WHAT IS A FROG?

Frogs and toads comprise the order Anura, by far the largest of the three orders that make up the vertebrate class Amphibia, animals that typically live part of their lives on land, part in water. The name Anura means "without a tail" and refers to

the most obvious difference between frogs and toads and the other amphibian orders, the Caudata (salamanders and newts) and the Gymnophiona (caecilians), both of which have long, flexible tails. Anurans make up 88 percent of all amphibian species.

Anurans share with other amphibians a number of features that have a profound influence on their natural history and distribution. Like all animals, except birds and mammals, they are ectothermic, meaning that they derive body heat from their environment, directly or indirectly from the sun. As a result, they are absent from the coldest parts of the Earth. For example, only one species, the Wood Frog (Rana sylvatica; here), is found north of the Arctic Circle, in North America. Being ectothermic also means that their reproductive activity is limited to warmer parts of the year, when their external environment provides the energy they require to sustain vigorous activities, such as calling and fighting. Amphibians differ from reptiles in having a thin, soft skin that is very permeable to water. This means that they are restricted to habitats where conditions are always moist or wet, and that they typically breed during times of the year when rainfall is high.

LIFE CYCLE

Like other amphibians, frogs exhibit a complex life cycle, that involves four distinct phases: egg, larva, juvenile, and adult. The larvae of frogs, known as tadpoles, are wholly aquatic creatures that obtain their oxygen from the water they live in via a pair of external gills; they swim by beating their tails. The transition from a tadpole to an adult, a process called metamorphosis, is perhaps the most familiar and remarkable characteristic of frogs. It involves major anatomical changes, including the loss of the larval tail, the development of four limbs, and a switch from breathing oxygen dissolved in water through gills to breathing atmospheric oxygen via lungs and the skin.

In the majority of frogs, the eggs are laid in water, where they hatch into swimming tadpoles, making these species dependent on water for breeding. Some frogs, however, have become emancipated from water by laying their eggs on land, contained within a waterproof capsule, or within their own bodies. Frogs are associated only with fresh water. While some can survive in brackish water, there are no anurans that have colonized the sea.

Anurans have a stout body and a very short backbone, containing only eight or fewer vertebrae. The hind limbs are typically longer than the forelimbs, largely due to the tarsal (ankle) bones being greatly lengthened. At rest, the hind limbs are folded under the body, but they can be rapidly straightened, enabling many frogs to lunge at their prey or escape from an enemy by jumping. They are often very muscular, making frogs' legs a very attractive food source for humans in some parts of the world. The head is wide, the neck is inflexible, and the mouth can typically be opened very wide, enabling many anurans to consume surprisingly large prey. Virtually all frogs are carnivores, feeding on living prey, particularly insects. Important for detecting moving prey, their eyes are usually very large. In fact, they may be so large that they bulge into the mouth — some frogs assist the passage of food from mouth to gut by "rolling their eyes."

Frogs typically have four fingers on their forelimbs and five toes on their feet. The shape and length of the fingers and toes vary a great deal from one species to another and largely reflects their mode of locomotion. For example, very aquatic frogs have webbing between their toes, and often also between their fingers, enabling them to swim more effectively. Tree frogs typically have disk-like tips to their fingers and toes that act as adhesive pads, enabling them to climb up smooth, vertical surfaces. Many burrowing frogs have horny tubercles on their hind feet that act as spades for moving soil.

In many anurans, the eardrum, called the tympanum, is visible just behind the eye. Hearing is of great importance to frogs because they communicate with one another by calling. These calls are produced by passing air back and forth between the lungs and the mouth, and in many species the sound is amplified by one or two inflatable vocal sacs that open into the mouth. The primary function of the lungs is to breathe in air, but the majority of frogs actually obtain most of the oxygen they need through their moist skin.

The skin of frogs is thin and contains numerous glands that secrete mucus to keep the surface moist. There are also glands that secrete toxic compounds. These may be very mild in their effects but, in some frogs, they are lethal to any animal that ingests them. The skin of frogs contains numerous cells that consist of a variety of colors. In poisonous frogs, these colors are typically very conspicuous, providing a warning to potential enemies. In most frogs, however, the skin colors and patterns provide very effective camouflage. Many frogs can change the color of their skin, usually rather slowly, though some species are capable of changing color within a few minutes.

A distinction is often made between "frogs" and "toads," but this can be misleading because the two terms are used differently in different parts of the world. In Europe and North America, anurans with smooth skins and aquatic habits are known as frogs, and those with warty skins and more terrestrial habits as toads. However, anurans belonging to the African genus Xenopus (see here) are often called clawed toads, despite being wholly aquatic and having very smooth skins. The distinction between frogs and toads is biologically meaningless; in this book, all anurans are referred to as frogs even though many are known colloquially as toads.

COMPLEX LIFE CYCLE

In common with other amphibians, frogs have a life history that consists of four distinct stages: egg, larva or tadpole, juvenile, and adult. The length of time spent in each of these stages varies enormously, both within and between species.

EGGS

Amphibians produce eggs similar to those of fish. They have no shell but are bounded by a permeable membrane that encloses a spherical blob of protective jelly. The eggs thus require wet conditions; otherwise they will soon dry out and die. Each egg is supplied with a quantity of yolk on which, after hatching, the tadpole feeds until it can feed itself. The amount of yolk allocated varies greatly between species depending on their mode of reproduction. Frogs that lay their eggs in large waterbodies containing lots of food tend to...