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The Book of Frogs
A Life-Size Guide to Six Hundred Species from Around the World
By Tim Halliday, Sandra Pond The University of Chicago Press
Copyright © 2016 Ivy Press Limited
All rights reserved.
ISBN: 978-0-226-18465-4
CHAPTER 1
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.
CHAPTER 2
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 produce many small eggs, each containing only a little yolk. Conversely, those laying their eggs in tiny pools in the leaf axils of plants, where there is very little for tadpoles to eat, lay a few, large eggs, each containing sufficient yolk to see the tadpole through most or all of its development. Likewise, frogs that show parental care of their eggs typically lay far fewer, larger eggs than frogs that release their eggs into water to fend for themselves.
Water is a hazardous environment for frogs' eggs and tadpoles, containing many fishes, insect larvae, and other predators that readily eat them. Some frogs have evolved modes of egg-laying that emancipate them from open water and thus reduce this predation risk. Numerous tree frogs attach their eggs to leaves high up in trees, for example. Several frog species lay their eggs in foam nests that not only protect them from predators but also provide a reliably moist environment in which the tadpoles can develop. Some species lay their eggs on land and some carry them about on their backs. Most remarkable of all are those frogs that carry their eggs within some part of their body, such as their skin, their gut, their vocal sacs, or their oviducts.
In many tropical terrestrial frogs, the eggs are large, are laid on land, and hatch to produce tiny froglets. Called "direct development," this life history appears to have eliminated the tadpole stage, but this is not the case. The egg hatches into a tadpole that completes its development within the confines of a capsule, feeding only on yolk provided by the mother. Direct-developing frogs have thus evolved a life cycle that makes them independent of ponds or streams, though they are still restricted to damp habitats.
The clutch size of frogs ranges from several thousand in larger, aquatic frogs to single figures in small, terrestrial species with parental care. Within a species, the clutch size of an individual female is typically related to her size, larger females producing more eggs and, in some species, eggs containing more yolk.
The eggs of frogs that are laid in open water tend to be black because they have a layer of the pigment melanin around them. This protects the genetic material within the egg, DNA, from being broken up by ultraviolet radiation in sunlight. Frogs' eggs that are laid underground or in shady places tend to be white or cream in color.
TADPOLES
Tadpoles are rather simple creatures that swim, eat continuously, and grow as fast as possible. Most tadpoles are very palatable to many predators, and the sooner they can reach metamorphosis and escape to the relative safety of land, the better. Rapid development is also vital for tadpoles living in ephemeral ponds, which often lack predators but may soon dry out. The tadpoles of some desert-living frogs can reach metamorphosis just eight days after hatching from the egg. Typically, tadpoles live in water and obtain oxygen by means of gills. They are usually herbivorous and their rate of growth is dependent on temperature as well as food supply. A herbivorous diet requires a long gut — the distinctive globular body shape of tadpoles is primarily due to their long, coiled gut. Many tadpoles feed by scraping algae off plants, stones, and other objects, and to help them in this their mouths are surrounded by rows of tiny teeth made of keratin (the material that forms fingernails in humans). The tadpoles of some species live in open water, often in shoals. They usually lack teeth and feed by swimming around open-mouthed, filtering tiny food items from the water. The mouths of the tadpoles of stream-breeding frogs are often surrounded by a sucker, which enables them to attach themselves to rocks to avoid being swept away by the current. In many species, the tadpole diet shifts from plant material to animal tissue as metamorphosis approaches. They become scavengers, feeding on the corpses of dead animals, including one another. In a few species, larger tadpoles become predators, hunting and feeding on the tadpoles of their own and other species.
Tadpoles appear to be simple, unsophisticated animals, but research has shown that they are capable of altering their behavior in response to changes in their environment. For example, in many species, tadpoles gather together in dense schools when they sense the presence of a predator. They do this is by detecting waterborne chemicals that are released when a tadpole is attacked. Most remarkably, the tadpoles of some species preferentially cluster together with their own immediate kin. This kin recognition is probably mediated by means of odor.
At metamorphosis, the larval anatomy is totally transformed into the adult form, with the acquisition of lungs and legs, and the loss of gills and the tail. There is also a radical change of diet, with froglets developing the ability to detect and catch small insects and other moving prey. This involves development and enlarging of the eyes.
JUVENILES
After metamorphosis, young frogs live on land, sometimes for several years, before returning to water to breed. This juvenile phase of a frog's life cycle is the least well known, froglets being small and very secretive in their habits. In some frogs, the juvenile phase lasts for only a few months, but in others it may last for several years. In the Pacific Tailed Frog (Ascaphus truei; here), for example, adulthood is achieved at four years of age.
ADULTS
Adult frogs continue to grow throughout life, albeit at a slower and slower rate, producing much variation in adult body size. This is why, in this book, the body length of frogs is usually given as a range rather than a single value. In many frogs, females are larger than males; the reverse is true in only a few frog species. In some species, such as the European Common Toad (Bufo bufo; here), the size difference arises because males reach sexual maturity one year earlier, and at a smaller size than females. This difference in the age of maturity also produces a biased sex ratio, with adult males greatly outnumbering adult females.
PARENTAL CARE
In many frogs and toads, the female makes a modest contribution to the survival of her eggs and the resultant tadpoles. The eggs are deposited in water during mating and are then left to fend for themselves. Eggs and larvae typically suffer very high mortality through predation, desiccation, freezing, or disease, and the chances that at least some will survive are increased by producing eggs in very large numbers. However, several frogs and toads have evolved an alternative strategy, in which fewer eggs are produced, and one or both parents provide some form of care that enhances each egg's chances of survival.
In almost all frogs, the eggs are fertilized externally, the male shedding sperm onto them as they are ejected from the female's body. In about 12 species, however, fertilization is internal, the male inserting sperm into the female's oviduct. In such species, the young can be retained in the female's body for part or all of their development, providing a high degree of protection. In a recently discovered fanged frog from Sulawesi (Limnonectes larvaepartus), the young are born as tadpoles; in the Kihansi Spray Toad (Nectophrynoides asperginis; here), they are born as tiny froglets.
Several tropical frogs protect their eggs by laying them in a foam nest, made from a secretion, usually produced by the female and whipped into a foam by both male and female beating their legs. Some frogs, such as the African leaf-folding frogs (Afrixalus; see here), protect their eggs by folding a leaf around them. In the midwife toads (Alytes; see here) of Europe, the eggs are laid in strings, fertilized externally, and then gathered up by the male. He carries the eggs, twisted around his legs, until they are ready to hatch, keeping them moist by occasionally entering a pool of water.
In most species with direct development, one or both parents remain close to the eggs, defending them against predators and keeping them clear of fungal infection.
In a number of poison frogs, such as the Yellow-headed Poison Frog (Dendrobates leucomelas; here), the male provides parental care, defending the eggs until they hatch, and then carrying them on his back to tiny pools in the axils of plants, called phytotelmata, where they complete their development. In poison frogs of the genus Ranitomeya (see here), males carry the tadpoles to phytotelmata and females then feed them with unfertilized eggs. In at least one species, the Imitating Poison Frog (R. imitator; here), male and female form a stable, monogamous relationship that lasts for the whole time it takes them to produce and rear their young.
There are frogs that hold their developing young within their bodies, but not in their reproductive tract. The male of the Hip-pocket Frog (Assa darlingtoni; here) has a pouch of skin on each side of its body in which the tadpoles develop. In the now extinct Southern Gastric Brooding Frog (Rheobatrachus silus; here), the tadpoles developed in their mother's stomach. In the endangered Darwin's Frog (Rhinoderma darwinii; here) the tadpoles develop in the male's vocal sac. Most remarkably, the young of the Surinam Toad (Pipa pipa; here) complete their entire development, from egg to froglet, in pockets in the skin on their mother's back.
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Excerpted from The Book of Frogs by Tim Halliday, Sandra Pond. Copyright © 2016 Ivy Press Limited. Excerpted by permission of The University of Chicago Press.
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