(Pipidae)
Class: Amphibia
Order: Anura
Family: Pipidae
Thumbnail description
Fully aquatic anurans distinguished by having a dorsoventrally depressed body; holding their limbs in a laterally sprawled position; having fully webbed feet and tiny, dorsally placed eyes; lacking a tongue; and retaining the lateral-line system as adults
Size
Small-to-medium sized anurans ranging from 0.8 to 1.2 in (20 to 30 mm) long up to 4.1–6.7 in (104–170 mm) long
Number of genera, species
5 genera; 30 species
Habitat
All pipids are aquatic and leave the water only under duress; in Africa and South America, they are found in almost every kind of water body, including lakes, rivers, swamps, forest ponds, and varieties of human-made bodies of water
Conservation status
Vulnerable: 1 species
Distribution
Africa south of the Sahara in the Old World; in extreme lower part of Central America, the Amazon Basin of South America, and coastal areas of Venezuela, Guyana, French Guiana, Suriname, and Brazil in the New World
Evolution and systematics
The family Pipidae is distinguished by a few soft anatomical characters, their larvae, and many skeletal features that involve the structure of the skull and the vertebral column. Living representatives are placed into two subfamilies—Xenopodinae, comprising Xenopus and Silurana in Africa, and Pipinae, comprising Pipa in the New World and Hymenochirus and Pseudhymenochirus in Africa. Pipids are indisputably basal but highly derived anurans; their relationships to other archaeobatrachian frogs are controversial.
The family has an extensive fossil record that spans about 90 million years and two continents—Africa and South America. The most ancient fossil is Pachybatrachus taqueti, which lived 84–90 million years ago (Upper Cretaceous) in what is now the Republic of Niger. Pachybatrachus is related closely to the living Hymenochirus and Pseudhymenochirus. A Cretaceous pipid 71–84 million years old, Saltenia ibanezi, lived in southern South America (Argentina), and a Paleocene pipid that is 60 million years old, "Xenopus" romeri, is known from Brazil. (The generic name Xenopus is placed inside quotation marks because scientists are not certain that it is a member of the living genus of that name.) There are two slightly younger (34–55 million years old) Eocene fossil pipids from Argentina—Shelania pascuali and S. laurenti. A contemporaneous Eocene fossil pipid, Eoxenopoides reuningi, is known from South Africa.
Two species of Xenopus, X. hasaunas and X. arabiensis, were described from the Lower Oligocene of Libya (30–34 million years ago) and Late Oligocene (26–30 million years ago) of the Republic of Yemen, respectively. The youngest pipid fossils are Xenopus stromeri, from the Lower Miocene (16–23 million years ago) of South Africa, and, from the Miocene of Morocco, Silurana tropicalis—a fossil that, if correctly identified, is represented by living frogs in central and west Africa today. It seems reasonable to speculate that Pipidae originated on the southern, Gondwanan landmass and was well established before South America completely separated from Africa about 80 million years ago, in the Late Cretaceous. From the Early Triassic through the Jurassic, the southern parts of the incipient continents experienced warm-temperate climatic conditions, not unlike those of eastern temperate Africa today, and during the Cretaceous, tropical conditions prevailed in the northern parts of both Africa and South America, where living pipids are found today.
Physical characteristics
Pipid frogs are medium-sized to large anurans with extra-ordinarily depressed bodies and flat to wedge-shaped heads with small, dorsally placed eyes. All lack tongues and tympana. The adults retain lateral-line organs, visible on the head and body as a series of "stitches." Usually the forelimbs are small; in all pipids except Hymenochirus and Pseudhymenochirus, the slender fingers are not webbed. The hind limbs are robust and the feet fully webbed. In all pipids except Pipa pipa and P. snethlageae, the inner three toes bear keratinized "claws." Xenopus species typically have smooth skin, whereas Pipa and the hymenochirines have tuberculate skin. The anurans usually are tan to olive brown to gray, with darker spots and mottling dorsally and paler coloring ventrally with darker mottling.
Like other anurans, pipids communicate acoustically, but under water rather than in the air. They lack vocal cords and vocal sacs and have a highly modified laryngeal apparatus to produce the typical "clicking" call. The small, cup-shaped arytenoid cartilages inside the larynges of other anurans are represented by a pair of large, cartilaginous discs, the medial surfaces of which are tightly opposed when the frog is not calling. Contraction of the laryngeal muscles separates the discs and produces the "click"—a sound that is thought to result from implosion of air rushing into the cleft that abruptly opens between the discs. Although a tympanum is absent, there is a large, circular cartilaginous disc located beneath the skin on the side of the head. This is part of the stapes (middle ear bone) that transmits sound vibrations received through the water to the inner ear of the frog.
Distribution
Pipids occur in lowland, tropical South America and sub-Saharan Africa. In Africa they are found from sea level to elevations of 9,000 ft (2,780 m). Introduced populations of Xenopus laevis occur in the United Kingdom, Europe, South America, and the United States.
Habitat
Pipids are totally aquatic, occupying temporary and permanent bodies of water, including swamps, reservoirs, and slow-moving streams and rivers.
Behavior
Little is known about diel (daily/nightly) activity or inter- and intraspecific territoriality of pipids, because they are exceptionally difficult to observe in nature. Apparently, they remain in a body of water as long as it is suitable. During droughts they burrow into the mud at the bottom of the pond or swamp and estivate up to several months, and during rains they undertake short overland excursions at night, moving from one body of water to another.
Feeding ecology and diet
Because pipids lack a tongue, prey capture is quite different from that in other anurans and is best known in Xenopus laevis. Like all other pipids, Xenopus species eat anything that they can catch, from aquatic invertebrates to fish, birds, and mammals, as well as their own larvae. Having teeth, Xenopus is able to grip its prey. Observations suggest that the frogs bite their prey. While holding it, they use their powerful hind limbs to claw at the prey and shred it and their forelimbs to shove the prey into the mouth. Adult Xenopus species have been reported to attack prey in groups and collectively tear the body of the prey into fragments that can be ingested. Edentate pipids, those with no teeth, (e.g., Pipa pipa) lunge at prey and suck it into the mouth using their forelimbs as Xenopus does. Pipids detect prey by chemosensory cues in the water, vision, and vibrations detected by the lateral-line system.
Reproductive biology
Breeding in pipids seems to coincide with the onset of rains and, in this sense, is opportunistic; under appropriate conditions, however, the frogs seem to be capable of breeding throughout the year. Both sexes vocalize and have repertoires of three to six types of clicking calls that are emitted under water. The advertisement calls of different species are distinguished by their temporal frequencies and the dominant frequency of the call. For example, an isolated male Xenopus borealis advertises his presence by two to four single clicks per second. A male of the same species, approaching a female, emits a call of 10 clicks per second, and frogs of both sexes emit a release call consisting of 20 clicks per second.
Old World pipids (Xenopus, Silurana, Pseudhymenochirus, and Hymenochirus) deposit their eggs in water. Free-swimming tadpoles hatch from the eggs and undergo their development in the water. All larvae lack keratinous mouthparts; those of Xenopus and Silurana bear sensory barbels at the periphery of the mouth. The larvae of Xenopus and Silurana are exclusively filter feeders, whereas those of Pseudhymenochirus and Hymenochirus are predaceous feeders on aquatic insect larvae and ostracods. Among New World pipids, the eggs are deposited on the backs of the females in Pipa, although this is not verified in one species, Pipa myersi. Two species have free-swimming larvae that lack barbels around the mouth (P. myersi and P. parva). In the remaining five species, the tadpoles undergo their development on the backs of the females and emerge from her back as miniatures of the adults.
Conservation status
Xenopus gilli (Gill's plantanna) is listed as Vulnerable by the IUCN, and as Endangered according to the South African Red Data Book. Many species in the lowland rainforests of sub-Saharan Africa, however, are threatened by habitat destruction.
Significance to humans
As discussed in the accounts that follow, pipids are tremendously important to humans, because of their use as biomedical experimental animals and their popularity in the pet trade. Medical researchers are investigating several substances found in the skin of some pipids. Among them are magainins (antimicrobial peptides that inhibit the growth of numerous bacteria and fungi) and other peptides, such as xenopsin and caerulein, that cause predators to vomit if they eat these frogs. Pipids are of particular interest to systematic biologists because of their widespread distribution and extensive fossil record.
Species accounts
Common plantannaMüller's plantanna
Tropical clawed frog
Surinam toad
Resources
Books:Channing, Alan. Amphibians of Central and Southern Africa. Ithaca, NY: Comstock Publishing Associates, Cornell University Press, 2001.
Rödel, Mark-Oliver. Herpetofauna of West Africa. Vol. 1, Amphibians of the West African Savanna. Frankfurt: Edition Chimaira, 2000.
Tinsley, R. C., and H. R. Kobel, eds. The Biology of Xenopus. London: Clarendon Press, 1996.
Periodicals:Báez, Ana María, and Lourdes Analía Púgener. "A New Paleogene Pipid Frog from Northwestern Patagonia." Journal of Vertebrate Paleontology 18 (1998): 511–524.
Cannatella, David C., and Linda Trueb. "Evolution of Pipoid Frogs: Intergeneric Relationships of the Aquatic Frog Family Pipidae (Anura)." Zoological Journal of the Linnean Society 94 (1988): 1–38. ——. "Systematics, Morphology, and Phylogeny of Genus Pipa (Anura: Pipidae)." Herpetologica 42 (1986): 412–449.
Henrici, Amy C., and Ana María Báez. "First Occurrence of Xenopus (Anura: Pipidae) on the Arabian Peninsula: A New Species from the Late Oligocene of the Republic of Yemen." Journal of Paleontology 75 (2001): 870–882.
Rabb, George B., and Mary S. Rabb. "On the Mating and Egg-Laying Behavior of the Surinam Toad, Pipa pipa." Copeia 4 (1960): 271–276. ——. "Additional Observations on Breeding Behavior of the Surinam Toad, Pipa pipa." Copeia 4 (1963): 636–642.
Trueb, Linda, and David Massemin. "The Osteology and Relationships of Pipa aspera (Amphibia: Anura: Pipidae), with Notes on Its Natural History in French Guiana." Amphibia-Reptilia 22 (2001): 33–54.
[Article by: Linda Trueb, PhD]
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