Amero-Australian Treefrogs (Hylidae)

(Hylidae)

Class: Amphibia

Order: Anura

Family: Hylidae

Thumbnail description
Small to large primarily arboreal frogs with expanded, adhesive discs on the digits

Size
0.8–4.8 in (20–120 mm)

Number of genera, species
42 genera; 854 species

Habitat
Primarily tropical and subtropical forests, some savannas, grasslands, and deserts; a few species inhabit elevations above the tree line

Conservation status
Critically Endangered: 6 species; Endangered: 5 species; Vulnerable: 5 species; Lower Risk/New Threatened: 5 species; Data Deficient: 8 species

Distribution
Most of the New World, Australia, and New Guinea, and discontinuously in Eurasia

Evolution and systematics

The earliest fossil Hylidae are from the Paleocene of Brazil; elsewhere, fossil hylids are known from as early as the Miocene in Australia, the Oligocene in North America, the Miocene in Europe, and the Pleistocene in Japan. The meager fossil data are consistent with a Gondwanan origin of the family, presumably in South America after its separation from Africa. Independent dispersals from South America were to Australia via Antarctica and to North America and subsequently to Eurasia.

Treefrogs of the family Hylidae presumably are related most closely to other families of frogs in the New World that also have the two halves of pectoral girdle overlapping ventrally (arciferal conditions and intercalary elements between the penultimate and terminal claw-shaped phalanges). These families include Centrolenidae, which differ by having T-shaped terminal phalanges, tarsal elements fused throughout their lengths (fused proximally and distally in Hylidae), and 10 pairs of chromosomes (11 or more in Hylidae). The monotypic Allophrynidae differs by lacking teeth on the maxillaries and premaxillaries and intercalary elements in the digits, but the T-shaped terminal phalange is offset ventrally, as in Hylidae.

Five subfamilies are recognized:

Hemiphractinae

The eggs are brooded on the back of, or in a dorsal pouch of females; the embryos have large, sheet-like gills that at least partially envelop them. Most species have direct development. In those in which the eggs hatch as tadpoles, the spiracle is ventrolateral in position but moves to a lateral position in Gastrotheca. The intercalary elements are cartilaginous, and the terminal phalange is offset ventrally. The constricted pupil of the eye is horizontally elliptical. The diplod number of chromosomes is 26 (28 and 30 in some species of Flectonotus).

The subfamily contains five genera and 71 species; it is distributed principally in montane regions from Costa Rica to northwestern Argentina, the Guiana Highlands, and eastern Brazil.

Hylinae

The eggs are deposited in water, on vegetation above water, or in subterranean nests near water; all eggs hatch as free-swimming tadpoles, which have a lateral spiracle. The intercalary elements are cartilaginous, and the terminal phalange is offset ventrally. The constricted pupil of the eye is horizontally elliptical. The diploid number of chromosomes is 24, but this number is reduced to 22 in Acris and increased to 30 in many species of Hyla and to 34 in Osteopilus brunneus. The subfamily contains 26 genera with more than 500 species; it has the same distribution as the family, except that it is absent in the Australo-Papuan region.

Pelodryadinae

The eggs are deposited in water or, in a few species, on vegetation above water. The free-swimming tadpoles have filamentous gills and a lateral spiracle. The intercalary elements are cartilaginous, and the terminal phalange is offset ventrally. The constricted pupil of the eye is horizontally elliptical (vertically elliptical in Nyctimystes). The diploid number of chromosomes is 26 (24 in Litoria infrafrenata and 30 in Litoria angiana). The subfamily contains three genera with about 150 species; it is widespread in Australia and New Guinea. Two species are endemic to the Bismarck Archipelago and Solomon Islands, and three Australian species have been introduced into New Zealand and New Caledonia.

Phyllomedusinae

The eggs are deposited on vegetation above water; the embryos have large, branched gills; and the free-swimming tadpoles have a ventrolateral spiracle. The intercalary elements are cartilaginous, and the terminal phalange is offset ventrally. The constricted pupil of the eye is vertically elliptical. The diploid number of chromosomes is 26. The subfamily contains five genera with 70 species; it is widely distributed in tropical parts of Mexico, Central America, and South America.

Pseudinae

These aquatic frogs deposit eggs in water; the free-swimming tadpoles have feathery gills and a lateral spiracle. The intercalary elements are elongated and ossified; the terminal phalange is essentially in the same plane as the other phalanges. The constricted pupil of the eye is horizontally elliptical. The diploid number of chromosomes is 24. Two genera contain seven species; the subfamily is distributed widely east of the Andes in tropical South America and on the island of Trinidad.

Physical characteristics

The family is characterized by a suite of internal features that distinguish Hylidae from other families. The two halves of the pectoral girdle overlap (arciferal condition); the vertebrae are procoelous, and the first two presacral vertebrae are not fused. The coccyx has a bicondylar (two-headed) articulation with the expanded sacrum. Neopalatine and quadratojugal bones usually are present. An intercalary element (usually cartilaginous) is present between the terminal and penultimate phalanges in each digit of the hand and foot, and, except for Pseudinae, the terminal segment of each digit is offset ventrally. The terminal phalanges are claw-shaped, and the terminal segment of each digit typically is expanded into an adhesive disc.

Several hylid frogs have casque heads, in which the skin on the head is co-ossified with expanded underlying dermal bones in the skull. Casque heads are especially well developed in Aparasphenodon, Corythomantis, Hemiphractus, Trachycephalus, and Triprion. In some of these frogs (e.g., Aparasphenodon and Triprion), the upper lips are flared outward, and an additional bone, the prenasal, is present; a different bone, the internasal, is present in Pternohyla. Some species of Gastrotheca and Osteocephalus have bony ridges on the skull, and Anotheca spinosa has bony spines directed upward on the back of the skull. All hylids have teeth on the maxillae and premaxillae, and most have teeth on the vomers. Gastrotheca guentheri is the only frog known to have true teeth on the dentary bones in the lower jaw, but Hemiphractus and Phyllodytes have bony projections (odontoids) on the anterior ends of the dentaries in the lower jaw.

Prominent glands are present on the top of the head in some species of Litoria, especially Litoria splendida, and parotoid glands in the shoulder region are present in many species of Phyllomedusa. The dermis of the dorsal skin in some arboreal hylids (e.g., Gastrotheca weinlandii, Phyllomedusa bicolor, and P. vaillanti) contains small, vascularized bony plates (osteoderms) from which small lamellar spines protrude into the epidermis. Presumably, these structures impede water loss through the skin on the body.

Most treefrogs have rather slender bodies and long limbs. The terrestrial and fossorial Cyclorana in Australia and the carnivorous Hemiphractus in South America, however, have robust bodies and proportionately shorter limbs. All hylid frogs, except pseudines, have ventrally offset terminal discs on their digits; these discs are expanded and adhesive in arboreal species. With the exception of most phyllomedusines, the feet are at least one-half webbed. The fingers may be webbed or not. The fingers are fully webbed in several arboreal species, some of which (e.g., Agalychnis craspedopus and Hyla miliaria) bear thin flaps of skin at the outer edges of the limbs. Other dermal modifications include a fleshy proboscis in Hemiphractus and eyelid "horns" in some species of Gastrotheca.

Most hylids have a prominent tympanum (eardrum). Males of most hylid frogs have a single, subgular vocal sac, which is inflated while they perch on the ground or vegetation. Osteocephalus, Phrynohyas, and Trachycephalus call while floating on water. In these frogs the vocal sacs are paired and located behind the angles of the jaws; when inflated, they form balloon-like structures that extend above the head and thus do not inhibit the floating frog. Males of most species develop nuptial excrescences in the breeding season. Commonly, they are keratinized, and in some stream-breeding species (e.g., Ptychohyla) they take the form of clusters of spines; a cluster of spines also is present on the humerus in Hyla armata. Male Plectrohyla and gladiator frogs (Hyla boans group) have a sharp spine at the base of the thumb. Burrowing hylids (Cyclorana and Pternohyla) have enlarged, spade-like inner metatarsal tubercles.

Treefrogs vary tremendously in size and coloration. With a few exceptions, females are larger than males. Several species have snout-vent lengths of less than 1 in (25 mm). The smallest is Litoria microbelos in northern Australia; adults of both sexes attain snout-vent lengths of only 0.65 in (16 mm). The largest species is Hyla vasta on the island of Hispaniola in the West Indies; females are known to exceed 4.8 in (142 mm). The exceptions are the gladiator frogs and relatives in South and Central America, males of which aggressively defend their nesting sites from other males.

In most hylid frogs the dorsum is brown or green, usually with darker markings. Others have a yellow or gray dorsum, and some, such as Hyla picturata with a gaudy red-and-yellow dorsum, are more boldly marked. The ventral surfaces typically are white or pale yellow, but many species have brown or black spots or mottling on the belly; males of many species have bright yellow or dark gray vocal sacs. The most striking aspects of coloration are the so-called flash colors on the flanks and surfaces of the hind limbs, which are not visible when the frogs are in a resting position. These flash colors are especially colorful in some species of Agalychnis and Phyllomedusa, in which the flanks are marked variously with black, blue, yellow, and white bars. Others, such as several species of Scinax, have bright yellow or red bars or spots on the posterior surfaces of the thighs.

With the exception of some Hemiphractinae, all hylid frogs have aquatic, free-swimming, feeding tadpoles, which have a sinistral spiracle and keratinized jaw sheaths. The oral disc usually is directed anteroventrally and lacks marginal papillae on the median part of the upper lip; elsewhere the lips typically bear one or two rows of marginal papillae. Tadpoles of many species that develop in streams have enlarged suctoral oral discs, but the tadpoles of Duellmanohyla and Phasmahyla have upturned, umbelliform oral discs in the form of an inverted umbrella. Labial teeth generally are present, but they are absent in one group of South American Hyla; the labial teeth are reduced greatly in oophagous (egg-eating) tadpoles. Most tadpoles that develop in ponds have two upper rows and three lower rows of labial teeth; the number of rows is increased greatly in many tadpoles that develop in torrential streams. The maximum is 17 upper rows and 21 lower rows of labial teeth.

Most hylid tadpoles have total lengths of less than 2 in (50 mm); in those that develop in ponds, the body is about one-third of the total length. The largest known tadpole is that of Pseudis paradoxa, which reaches a total length of more than 10 in (25 cm). Tadpoles that develop in streams have proportionately longer, more muscular tails with lower fins than those that mature in ponds.

Distribution

The family has a continuous distribution throughout most of the New World, including the West Indies but excluding Arctic North America and southern South America. It is distributed widely in Australia and New Guinea, and two species also inhabit the Bismarck Archipelago and the Solomon Islands. A few species of Hyla occur in Europe, southwestern Asia, discontinuously in eastern Asia (including the Japanese Archipelago), and Mediterranean North Africa as well as on the Azores, Madeira, and the Canary Islands in the Atlantic Ocean. Some Australian species have been introduced into New Caledonia and New Zealand, and some North American species have been introduced onto islands in the West Indies. One West Indian species has been introduced into Florida.

Habitat

Hylid frogs are most diverse in tropical and subtropical humid forests, especially in the Amazonian rainforest, where as many as 40 species may occur together. Hylids also are numerous in montane cloud forests, especially in Mexico, Central America, and New Guinea, as well as in the coastal forests of southeastern Brazil and the lowland forests of northern Australia and New Guinea. In Australia some species of Litoria and all species of Cyclorana, most of which inhabit grasslands and deserts, are terrestrial or even fossorial, a habit also characteristic of Pternohyla in Mexico. Members of the sub-family Pseudinae are aquatic. By day most hylids secrete themselves in arboreal situations, such as under the loose bark of trees, on the undersides of leaves, and in bromeliads. A few that breed in mountain streams seek diurnal shelter under rocks at the edges of streams or in rock crevices.

Behavior

Nearly all species are nocturnal; Acris in North America also is active by day, and some montane species are active by day. In the latter category are the Andean Hyla labialis and the Guatemalan Plectrohyla glandulosa, both of which bask on bushes or rocks. Thus, hylids are encountered mostly at night, especially after rains, when they feed and breed. Although adults may spend the day in seclusion, most treefrogs perch on branches, leaves, or grasses at night. Aside from natural diurnal retreats, treefrogs also utilize human-made structures, including window shutters, thatch roofs, water tanks, and cisterns.

Some hylids that live in arid regions survive long dry periods by special behaviors to prevent desiccation. In the Australian deserts Cyclorana dig burrows with spadelike tubercles on the hind feet; they remain underground for many months. Dehydration is prevented by shedding layer upon layer of skin, which hardens into an impermeable cocoon. Some Phyllomedusa in dry regions of South America have lipid glands in the skin. The secretion from these glands is wiped by the hands and feet over the entire body so as to provide an almost impermeable covering that allows the frogs to remain exposed to air for long periods of time. Some of the casqueheaded treefrogs (e.g., Gastrotheca, Trachycephalus, and Triprion) back into bromeliads, where water exists at the bases of the leaf axils, or tree holes with water inside; they plug the openings with their bony heads. Treefrogs living in temperate regions hibernate below ground. At least two species (Hyla versicolor and Pseudacris crucifer) have large quantities of glycerol in their tissues, which acts as an antifreeze; these frogs can tolerate temperatures well below freezing.

Territorial behavior in hylids mostly is acoustic; males of many species are known to emit territorial or aggressive calls in the presence of conspecific males. Such calls usually define a given calling site; in cases where calling fails, males have been observed to grapple or even bite one another. Male gladiator frogs in the American tropics defend their excavated nests by attacking intruders with the sharp spines at the bases of their thumbs. Such attacks may result in deep cuts, punctured eardrums, or even death. At least some of the stream-breeding Plectrohyla in Central America presumably also incur damage with their thumb spines, inasmuch as some males have scarred bodies. Captive Anotheca spinosa have been seen to puncture the body of another individual in the same tree hole with the sharp spines on their heads.

Hylid frogs are prey for many kinds of animals, especially snakes. Avoidance of predation principally is by the escape behavior of leaping to another branch or leaf; this is carried to an extreme by some species (e.g., Agalychnis moreletii and

Anotheca spinosa) by "parachuting" for a long distance from a high limb. The terrestrial Litoria nasuta in Australia escapes by a series of long leaps. Some small species with fully webbed feet are capable of skittering across the surface of the water. Acris crepitans skitters after an initial leap from land, and Scarthyla goinorum is capable of leaping off a low bush to skitter on the water and then jump up onto another bush.

In an encounter with a potential predator, some Hyla and Phyllomedusa feign death by tucking their limbs close to the body and remaining motionless on their backs. In contrast, Hemiphractus turn their heads up, open their mouths so as to display an orange tongue, and even snap at a potential predator. The volatile, alkaline skin secretions of Phrynohyas are insoluble in water and have a deleterious effect on mucous membranes of the eyes and mouth; consequently, most predators avoid these frogs.

Tadpoles of most species seem to exist independently from conspecifics, but tadpoles of Hyla geographica and Phyllomedusa vaillanti form schools of hundreds of individuals. This behavior may result in less predation. Otherwise, tadpoles avoid predation by either remaining motionless or rapidly hiding amidst aquatic vegetation.

Feeding ecology and diet

All hylids seem to be sit-and-wait predators that feed on a wide variety of arthropods; the selection of food depends primarily on the size of the prey. A few species are specialists on certain kinds of insects. Sphaenorhynchus lacteus feeds almost exclusively on ants, and Hyla leucophyllata feeds mostly on moths. The large-headed, broad-mouthed Hemiphractus eat large insects and other frogs.

Reproductive biology

Throughout temperate regions and the lowland tropics, hylid frogs respond to rains by moving to breeding sites, either temporary or, less frequently, permanent ponds. The length of the breeding season is determined by the period of rainfall; some northern species (e.g., Pseudacris crucifer) even call from the edges of ponds with ice on the water and snow on the banks. Species in dry regions tend to be explosive breeders that are active for only a day or so after heavy rains form temporary ponds. In contrast, hylids inhabiting humid rainforests and montane cloud forests may breed throughout the year in streams and ponds.

In those species that breed in ponds and streams, males congregate for breeding; after a heavy rain in tropical regions, breeding sites may have hundreds of individuals of several species calling at the same time. The calls vary from soft "peeps" to loud "growls." The calls of some species consist of only one note repeated at intervals of a few seconds to several minutes; other calls are a series of notes. In those species that call from bromeliads or tree holes, males usually are solitary. Females are attracted to the breeding site by the calls. Amplexus is axillary.

Diverse reproductive modes are employed by hylid frogs:

• Eggs are deposited in water (ponds or streams), and tadpoles develop in water: most Hylinae and Pelodryadinae and all Pseudinae.
• Eggs are deposited, and early-stage tadpoles develop in natural or constructed basins; subsequent flooding releases tadpoles into ponds or streams: Hyla boans group.
• Eggs are deposited in a foam nest floating on water in a pond; tadpoles develop in the pond: Scinax rizibilis.
• Eggs are deposited, and tadpoles develop in subterranean nests near ponds; subsequent flooding releases feeding tadpoles into ponds: Hyla leucopygia.
• Eggs are deposited on vegetation above water; feeding tadpoles develop in ponds or streams: all Phyllomedusinae and a few Hylinae and Pelodryadinae.
• Eggs are deposited, and tadpoles develop in bromeliads or cavities in trees: several species of Hylinae.
• Eggs are deposited in a pouch on the dorsum of the female; feeding tadpoles live in ponds: some Gastrotheca.
• Eggs are deposited in the dorsal pouch or on the back of the female; nonfeeding tadpoles live in bromeliads or tree holes: Flectonotus.
• Eggs are deposited in the dorsal pouch or on the back of a female; eggs hatch as froglets: Cryptobatrachus, Hemiphractus, Stefania, and some Gastrotheca.

At high latitudes and high elevations, as well as in arid environments, females usually deposit only one clutch of eggs per year, but at lower latitudes, especially in the lowland humid tropics, females may lay several clutches per year. Clutch size correlates with female body size within a given reproductive mode. Females of large species, such as Hyla rosen-bergi and Phrynohyas venulosa, that deposit small eggs in water have clutches in excess of 2,000 eggs, whereas in small species, such as Pseudacris ocularis, clutches consist of only about 100 eggs. Species that deposit eggs on vegetation over water have smaller clutches, ranging from 10 in the small Hyla thorectes to more than 250 in the large Phyllomedusa bicolor. Clutch size is less than 100 eggs in those species of Gastrotheca that transport eggs that hatch as tadpoles, whereas in those hemiphractines that carry eggs that hatch as froglets, clutches typically contain fewer than 15 proportionately much larger eggs.

No parental care exists among most hylid frogs, but female hemiphractines carry their eggs several weeks or months, depending on the stage at which the eggs hatch. The ultimate in parental care exists in several species that deposit their eggs in bromeliads or tree holes, where food is scarce. After deposition of a clutch of fertilized eggs, the female, accompanied or not by the male, returns to the breeding site and deposits additional fertilized or unfertilized eggs, which are eaten by the tadpoles. This behavior is known only in a few hylines (Anotheca spinosa, Osteocephalus oophagus, Osteopilus brunneus, and Phrynohyas resinifictrix) in tropical America.

Conservation status

According to the IUCN, six species are Critically Endangered; five are Endangered; five are Vulnerable; five are Lower Risk/Near Threatened; and eight are Data Deficient.

Habitat destruction imperils many species of hylid frogs. This is especially evident in montane regions, where many species have limited distributions. Some of the species of Hyla, Plectrohyla, and Ptychohyla have not been seen in recent years in areas where they were common before logging and stream pollution. Likewise, the conversion of dry tropical forests to agriculture seems to have limited greatly the distributions of such species as Triprion spatulatus. Chytrid fungus may be responsible for drastic declines or the extinction of many species, such as Hyla calypsa and H. xanthosticta in Central America and Nyctimystes dayi and at least three species of Litoria in northeastern Australia.

Significance to humans

Hylid frogs are not among those species commonly eaten by Europeans and North Americans, but many indigenous peoples in the American tropics and in the Australo-Papuan region catch and eat a variety of larger hylids, especially Hyla boans and Osteocephalus taurinus in the Americas and Nyctimystes in New Guinea. Indigenous people in New Guinea also eat tadpoles of Litoria and Nyctimystes, and the large tadpoles of Pseudis paradoxa are eaten in South America. Australian Aborigines unearth estivating Cyclorana platycephala and squeeze water out of them before replacing the frog in its burrow. Before going on a hunt, some indigenous people in the upper Amazon Basin lick the skin secretions of Phyllomedusa bicolor; this has a hallucinogenic effect.

Species accounts

Resources

Barker, John, Gordon Grigg, and Michael J. Tyler. A Field Guide to Australian Frogs. Chipping Norton, Australia: Surrey Beatty and Sons, 1995.

Duellman, William E. Hylid Frogs of Middle America. Ithaca, NY: Society for the Study of Amphibians and Reptiles, 2001.

Lescure, Jean, and Christian Marty. Atlas des Amphibiens de Guyane. Paris: Muséum National d'Histoire Naturelle, 2001.

Duellman, William E. "The Biology of an Equatorial Herpetofauna in Amazonian Ecuador." Miscellaneous Publications, Museum of Natural History of the University of Kansas 65 (1978): 1–352.

Rodríguez, Lily O., and William E. Duellman. "Guide to the Frogs of the Iquitos Region, Amazonian Peru." Special Publications, Museum of Natural History of the University of Kansas 22 (1994): 1–80.

Trueb, Linda. "Evolutionary Relationships of Casque-headed Tree Frogs with Co-ossifed Skulls (Family Hylidae)." Miscellaneous Publications, Museum of Natural History of the University of Kansas 18, no. 7 (1970): 547–716.

[Article by: William E. Duellman, PhD]