Amphibian

Amphibians are a fascinating group of animals that include frogs, toads, salamanders, and caecilians. They are cold-blooded, four-limbed vertebrates that usually start life in water as larvae called tadpoles. They then change into adults through metamorphosis. Unlike reptiles, amphibians need moist places to live and breed, and they often use their skin to help breathe.

These animals live in many different places, from forests to wetlands. They are important in ecosystems because they eat other animals and also become food for others. Because their skin is very sensitive, amphibians can show if the environment is healthy.

Amphibians first appeared millions of years ago during the Devonian period, evolving from ancient lobe-finned fish. Today, there are about 8,000 known species, with frogs making up most of them. The smallest amphibian is a tiny frog from New Guinea, while the largest living amphibian is the South China giant salamander. The study of these creatures is called batrachology, and the study of both amphibians and reptiles is known as herpetology.

Classification

The word amphibian comes from an Ancient Greek term meaning "both kinds of life." This name refers to animals that can live both on land and in water. Today, all living amphibians belong to a group called Lissamphibia. This group has three main orders: Anura (frogs and toads), Urodela (salamanders), and Gymnophiona (caecilians).

There are about 8,000 known amphibian species, and most of them are frogs. Amphibians are all part of the Lissamphibia group, which came from one common ancestor long ago. Fossils show that frogs, salamanders, and caecilians have been around since the Jurassic period.

Evolutionary history

Main article: Evolution of tetrapods

See also: List of prehistoric amphibian genera

Amphibians first appeared during the Devonian period, about 370 million years ago. They evolved from special fish called lobe-finned fish, like today's coelacanth and lungfish. These fish had fins that helped them move on the sea floor and even onto land. Over time, their fins changed into limbs. This led to the first animals with four legs, called tetrapods. These early amphibians could live in water and on land, but they still needed water to lay their eggs.

As amphibians kept changing, they grew better lungs for breathing air and stronger bodies for life on land. They grew limbs with digits to help them move. By the Carboniferous period, amphibians were the top land animals, and some grew very big. Later, reptiles became more common, and amphibians became smaller. Today’s amphibians, like frogs, salamanders, and caecilians, come from these ancient animals. They still need water for part of their lives.

Characteristics

Amphibians are special animals that need both land and water to live. They have four limbs, but their eggs and early development usually happen in water. Unlike reptiles, birds, and mammals, amphibians don’t have protective coverings around their eggs, so they often lay them in water. Some amphibians have found clever ways to protect their eggs.

Amphibians come in many sizes. The tiniest is a frog from New Guinea that measures just 7.7 mm long, while the largest is the Chinese giant salamander, which can grow up to 1.8 meters. They are cold-blooded, meaning they rely on the sun or warm places to control their body temperature. Most have moist skin without scales.

Anatomy and physiology

Amphibians have special skin that stays moist and lets them breathe through it. This skin can change color and often has glands that make sticky stuff or poisons to scare away predators. Their bodies have a light skeleton with hollow bones, and most have four limbs, but some, like caecilians, have no limbs.

Amphibians move in unique ways. Frogs jump with their strong back legs, while salamanders walk or climb. Some amphibians can grow back lost body parts, like tails. Their circulatory system changes from when they are young to when they are adults, growing lungs to breathe air. They also have special senses, like hearing with an eardrum and seeing colors, which develop as they change from tadpoles to adults.

Reproduction

See also: Sexual selection in amphibians

Most amphibians need fresh water to reproduce, but some lay their eggs on land and keep them moist. A few can live in brackish water, but none live in the sea. Some frogs in tropical rainforests don’t need water at all to breed. Their eggs hatch into tiny versions of the adult, skipping the tadpole stage.

In warm places, amphibians can breed at any time of year. In colder areas, breeding usually happens in the spring when days get longer and temperatures rise. Males often arrive at breeding sites before females and call to attract them. Most frogs use external fertilisation, where the male and female hold each other closely while the female lays eggs and the male adds sperm. Some salamanders and one type of frog use internal fertilisation, where the male places sperm inside the female.

Life cycle

Most amphibians go through metamorphosis, a big change in their body after they are born. They usually start life as eggs in water, hatching into larvae that live in water. Frogs, toads, and salamanders all begin as larvae with gills to breathe underwater.

Amphibians change because of hormones in their blood. Some of these changes help them live on land later. Tadpoles, the larval stage of frogs, have special features like skin extensions or fins to help them survive in water. After metamorphosis, these special features go away because they are no longer needed.

Amphibians lay their eggs in different places such as water, mud, or even on land. Some frogs build nests from foam to protect their eggs. The larvae, called tadpoles, have tails and gills and usually eat plants, but some eat other animals. As they change into adults, their bodies transform, losing their tails and gills and developing lungs to live on land.

Genetics and genomics

Amphibians have many different chromosomes and genes, which scientists find interesting. Researchers have looked at the chromosomes of more than 1,193 amphibian species, such as frogs, salamanders, and caecilians. These animals usually have 20 to 26 chromosomes that hold their genetic material.

Amphibians also have big genomes compared to other animals. For example, the genome of a small frog called Xenopus tropicalis is 1.7 GB. The genome of the Mexican axolotl, a type of salamander, is 32 GB—more than ten times larger than the human genome.

Feeding and diet

Adult amphibians are predators. They eat small moving creatures like beetles, caterpillars, earthworms, and spiders. Some, like the sirens, also eat aquatic plants. Others, such as the Mexican burrowing toad, have special tongues to catch ants and termites. They usually catch their food by sight, but toads, salamanders, and caecilians can also use their sense of smell.

Amphibians often swallow their food whole but may chew it a little first. They have special small, hinged teeth. When they are young, frog larvae eat the yolk of their egg. Later, they feed on tiny particles in the water. Some have special mouthparts to help them catch and eat different kinds of food.

Vocalization

Caecilians and salamanders don't make many sounds, mostly just soft squeaks or grunts. Some salamanders, like the California giant salamander, can make rattling or barking sounds. Frogs are more vocal, especially when they want to find mates. They make sounds by pushing air from their lungs over their vocal cords. Males call loudly to attract females and use quieter sounds when a female is near.

Territorial behaviour

Some frogs and salamanders protect areas they use for food, breeding, or hiding. Males usually do this, but sometimes females and young ones do too. They might have special features like bigger teeth or spines to help defend themselves.

Salamanders protect their areas by showing strong postures and, if needed, chasing or biting others. They might even lose a tail in these fights. Red back salamanders stay near their hiding spots and leave scent marks to warn others away.

In frogs, males often protect breeding areas by making calls. A deeper voice can show strength and scare away smaller males. If needed, they might push or bite to protect their area.

Defence mechanisms

Amphibians have soft bodies and thin skins, but they have clever ways to protect themselves. Many, like salamanders and frogs, make a slippery mucus that makes them hard to catch. Some of this mucus can be toxic, helping them escape from predators.

Some amphibians are brightly coloured to warn predators that they are poisonous. Others hide during the day or blend into their surroundings with camouflage. A few even lose part of their tail to get away, and grow it back later. These smart tricks help amphibians stay safe.

Main article: Coevolution

Cognition

Amphibians can learn and remember things. They can get used to new situations and tell the difference between things.

In experiments, salamanders chose bigger fruit flies when given a choice. Frogs can tell the difference between small groups of prey, like one or two flies, and larger groups, like three or six flies.

Conservation

Main article: Decline in amphibian populations

Amphibian numbers have been going down a lot since the late 1980s. This is a big problem for nature. Many things may be causing this, like losing homes, pollution, new animals moving in, and diseases.

When there are fewer amphibians, it can affect many other animals and plants. For example, snakes that eat frogs may have trouble finding food. This can then affect birds and other animals that eat snakes. Fewer tadpoles in water can let more algae grow, which can hurt other water creatures. Scientists and groups are working to protect amphibians by making safe places in zoos and helping countries fight diseases that harm these animals.