| Poison dart frogs (Dendrobatidae) | |
|---|---|
| Dendrobates azureus (top) and Dendrobates leucomelas | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Amphibia |
| Order: | Anura |
| Suborder: | Neobatrachia |
| Superfamily: | Dendrobatoidea |
| Family: | Dendrobatidae Cope, 1865 |
| Subfamilies and genera | |
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| Distribution of Dendrobatidae (in black) | |
Poison dart frog (also dart-poison frog, poison frog or formerly poison arrow frog) is the common name of a group of frogs in the family Dendrobatidae which are native to Central and South America. These species are diurnal and often have brightly-colored bodies. Although all wild dendrobatids are at least somewhat toxic, levels of toxicity vary considerably from one species to the next and from one population to another. Many species are critically endangered. These amphibians are often called "dart frogs" due to the Amerindians' indigenous use of their toxic secretions to poison the tips of blowdarts.[2] However, of over 175 species, only three have been documented as being used for this purpose (curare plants are more commonly used), and none come from the Dendrobates genus, which is characterized by the brilliant color and complex patterns of its members.[3][4]
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Most species of poison dart frogs are small, sometimes less than 1.5 centimetres (0.59 in) in adult length, although a few grow up to 6 centimetres (2.4 in) in length. They weigh about 2 grams, depending on the size of the frog. Most poison dart frogs are brightly colored, displaying aposematic patterns to warn potential predators. Their bright coloration is associated with their toxicity and levels of alkaloids. Frogs like the ones of Dendrobates species have high levels of alkaloids, whereas the Colostethus species are cryptically colored and are nontoxic.[5] When born and raised in captivity, poison frogs do not produce the skin toxins which they attain in their native habitats.[6]
Poison dart frogs are an example of an aposematic organism. Their bright coloration advertises unpalatability to potential predators. Aposematism is currently thought to have originated at least four times within the poison dart family according to phylogenetic trees, and dendrobatid frogs have since undergone dramatic divergences- both interspecific and intraspecific- in their aposematic coloration. This is surprising given the frequency-dependent nature of this type of defense mechanism.[7][8]
Adult frogs lay their eggs in moist places, including on leaves, in plants, among exposed roots, and elsewhere. Once the eggs hatch, the adult piggybacks the tadpoles, one at a time, to suitable water, either a pool, or the water gathered in the throat of bromeliads or other plants. The tadpoles remain there until they metamorphose, fed by unfertilised eggs laid at regular intervals by the mother.[9]
Poison dart frogs are endemic to humid, tropical environments of Central and Latin America (South America).[3] These frogs are generally found in tropical rainforests, including in Bolivia, Costa Rica, Brazil, Colombia, Ecuador, Venezuela, Suriname, French Guyana, Peru, Panama, Guyana, Nicaragua, and Hawaii (introduced).[3][10]
Natural habitats include subtropical or tropical, moist, lowland forests, subtropical or tropical high-altitude shrubland, subtropical or tropical, moist, montanes and rivers, freshwater marshes, intermittent freshwater marshes, lakes and swamps. Other species can be found in seasonally wet or flooded lowland grassland, arable land, pastureland, rural gardens, plantations, moist savanna and heavily degraded former forest. Premontane forests and rocky areas have also been known to hold frogs. Dendrobatids tend to live on or close to the ground, but also in trees as much as 10 metres (33 ft) from the ground.[11]
Many species of poison dart frog are dedicated parents. The red-and-blue poison-arrow frogs (Dendrobates pumilio) carry their newly hatched tadpoles into the canopy; the tadpoles stick to the mucus on the backs of their parents. Once in the upper reaches of the rainforest trees, the parents deposit their young in the pools of water that accumulate in epiphytic plants, such as bromeliads. The tadpoles feed on invertebrates in their arboreal nursery, and their mother will even supplement their diet by depositing eggs into the water. Other poison frogs lay their eggs on the forest floor, hidden beneath the leaf litter. Poison frogs fertilize their eggs externally, that is to say, the female lays a clutch of eggs and a male fertilizes them afterward, in the same manner as most fish (external fertilization). Poison frogs can often be observed clutching each other, similar to the manner most frogs copulate. However, these demonstrations are actually territorial wrestling matches. Both males and females frequently engage in disputes over territory. A male will fight for the most prominent roosts from which to broadcast his mating call; females fight over desirable nests, and even invade the nests of other females to devour competitor's eggs.[12]
The operational sex ratio in the poison dart frog family is mostly female biased. This leads to a few characteristic behaviors and traits found in organism with an uneven sex ratio. In general, females have a choice of mate. In turn, males show brighter coloration, are territorial, and are aggressive toward other males. Females select mates based on coloration (mainly dorsal), calling perch location, and territory.[13]
Dart frogs are the focus of major phylogenetic studies, and undergo taxonomic changes frequently.[1] Family Dendrobatidae was revised taxonomically in 2006 and contains 12 genera, with about 170 species.[14][15]
Some poison dart frogs species include a number of conspecific color morphs that emerged as recently as 6,000 years ago.[16] Therefore, species such as Dendrobates tinctorius, D. pumilio, and D. granuliferus can include color pattern morphs that can be interbred (colors are under polygenic control, while the actual patterns are probably controlled by a single locus).[17] Differing coloration has historically misidentified single species as separate, and there is still controversy among taxonomists over classification.[18]
Variation in predation regimens may have influenced the evolution of polymorphism in D. granuliferus,[19] while sexual selection appears to have contributed to differentiation among the Bocas del Toro populations of D. pumilio.[20][21][22]
Many poison dart frogs secrete lipophilic alkaloid toxins through their skin. Alkaloids in the skin glands of poison frogs serve as a chemical defense against predation, and they are therefore able to be active alongside potential predators during the day. About 28 structural classes of alkaloids are known in poison frogs.[3][23] The most toxic of poison dart frog species is Phyllobates terribilis. It is argued that dart frogs do not synthesize their poisons, but sequester the chemicals from arthropod prey items, such as ants, centipedes and mites - the dietary hypothesis.[24] Because of this, captive-bred animals do not contain significant levels of toxins. Despite the toxins used by some poison dart frogs, some predators have developed the ability to withstand them, including the Amazon ground snake (Liophis epinephelus).[25]
Chemicals extracted from the skin of Epipedobates tricolor may be shown to have medicinal value.[26] One such chemical is a painkiller 200 times as potent as morphine, called epibatidine, that has unfortunately demonstrated unacceptable gastrointestinal side effects in humans.[27] Secretions from dendrobatids are also showing promise as muscle relaxants, heart stimulants and appetite suppressants.[28] The most poisonous of these frogs, the golden poison frog (Phyllobates terribilis), has enough toxin on average to kill ten to 20 men or about ten thousand mice.[29] Most other dendrobatids, while colorful and toxic enough to discourage predation, pose far less risk to humans or other large animals.
All species of poison dart frogs are neotropical in origin. Wild-caught specimens can maintain toxicity for some time, so appropriate care should be taken when handling such animals.[30] While scientific study on the lifespan of poison dart frogs is scant, retagging frequencies indicate it can range from one to three years in the wild.[31] However, these frogs typically live for much longer than that in captivity, having been reported to live as long as 25 years. These claims also seem to be questionable, since many of the larger species take a year or more to mature, and Phyllobates species can take more than two years. In captivity, most species thrive where the humidity is kept constant at 80 to 100% and where the temperature is around 72 °F (22 °C) to 80 °F (27 °C) during the day and no lower than 60 °F (16 °C) to 65 °F (18 °C) at night. Some species tolerate lower temperatures better than others.
Many species of poison dart frogs have recently experienced habitat loss (due to logging and farming), as well as chytrid diseases. Some are listed as threatened or endangered as a result.[32] Zoos have tried to counteract this disease by treating captive frogs with an antifungal agent that is used to kill athlete's foot in humans.[33]
In the TV series Penguins of Madagascar, one of the characters by the name of Barry is a poison-dart frog.
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