Frogs and Toads

by Dave Showler, Barry Croucher (Illustrator)

Overview

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A Golden Guide ® FROM ST. MARTIN'S PRESS
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For many naturalists, woodland frogs and toads represent some of their first encounters with wildlife. Now this Golden Guide from St. Martin's Press offers fascinating insights into the mysterious world of frogs and toads.

* Covers everything from the deadly poisonous frogs of South America to our common woodland species
* The ways of distinguishing frogs and toads
* Learn about their growth, reproduction, and way of life

Includes gorgeous, accurate, full-cover illustrations and little-known facts about these extraordinary, overlooked animals.

Product Details

ISBN-13:	9781466864702
Publisher:	St. Martin's Publishing Group
Publication date:	02/25/2014
Series:	A Golden Guide from St. Martin's Press
Sold by:	Macmillan
Format:	eBook
Pages:	160
File size:	11 MB
Note:	This product may take a few minutes to download.

About the Author

Golden Guides first appeared in 1949 and quickly established themselves as authorities on subjects from Natural History to Science. Relaunched in 2000, Golden Guides from St. Martin's Press feature modern, new covers as part of a multi-year, million-dollar program to revise, update, and expand the complete line of guides for a new generation of students.

Dave Showler is the author of Frogs and Toads: A Golden Guide.
Barry Croucher is a professional illustrator who specializes in wildlife, technical, and historic illustration using a variety of traditional techniques and media. He won the 2012 Association of Illustrators Silver Award for Design.

Read an Excerpt

CHAPTER 1

INTRODUCTION

Few animals induce such mixed emotions as frogs and toads. Some of us love them, many dismiss them as horrible slimy creatures, and they are often maligned and misunderstood. The eighteenth-century Swedish scientist Linnaeus, responsible for the modern-day classification system, considered them "foul and loathsome animals." He obviously didn't appreciate what are truly fascinating and sometimes bizarrely spectacular animals.

Frogs and toads, collectively called anurans, are found throughout most of the world except for the polar regions and oceans. There are about 4,750 living species, ranging in size from the South American Two-Fingered Toad measuring less than 3/8 inches (1 cm) in length, to the African Goliath Frog which reaches 14 in (35 cm). The record for the most long-lived anuran probably goes to the Eurasian Common Toad, at 40 years old.

Many have an "explosive" breeding strategy and can be encountered in large numbers for a few days each year as they migrate en masse to breeding ponds, where they lay eggs in huge quantities as gelatinous mats, clumps, or strings of spawn. The eggs hatch as tadpoles, which, as they mature, undergo the process known as metamorphosis.

ANURAN CHARACTERISTICS

Anurans are tailless amphibians, as opposed to their tailed amphibian relatives, salamanders, newts, and the burrowing, wormlike caecilians. They typically have long, enlarged hind legs for jumping and swimming, webbed toes, a squat stance, no neck, a large head, and protruding eyes. Like other amphibians they have a soft, moist, permeable glandular skin and their eggs are covered in a flexible membrane. These features distinguish them from reptiles, which have a dry, scaly skin and eggs covered in a tough, leathery shell. They also differ from reptiles in that, with the exception of 11 or so species, fertilization is external. However, only two of these species, the "tailed frogs," Ascaphus, have true copulation – what appears to be a short tail actually is the male copulatory organ.

Most anurans have a characteristic life cycle, eggs that are laid in water hatching into an aquatic, pre-adult, larval stage, known as the tadpole. The tadpole undergoes metamorphosis, which includes the development of hind limbs, then forelimbs, and the resorption of the tail. The young then emerges as a miniature of the adult.

However, many anurans, especially from tropical regions, are not reliant upon water to breed. They lay eggs on land, the tadpole stage passing within the egg, from which the young emerge fully formed. A few have evolved highly specialized strategies – some tadpoles, for example, develop on the back of a parent. There are even two species that give birth to live young.

Amphibians are "cold-blooded" (exothermic) or, more correctly, they have variable temperatures dependent on the environment around them. One disadvantage of this is that at low temperatures they are unable to function and therefore some species have to hibernate during the cold winter months. A distinct advantage, however, is that unlike the "warm-blooded" (endothermic) birds and mammals that need to consume relatively large amounts of food to maintain a constant high body temperature (around 98.6°F or 37°C in humans), amphibians need to produce only a little or no internal heat, so in comparison can survive on a much reduced food intake. As such, they can occur at high densities in areas where food is quite scarce.

WHAT IS THE DIFFERENCE BETWEEN A FROG AND A TOAD?

Originally, the English vernacular or common names "frog" and "toad" were coined simply to describe the two basic kinds of anuran found in Britain: the Common Frog, Ranatemporaria Ranidae, and two toads, the Common Toad, Bufo bufo, and the much rarer Natterjack, B. calamitaBufonidae. The Common Frog is easily distinguished from these toads, being much smoother and wetter-skinned, with long hind limbs that enable it to leap powerfully. It also has small teeth, known as vomerine teeth, in the upper jaw. The toads in comparison are warty and quite dry-skinned, with a pair of large swellings on the sides of the head – the paratoid glands, which produce toxins to deter predators. They are rather stout and move over land with a series of short hops, or by walking, or, in the case of the Natterjack, often with short running spurts. They also lack vomerine teeth.

Subsequently, the names "frog" and "toad," or their non-English equivalents, e.g., French: frog = grenouille, toad = crapaud; Spanish: frog = rana, toad = sapo, have been applied to anurans in different families throughout the world on the basis of which of the two archetypal forms they happen to most closely resemble. However, many of these species, unless they belong to the Ranidae or Bufonidae families, are neither typical Rana frogs nor typical Bufo toads, and have traits unique to themselves. Thus there is no scientific basis for the terms "frog" or "toad," and as such there are no hard and fast rules that can be applied to distinguish between them. For example, no bufonids (toads in the family Bufonidae) have teeth, but the absence of teeth is not always a distinguishing feature, as a few non-bufonids also lack them.

Even within the family Bufonidae itself, things are not straightforward. While most are referred to as "toads," as one would reasonably expect, the members of one genus, Atelopus, are usually referred to as "harlequin frogs" and occasionally "harlequin toads." Although some Atelopus species are warty and have fairly prominent paratoid glands, thus resembling a Bufo toad, others are smoother skinned and superficially more similar in appearance to a Rana frog.

KEY FEATURES OF FROGS AND TOADS

FROGS pupil shape smooth skin thinner body ear drum/tympanum long hind legs usually more webbing on feet

TOADS dry, warty skin stockier body dorsal lateral fold short hind legs

EYES AND SIGHT

Most anurans have large, protuberant eyes positioned on the sides or toward the top of the head, allowing them good, almost all-round vision. The eyes of the majority of anurans are well developed and many can detect approaching danger at a considerable distance. Most are thought to have excellent color vision, critical in species in which color plays a role in, for example, territorial defense, mate attraction, and prey palatability, although there is little evidence that color discrimination is used widely. In many species there is overlap in the forward-looking field of view, this "binocular vision" giving them good depth perception that allows them to target moving prey accurately. Some can judge distance to their prey equally well using a single eye. The importance of sight to the aquatic South American and African pipid frogs in the sometimes dark or murky waters that they inhabit may be limited, and in this family other sensory organs, such as their highly developed lateral line system and sense of smell, are utilized in conjunction with sight in prey location and mate finding.

In some species the eyes stick up prominently above the head and the nostrils are located on top of the snout, which enables them to submerge themselves in water or bury themselves in leaf litter or loose soil, leaving only the eyes and nostrils above the surface. This has the obvious advantages of concealing them from predators and allowing them to approach or ambush prey unnoticed.

In many species the iris is brightly colored and may be the most outwardly striking feature of the animal. The iris is sometimes beautifully metallic in appearance, for example in Bufo toads, and is often intricately marbled or reticulated with fine venation. In at least one anuran, the Red-Eyed Tree Frog, Agalychnis callidryas, the eyes may play a role as a predator deterrent. When the frog is at rest during the day its eyes are closed, but if disturbed it flashes them quickly open. The sudden appearance of the large, brilliant red eyes may startle a predator just long enough to give the frog time to leap to safety.

Pupil shape varies depending on species but is most commonly vertically elliptical (catlike), as in spadefoot toads, Pelobatidae, or horizontally elliptical, as in ranid frogs. However, they can be round, as in narrow-mouthed frogs, Gastrophryne, rounded-triangular, as in painted frogs, Discoglossus, or heart-shaped, as in fire-bellied toads, Bombino. The vertical pupils of some species can contract and dilate to an extremely high degree, from a narrow hairline slit in bright light, to wide open and rounded (resulting in the almost total disappearance of the iris) on dark nights.

The eyes of most anurans are protected by moveable eyelids, the lower one more mobile than the upper. There is a thin transparent or translucent "third eyelid," or nictitating membrane, fused to the bottom of the lower lid. This protective membrane can be drawn completely over the eye when the anuran is jumping or swimming, and also cleans and moistens the eyes when it is on land.

One unusual function of anuran eyes is that in some species they assist in the swallowing of food. The eyeballs are retracted downward through the sockets and help push the food down the gullet.

NOSE AND OLFACTORY SENSE

Anurans have a well-developed sense of smell, which is probably important for locating breeding sites, especially in species that undergo seasonal breeding migrations, and also for homing orientation. Anurans have two areas, both located in the nose, that respond to chemicals in the environment: the lining of the nostrils known as the olfactory epithelium, which probably mainly detects air and waterborne odors; and the Jacobson's Organs, located close to the internal nostrils, which detect or taste substances in the mouth. It is thought important in free-swimming tadpoles as, in some species at least, the larvae appear to be imprinted with the characteristic odor of their natal site, returning to it when ready to breed as adults.

HEARING AND VOCALIZATIONS

Hearing is well developed and the eardrum, or tympanum, of many species is visible as a flat, smooth, often circular area of skin situated behind the eye.

Many anurans are highly vocal. Males of many species emit calls, often very loud, to defend a territory and attract a mate, and when threatened or attacked by a predator, a distress cry may be uttered. The deep "br-wum" of the American Bullfrog, Rana catesbiana is audible to humans at a distance of around one mile (1–2 kilometers). Calls vary tremendously, and are often amplified by vocal sacs, which are soft, thin-skinned structures that become balloonlike resonators when inflated. Dependent upon species, there may be a single sac situated on the throat, or a pair, with one either side of the throat or mouth. Some anurans also inflate their body when calling, the air being pushed forward over the vocal chords and into the vocal sac. Some species lack vocal sacs altogether – for example, tailed frogs, Ascaphus, which inhabit fast-flowing streams – perhaps as a evolutionary consequence, as vocalizations would be drowned out by the sound of rushing water.

Calls are specific to a species, which is very important when more than one is breeding in the vicinity. It enables a female to locate the "correct" male, and when closely related species breed together, the differences in calls tend to prevent hybridization. In some species, the male may call singly or In chorus with large numbers of others in communal breeding grounds. The calls maintain a minimum distance between each frog. Male anurans are often very unselective about what they grab hold of during the mating frenzy. Frequently other males are grasped, but are quickly freed upon uttering a short, distinctive release call.

Weather conditions may influence calling. In temperate areas, for example, if spring breeding is interrupted by a cold snap, the frequency of calling may become much reduced or cease.

Calls are a good way to identify and survey species, especially if the animals themselves are difficult to locate. In the case of some very similar-looking species their distinctive calls are perhaps the only reliable way to distinguish them in the field.

LEGS AND FEET

Anurans have four limbs, the greatly enlarged hind legs of most species being an adaptation for jumping and swimming. The forelegs are usually considerably smaller and in many the forefeet are utilized like hands to eat. Those with short legs, as opposed to long-legged leapers, are short hoppers and tend to be terrestrial or burrowers.

The amount of webbing between the toes varies with species. There is usually little or no webbing between the fingers but there are exceptions, and is often indicative of lifestyle. Those with long toes and extensive webbing, such as the pipa toads (Pipidae), are usually highly aquatic. Those with little or no webbing are mostly terrestrial. A few highly adapted arboreal frogs, like the Wallace's Flying Frog, (Rhacophorus nigropalmatus have), have long, webbed fingers and toes and, with digits splayed wide, are able to perform a remarkable downward glide or "parachute" when leaping from up high.

Most climbing anurans, including tree frogs and glass frogs, have expanded adhesive pads on the digit tips. Mucous glands open on the flat undersides of the pads and adhesion is primarily by surface tension. This allows them to climb smooth vertical or near-vertical surfaces. Many arboreal species have digits that are able to grasp small stems or twigs. Some terrestrial species have the undersides of their hands and feet covered in tough tubercles to reduce abrasion and increase grip. Spadefoot toads and Australian burrowing frogs, have a sharp-edged, hardened "spade" on the edge of the hind foot, used for burrowing.

EGGS

An anuran egg is made up of the ovum contained within one or several transparent or translucent gelatinous capsules, coated by a flexible membrane. In most eggs laid in water, the upper part of the ovum is protected from harmful ultraviolet light by a dark brown pigment, melanin. This pigment also has heat-absorption properties, concentrating warmth from solar radiation and so speeding up embryo development, important in temperate species where temperature may be a limiting factor in reproduction.

When first laid, the jelly layers of eggs deposited in water rapidly absorb water and swell so that within a few hours they have attained their full size. The size and number of eggs vary immensely and are related to reproductive strategy. Typically, eggs laid on land are larger, have a reduced gelatinous coat and a tougher outer membrane.

Almost all European and North American anurans lay eggs in still or slow-flowing water of ponds, marshes, and occasionally rivers. In subtropical and tropical regions most species also lay eggs in water and have free-swimming tadpoles. Many subtropical and tropical anurans lay their eggs on land, where they undergo direct development, the tadpole stage passing within the egg and the young emerging as miniature adults. Most temperate species spawn once a year, typically in the spring when many undergo short migrations to their breeding grounds. Two species – the West African Live-bearing Toad, Nectophrynoides occidentalis and the Golden Coqui, Eleutherodactylus jasperi – are unique in that they give birth to fully developed young.

TADPOLES

Upon hatching, a typical aquatic tadpole (or larva) in still or slow-flowing water has external gills visible as feathery growths on the sides of the head. They are very poor swimmers, able to do little more than wriggle weakly, and have a suctorial and/or adhesive organ on the underside of the head, which produces a sticky mucus that enables them to cling to the disintegrating gelatinous egg mass, submerged vegetation, or other objects. The gills are soon concealed by a covering of skin and are aerated by a flow of water entering through the mouth and exiting via a small hole in the body wall, the spiracle.

As aquatic tadpoles grow and become more adept at swimming, the attachment organs are generally lost. The mouth of most tadpoles is bordered by a hardened horny beak and contains microscopic rows of labial (lip) teeth. The arrangement of these is unique to a particular species and therefore very useful for identification purposes. The mouth is fringed on either side, or more or less surrounded, by sensory papillae. Mouth parts are adapted to diet, some tadpoles being vegetarian, others carnivorous, and many omnivorous. They may be microphagous (feeding on small particles), grazing on algae, leaves, or rotting detritus. Many are macrophagous (eating larger food items), including carnivorous species with specialized mouthparts for cutting, rasping, or even swallowing prey whole. Many species have cannibalistic tendencies and will attack and consume tadpoles of their own kind.

(Continues…)

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Excerpted from "Frogs and Toads"
by .
Copyright © 2004 St. Martin's Press.
Excerpted by permission of St. Martin's Press.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

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Table of Contents

Introduction,
Anuran Characteristics,
What is the Difference Between a Frog and a Toad?,
Key Features of Frogs and Toads,
Eyes and Sight,
Nose and Olfactory Sense,
Hearing and Vocalizations,
Legs and Feet,
Eggs,
Tadpoles,
Predators and Defense,
Hunting and Prey,
Habitats and Niches,
Conservation,
Classification,
Family and Species Accounts,
Glossary,
Index,