Frogs of the United States and Canada, 2-vol. set

Introduction

I have always liked frogs . . . I like the looks of frogs, and their outlook, especially the way they get together in wet places on warm nights and sing about sex. Archie Carr, The Windward Road As this book is completed (April 2013), there are >6,800 species of frogs known worldwide with new species being described at a rapid pace. North of the United States–Mexico border, however, only 100 species are known (1.7% of the world total), and 10 of them barely enter the United States. All of Hawaii’s frogs are introduced species, and only 27 species occur in Canada, with many only just entering the country. Only the genera Acris, Ascaphus, Pseudacris, and Spea may be considered endemic to our region, although Pseudacris and Spea also cross the border into northern Mexico. The regions of temperate North America with the greatest species richness of frogs are the south Atlantic Coastal Plain, south Texas, and the Pacific Northwest. A surprising diversity of frogs is found in the arid Southwest. Although it might appear that the diversity of North American frogs is reasonably well known, one new species from the Southeast will be described, perhaps by the time this book is published (see Lithobates catesbeianus account) and a number of subspecies or isolated populations are likely to be reevaluated as full species, especially in the Anaxyrus boreas complex. No one can mistake a frog for any other vertebrate . All frogs are tetrapods and have the same basic body plan (Figs. 1, 2), with short heads, large bulging eyes (reduced in some fossorial forms), little or no neck, and a short compact body. The front legs have four toes on each foot; the rear legs have five toes on each foot and are often webbed to some extent except in terrestrial Introduction Fig. 1. Basic body plan of an aquatic frog. Illustration by Camila Pizano Fig. 2. Basic body plan of a toad. Illustration by Camila Pizano xii INTRODUCTION which the frog walks, enabling it to hold on and climb. The largely ground-dwelling chorus frogs do not need such expanded toe pads. Aquatic species usually have a membranous web between their hind toes that facilitates swimming, but Xenopus has webs between the front toes as well. A comparison of the hind feet of bufonids, hylids, and ranids is provided in Fig. 4. Frogs do not have external ears, although they are very attuned to sound. The opening to the inner ear is covered by a thin, membranous sheath of skin, the tympanum, located behind the eyes. The tympanums of American Bullfrogs, for instance, are rounded and large; they are conspicuous on either side of the head. The tympanums of some other species are less easily seen or may not be present (Ascaphus). Many frogs have “warts,” bumps or ridges on the back (the dorsum), dorsolaterally, or on the upper portions of the limbs. These bumps and ridges usually contain mucous or granular glands and are important for moisture retention and defense. The parotoid glands of toads are kidney bean– shaped structures located on the head behind the eyes, and their shape and size are important, in conjunction with the configuration of the cranial crests, for the identification of the different species of toads (see Fig. 2). Male frogs of many species can be distinguished from females by the presence of vocal pouches and a darkened coloration on their throats, at least during the breeding seasons. Males also develop enlarged and roughened thumbs during the reproductive season, which are useful while amplexing females (that is, when the male clasps the female during courtship). Female frogs are often much larger than males, and usually do not produce the “warning croak” when picked up. Eggs may be visible through the ventral body wall. Outside the breeding season, differentiating the sexes may be difficult. Juveniles usually resemble miniature adults. Anuran Evolution The first vertebrates to leave the water did so during the Devonian Period, some 350 to 370 million years ago. These animals were transitional between the lobe-finned fishes and the true Amphibia . These amphibian ancestors moved around on land, based on the fossilized trackways that forms. Frogs, of course, lack tails as adults. Their short powerful bodies with specialized hind legs are made for hopping, jumping (sometimes in great leaps), running, climbing, swimming, or burrowing. Toads (Anaxyrus, Ollotis, Rhinella), for example, usually walk or hop, whereas the so-called true frogs (Lithobates, Rana) and chirping frogs...