True Toads, Harlequin Frogs, and Relatives (Bufonidae)

(vertebrate zoology) A family of toothless frogs in the suborder Procoela including the true toads (Bufo).

(Bufonidae)

Class: Amphibia

Order: Anura

Family: Bufonidae

Thumbnail description
These are tiny to very large, generally warty or dry-skinned frogs, with usually unspecialized digital tips; most have parotoid glands

Size
0.6–9.8 in (15–250 mm)

Number of genera, species
33 genera; 344 species

Habitat
Deserts, savanna, dry and humid forests, from sea level to 16,404 ft (5,000 m)

Conservation status
Critically Endangered: 1 species; Endangered: 6 species; Vulnerable: 6 species; Lower Risk/Near Threatened: 2 species; Data Deficient: 3 species

Distribution
Worldwide, except for Madagascar, Australia (introduced), and New Guinea; bufonids just barely cross Wallace's Line to the east, and are present on the Indonesian island of Sulawesi

Evolution and systematics

Bufonidae currently contains 33 genera. Subfamily names have been proposed, but these were based on geographic distribution rather than evolutionary relationships and have not been widely accepted.

Scientists have not determined which are the closest relatives of Bufonidae. Although many data from DNA sequences have been accumulated recently, no other group of frogs has emerged as a close relative to Bufonidae. Although not known with certainty, South America is generally believed to be the continent of origin for the group.

Bufo is known as far back as the Oligocene (Whitneyan) of North America, the Miocene of Europe, western Asia, and North Africa, and questionably from the Middle Paleocene (with certainty from the Miocene) of South America.

Evolutionary novelties that unite the species of Bufonidae include the presence of Bidder's organ; a unique pattern of insertion of the rectractor muscle of the tongue (hyoglossus); the loss of the posterior constrictor muscles of the larynx; the absence of teeth; and the presence of the "otic element," an independent bone in the temporal region that fuses indistinguishably to the posterior arm of the squamosal bone.

Physical characteristics

The term toad is usually applied to frogs in the family Bufonidae. Sometimes "toad" is used for any frog that is rough-skinned, regardless of its evolutionary relationships. More often, toad is used to describe any member of the family Bufonidae. English is not the only language to recognize frogs and toads; the distinction is made in languages as diverse as French, German, Quechua, and Bahasa Indonesia.

Toads in the family Bufonidae are relatively diverse in their appearance. Yet all of them share certain structural characteristics that unite them into this taxon. Most frogs have teeth on the upper jaws, but all bufonids lack them. At the turn of the twentieth century, the presence or absence of teeth was considered to be a significant character for classification, and several species that herpetologists now know are unrelated were grouped into Bufonidae simply because they lacked teeth. Although several other groups of frogs have independently lost teeth, the absence of teeth in bufonids remains a diagnostic feature of the group.

Only the Bufonidae among frogs have a Bidder's organ. In amphibians, the testis and ovary develop from an undifferentiated mass of gonadal tissue. During larval development, the gonadal tissues of future males secrete testosterone, which causes the animal to develop as a male. In the absence of testosterone production, the animal will become a female. In other words, being female is the default sex. The organ of Bidder is a bit of gonadal tissue that apparently retains its female attributes in male toads, which also develop normal testes. If the testes of adult toads are removed surgically, Bidder's organ will transform into a functional ovary. Thus it seems that the presence of a functional testis, which produces male hormones, suppresses the development of the ovarian tissue of the bidder's organ. It is not known whether Bidder's organ has an adaptive or functional role in the natural life of toads.

Bufonids are basically hoppers and walkers, never leapers. The joint between the hip bones (pelvic girdle) and vertebral column (at the sacrum) is modified such that the range of motion is not in the longitudinal vertical plane, as in leapers such as Rana, but rather movement is from side to side. Also, several genera of bufonids apparently have evolved skeletal modifications that perhaps reflect the reduced locomotor abilities of these toads. They have seven or fewer vertebrae (rather than eight as in most frogs). The coccyx is fused to the sacrum, rather than having a flexible joint. The left and right halves of the shoulder girdle are fused to each other, rather than having a flexible joint at midline. The number of bones in the hands and feet is reduced; the lengths of the fingers and toes are correspondingly shorter; and the hands and feet look more like a mitten rather than a glove (the name Atelopus means "incomplete foot"). All of these modifications suggest a reduced ability to jump. In fact, this is true. But also, these toads are all rather small, and the skeletal modifications might also result from a smaller size.

Skin glands are present in almost all amphibians and are generally widely distributed throughout the skin as small structures that are not obvious. In contrast, the parotoid gland consists of closely spaced skin glands concentrated into a prominent organ behind the ear. Within bufonids, one can distinguish two groups, those with parotoid glands and those without. Parotoid glands are found in all species of Bufo, as well as members of several of the non-Bufo genera. However, several non-Bufo genera lack parotoid glands. In some species the glands are difficult to distinguish without a close examination of the skin in cross section. When disturbed, the toad can discharge a milky venom from the glands, sometimes through the air. The secretions of large toads have been known to kill predators such as dogs.

Distribution

The genus Bufo has a world-wide distribution, with radiations in North America, Central America, South America, the West Indies, Africa (but not Madagascar), Europe, and all of Asia including Japan, the Philippines, Southeast Asia, and Sulawesi east of Wallace's Line.

The non-Bufo genera in the Neotropics include Crepidophryne and Atelophryniscus, which are endemic to Central America; Atelopus in Central and South America; and Andinophryne, Atelopus, Dendrophryniscus, Frostius, Melanophryniscus, Metaphryniscus, Osornophryne, Oreophrynella, Truebella, and Rhamphophryne in South America. The other genera endemic to Africa are Altiphrynoides, Capensibufo, Didynamipus, Laurentophryne, Mertensophryne, Nectophrynoides, Nectophryne, Nimbaphrynoides, Schismaderma, Spinophrynoides, Stephopaedes, Werneria, and Wolterstorffina. Some genera are found in Southeast Asia: Ansonia, Leptophryne, Pedostibes, Pelophryne, and Pseudobufo. Genera endemic to the Indian subcontinent include Adenomus and Bufoides.

Habitat

It is difficult to make general statements for a group with as many species and with as broad a geographic range as Bufonidae. Species may be found in near-desert to primary tropical rain-forest habitats, from sea level to 16,400 ft (5,000 m) in treeless alpine environments. The genus Bufo occupies the greatest range of latitudes and altitudes of any frog. Most bufonids would be termed terrestrial; very few are fully aquatic or arboreal.

Behavior

Behavior in this diverse group of toads varies. Very little is known about some species, while others are more well-studied. Atelopus varius is known to have exceptional homing ability.

Feeding ecology and diet

Like most frogs, bufonids feed mainly on a diet of arthropods. Ants form a large part of the diet in tropical areas. Despite their size, large toads such as Bufo marinus are not decidedly carnivorous, although they are capable of eating small mammals such as mice.

Reproductive biology

The mating call of most species is a trilled call emitted at a rather steady pitch, rather than an untrilled or pure tone that might rise or drop in pitch. Most bufonids lay numerous, small pigmented eggs that are enclosed single file in strings of jelly, rather than in a discoid or globular egg mass. These egg masses are usually laid in temporary ponds rather than large bodies of water or streams. Typically, the eggs develop quickly, with tiny tadpoles hatching out in large numbers. Most of these die, and the few that make it through metamorphosis do so as very tiny toadlets, regardless of the ultimate size of the adult. Even Bufo marinus has small toadlets; therefore the total weight increase over the life of the animal may be several orders of magnitude. Tiny toadlets are notoriously difficult to identify to the exact species. A few species of bufonids deviate from this general pattern and are presumed to have direct development, because the eggs observed in dissected specimens are large, few in number, and not pigmented. Few toads are known to exhibit parental care.

Many species of toads are known to hybridize in nature, and hybrids have been produced in the laboratory between species that are very distantly related, even from different continents.

Conservation status

The IUCN lists 1 species as Critically Endangered (Bufo periglenes), 6 as Endangered, 6 as Vulnerable, 2 as Lower Risk/Near Threatened, and 3 as Data Deficient. Although many species are not threatened and some are so common in human settlements as to be considered pests, several species appear to have suffered dramatic and mysterious population declines since the 1980s, along with many other amphibians.

Significance to humans

Because of their ubiquity, toads have been the subject of myth and lore.

Species accounts

Resources

Crump, Martha L. In Search of the Golden Frog. Chicago: University of Chicago Press, 2000.

Duellman, William E., and Linda Trueb. Biology of Amphibians. New York: McGraw-Hill, 1986.

Inger, Robert F., and Robert B. Stuebing. A Field Guide to the Frogs of Borneo. Kota Kinabalu: Natural History Publications, 1997.

Sanchíz, Borja. Salientia. Part 4: Encyclopedia of Paleoherpetology. Munich: Verlag Dr. Friedrich Pfeil, 1998.

Savage, Jay M. The Amphibians and Reptiles of Costa Rica. Chicago: University of Chicago Press, 2002.

Crump, Martha L. "Homing and Site Fidelity in a Neotropical Frog, Atelopus varius (Bufonidae)." Copeia 1986 (1986): 438–444.

Daly, John W., Robert J. Highet, and Charles W. Myers. "Occurrence of Skin Alkaloids in Non-dendrobatid Frogs from Brazil (Bufonidae), Australia (Myobatrachidae) and Madagascar (Mantellinae)." Toxicon 22 (1984): 905–919

Ford, Linda S., and David C. Cannatella. "The Major Clades of Frogs." Herpetological Monographs 7 (1993): 94–117.

Graybeal, Anna. "Phylogenetic Relationships of Bufonid Frogs and Tests of Alternate Macroevolutionary Hypotheses Characterizing their Radiation." Zoological Journal of the Linnean Society 119 (1997): 297–338.

Graybeal, Anna, and David C. Cannatella. "A New Taxon of Bufonidae from Peru, with Descriptions of Two New Species and a Review of the Phylogenetic Status of Supraspecific Bufonid Taxa." Herpetologica 51 (1995): 105–131.

McDiarmid, Roy W. "Comparative Morphology and Evolution of Frogs of the Neotropical Genera Atelopus, Dendrophryniscus, Melanophryniscus, and Oreophrynella." Science Bulletin of the Los Angeles County Museum of Natural History 12 (1971): 1–66.

McDiarmid, Roy W., and Stefan Gorzula. "Aspects of the Reproductive Ecology and Behavior of the Tepui Toads, Genus Oreophrynella (Anura, Bufonidae)." Copeia 1989 (1989): 445–451.

Roessler, Martha K. P., Hobart M. Smith, and David Chiszar. "Bidder's Organ: A Bufonid By-product of the Evolutionary Loss of Hyperfecundity." Amphibia-Reptilia 11 (1990): 225–235.

Wake, Marvalee H. "The Reproductive Biology of Nectophrynoides malcolmi (Amphibia: Bufonidae), with Comments on the Evolution of Reproductive Modes in the Genus Nectophrynoides." Copeia 1980 (1980): 193–209.

[Article by: David Cannatella, PhD]