Strange Amphibians - Darwin's Frogs, Midwife Toads and a Fungus
Two Unusual Amphibians
Darwin’s frogs and midwife toads have strange and interesting methods of reproduction. Once the female has released her eggs, the male fertilizes them and then picks them up. He carries the eggs around in or on his body until the youngsters have developed. This degree of egg care is unusual for amphibians.
Darwin's frogs live in South America. After the female's eggs have been laid and fertilized, the male guards them until the tadpoles - the first stage of the young frogs’ lives - move inside the eggs. The male then picks the eggs up with his tongue and places them in his vocal sac, which normally functions to amplify his sounds. Here the youngsters live until they have become tiny froglets. At this point they leave the vocal sac to lead independent lives.
Midwife toads are found in Europe and North Africa. The female lays a string of eggs. Once the eggs are fertilized, the male wraps the eggs around his hind legs and carries them around until the tadpoles are ready to be released. He dips the eggs in water if necessary to keep them moist.
Differences Between Frogs and Toads
Frogs have smooth and moist skin, slender bodies and long legs. They move by jumping. They live near water and lay their eggs in clusters. Toads have dry and warty skin, a stouter body and shorter legs. Glands in their skin produce a venom. They move by walking or hopping. Toads live on land and lay their eggs in a string. There are exceptions to these rules, however.
The Darwin's Frog and its Reproduction
The Darwin’s frog lives in Chile and Argentina and has the scientific name Rhinoderma darwinii. The animal is named after Charles Darwin, the famous scientist who discovered the frogs by Chilean forest streams. It's also known as the Southern Darwin's frog
A Darwin's frog is a tiny creature and has a maximum size of about 3 cm, or 1.2 inches. It has a long, pointed snout (technically called a proboscis), which gives its head a triangular appearance. Its upper surface is bright green, pale green or brown. Some frogs have green and brown areas arranged in an attractive pattern. The lower surface is light or medium brown with black and white patches.
The male has a very large vocal sac which extends from his throat to the end of his abdomen. This vocal sac houses the young frogs as they develop.
The Darwin's frog or Southern Darwin's frog lives in Chile and Argentina. The Chile or Northern Darwin's frog lives only in central Chile (if it still exists).
The Lives of Darwin's Frogs
Darwin's frogs are active during the day. They spend most of their time on land in the leaf litter around forest streams and bogs. They feed mainly on insects but eat other small invertebrates as well. Their colouration helps to camouflage them against the leaf litter and to protect them from predators.
Darwin's frogs often respond to danger by feigning death. They turn upside down and stay still, on land or in water. They sometimes jump into the water to protect themselves, turning upside down to display their patterned underside and drifting in the water as though they are dead.
Young Frogs Moving Inside the Vocal Sacs of the Darwin's Frog
Darwin's frogs have a unique method of reproduction. The female deposits about forty eggs on the leaf litter and then leaves. The male stays to fertilize and protect the eggs. After about three weeks, the tadpoles that have survived are moving inside the eggs. Just before they are ready to hatch, the male picks the eggs up with his tongue and guides them through slits linking his mouth to his vocal sac. The male's vocal sac can hold up to nineteen tadpoles. He doesn't vocalize while he is brooding the eggs.
As the tadpoles develop, they frequently move around and cause the vocal sac to ripple. They feed on yolk from the egg and on a secretion produced by the male. Metamorphosis, the process in which a tadpole changes into a frog, takes place inside the vocal sac. The froglets are able to leave the sac around six to eight weeks after the eggs entered it. The male opens his mouth and the youngsters jump out.
Darwin's Frog Population Status
The Darwin's frog population is classified in the "vulnerable" category of the IUCN (International Union for Conservation of Nature) Red List. This list consists of seven categories denoting how close an animal population is to extinction. From the least serious state to the most serious state, the categories are Least Concern, Near Threatened, Vulnerable, Endangered, Critically Endangered, Extinct in the Wild and Extinct.
Darwin's frogs are threatened by habitat loss due to agriculture and forestry plantations. A chytrid fungus which has been discovered in Chile is worrying conservationists and may also be affecting the frogs. This fungus is believed to be at least partly responsible for the worldwide decrease in amphibian populations. It's called Batrachochytrium dendrobatidis, or Bd. One of the scientists in the video below says that he expects 40% of amphibian species to become extinct during his lifetime due to the presence of the fungus.
Bd can cause a disease called chytridiomycosis. It infects the frog's skin and causes it to thicken. This is dangerous because water and electrolytes such as sodium and potassium salts are normally absorbed through the skin. The electrolytes are important for heart function. If the skin is too thick to allow enough electrolytes to enter the frog's body, the frog's heart will stop beating.
The Chytrid Fungus and Amphibians
In early 2015 it was announced that the chytrid fungus had been found in several places in Madagascar. The island is home to over 500 species of frogs, many of which live nowhere else on Earth. It's unknown if the fungus has infected the Madagascan frogs.
The Chile Darwin's Frog
In recent times, another species of Darwin's frog lived in Chile. The frog was called the Chile Darwin's frog or the northern Darwin's frog and had the scientific name Rhinoderma rufum. The IUCN Red List classifies this frog as critically endangered, but no members of the species have been seen since around 1980. Many researchers believe that the frog is extinct.
The reasons for the frog's disappearance are uncertain, but habitat loss and disease may have played a role. Sometimes an animal that is thought to be extinct is actually living in very small and remote populations and is eventually rediscovered. It would be wonderful if this was the case for the Chile Darwin's frog, but it's unlikely. Thirty-five years without a sighting is a very long time. The amazing case of the Mallorcan midwife toad described below offers hope, however.
Midwife Toads in Wales and Their Call
There are five species of midwife toads, all belonging to the genus Alytes. (The genus is the first part of the scientific name for an organism.) The common midwife toad has the scientific name Alytes obstetricans and is native to countries in western and central Europe. The toad also lives in Britain, where it's an introduced species.
The common midwife toad is brown or grey in colour and is covered by darker bumps. Its underside is light grey or white. Midwife toads are small, but they are larger than Darwin’s frogs. They may reach 5.5 cm in length (2.2 inches).
Unlike the long, thin tongue of many other amphibians, the tongue of midwife toads is round and flattened. Midwife toads belong to a family called Discoglossidae, which also includes the painted frogs.
Life of the Common Midwife Toad
Midwife toads are nocturnal, spending their days in burrows or under logs or rocks. They spend most of their time on land, burrowing into the ground if they start to dry out. They feed on insects and small invertebrates like spiders, millipedes, worms and slugs. During winter the common midwife toad hibernates, usually in a burrow.
When a toad is alarmed, such as by being attacked or handled, the "warts" on its skin produce a poison with a strong and unpleasant smell. This poison helps to protect the toad from its predators. It doesn’t seem to affect humans, although it’s not a good idea for someone to touch their eyes right after handling a midwife toad.
The first midwife toad video in this article includes the high pitched peeping sound made by the toad. It's often described as a ringing or bell-like call. A frog or toad makes sounds by moving air through the larynx, which is often called the voice box in humans. The common midwife toad has no vocal sac to amplify the sound, but its call is still very audible. During the breeding season the male calls to attract a female and she answers back.
The Mallorcan midwife toad lives only on the island of Majorca. The toad is making a comeback after being close to extinction.
Eggs and Tadpoles
During mating, the male common midwife toad climbs on top of the female and wraps his front legs around her. This position is known as amplexus. He also strokes her body with his back legs. These movements stimulate the female to release her eggs. The female catches the eggs between her back legs and the male fertilizes them. The fertilized eggs are then transferred to the male’s back legs.
There are about sixty eggs in a string. The male carries the eggs around for twenty to fifty days. If the weather is very dry he may dip the eggs into water to moisten them. The male may mate with more than one female and carry around more than one strand of eggs. When the eggs are ready to hatch, the male enters the water. The tadpoles then emerge from the eggs and swim away.
Common midwife toad tadpoles grow to a very large size and become bigger than the adult. Unlike the adults, the tadpoles are vegetarians. They change into an adult frog after about eight months.
Midwife Toad Research in Majorca
Conservation Status of Midwife Toads
The common midwife toad in classified in the "Least Concern" category of the IUCN Red List, but the other four species are classified in the "Vulnerable" or "Near Threatened" categories. These species include the Moroccan midwife toad, the Betic midwife toad in Spain, the Iberian midwife toad in Spain and Portugal and the Mallorcan midwife toad in Majorca.
The Mallorcan midwife toad, also called the Majorcan midwife toad (Alytes muletensis) is found in the wild only in Majorca, where it lives in limestone gorges in remote areas. Before 1980 the species was thought to have been extinct for two thousand years and was known only from fossils. Its population was believed to have been eliminated by introduced predators and competitors.
The Durrell Zoo in Jersey has established a successful breeding program for Mallorcan midwife toads and has repopulated wild areas with the toad. Other organizations are also involved in the toad conservation program. In 1996 the toad was classified in the "Critically Endangered" Red List category, but its population status has since been upgraded to the "Vulnerable" category.
Despite the success with the Mallorcan midwife toad, there are concerns about midwife toads in general. Some populations have died from chytrid fungal infections.
The Class Amphibia includes salamanders and newts as well as frogs and toads. All of these animals can be infected by the chytrid fungus.
The Future for Frogs, Toads and Other Amphibians
Although the Mallorcan midwife toad isn't completely safe yet, the conservation efforts involving the toad show what can be done when people are determined. It would be great if this effort could be applied to other amphibians as well.
The combination of human activities and the chytrid fungus is very worrying with respect to the future of amphibians. Interestingly, although the fungus is having a devastating effect on many amphibians, some species seem to be immune to it or recover once they are infected. If scientists can find the reason for these observations, they may be able to help amphibians. Many fascinating and strange creatures belong to the Class Amphibia. It would be a great shame to lose this diversity from the Earth.
© 2011 Linda Crampton
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