The size of the ovary varies with the seasons more than does the size of the testis. Contributors Badam Smith WebGuru. Frogs may leave fertilized eggs at the fertilization site like toads, but they may also bury them, or one of the adults will carry them on their body until they hatch. It is thought that these may benefit the developing larvae by providing them with extra oxygen through photosynthesis. The Frog and Lizard". For Rana pipiens this period of most active feeding comes during the summer when the natural foods, insects, worms, etc. The Origin of the Universe and
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. The mountain chicken frog , so-called as it tastes of chicken is now endangered, in part due to human consumption and was a major food choice of the Dominicans.
Coon, possum, partridges, prairie hen, and frogs were among the fare Mark Twain recorded as part of American cuisine. Frogs are used for dissections in high school and university anatomy classes, often first being injected with coloured substances to enhance contrasts among the biological systems. This practice is declining due to animal welfare concerns, and "digital frogs" are now available for virtual dissection.
Frogs have served as experimental animals throughout the history of science. Eighteenth-century biologist Luigi Galvani discovered the link between electricity and the nervous system by studying frogs. Stannius used a frog's heart in a procedure called a Stannius ligature to demonstrate the ventricle and atria beat independently of each other and at different rates. A sample of urine from a pregnant woman injected into a female frog induces it to lay eggs , a discovery made by English zoologist Lancelot Hogben.
This is because a hormone, human chorionic gonadotropin , is present in substantial quantities in the urine of women during pregnancy. King cloned a frog by somatic cell nuclear transfer. This same technique was later used to create Dolly the sheep , and their experiment was the first time a successful nuclear transplantation had been accomplished in higher animals. Frogs are used in cloning research and other branches of embryology. Although alternative pregnancy tests have been developed, biologists continue to use Xenopus as a model organism in developmental biology because their embryos are large and easy to manipulate, they are readily obtainable, and can easily be kept in the laboratory.
The genome of X. Because frog toxins are extraordinarily diverse, they have raised the interest of biochemists as a "natural pharmacy". The alkaloid epibatidine , a painkiller times more potent than morphine is made by some species of poison dart frogs, although it can also cause death by lung paralysis. Other chemicals isolated from the skins of frogs may offer resistance to HIV infection.
It has long been suspected that pre-Columbian Mesoamericans used a toxic secretion produced by the cane toad as a hallucinogen , but more likely they used substances secreted by the Colorado River toad Bufo alvarius.
These contain bufotenin 5-MeO-DMT , a psychoactive compound that has been used in modern times as a recreational drug. Typically, the skin secretions are dried and then smoked.
Exudations from the skin of the golden poison frog Phyllobates terribilis are traditionally used by native Colombians to poison the darts they use for hunting.
The tip of the projectile is rubbed over the back of the frog and the dart is launched from a blowgun. The combination of the two alkaloid toxins batrachotoxin and homobatrachotoxin is so powerful, one frog contains enough poison to kill an estimated 22, mice. These are less toxic and less abundant than the golden poison frog.
They are impaled on pointed sticks and may be heated over a fire to maximise the quantity of poison that can be transferred to the dart.
Frogs feature prominently in folklore , fairy tales , and popular culture. They tend to be portrayed as benign, ugly, and clumsy, but with hidden talents. Examples include Michigan J. Frog, that will only dance and sing for the demolition worker who opens his time capsule, but will not perform in public. Toads have a more sinister reputation. It was believed in European folklore that they were associated with witches as their familiar spirits and had magical powers.
The toxic secretions from their skin was used in brewing evil potions, but was also put to use to create magical cures for human and livestock ailments. They were associated with the devil; in John Milton 's Paradise Lost , Satan was depicted as a toad pouring poison into Eve 's ear.
The Moche people of ancient Peru worshipped animals, and often depicted frogs in their art. Some believed when one of these frogs died, it would turn into a golden talisman known as a huaca.
Today, despite being extinct in the wild, Panamanian golden frogs remain an important cultural symbol and are illustrated on decorative cloth molas made by the Kuna people. They also appear as part of the inlaid design on a new overpass in Panama City , on T-shirts, and even on lottery tickets.
From Wikipedia, the free encyclopedia. This article is about the group of amphibians. For other uses, see Frog disambiguation. Sexual selection in frogs. Larva of the common frog Rana temporaria a day before metamorphosis. Metamorphosis stage with deforming jaws, large eyes, and remains of gill pouch.
Young frog with a stumpy tail, metamorphosis nearly complete. Decline in amphibian populations. Animal testing on frogs. Archived from the original PDF on Tree of Life Web Project. University of California, Berkeley. Biology of Amphibians , p. Tree of Life web project. Phylogenetic analysis and revision". Bulletin of the American Museum of Natural History.
Dinosaur distribution Early Jurassic, North America: The Dinosauria 2nd edition. University of California Press. Proceedings of the National Academy of Sciences. Retrieved 3 July Terrarana from South America and an expanded direct-developing clade revealed by molecular phylogeny" PDF. Alexander Pyron; John J. Molecular Phylogenetics and Evolution. Journal of Experimental Biology. The museum of science, art and human perception.
Biological Journal of the Linnean Society. Rhacophorus from Sumatra and Java". A Natural History of Amphibians. Amphibians and Their Ways. North American Frogs and Toads. An Introductory Biology of Amphibians and Reptiles. Archived from the original on The Frog and Lizard".
Binocular Vision and Stereopsis. Ohio Department of Natural Resources. Annual Review of Ecology and Systematics. Call of the Amorous Bullfrog. Why don't they freeze to death? Regulatory, Integrative and Comparative Physiology. Environmental Physiology of Animals. Physiological and Biochemical Zoology. Advances in Physiology Education. University of Kansas Publications. Museum of Natural History. From tadpole to adult". Scaphiophryne Boulenger, ". Journal of the Royal Society Interface.
Living Amphibians of the World. Amphibians of East Africa. Bufonidae , and reassessment of its taxonomic status" PDF. Integrative and Comparative Biology.
American Society of Ichthyologists and Herpetologists. An investigation on the diet of Xenohyla truncata Lissamphibia: Archived from the original PDF on September 27, American Journal of Physiology. Advances in the Study of Behavior. Journal of Evolutionary Biology.
Dendrobatidae , with a description of the tadpole". Repties et Amphibiens de France. University of Michigan Museum of Zoology. Pharmacology Biochemistry and Behavior. The Amphibians and Reptiles of Costa Rica. University of Chicago Press. Archived from the original PDF on July 4, Journal of Natural Products.
American Museum of Natural History. Reptiles and Amphibians of Britain and Europe. The Encyclopedia of New Zealand. The Atlas of Global Conservation: Changes, Challenges, and Opportunities to Make a Difference. Alan; Fogden, Michael P.
How big is the problem, what are the causes and what can be done? Potential sources of transmission within and between ponds". Journal of Wildlife Diseases. Tracking the Vanishing Frogs. Frontiers in Ecology and the Environment. Year of the Frog". Fisheries and Aquaculture Department. The Writings of Mark Twain [pseud.
The science of common things: For schools, families, and young students. Bringing in the Clones". Journal of Psychoactive Drugs. International Journal of Addiction. Three Cartoons — ". East of the Web. Witch Hunts in Europe and America: The Spirit of Ancient Peru: Panama Amphibian Rescue and Conservation Project. Inside their Remarkable World. Contemporary Research on Major Problems. University of Missouri Press, Columbia. A case study of congruent phylogenetic reconstruction using amino acid and nucleotide characters".
Fossil Frogs and Toads of North America. Australian Frogs A Natural History. Extant anuran families by suborder. Ascaphidae Bombinatoridae Discoglossidae Leiopelmatidae. Extant amphibian positions by subclass. These are closely packed, oval-shaped sacs, which are separated from each other by thin partitions septula of supporting connective tissue known as interstitial tissue.
This tissue presumably has some endocrine function. The thickness of this tissue is much reduced immediately after breeding or pituitary stimulation.
The interstitial tissue is continuous with the covering of the testes known as the tunica albuginea, and the whole testis is enclosed in the thin peritoneal epithelium. Shortly after the normal breeding season in the spring for Rana pipiens, the spermatogonium, which has ceased all mitotic activity, enters upon a period of rest but not inactivity.
During this period the nucleus passes through a sequence of complex changes which represent an extended prophase. This is in anticipation of the two maturation divisions that finally produce the haploid spermatid which metamorphoses into a spermatozoon. The nucleus of the spermatogonium contains chromatin which appears as relatively coarse lumps distributed widely over an achromatic reticulum. Both the cytoplasm and the nucleus grow and the chromatic granules become finely divided and arranged into contiguous rows, bound by an achromatic thread, together known as chromosomes.
This is the leptotene stage of spermatogenesis. Shortly the chromosomes become arranged in pairs which converge toward that side of the nucleus where the centrosome is found. The opposite ends of the paired chromosomes merge into the general reticulum. This is the synaptene stage. The chromatin granules become telescoped together on the filaments so that the aggregated granules, known as chromosomes, appear much shorter and thicker.
Pairs of chromosomes become intertwined and the loose terminal ends become coiled and tangled together. This is the contraction or synizesis stage. Then the members of the various pairs become laterally parabiotically fused. While there is no actual reduction in total chromatin, there is a temporary and an apparent but not real reduction in the total number of chromosomes to the haploid condition, because of this fusion.
There is no actual reduction in the total amount of chromatin material, nor is there any permanent reduction at this stage in the number of chromosomes. Their identity is lost only temporarily. This is known as the pachytene stage. The members of each pair then separate again. It must be remembered, however, that a the separation need not be along the original line of fusion and that b an exchange of homologous sections of the chromosomes may occur without any cytological evidence.
In any case, the diploid number of chromosomes reappears and this is then known as the diplotene stage. During these changes in the chromatin material of the nucleus, the volume of the nucleus and the cytoplasm are considerably increased, the nuclear membrane breaks down, and the chromosomes assume bizarre shapes and various sizes.
They may be paired, curved, or straight; "V" and "C" and reversed "L" shapes, figure 8's, and grouped as tetrads. This is known as the diakinesis stage. The chromosomes are then lined up on a spindle in anticipation of the first of the two maturation divisions.
Spermatogenesis in the frog is seasonal and is completed within the testes. The walls of the seminiferous tubules produce spermatogonia which go through mitotic divisions and then the series of nuclear changes described above without mitosis. This results in the appearance, toward the lumen of each tubule, of clusters of mature spermatozoa.
By the time of hibernation October all the spermatozoa that are to become available for the following spring breeding season will have matured.
At this time the testis will exhibit only these spermatozoa and relatively few spermatogonia, without the intervening maturation stages. The spermatogonia are found close to the basement membrane of the seminiferous tubule. These then await their turn to undergo the maturation changes necessary for the production of spermatozoa which will be ready for the breeding season a year and a half thereafter.
The elongated and filamentous tails of the clustered mature spermatozoa project into the lumen of each seminiferous tubule. If one studies the July or August testis, which organ is then in its height of spermatogenetic activity, he can find all the stages of maturation from the spermatogonium to the spermatozoon. The spermatogonia are always located around the periphery of the seminiferous tubule and are small, closely packed cells, each with a granular, oval nucleus. In between the spermatogonia may be found occasional very large cells, the primary spermatocytes.
These tend to be irregularly spherical, possessing large and vesicular nuclei. The cells are so large that they may be seen under low power magnification X of the microscope. Apparently they divide to form secondary spermatocytes almost immediately, for they are so few and far between. The secondary spermatocytes which develop as the result of the first division are about half the size of the primaries, and lie toward the lumen of the tubule.
They generally have a darkly staining nucleus, and the cytoplasm may be tapered toward one side. The spermatid, following another division, is even smaller and possesses a condensed nucleus of irregular shape.
Clusters of spermatids appear as clusters of granules, the dark nucleus being almost as small as the cross section of a sperm head. The metamorphic stages from spermatid to spermatozoon are difficult to identify with ordinary magnification, and are often confused with the spermatids themselves.
During this change the inner of two spermatid centrioles passes into the nucleus while the outer one gives rise to the tail-like flagellum. The mature spermatozoon averages about 0. It has an elongated, solid-staining head nucleus with an anterior acrosome, pointing outwardly toward the periphery of the seminiferous tubule. The short middle piece generally is not visible but the tail appears as a gray filamentous extension into the lumen, about four or more times the length of the sperm head.
In any cross section of the testis, bundles of sperm heads or tails may be cut at right angles or tangentially, giving misleading suggestions of structure. The mature spermatozoon is dependent upon external sources of nutrition so that it joins from 25 to 40 other spermatozoa, all of whose heads may be seen converging into the cytoplasm of a relatively large, columnar-type basal cell known as the Sertoli cell.
This is functionally a nurse cell, supplying nutriment to the clusters of mature spermatozoa until such time as they may be liberated through the genital tract to function in fertilization. In observing a section of the summer testis of the frog under low power magnification, it is readily apparent that each seminiferous tubule may contain all the stages of maturation and that each stage is found in a cluster or group within the tubule.
Each group of similar cells is derived presumably from a single original spermatogonium, by the processes of mitosis and meiosis. This is reminiscent of the condition found in the grasshopper Rhomaleum testis.
Maturation of the germ cells occurs in groups so that when the spermatid stage is reached, the tips of the metamorphosing spermatozoon heads are all gathered together into the cytoplasm of the Sertoli cell.
Spermatozoa may remain thus throughout the entire period of hibernation only to be liberated under the influence of sex-stimulating hormones during the early spring. These spermatozoa are functionally mature, as can be demonstrated by dissecting them from the testes and using them to fertilize frogs' eggs artificially at any time from late in August until the normal breeding season in April or May.
It has been proved definitely that the anterior pituitary hormone causes the release of the mature spermatozoa from the testis. But this hormone also releases other maturation stages. It is therefore probable that there are smooth muscle fibers, either among the interstitial cells or in the tunica albuginea of the testes, which fibers contract to force the spermatozoa from the seminiferous tubules.
It would be as difficult to physiologically demonstrate the presence of these fibers in the testis as it is simple to demonstrate them in the contracting cyst wall of the ovary. 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.
These collecting tubules are small and are lined with closely packed cuboidal cells. They join the vasa efferentia which leave the testis to pass between the folds of the mesorchium and thence into the Malpighian corpuscles of the kidney.
From this point the spermatozoa pass by way of the excretory ducts, the uriniferous tubules, and into the mesonephric duct ureter which may be found attached to the lateral margin of the kidney. Within the excretory system the spermatozoa are immotile, due to the slightly acid environment. They are carried passively down the ureter to the slight dilation near the cloaca, known as the seminal vesicle. Within the vesicle the spermatozoa are stored briefly in clusters until amplexus and oviposition occur.
At oviposition the male ejaculates the spermatozoa into the neutral or slightly alkaline water where they are activated and then are able to fertilize the eggs as they emerge from the cloaca of the female. During the normal breeding season amplexus is achieved as the females reach the ponds where the males are emitting their sex calls.
During amplexus there are definite muscular ejaculatory movements on the part of the male frog, coinciding with oviposition on the part of the female. Amplexus may be maintained by the male for many days, even with dead females. As soon as the eggs are laid and the male has shed his sperm, he goes through a brief weaving motion of the body and then releases his grip to swim away. The frogs completely neglect the newly laid eggs.
In the male frog the ureter is not directly connected with the bladder, as it is in higher vertebrates. It is possible that the bladder in the Anura may be an accessory respiratory and hydrating organ, particularly in the toads, where water may be stored during migrations onto land.
The male frog also has a duct, homologous to the oviduct of the female, known as the "rudimentary oviduct" or Miillerian duct. This duct normally has no lumen, and is very much reduced in size so that it may be difficult to locate.
There is experimental evidence that this duct may be truly a vestigial oviduct since it responds to ovarian or female sex hormones by enlarging and acquiring a lumen. At the anterior end of the testes of some Anura e. This structure is said to respond to the removal of the adjacent testis or to the injection of female sex hormones by enlarging to become structurally like an ovary. Occasionally isolated ova have been found within the seminiferous tubules of an otherwise normal testis, suggesting the similar origin and the fundamental similarity of the testis and the ovary.
Finally, attached to the anterior end of the testis of the hibernating frog may be seen finger-like fat bodies corpora adiposa which represent stored nutrition for the long period of hibernation, and for the pre-breeding season when food is scarce.
Under the microscope these fat bodies appear as clusters of vacuolated cells, and are not to be confused with the mesorchium. It is believed that they, as well as the gonads, arise from the genital ridges of the early embryo. The fat bodies tend to be reduced immediately after the breeding season, only to be built up again as the time for hibernation approaches.
The mature female frog is generally larger than the male of the same age and species, the Rana pipiens female measuring from 60 to 1 10 mm. The sexually mature female has a body length of at least 70 mm.
It can be identified by the absence, at any season, of the dark thumb pad; the inability to produce lateral cheek pouches resulting from the croaking reaction; a flabby and distended abdomen; and the presence of peritoneal cilia.
These cilia are developed in the female in response to the prior development and secretion of ovarian hormones.
The ovaries of the frog are paired, multi-lobed organs, attached to the dorsal body wall by a double-layered extension of the peritoneum known as the mesovarium. This peritoneum continues around the entire ovary as the theca externa. Each lobe of the ovary is hollow and its cavity is continuous with the other 7 to 12 lobes. The ovaries of the female are found in the same relative position as the testes of the male but the peritoneum extends from the dorso-mesial wall rather than from the kidneys, as in the male.
The size of the ovary varies with the seasons more than does the size of the testis. 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.
While all of these animals reproduce sexually meaning that the species consists of males and females and mating involves the fetilization of eggs by sperm , reptiles and mammals reproduce through internal fertilization inside the female whereas amphibians practice external fertilization.
Mating for amphibians must nearly always occur in fresh water. It consists of the joining of the male and female frog, and at this time, the female lays eggs while the male releases sperm. The fertilized eggs are now left alone by both parents. The young that hatch will survive on their own. Amphibian eggs are protected by a jelly-like substance before hatching, unlike the hard or semi-hard shells of reptiles and birds. With the exception of some rainforest frog species, amphibians are not hatched into tiny replicas of their parents.