|Poison dart frogs|
Red and Blue "Blue Jeans" Dendrobates pumilio Strawberry Poison Dart Frog
Distribution of Dendrobatidae (in black)
Poison dart frog (also poison arrow frog, dart frog, or poison frog) is the common name for any of the very small, diurnal frogs of the Dendrobatidae family. Their common name refers to the poisons contained within their skin that are used by local tribes to coat their blowgun darts. The family name Dendrobatidae and the genus name Dendrobates are derived from the Greek words dendro, meaning "tree," and bates, meaning "walker"; thus literally "tree walker" (Schlager 2003).
Many of these poison frogs are brightly colored with combinations of orange, blue, red, yellow, or black pigments; however, the majority are not brightly colored and are referred to as cryptically colored. Although the secretions from all of these frogs are poisonous, only a few species have enough poison to kill a human being (Patocka et al. 1999).
Many new poisonous alkaloids found within their skin are now being used to study the neurophysiology of nerve and muscle transmission. The discovery of epibatidine as a pain reliever 200 times more potent than morphine has stimulated research on a new class of compounds that may help many people overcome pain without the side effects of addiction.
In addition to practical use for Native Americans to capture game, these frogs contribute to humans a certain intrinsic beauty in their shape and coloration, as well as in the way they call for their mates and take care of their young. The poison dart frogs are a good example of how even the smallest, most toxic, most remote, and difficult to obtain organism can bring joy to humanity. These frogs have become so famous for their combination of vivid coloration and highly toxic poisonous skin that they are in demand as pets.
Because of their poisonous skin, these frogs do not appear to be consumed by other animals, with the exception of one species of snake. The main predators of these frogs are now human beings, which want to use their poison, either to catch food or to do research in toxicology. Thus, it is somewhat ironic that their very existence is threatened because of their highly toxic skin, which instead of repelling predators is now attracting them; and their bright coloration, which instead of warning others to stay away, makes them easy to locate.
In this article, a member of Dendrobatidae is referred to as a dendrobatid, while a member of the genus Dendrobates, the second largest of 11 genera in the Dendrobatidae family, is referred to as a dendrobate.
The adults are usually 0.5 to 2.5 inches in length from snout to anus, with most species 0.75 to 1.5 inches (Schlager 2003) or no larger than an adult human thumbnail. One of the largest of these is D. tinctorius, which can reach 2.5 inches (SNZP 2007).
Male and female frogs look similar. The females are usually slightly longer. The easiest way to determine the sex of a particular species of poison dart frog is by observation in the wild. Mature male frogs will usually make a mating call after eating or after a heavy misting of water. The sound is similar to that of a series of high-pitched "clicks." In juvenile frogs, the sex can sometimes be determined by the profile of the amphibian. The backs of males usually slope down with less of a break than females. Females are usually rounder and show a bigger break.
Another simpler way to sex these frogs is by their toes. A female dendrobate will have narrow toes all the way down and a male's toes get very wide at the ends. Some mature males have a small section of grey skin on their neck.
Only a few of the poison frogs are brightly colored. Their skin may contain combinations of red, orange, and blue skin pigments. Some have multiple bright colors with black stripes or spots. Others are green with black stripes or are mottled with black and yellow. Their coloration is very striking and does not blend into the surrounding vegetation. This is often referred to as warning coloration or aposematism. These brightly colored members of the Dendrobatidae usually have the most toxic skin (Summers 2003).
Most members of the Dendrobatidae, such as those in the genus Colostethus, do not have brightly colored skin and are said to be cryptically colored, or able to hide and be camouflaged. Most of the cryptically colored frogs do not contain as much skin toxins as the brightly colored frogs (Summers 2003).
The family Dendrobatidae consists of 164 species in 11 genera (Frost 2007):
Poison dart frogs live primarily in the neotropical rainforests or cloud forests of Central and South America. Their home range is from as far north as Nicaragua and proceeds southward to Costa Rica, Panama, and then as far south as southern Brazil and into Bolivia. Most species have a very small range in which they are found.
Some poison dart frogs live outside of Central and South America. Dendrobates auratus was transported to Oahu, Hawaii in 1932 by people and continues to thrive there. Some poison dart frogs are also found on the Caribbean island of Tobago (SNZP 2007). The cryptically colored poison dart frogs of the genus Colostethus, the largest genus in Dendrobatidae, are usually found on the floor of the rainforest in the leaf litter near pools of water or a stream. Many of the brightly colored members, such as those belonging to the genus Dendrobates, are usually found in trees or vines. D.auratus and D.tinctorius live at elevations below 2,600 feet and spend much of their time on the forest floor (SNZP 2007).
All of the poison dart frogs have a diurnal lifestyle, which means they are more active during the day than at night. They become more noticeable earlier in the day rather than later. Anytime it rains they will become more conspicuous. Naturally, these frogs are easier to observe during the rainy season than the dry season.
Mating usually occurs during the rainy season. Male frogs, in preparation for mating, will fight among themselves to establish their territory. Then each species will make their characteristic calls, usually early in the morning at first daybreak to attract mates (Schafer 1999). Surprisingly, in several members of the genus Colostethus, it is not the male but the female that establishes the territory (Schlager 2003). In D. auratus, the females will protect their male and attack any other female that approaches.
In most species, eggs are laid on or near the forest floor, usually in a sheltered and moist spot. Typically, the female lays infertile eggs and the male fertilizes them externally in a process called oviparity. The number of eggs laid (the clutch size) varies among genera. In the genus Colostethus, the size will vary from 25 to 35 eggs. In the genus Dendrobates, only 2 to 6 eggs will be laid (Schlager 2003). Once the eggs are laid and fertilized, one parent (generally the male) guards them until they hatch. Because female poison frogs are both extremely territorial and competitive, a parent must guard constantly its eggs in order to prevent a rival female from devouring them. The eggs will hatch in about two weeks.
The newly hatched tadpoles wriggle and climb onto the back of either the male or female parent, who transports them in a sticky mucous film to a slow moving stream or a small pool of water. This may be water trapped in the axil of a Bromeliad plant, a tree hole, the capsule of a Brazil nut, or some other type of water-holding plant (Schlager 2003). Although the adult skin is so toxic that touching it can cause poisoning, the young tadpoles are not harmed, possibly because of the mucous film.
Within the Dendrobate genus, the parents will transport the hatched tadpoles one at a time to their own separate pool of water. Most species of dendrobatid tadpoles feed on aquatic plants such as algae, but those members of the genus Dendrobates are carnivores and must have meat. Dendrobate tadpoles will eat each other and must be raised isolated from each other. In these species, the female returns to the tadpoles about every 5 days to lay infertile eggs for them to feed upon; these species are called "obligate egg feeders." In a few species, this is the only source of food for the tadpoles until they undergo metamorphosis into sub-adult froglets. It takes about 6 weeks for the tadpoles to develop into adult frogs.
Poison frogs have been used since ancient times by the Embera and Noanama Choco tribes to provide poison for their blowgun darts.
The frogs are stressed by running a small wooden splinter from the mouth to the leg. This causes the skin poison sacs to exude poison. It has been said that as many as 50 darts can be coated with the poison from one frog (Stewart 2007). These darts are used for hunting small game. The poison acts quickly enough so that the game does not have time to run or fly away. Surprisingly, this poisoned quarry can be eaten without any ill effects.
The skin of the poison dart frogs has been found to contain almost 500 different lipophilic alkaloids spanning 20 different structural classes (Weldon et al. 2006). The types of toxins include batrachotoxins, pumilioxins, allopumiliotoxins, homopumiliotoxins, gephyrotoxins, and histrionicotoxins. The most potent among these are the batrachotoxins and pumiliotoxins.
The most poisonous of all the poison dart frogs, Phyllobates terribilis, lives in the rainforests of the Cauca region of Colombia. It is considered by many to be the most poisonous animal in the world, with a very high concentration of batrachotoxin. The adult frog can secrete 700 to 1900 ug of batrachotoxin/homobatrachotoxin. Homobatrachotoxin is slightly less toxic than batrachotoxin. The minimum amount of this compound required to kill a 20-gram white mouse (lethal dose), is only 0.05 micrograms subcutaneously (s.c.) (Patocka et al. 1999). This amounts to a level of 2.5 ug/kg. Thus it would take 125 ug to kill a 50 kg person, assuming mice and human beings have the same sensitivity.
Another group of toxins that occur in the skin of the Dendrobatidae are the pumiliotoxins. They are known to occur in all members of the genera Dendrobates and Phyllobates. There are more than 180 different pumiliotoxins that can be roughly categorized as three types: pumiliotoxin A, pumiliotoxin B, and pumiliotoxin C. The least toxic of these is the C type. Pumiliotoxin A and B have s.c. toxicities in mice of about 1–3 mg/kg, or are about 1,000 times less toxic than batrachotoxin.
The poisonous alkaloids in the frog skin are categorized as neurotoxins. They affect nerves and muscles by causing an irreversible depolarization that blocks signal transmission. The depolarization is caused by opening the sodium channel and allowing sodium ions to rush into the cell and thus eliminating the resting membrane potential necessary for electrical transmission. This results in cardiac arrhythmia, neuromuscular blockage, and death. Batrachotoxin and pumiliotoxin act in slightly different way to achieve their neurotoxic effects.
The only known predator of these highly poisonous frogs is a snake, Liophis (Leimadophis) epinephelus.
It is thought that poison dart frogs do not actually manufacture any of the toxic alkaloids they use to defend themselves.
In captivity, when the frogs are fed insects such as fruit flies and crickets that do not represent their diet in the wild, and are not rich in the required alkaloids, poison frogs stop producing toxins. In fact, many hobbyists and herpetologists have reported that most dart frogs will not consume any ants in captivity, though ants comprise the larger portion of their diet in the wild. Though all poison frogs lose their toxicity when deprived of certain foods, and captive-bred poison frogs are born harmless, a poison frog caught in the wild can retain alkaloids for years.
Three toxic species of poison dart frogs had a diet consisting of 50–73 percent ants. Five nontoxic dart frogs have diets consisting of only 6–16 percent ants (Schlager 2003). Several alkaloids found in Dendrobatid frogs have also been found in Myrmicine ants, with the greatest number found in the genus Solenopsis (Carr 2000). Thus far, none of the most toxic alkaloids have been found in a food source (Daly et al. 2000).
Some poison frogs not only absorb the alkaloids of the ants they consume, but also have the ability to chemically modify certain other toxins and thus create more toxic variants. For example, while Dendrobates auratus consumes pumiliotoxin 251D and merely stores it, some members of the Dendrobatidae family are able to convert 80 percent of ingested pumiliotoxin (+)- 251D to allopumiliotoxin (+)- 267A, which is five times more toxic than the starting material (Daly et al. 2003).
In New Guinea, the locals avoid eating birds of the Pitohui genus. Ingestion of this bird causes a numbing and burning sensation. Dr. Jack Dumbacher and his colleagues discovered in 1992 that the skin and feathers of these birds, and birds of the genus Ifrita, contained batrachotoxin. Pitohui birds had Choresine beetles in their stomach. These beetles, which belong to the Melyridae family, were also found to contain batrachotoxin. Members of this family are also found in the Colombian rainforest and could be a dietary source of poison for the dendrobatid frogs (Stone et al. 2004).
Poison frogs are creatures of great scientific interest to biologists. The frog's intriguing ability to resist, store, and manipulate toxins, along with its role in the food chain pose many important questions in the study of food-chain evolution. Biologists have speculated that the frogs may have first evolved a resistance to the alkaloids in their food sources. Their ability to digest toxic foods may have allowed them capitalize on unwanted supplies of foods. Did the development of bright coloration occur at the same time as the development of their toxic skin?
In captivity, poison dart frogs have a lifespan of 5 to 12 or more years, but little data exists for wild frogs. Poison dart frogs are commonly bred in captivity to be non-toxic. Most species reach maturity around 1.5 to 2.5 years of age.
In captivity, most species thrive where the humidity is kept constant at 80–100 percent and where the temperature is around 75–80°F (24–27°C) during the day and no lower than 60–65°F (16–18°C) at night.
Some species include a number of sub-species "morphs." Different morphs represent geographically separated populations of the same species in the wild, showing different coloration. For example, the species Dendrobates tinctorius includes at least a dozen morphs.
The most obvious contribution of the poison frogs is to the native populations. The Embera Indians use the poison frogs to make their weapons more effective in bringing down prey and thereby providing food. Although the poison kills the small animals that they hunt, it is apparently nontoxic when consumed by the tribesmen and their families.
The most potent poison of these frogs is batrachotoxin. In studying the mechanism of its toxicity, it was found to be a potent neurotoxin that acted as a sodium blocker in excitable tissues such as nerve and muscle. Batrachotoxin is now a valuable research tool in neurophysiology.
One of the most clinically exciting discoveries came from the poison dart frog Epipedobates tricolor, which is living in Ecuador near the Peruvian border. The skin of this frog contains an opioid compound with a unique structure, which gives it analgesic properties 200 times greater than morphine (Daly et al. 2000). This compound was named epibatidine. Abbott Laboratories began making analogues of this compound and are now testing ABT-594, a promising new painkiller drug with fewer side effects than opiates.
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