaws.

Their lower jaws usually have three rows of keradonts surrounded by a horny beak, but the number of rows can vary and the exact arrangements of mouth parts provide a means for species identification. This has been suggested as an adaptation to their lifestyles; because the transformation into frogs happens very fast, the tail is made of soft tissue only, as bone and cartilage take a much longer time to be broken down and absorbed.

The tail fin and tip is fragile and will easily tear, which is seen as an adaptation to escape from predators which tries to grasp them by the tail.

Tadpoles are typically herbivorous , feeding mostly on algae , including diatoms filtered from the water through the gills. Some species are carnivorous at the tadpole stage, eating insects, smaller tadpoles, and fish. The Cuban tree frog Osteopilus septentrionalis is one of a number of species in which the tadpoles can be cannibalistic. Tadpoles that develop legs early may be eaten by the others, so late developers may have better long-term survival prospects. Tadpoles are highly vulnerable to being eaten by fish, newts , predatory diving beetles , and birds, such as kingfishers.

Some tadpoles, including those of the cane toad Bufo marinus , are poisonous. The tadpole stage may be as short as a week in explosive breeders or it may last through one or more winters followed by metamorphosis in the spring.

At the end of the tadpole stage, a frog undergoes metamorphosis in which its body makes a sudden transition into the adult form. This metamorphosis typically lasts only 24 hours, and is initiated by production of the hormone thyroxine.

This causes different tissues to develop in different ways. The principal changes that take place include the development of the lungs and the disappearance of the gills and gill pouch, making the front legs visible. The lower jaw transforms into the big mandible of the carnivorous adult, and the long, spiral gut of the herbivorous tadpole is replaced by the typical short gut of a predator. The eardrum, middle ear, and inner ear are developed. The skin becomes thicker and tougher, the lateral line system is lost, and skin glands are developed.

At this time, the tail is being lost and locomotion by means of limbs is only just becoming established. After metamorphosis, young adults may disperse into terrestrial habitats or continue to live in water. Almost all frog species are carnivorous as adults, preying on invertebrates, including arthropods , worms , snails , and slugs.

A few of the larger ones may eat other frogs, small mammals , and fish. Some frogs use their sticky tongues to catch fast-moving prey, while others push food into their mouths with their hands. A few species also eat plant matter; the tree frog Xenohyla truncata is partly herbivorous, its diet including a large proportion of fruit, [] Leptodactylus mystaceus has been found to eat plants, [] [] and folivory occurs in Euphlyctis hexadactylus , with plants constituting The northern leopard frog Rana pipiens is eaten by herons , hawks , fish, large salamanders , snakes , raccoons , skunks , mink , bullfrogs, and other animals.

Frogs are primary predators and an important part of the food web. Being cold-blooded , they make efficient use of the food they eat with little energy being used for metabolic processes, while the rest is transformed into biomass.

They are themselves eaten by secondary predators and are the primary terrestrial consumers of invertebrates, most of which feed on plants. By reducing herbivory, they play a part in increasing the growth of plants and are thus part of a delicately balanced ecosystem. Little is known about the longevity of frogs and toads in the wild, but some can live for many years.

Skeletochronology is a method of examining bones to determine age. Using this method, the ages of mountain yellow-legged frogs Rana muscosa were studied, the phalanges of the toes showing seasonal lines where growth slows in winter. The oldest frogs had ten bands, so their age was believed to be 14 years, including the four-year tadpole stage.

The cane toad Bufo marinus has been known to survive 24 years in captivity, and the American bullfrog Rana catesbeiana 14 years. Those that breed in smaller water bodies tend to have greater and more complex parental care behaviour. Once this happened, the desiccating terrestrial environment demands that one or both parents keep them moist to ensure their survival. In small pools, predators are mostly absent and competition between tadpoles becomes the variable that constrains their survival.

Certain frog species avoid this competition by making use of smaller phytotelmata water-filled leaf axils or small woody cavities as sites for depositing a few tadpoles.

Frog species that changed from the use of larger to smaller phytotelmata have evolved a strategy of providing their offspring with nutritive but unfertilized eggs.

The male frog guards them from predation and carries water in his cloaca to keep them moist. When they hatch, the female moves the tadpoles on her back to a water-holding bromeliad or other similar water body, depositing just one in each location.

She visits them regularly and feeds them by laying one or two unfertilized eggs in the phytotelma, continuing to do this until the young are large enough to undergo metamorphosis. Many other diverse forms of parental care are seen in frogs.

The tiny male Colostethus subpunctatus stands guard over his egg cluster, laid under a stone or log. When the eggs hatch, he transports the tadpoles on his back to a temporary pool, where he partially immerses himself in the water and one or more tadpoles drop off. He then moves on to another pool. He keeps them damp in dry weather by immersing himself in a pond, and prevents them from getting too wet in soggy vegetation by raising his hindquarters.

After three to six weeks, he travels to a pond and the eggs hatch into tadpoles. The foam is made from proteins and lectins , and seems to have antimicrobial properties. The eggs are laid in the centre, followed by alternate layers of foam and eggs, finishing with a foam capping. Some frogs protect their offspring inside their own bodies. Both male and female pouched frogs Assa darlingtoni guard their eggs, which are laid on the ground.

When the eggs hatch, the male lubricates his body with the jelly surrounding them and immerses himself in the egg mass. The tadpoles wriggle into skin pouches on his side, where they develop until they metamorphose into juvenile frogs. She ceases to feed and stops secreting stomach acid. The tadpoles rely on the yolks of the eggs for nourishment.

After six or seven weeks, they are ready for metamorphosis. The mother regurgitates the tiny frogs, which hop away from her mouth. When the tadpoles are about to hatch, they are engulfed by the male, which carries them around inside his much-enlarged vocal sac. Here they are immersed in a frothy, viscous liquid that contains some nourishment to supplement what they obtain from the yolks of the eggs.

They remain in the sac for seven to ten weeks before undergoing metamorphosis, after which they move into the male's mouth and emerge. At first sight, frogs seem rather defenceless because of their small size, slow movement, thin skin, and lack of defensive structures, such as spines, claws or teeth. Many use camouflage to avoid detection, the skin often being spotted or streaked in neutral colours that allow a stationary frog to merge into its surroundings.

Some can make prodigious leaps, often into water, that help them to evade potential attackers, while many have other defensive adaptations and strategies. The skin of many frogs contains mild toxic substances called bufotoxins to make them unpalatable to potential predators.

Most toads and some frogs have large poison glands, the parotoid glands , located on the sides of their heads behind the eyes and other glands elsewhere on their bodies. These glands secrete mucus and a range of toxins that make frogs slippery to hold and distasteful or poisonous. If the noxious effect is immediate, the predator may cease its action and the frog may escape. If the effect develops more slowly, the predator may learn to avoid that species in future.

The poison dart frogs in the family Dendrobatidae do this. They are typically red, orange, or yellow, often with contrasting black markings on their bodies.

Allobates zaparo is not poisonous, but mimics the appearance of two different toxic species with which it shares a common range in an effort to deceive predators. They "flash" this when attacked, adopting a pose that exposes the vivid colouring on their bellies. Some frogs, such as the poison dart frogs , are especially toxic.