Poison Frogs (Dendrobatidae)

(Dendrobatidae)

Class: Amphibia

Order: Anura

Family: Dendrobatidae

Thumbnail description
Small, agile frogs that occur in rainforests of the New World tropics; more primitive genera are cryptically colored with nontoxic skin, whereas the derived genera are brightly colored poison frogs

Size
Most species are 0.75–1.5 in (15–35 mm) in length; a few species reach 2.5 in (62 mm)

Number of genera, species
9 genera; 207 species

Habitat
Rain and cloud forest

Conservation status
No species listed by the IUCN

Distribution
Southern Central America through tropical South America

Evolution and systematics

The relationship of the poison frog family to other frogs remains the subject of controversy. Dendrobatidae lies within a clade of frogs, the neobatrachians, that diverged in the early Cretaceous or late Jurassic. Within this clade, two major groups, the hyloids and ranoids, diverged. Since 1959 numerous studies have placed dendrobatids inconsistently within these two groups. Most recent studies have shown that placement within the hyloids, specifically the leptodactylid/bufonid clade, is most likely.

The largest dendrobatid genus, Colostethus, comprises about 100 species, whereas the more derived genera, including Dendrobates and Phyllobates, contain about 36 and five species, respectively. Relationships at the generic level are fairly well understood. In 1991 the most primitive dendrobatid known, Aromobates, was described. Colostethus and Mannophryne are basal or primitive groups closely related to Aromobates, whereas Dendrobates is the most derived, or advanced, genus.

Relationships of species within each genus are not well worked out. Partly this is because many species have small ranges and occur in areas that cannot be reached easily; thus, many species have not been studied in detail. The genus Phyllobates is best understood. This genus contains five species that occur from Colombia to southern Nicaragua and are united by being the only species that have a unique alkaloid (batrachotoxin) in the skin. Other genera, such as Colostethus and Epipedobates, may be composed of groups of species that are not related closely; thus, it is probable that these genera will be subdivided into smaller genera in the future. Many species in the poison frog family have yet to be discovered. At least three or four new species in this group are described each year. No subfamilies are recognized.

Physical characteristics

Poison frogs are typically small frogs, less than 1 in (2.5 mm) in body length. The name of one genus, Dendrobates, is derived from the Greek dendro (tree) and bates (walker), an allusion to the fact that they can walk or hop up trees. Dendrobatids have short but powerful hind limbs and are agile jumpers and, in some cases, climbers. They are characterized by the presence of divided scutes (thick pads of skin) on the upper surfaces of the fingers and toes.

Poison frogs derive their common name from the fact that the more derived, or advanced, frogs in the group are brightly colored and have toxic skin. All frogs have glands in the skin that produce a variety of noxious substances serving to protect them from predators. However, poison frogs are unique, because their skin glands contain a large array of alkaloids, which are especially toxic nitrogen-containing chemicals once believed to be produced only by plants. Investigations of these chemicals have found hundreds of different alkaloids in the four derived genera of dendrobatids. Individual species or populations of frogs have different alkaloids, and within a population individual frogs may have different combinations of alkaloids. Some of these alkaloids are encountered in only one or a few species of frogs, whereas others may be present in numerous species. Examples are batrachotoxin, found in only four of the five species of the genus Phyllobates, and epibatidine, found in just one species in the genus Epipedobates. It was once thought that the frogs produced these chemicals, but it is now believed that they are obtained from the frogs' diet.

The derived poisonous species of dendrobatids are brightly colored. Some species of Dendrobates are bright yellow with black spots and black and blue legs, whereas others are a brilliant green with black markings. At least one species, Dendrobates pumilio, has numerous differently colored individuals throughout its range. Some populations are bright red, others are yellow with black spots, and still others are blue. The more primitive species, such as those in the genera Aromobates, Colostethus, and Mannophryne, are cryptically colored, generally brown frogs that lack alkaloids in their skin.

Distribution

Poison frogs occur in the neotropics from Nicaragua south through Costa Rica, Panama, and northern South America to southern Brazil and Bolivia. Within their range, most species are found only in undisturbed primary rainforest or cloud forest, although a few species occur in converted pastureland, cerrado (a savanna-like habitat) in southern Brazil, or cacao plantations in Central America.

No dendrobatids are known to be extinct at present, but many dendrobatids have small distributions within the over-all range of the family, so information on the status of these species is difficult to obtain. Some species seem to be on the verge of extinction (e.g., D. mysteriosus in the Cordillera del Condor of Peru), and others have not been seen in the wild for years and may be extinct (e.g., D. speciosus from Panama). Only one species, Dendrobates auratus, has become established outside its natural range. This species was introduced into Hawaii.

Habitat

Throughout the range, most species of dendrobatids occur in primary rainforest. The more primitive species, such as those in the genus Colostethus and Mannophryne, occur strictly in leaf litter on the forest floor. Frogs in the genus Dendrobates are partially or completely arboreal, depending on the species. For example, some species of Dendrobates, such as D. auratus, D. pumilio, and D. castaneoticus, largely inhabit the forest floor leaf litter but frequently climb trees and vines. Dendrobates vanzolinii, a species in western Brazil, lives in the lower canopy of the forest and avoids the forest floor. Other species, such as D. arboreus in Panama, seem to have an almost entirely arboreal existence.

Nearly all species of dendrobatids deposit eggs on land. Among the more primitive species, tadpoles are transported to small, slow-moving streams or pools on the forest floor. The more derived species transport their tadpoles to container habitats, such as bromeliads, tree holes, Heliconia bracts, Brazil nut capsules, or other types of small phytotelmata (water-holding plants). Although most of these container habitats are arboreal, some, such as Brazil nut capsules and other seed husks, are on the forest floor.

Behavior

All species of dendrobatids, except the most primitive, Aromobates, are diurnal. They tend to be most active in early morning at first light and in late afternoon, particularly on rainy days. Dendrobatids are also most active and conspicuous during the rainy season. Their collective range encompasses a large area of the neotropics, and consequently the rainy season in any one area may be longer or shorter or occur earlier or later in the year compared with other areas. Dendrobatids may be found during the dry season, but they are generally less active during this time.

Many species of dendrobatids seem to be territorial. Territoriality typically is associated with reproduction. In most species, males are territorial, and females are not. However, in several species of Colostethus females are territorial.

Feeding ecology and diet

Dendrobatids usually feed on tiny arthropods, ranging in average size from 0.03 to 0.07 in (0.8 to 1.7 mm) in length. Prey include ants, mites, small beetles, small flies, springtails, and tiny spiders, among others.

Evidence is accumulating that toxic dendrobatids obtain alkaloids from their diet. When frogs collected as tadpoles were raised under similar conditions and fed either leaf-litter arthropods or fruit flies, only those fed on arthropods developed alkaloids in the skin. Poison frogs fed fruit flies dusted with alkaloids have been shown to absorb those toxins from the diet and secrete them from the skin glands. Some species of ants have the same alkaloids as those in dendrobatid skin; thus, ants may be one of the main sources of alkaloids. Further work on species of dendrobatid frogs representing both primitive and derived clades showed that three derived species (which are also toxic) had diets consisting 50–73% of ants, whereas the diets of five nontoxic species contained only 6–16% ants. Thus, the diet of the derived, toxic, brightly colored dendrobatids is composed of a much higher percentage of ants than the primitive, cryptically colored, nontoxic species.

Reproductive biology

Dendrobatid frogs breed primarily during the rainy season. Males typically call intensely at first light for a period of several hours and then are quiet during midday, unless rainfall occurs. They may call again in late afternoon, especially on wet days. Like other frogs, each species has a unique call. Call characteristics are important in distinguishing closely related species that may be very similar in morphologic features.

All species of dendrobatids (with the possible exception of Aromobates nocturnus) deposit eggs on land. Depending on the species, eggs may be deposited on leaves in leaf litter on the forest floor, or they may be attached above the waterline to the inside of a tree hole or a bromeliad tank.

The eggs are attended by the male or female parent, depending on the species. When the eggs have developed into tadpoles, the transporting parent sits among the tadpoles in the nest, and they wiggle up the parent's leg and onto the back. The tadpoles are attached to the body of the parent by mucopolysacchrides, chemicals that dissolve easily in water. The parent frog then carries the tadpoles to a body of water, such as a small stream or pool or a bromeliad tank. The parent submerges the back half of its body in the water, and the tadpoles gradually dislodge and swim away, thus ending the period of parental care for most species. In some species, parents spend substantial amounts of time searching for suitable pools.

In terms of reproduction, differences exist between the primitive and the advanced dendrobatids and even within these groups. Among the primitive species, clutch size is larger, and all tadpoles are transported at once. In some species of Colostethus, for example, clutch size may be from 25 to 35 offspring. Clutch size in Dendrobates is much smaller, generally from three to six eggs. In these species, depending on aspects of the biology, each tadpole may be transported singly by the parent to a different aquatic site, usually a tree hole or some other type of phytotelmata.

Parental care is more complex in some species of Dendrobates. In at least one species, D. vanzolinii, males and females may remain together as pairs and care for their offspring together. Eggs are deposited on the inside of tiny tree holes or vine holes above the water level. After the tadpoles develop, the male parent carries each one to another tiny tree hole, in which it completes its development. About every five days, the male and female court, which appears to trigger ovulation in the female. However, instead of depositing fertilized eggs above the waterline, the pair returns to the site of their tadpole. The tadpole goes through a stereotypic movement in which the body stiffens and vibrates. The female parent appears to respond to this movement by the tadpole by backing into the water and depositing one or two unfertilized eggs for the tadpole to consume. This type of parental care presumably evolved in response to the lack of nutrients in the tree holes; the tadpoles are dependent on the nutritive eggs produced by the parent to survive.

In another species of Dendrobates, D. pumilio, the female rather than the male transports the tadpoles from the nest in the leaf litter to individual leaf axils that contain a tiny bit of water. The female then returns about every five days to deposit eggs for the tadpole to consume. Although the male parent appears not to be involved, as is the case for D. vanzolinii, there is some suggestion that the female seeks the calling male and remains near him for a period of time, possibly to stimulate ovulation.

Tadpoles of all species of dendrobatids, except those in Dendrobates, are typical herbivores that graze on algae and detritus. In contrast, those of Dendrobates are predaceous. This trait may have evolved in response to confinement of the tadpoles in small, unproductive habitats, where the ability to kill and eat small macroinvertebrates, such as mosquito larvae, would have been advantageous. In some species, tadpoles of Dendrobates readily kill and eat smaller tadpoles of the same and different species; thus, as discussed earlier, parent frogs in some species transport only one tadpole at a time and place it singly at an isolated site for development so that it will not be consumed by one of its larger siblings.

Conservation status

Most species of dendrobatid frogs occur in rainforest habitats that are vulnerable to deforestation. In addition, the extent of the distribution ranges of many species is unknown, because areas where they may occur are unexplored. These facts make determination of the conservation status difficult. In Ecuador five species of Colostethus and one species of Dendrobates are declining in numbers; all are Andean species that occur above 3,940 ft (1,200 m). At present, no species are cited as threatened on the IUCN Red List. In contrast, all species, except those in the genera Colostethus, Mannophryne, and Nephelobates, are listed on CITES Appendix II.

Significance to humans

Derived species of dendrobatids produce large numbers of alkaloids in the skin. The toxins in most species have not been studied thoroughly, and little is known about the potential pharmacological uses. An alkaloid produced by one species of Epipedobates is a painkiller 200 times more potent than morphine. Some Indian tribes in Colombia have used skin toxins of three species in the genus Phyllobates as poison for their blowgun darts; blowguns were used in hunting small game.

Species accounts

Resources

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[Article by: Janalee P. Caldwell, PhD; Kyle B. Summers, PhD]