Frog

A frog is any member of a diverse group of short-bodied, tailless amphibians known as the order Anura. Frogs are found all over the world, from the tropics to subarctic regions, but most live in tropical rainforest and wetlands. They make up around 88% of all living amphibian species and are one of the five most diverse groups of vertebrates.

Adult frogs have strong bodies, big eyes, and long tongues. They live both in fresh water and on land, with some even living in trees or underground. Their skin can be many colors, helping them hide from predators or warn others they are dangerous. Frogs make many different vocalisations, especially when they want to find a mate.

Frogs start life as eggs laid in water, which hatch into swimming tadpoles. Tadpoles grow and change, or metamorphose, into adult frogs. Adult frogs eat small animals, mostly insects, catching them with their sticky tongues. They are important in nature because they eat many pests and are food for larger animals.

Frogs are also important to humans. In many places, people eat frog legs, and frogs often appear in stories and symbols. However, many frog species are in danger because of diseases and changes to their homes. Scientists are working hard to protect these amazing animals.

Etymology and taxonomy

All members of the order Anura are considered frogs, but only members of the family Bufonidae are called "true toads". The words "frog" and "toad" describe how the animals look and where they live, not their actual family relation. Frogs usually live in water or near it and have smooth skin, while toads live on land and have dry, warty skin. However, there are many exceptions to this rule.

The name "Anura" comes from Ancient Greek and means "tailless", referring to the fact that these amphibians do not have tails. The word "frog" has an uncertain origin but may come from old English words related to jumping. The word "toad" is unique to English and also has an unclear origin.

Evolution

The origins and evolutionary relationships between the three main groups of amphibians are still debated by scientists. Studies using molecular data suggest that salamanders and caecilians are more closely related to each other than they are to frogs. The split between these groups happened in the Paleozoic or early Mesozoic era, before the supercontinent Pangaea broke apart.

In 2008, a fossil called Gerobatrachus hottoni was discovered in Texas. It lived 290 million years ago and had features of both frogs and salamanders, making it a possible missing link between the two groups. Scientists also found early frog relatives like Triadobatrachus massinoti from Madagascar and Czatkobatrachus polonicus from Poland, dating back to about 250 million years ago. These early frogs looked somewhat like modern frogs but had longer bodies and more vertebrae. True frogs, such as Prosalirus bitis from the early Jurassic period, began appearing around 199 million years ago. These early frogs had lost most of their tails and were better adapted for jumping. By the Jurassic period, the evolution of modern frogs was likely complete.

Frog fossils have been found on every continent, including a discovery of 40 million-year-old helmeted frog fossils in Seymour Island on the Antarctic Peninsula, showing that frogs once lived in this region, related to those in South American Nothofagus forests today.

Morphology and physiology

Frogs have no tails, except when they are larvae. Most frogs have long hind legs, webbed toes, large eyes, and either smooth or warty skin. They range in size from very small to quite large.

A frog's foot and leg structure changes depending on where it lives—on the ground, in water, in trees, or in burrows. Frogs must move quickly to catch food and escape danger. Many frogs are good jumpers, with their legs and muscles adapted for leaping great distances.

Frogs have skin that helps protect them, control their temperature, and even breathe. Their skin can absorb water and oxygen, but it can also lose water. Some frogs have special skin features to help them stay camouflaged or to reduce water loss.

Frogs can breathe through their skin and also have lungs. Some aquatic frogs don’t have lungs at all and rely completely on their skin for breathing. Frogs have a three-chambered heart, which helps mix oxygenated and deoxygenated blood.

Frogs catch food using their sticky, cleft tongue, which they can shoot out quickly to grab insects. They have teeth along their upper jaw to hold food, but these teeth are weak and not used for chewing.

In female frogs, eggs pass through ovaries and oviducts to the outside. Male frogs have testes attached to their kidneys, and sperm is ejected directly onto the eggs when they mate. The male climbs on the back of the female in a position called amplexus to release sperm over the eggs.

Locomotion

Frogs move around in many ways, including jumping, running, walking, swimming, burrowing, climbing, and gliding. They are especially known for their jumping abilities. For example, the striped rocket frog can jump more than fifty times its own body length!

Frogs have special ways to move depending on where they live. Some frogs, like the red-legged running frog, run instead of jump. Others, like the purple frog, dig underground homes. Tree frogs can climb and even glide through the air using flaps of skin on their limbs.

Life history

Frogs have two main ways of reproducing: prolonged breeding and explosive breeding. In prolonged breeding, which is common, frogs gather at ponds, lakes, or streams to breed at certain times of the year. Many return to the same water where they started life as larvae. This can involve thousands of frogs migrating annually. In explosive breeding, frogs arrive at breeding sites quickly when triggered by things like rain in dry areas. Mating and egg-laying happen fast so the young can grow quickly before the water dries up.

Males usually arrive first at breeding sites and stay there, while females come later and leave after laying eggs. This means there are more males than females, and the males defend their areas and call to attract females. Females often choose mates based on the depth of their calls. Some males don’t have territories and instead try to mate with females that are attracted to calling males. Calling uses a lot of energy, so sometimes these males switch roles and become silent "satellite" males instead.

When females lay eggs, the male grabs the female and holds her tightly. The female releases eggs and the male puts sperm on them; this happens in the water. In some frogs, the male holds the eggs with his feet for a few minutes. A few special frogs give birth to live young instead of laying eggs. These include some West African frogs and a few others from Tanzania and Chile.

Frogs lay their eggs in different ways—clumps, films on the surface, strings, or singly. Some lay eggs in water, others on land. For example, the yellow-striped pygmy eleuth lays eggs singly in moist soil, while the red-eyed treefrog lays eggs on leaves above water, and the tadpoles drop into the water below. Some eggs develop with help from algae that give extra oxygen, and some eggs hatch early if they sense danger.

The young frogs, called tadpoles, hatch from eggs. Tadpoles don’t have eyelids or limbs and have gills and tails for swimming. They mostly eat plants, but some eat meat. Tadpoles change into adult frogs in a process called metamorphosis, which usually takes just one day. During this change, they develop lungs, lose their gills, grow legs, and their bodies change to be ready for life as adult frogs.

Adult frogs live near water but are not fully aquatic. Most eat small animals like insects, spiders, and worms. Some larger frogs eat other frogs, small mammals, or fish. A few eat plants too. Frogs catch their food with sticky tongues or by grabbing it with their mouths. They are eaten by many animals, including birds, snakes, and other frogs.

Frogs play an important role in nature because they eat many insects, helping to control their numbers and support plant growth. They are also food for many larger animals. Scientists think some frogs can live for many years, with some recorded living up to 40 years in captivity.

Some frogs care for their young. For example, the strawberry poison-dart frog lays eggs on the forest floor, and the male keeps them moist. When they hatch, the female carries the tadpoles to small pools and feeds them unfertilized eggs. Other frogs guard their eggs or carry them in special ways to keep them safe until they can develop into adult frogs.

Defence

Frogs may seem vulnerable due to their small size and lack of sharp teeth or spines, but they have many clever ways to defend themselves. Many frogs blend into their surroundings with their spotted or streaked skin, helping them hide from predators. Others can jump great distances, often into water, to escape danger.

Many frogs have special glands that produce toxins, making them taste bad or even poisonous to predators. These toxins can make predators sick, so they learn to avoid eating those frogs. Some poisonous frogs, like the poison dart frogs, have bright colors to warn predators of their toxicity. Other frogs use tricks like inflating their bodies or making loud noises to scare off attackers. Even when faced with predators, many frogs have strategies to stay safe.

Distribution

Frogs can be found on every continent except Antarctica, though some islands lack them. They are especially diverse in tropical areas, like rainforests, compared to cooler regions such as Europe. Some frogs live in dry places, like deserts, and have special ways to survive. For example, frogs in the Australian genus Cyclorana bury themselves underground and wait for rain to come out and breed. Other frogs, like the wood frog (Rana sylvatica), live in very cold areas, such as the Arctic Circle, and can survive freezing temperatures by freezing parts of their bodies.

Conservation

See also: Decline in amphibian populations

Frog populations have declined dramatically since the 1950s. More than one-third of frog species are considered threatened with extinction, and many have disappeared since the 1980s. Causes include habitat loss, pollutants, climate change, increased UVB radiation, and diseases like chytridiomycosis and ranavirus.

Amphibians, including frogs, are important indicators of ecosystem health because they are sensitive to changes in their environment. Some frogs have extra or missing legs due to various environmental stressors. In response, some places have started captive breeding programs to help save frog species.

Human uses

Culinary

Frog legs are eaten in many parts of the world. Indonesia is the largest exporter of frog meat, sending more than 5,000 tonnes each year to countries like France, Belgium and the United States. Frog farming has become important because wild frog populations decreased due to over-hunting.

Scientific research

See also: Animal testing on frogs and Frogs in captivity

Frogs have been important in scientific research. In 1970, NASA sent two bullfrogs into space to study the effects of weightlessness. Frogs are often used in schools for dissections, though this practice is decreasing due to concerns about animal welfare. Scientists have also used frogs to study electricity, heart function, and cloning. The African clawed frog helped scientists understand pregnancy and develop cloning techniques.

Pharmaceutical

Frog toxins interest scientists because they are very diverse. Some toxins from frogs could help develop new medicines, including possible treatments for diseases like HIV. Historically, some cultures used frog toxins for hunting or as traditional medicine.

Cultural significance

Frogs appear in many stories and cultures. In Chinese myths, a giant frog tries to swallow the moon during a lunar eclipse. Ancient Egyptians had a frog-shaped god named Heqet who protected newborns. In the Bible, Moses brought a plague of frogs to the Egyptians. Frogs also appear in fairy tales like The Frog Prince and in popular culture, such as Kermit the Frog and the game Frogger.