Strange Amphibians: Darwin's Frogs and Midwife Toads
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 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.
Frogs and toads belong to the class Amphibia and the order Anura. There are often differences between the two kinds of animals. Some species have features of both a frog and a toad, however.
Some Common Differences Between Frogs and Toads
smooth and moist skin
warty and dry skin
no parotid gland
parotid gland visible behind eye; the gland produces a toxin
comparatively slender body
move by jumping
move by hopping and walking
live in and near water
live mainly on land
lay eggs in clusters
lay eggs in strings
A Strange Way to Produce Offspring
The Darwin’s frog lives in Chile and Argentina and has the scientific name Rhinoderma darwinii. The frog is named after Charles Darwin, the famous scientist who discovered the animal by Chilean forest streams. It's also known as the Southern Darwin's frog
A Darwin's frog is a tiny creature that 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 or Southern Darwin's frog lives in Chile and Argentina. The Chile or Northern Darwin's frog lives only in central Chile (if the animal still exists).
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.
The 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 Male's Vocal Sac
As in many other amphibians, during the mating process the male climbs on top of the female and wraps his front legs around her. This position is known as amplexus. The contact stimulates the female to release her eggs, which the male fertilizes.
Darwin's frogs have a unique and very interesting aspect to their 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 move inside the eggs. Shortly 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 vocal sac can hold up to nineteen tadpoles. The male 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.
The Darwin's frog population is classified in the "Vulnerable" category of the Red List established by the IUCN, or International Union for Conservation of Nature. The list consists of seven (or sometimes more) 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 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-seven years without a sighting is a very long time. The amazing case of the Mallorcan midwife toad described below offers hope, however.
Features of Midwife Toads
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. The toads belong to the family 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 one of the animals.
The first midwife toad video in this article includes the high pitched peeping sound made by the amphibian. 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
After amplexus, the female releases her eggs and the male fertlizes them with his sperm. He then coils the string or strings of eggs around his hind legs and carries them around with him until they are ready to hatch.
There are about sixty eggs in a string. The male carries the eggs for twenty to fifty days. If the weather is very dry he may dip the eggs into water to moisten them. He may mate with more than one female and carry around more than one strand of eggs. When the eggs are ready to hatch, the toad enters the water. The tadpoles then emerge from the eggs and swim away.
Common midwife toad tadpoles grow to a very large size and may become bigger than the adult. Unlike the adults, the tadpoles are vegetarians. They change into an adult frog after about eight months.
Toad Research in Majorca
The common midwife toad is 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.
The Mallorcan or Marjorcan 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 animal. Other organizations are also involved in the 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 species, 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 Amphibians
Although the Mallorcan midwife toad isn't completely safe yet, the conservation efforts involving the animal 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 animals, 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