Amphibian Reproduction

Highlights

How Do Amphibians Reproduce?
The common features possessed by these proto-frogs include 14 presacral vertebrae modern frogs have eight or 9 , a long and forward-sloping ilium in the pelvis , the presence of a frontoparietal bone , and a lower jaw without teeth. Publisher Summary The female reproductive system of amphibians, as in most vertebrates, consists of paired ovaries and oviducts. Responding to sex stimulation, the spermatozoa become free from their Sertoli cells and are forced from the lumen of the seminiferous tubule into the related collecting tubule. This page was last modified on 12 April , at This duct emerges from the kidneys and finally opens into the cloaca.

Hormones and Reproduction of Vertebrates

The Variety of Amphibian Reproduction

Select a subject to preview related courses: Caecilians Caecilians are sometimes called blindworms; they are a legless amphibian that resembles a worm. Caecilians look a bit like worms. Lesson Summary We've learned that there are four groups of amphibians - toads, frogs, salamanders and newts, and caecilians, and each of them uses a slightly different method to reproduce.

Salamanders and newts use a spermatophore so that eggs can be fertilized internally without sexual intercourse. Toads and frogs use the amplexus position to signal the male and female to simultaneously release eggs and sperm so external fertilization can take place. Caecilians use internal fertilization and often give live birth. Register to view this lesson Are you a student or a teacher?

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Vork holds a bachelor's degree in music performance from St. By Lauren Vork; Updated April 24, Sexual Reproduction Amphibian reproduction has more in common with that of fish than it does of mammals or even reptiles. What Is a Horned Frog? How Do Amphibians Reproduce? 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.

The native peoples of South America extract poison from these frogs to apply to their weapons for hunting, [] although few species are toxic enough to be used for this purpose. At least two non-poisonous frog species in tropical America Eleutherodactylus gaigei and Lithodytes lineatus mimic the colouration of dart poison frogs for self-protection.

Many predators of frogs have become adapted to tolerate high levels of these poisons, but other creatures, including humans who handle the frogs, may be severely affected. Some frogs use bluff or deception. The European common toad Bufo bufo adopts a characteristic stance when attacked, inflating its body and standing with its hindquarters raised and its head lowered. This places the parotoid glands in the most effective position, the other glands on its back begin to ooze noxious secretions and the most vulnerable parts of its body are protected.

The gray tree frog Hyla versicolor makes an explosive sound that sometimes repels the shrew Blarina brevicauda. The strategy employed by juvenile American toads Bufo americanus on being approached by a snake is to crouch down and remain immobile. This is usually successful, with the snake passing by and the toad remaining undetected. If it is encountered by the snake's head, however, the toad hops away before crouching defensively. Frogs live on all the continents except Antarctica, but they are not present on certain islands, especially those far away from continental land masses.

Members of the Australian genus Cyclorana bury themselves underground where they create a water-impervious cocoon in which to aestivate during dry periods. Once it rains, they emerge, find a temporary pool, and breed. Egg and tadpole development is very fast in comparison to those of most other frogs, so breeding can be completed before the pond dries up.

The wood frog Rana sylvatica , whose habitat extends into the Arctic Circle , buries itself in the ground during winter. Although much of its body freezes during this time, it maintains a high concentration of glucose in its vital organs, which protects them from damage. In , of 4, species of amphibians that depend on water during some lifecycle stage, 1, This is likely to be an underestimate because it excludes 1, species for which evidence was insufficient to assess their status.

More than one-third of frog species are considered to be threatened with extinction , and more than species are believed to have become extinct since the s. The latter is of particular concern to scientists because it inhabited the pristine Monteverde Cloud Forest Reserve and its population crashed in , along with about 20 other frog species in the area.

This could not be linked directly to human activities, such as deforestation, and was outside the range of normal fluctuations in population size. Many environmental scientists believe amphibians, including frogs, are good biological indicators of broader ecosystem health because of their intermediate positions in food chains, their permeable skins, and typically biphasic lives aquatic larvae and terrestrial adults. Frog mutations and genetic defects have increased since the s.

These often include missing legs or extra legs. Various causes have been identified or hypothesized, including an increase in ultraviolet radiation affecting the spawn on the surface of ponds, chemical contamination from pesticides and fertilizers, and parasites such as the trematode Ribeiroia ondatrae.

Probably all these are involved in a complex way as stressors , environmental factors contributing to rates of disease, and vulnerability to attack by parasites. Malformations impair mobility and the individuals may not survive to adulthood. An increase in the number of frogs eaten by birds may actually increase the likelihood of parasitism of other frogs, because the trematode's complex lifecycle includes the ramshorn snail and several intermediate hosts such as birds.

In a few cases, captive breeding programs have been established and have largely been successful. The cane toad Bufo marinus is a very adaptable species native to South and Central America. In the s, it was introduced into Puerto Rico, and later various other islands in the Pacific and Caribbean region, as a biological pest control agent.

Initial results in many of these countries were positive, but it later became apparent that the toads upset the ecological balance in their new environments.

They bred freely, competed with native frog species, ate bees and other harmless native invertebrates, had few predators in their adopted habitats, and poisoned pets, carnivorous birds, and mammals.

In many of these countries, they are now regarded both as pests and invasive species , and scientists are looking for a biological method to control them. Frog legs are eaten by humans in many parts of the world. French cuisses de grenouille or frog legs dish is a traditional dish particularly served in the region of the Dombes département of Ain. Chinese edible frog and pig frogs are farmed and consumed on a large scale in some areas of China.

Frog legs are part of Chinese Sichuan and Cantonese cuisine. In Indonesia , frog-leg soup is known as swikee or swike. Indonesia is the world's largest exporter of frog meat, exporting more than 5, tonnes of frog meat each year, mostly to France, Belgium and Luxembourg. Originally, they were supplied from local wild populations, but overexploitation led to a diminution in the supply.

This resulted in the development of frog farming and a global trade in frogs.

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