Clawed frogs and Surinam toads

(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

List of Species

Common plantanna

Xenopus laevis

subfamily

Xenopodinae (= Dacylethrinae)

taxonomy

Bufo laevis Daudin, 1802, type locality not designated. Five subspecies are recognized.

other common names

English: Plantanna, African clawed frog, clawed toad, clawed frog, upland clawed frog; German: Glatter Krallenfrosch; Spanish: Rana de garras africana.

physical characteristics

Xenopus laevis varies in size over the range of the species; males, however, always are smaller than females within a given population. Female frogs range from 2.2 to 5.8 in (57 to 147 mm) and males from 1.8 to 3.8 in (45.6 to 97.5 mm) in snout-vent

length. The body and head are depressed, and the small, round eyes are located on top of the head. The skin is smooth. The hind limbs are long and robust. The three inner toes of the large, fully webbed feet bear small, black claws. The inner metatarsal tubercle is an elevated ridge. The subocular tentacle is minute, less than half the diameter of the eye, and there are 23–31 lateral-line bars between the eye and the vent. Although the dorsal coloration varies, it usually is dark—gray to greenish brown—and marked with darker blotches. The venter is pale and may bear irregular spots. The toe webbing usually is gray and occasionally is tinged with yellow.

distribution

Members of this group occupy mainly savannas of the Republic of South Africa north to Kenya, Uganda, and the Democratic Republic of Congo westward to Cameroon. These frogs are not found in the Congo Basin or the hotter lowlands of eastern Africa. Owing to the pet trade and common use of Xenopus as a laboratory animal, the frog (most likely X. laevis) has been introduced in Europe, the United States, and South America. Introduced populations thrive in the United Kingdom (Isle of Wight, South Wales, and southeast and southwest England), mainland Europe (Germany and the Netherlands), Chile (specific locality unknown), Ascension Island, and the United States (Tucson, Arizona; and Los Angeles, Orange, Riverside, and San Diego counties in California).

habitat

An extraordinary trait of this species is its apparent environmental tolerances and lack of discrimination with regard to its habitat, as long as there is a body of water for the frog to occupy. Doubtless this accounts for the success of the species in the laboratory and as invasive populations around the world. The species occurs in any kind of body of water, including rivers, lakes, reservoirs, swamps, flooded pits, ditches, and wells, and at elevations from sea level to about 9,000 ft (2,780 m) in the Drakensberg Mountains of South Africa. Water quality seemingly is not an issue. The species is common in stagnant, still waters and sluggish streams as well as fast-flowing waters. Similarly, it can be found in water occluded with organic detritus or clear water and in acidic or alkaline waters.

Unlike nearly all other anurans, the common plantanna can tolerate saline waters and has been known to survive in 25% seawater indefinitely and in 40% seawater for a few days. Temperature, as it affects larvae, may be a limiting factor for the species, although they display a remarkable tolerance; in laboratory tests, the critical lethal minimum and maximum temperatures for larvae were shown to be 50°F (10°C) and 95°F (35°C), respectively. Introduced populations survive in ice-covered ponds for several months a year as well as in ponds that are subject to the extreme summer temperatures of southern Arizona. In the introduced southern California populations, when temperatures reach 86°F (30°C), the frogs burrow into the pond bottom to depths of about 12–16 in (30–40 cm), where the temperature is relatively stable at 68°F (20°C).

behavior

Under good conditions, members of this group do not leave the water, although they may undertake short nocturnal excursions. Prolonged drought, however, forces them to estivate. They have been observed to estivate in the laboratory for up to eight months and doubtless are capable of the same in nature, where they burrow backward into drying mud and occupy a vertically oriented chamber. Physiological tests reveal that estivating X. laevis have a number of adaptations to survive desiccation and starvation. Rather than excreting ammonia, which is highly toxic, they excrete the less-toxic urea that is accumulated in the blood, liver, and muscle tissue. The frogs can reduce their oxygen consumption by 30%. Frogs maintained in water can survive a year without food and incur as much as a 35–45% loss in body weight. They survive by using stored carbohydrates and lipids for energy for the first four to six months, after which time, the frogs switch to protein catabolism (especially breakdown of body muscle) for energy. If they survive an extended period of drought, they are likely to migrate over land en masse with the onset of torrential rains.

feeding ecology and diet

The food preferences of this species are as wide ranging as are the aquatic habitats in which the species is found. Typically, the hatching of tadpoles coincides with algal blooms, and the larvae, which have a highly specialized filter-feeding mechanism, are able to extract algae and the finest suspended organic matter from the water. Recently metamorphosed froglets seem to specialize in small crustaceans and aquatic insect larvae. Adults eat insects and prey on or scavenge other vertebrates—anurans, fish, birds, and small mammals. They have been seen to leap out of the water to capture winged insects. Although the species typically is not found in bodies of water with high natural populations of fishes, it is known to prey occasionally on fish in constrained situations (e.g., isolated pools and hatcheries). Adult frogs feed on other species of anurans and are cannibalistic with respect to their own larvae and young. They will eat small birds and rodents that fall into the water. There is no evidence that the species feeds on land, and in this regard they are quite distinct from most other anurans.

reproductive biology

Under laboratory conditions, the common plantanna will breed throughout the year and attains sexual maturity in eight months (i.e., six months after metamorphosis). In nature, when these frogs are exposed to seasonal differences in temperature (e.g., South Africa), development probably is slower. Mating coincides with the onset of heavy rainfall and a temperature of about 68°F (20°C) and thus varies throughout the range of the species. Oogenesis is determined by food supply, and prey abundance correlates positively with rainfall; thus, the onset of rainfall indirectly stimulates egg production. Heavy rains wash fresh sediments into the breeding sites and enrich the nutrients; this, in turn, triggers phytoplankton blooms that provide food for developing larvae.

Both male and female frogs call, and the calls vary in length, pattern, and frequency throughout the range of the species. In South African common plantanna, the male advertisement call is described as a long trill composed of alternating fast-pulsed (43–66 pulses per second) and slow-pulsed (24–42 pulses per second) elements, with the fast element lasting 0.18–0.6 seconds and the slow element lasting 0.34–0.9 seconds. Calling continues for several minutes, and frequencies up to 2.3 kHz are emphasized. When males clasp females in inguinal amplexus, they utter a soft amplectant call. Typically about 1,000 eggs are laid at a time and attached to aquatic vegetation or other underwater objects. The pale brown eggs are about 0.05 in (1.15 mm) in diameter.

conservation status

Not threatened.

significance to humans

X. laevis is probably one of the most familiar frogs to humans, owing to its long use as a model system for biomedical research and its popularity in the pet trade. Until the 1940s these frogs were used for pregnancy tests; injection of a small amount of urine from a pregnant woman under the frog's skin causes the frog to lay eggs. Biomedical researchers discovered that the species is a convenient experimental organism. It is easy to maintain in aquariums, and it is robust and has a relatively short life cycle. Unlike most other amphibians, it can be induced to provide fertile embryos throughout the year, and the embryos and their cells are large, making them convenient subjects for experimental manipulation and molecular research. In the past, members of this species were used as a food source in Cameroon, Sierra Leone, Central African Republic, Uganda, Rwanda, and the Democratic Republic of the Congo. The indigenous people of these countries used baited wicker baskets or traps to capture the frogs or occasionally drained smaller bodies of water and collected the stranded frogs.

Müller's plantanna

Xenopus muelleri

subfamily

Xenopodinae (= Dacylethrinae)

taxonomy

Dactylethra mülleri Peters, 1844, "Mozambique."

other common names

English: Mueller's clawed frog, Müller's smooth clawed frog, tropical plantanna, northern tropical plantanna, northern plantanna.

physical characteristics

Xenopus muelleri is moderately large; females are 2.6–3.5 in (65–90 mm) long, and males are 2.1–2.8 in (52–72 mm) long. The body and head are depressed, and the small, round eyes are located on top of the head; the skin is smooth. The hind limbs are long and robust. The three inner toes of the large, fully webbed feet bear small, black claws. The inner metatarsal tubercle is a small, fingerlike projection. The subocular tentacle is long and conspicuous, equal to or more than half the diameter of the eye, and there are 22–27 lateral-line bars between the eye and the vent. Dorsally, the frog is gray and marked with darker blotches. The venter varies from pale gray to darkly marked and may be deep orange-yellow on the belly and legs. The toe webbing is orange-yellow.

distribution

Müller's plantanna has the widest range of any pipid species and occurs in two disjunct populations, usually below 2,625 ft (800 m). In eastern Africa it is found from southeastern Kenya through Tanzania, Zanzibar and the Mafia islands, Zambia, Malawi, Botswana, Zimbabwe, and Mozambique and in eastern South Africa south to the area of Saint Lucia/Empangeni (ca. lat. 28°S). The western population extends from Burkina Faso and Ghana eastward to southern Sudan and the northeastern part of the Democratic Republic of the Congo.

habitat

This frog is found in hot, dry lowlands exclusive of rainforests. It frequents a wide variety of bodies of water and prefers permanent ones, such as reservoirs, ponds, and quiet regions of rivers during the dry season. Only rarely is it found in the same body of water with another species of Xenopus.

behavior

Members of the western population of Müller's plantanna seem to pass the dry season in the bank zone of rivers, burrowed in the mud of permanent savanna ponds and occasionally beneath humid layers of leaf litter. The frogs migrate short distances between ponds on rainy nights.

feeding ecology and diet

Adults eat toad tadpoles and fish.

reproductive biology

Little is known about the breeding behavior of Müller's plantanna. The species uses temporary bodies of water for breeding. Females produce small (0.04 in [1.0 mm] in diameter), dark gray eggs, which are attached singly on aquatic plants and rocks. There are many different reported advertisement calls, suggesting that they vary within and between populations. In South Africa the call of Müller's plantanna is a single note that lasts 0.2 seconds and consists of five to seven pulses at a rate of 26–32 pulses per second and an emphasized frequency of 774–1,142 Hz. The western population is reported to have a call consisting of a repetition of two-pulsed notes at a rate of four to eight pulses per second; the emphasized frequency is unknown.

conservation status

Not threatened.

significance to humans

None known.

Tropical clawed frog

Silurana tropicalis (= Xenopus tropicalis)

subfamily

Xenopodinae (= Dacylethrinae)

taxonomy

Silurana tropicalis Gray, 1864, "West Africa, Lagos," Nigeria.

other common names

None known.

physical characteristics

Silurana tropicalis is moderately small; females have an average length of 1.7 in (43 mm), and males have an average length of 1.4 in (36.6 mm). The body and head are depressed, and the small, round eyes are located on top of the head; the dorsal skin is pustulose, especially on the heads of males. The hind limbs are short and robust. The inner three toes bear small, black claws, and the inner metatarsal tubercle is in the form of a claw. The subocular tentacle is minute, less than half the diameter of the eye, and there are 18–20 lateral-line bars between the eye and the vent. Dorsally, the frog is olive to brown, with fine gray and black marks that never coalesce into larger spots. The venter is white to gray with scattered black mottling.

distribution

The tropical clawed frog is found in western Africa from the Casamance River (Senegal) to the Cross River (Nigeria); the eastern limit of distribution is undetermined.

habitat

Confined to lowland tropical forest below 2,297 ft (700 m), the tropical clawed frog is found in still and running waters. Occasionally, it is found in savanna ponds near forests after heavy rainfalls.

behavior

During heavy rain, tropical clawed frogs move between ponds at night. In the dry season the species is found along riverbanks under flat stones, in holes in the banks, or under roots by day; at night, it is found in small rock pools along the river. If isolated in pools, the frog burrows into the mud at the bottom of the pool.

feeding ecology and diet

Little is known about the diet of the tropical clawed frog. The species apparently is an opportunistic feeder and has been reported to eat arthropods and tadpoles—whether its own or those of other species is unknown.

reproductive biology

Male frogs grasp females in the inguinal region and perform mating turnovers in the water before attaching eggs to aquatic plants in forest pools. Apparently, they breed throughout the year when it rains. Although the frogs prefer larger bodies of water, they will use small, water-filled holes in the forest; during the day, adult frogs hide nearby under dead trunks in shallow water. The advertisement call is described as a deep rattling trill that sounds like "roaroaroa" and lasts one to 10.5 seconds, with an emphasized frequency of about 1 kHz.

conservation status

Not threatened.

significance to humans

None known.

Surinam toad

Pipa pipa

subfamily

Pipinae

taxonomy

Rana pipa Linnaeus, 1758, "Surinami."

other common names

French: Pipa américain; German: Wabenkröte.

physical characteristics

Arguably, Pipa pipa is the most bizarre-looking frog known. The frog is large, with adult females being 4–7 in (105–171 mm) long and the slightly smaller males being 4–6 in (105–154 mm) long. It has an improbably flat, triangular head surmounting an extremely depressed, wide body equipped with short, muscular hind limbs and immense, fully webbed feet. The tip of each finger is divided into four lobes, each of which is distally bifurcate. The eyes are minute and sometimes covered with skin. The nostrils are located at the tip of the snout and are valvular and slitlike. The lateral-line organs around the mouth are elaborated into spine-shaped dermal appendages; those at the corner of the mouth are associated with an enlarged, flat, bifurcate flap of skin. The dorsal and lateral surfaces of the body are tuberculate. Although coloration varies from light tan to dark brown with variable mottling, a distinctive T-shaped mark always is present on the venter; the top of the "T" traverses the chest between the forelimbs, and the leg of the "T" runs down the middle of the abdomen.

distribution

The Surinam toad is distributed widely in the Amazon Basin, occurring in eastern Venezuela, Guyana, Surinam, Brazil, Colombia, Ecuador, Peru, and Bolivia. It also is known from the Guianan region and Trinidad.

habitat

These odd frogs are found in slow-moving streams and rivers and lowland rainforest ponds and swamps, the bottoms of which are covered with organic detritus.

behavior

Because the Surinam toad is extraordinarily difficult to observe in nature, most of its reported "natural history" is based on observations of captive individuals. The frogs usually are found lying immobile amidst the detritus on the bottom of ponds. On rainy nights they move from one pond to another.

feeding ecology and diet

In nature the Surinam toad has been observed to eat small fish and aquatic invertebrates. The frog lacks teeth. Typically, it lunges toward its prey, opens its mouth, and inflates its body, creating a vacuum into which the prey is sucked; the frog uses its forelimbs to push food into its mouth.

reproductive biology

Preliminary amplexus lasts 24–30 hours, with a male clasping a female around the waist while bobbing and pumping his body;

during this time the female's back becomes tumescent, and the lips of her vent swell. Amplectant pairs of Surinam toads conduct a complex, repeated ritual of midwater acrobatic turnovers, with the female leading the male, who clasps her around her waist. The 11–14-second sequence includes ascending to the surface, doing a complete turnover without breaking the surface of the water, and descending to the bottom. When the female is upside down and ascending through the water, she expels three to five eggs that are fertilized and caught on the male's belly. As the rollover is completed, the eggs drop on the female's back and are implanted there by pressure from the male's clasp. The entire oviposition sequence takes about three hours. The skin of the female's back gradually swells up around the individual eggs (about 50); after 10 days only a small portion of the top of the embryo is visible, and the outer membrane of the egg covers it. Fully developed froglets begin to emerge from the female's back within three to four months. The call of the Surinam toad is a metallic clicking noise produced at the rate of four per second for periods of 10–20 seconds.

conservation status

Not threatened.

significance to humans

The Surinam toad is a source of food for some indigenous Amazonian people.

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.

Linda Trueb, PhD