The Poison Dart Frog

Mar 20, 2011 Features / Columnists, Interesting Creatures in Guyana

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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.
Unlike most frogs, these species are active during the day and often have brightly-coloured 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. Actually, 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 colour and complex patterns of its members.
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 six centimetres (2.4 in) in length. They weigh about two grams, depending on the size of the frog.
Most poison dart frogs are brightly coloured, 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 coloured and are non-toxic.
Unlike most other frogs, they are diurnal, rather than being primarily nocturnal or crepuscular. When born and raised in captivity, poison frogs do not produce the skin toxins which they retain in their native habitat. Adult frogs lay their eggs in moist places, including on leaves, in plants, among exposed roots, and elsewhere.
Once the eggs hatch, the adults carry the tadpoles via piggyback to the nearest source of water, which is typically a pool, but can also be the water gathered in bromeliads or other plants. The tadpoles remain there until metamorphoses. They are as small as your finger nail, which makes them very hard to see.
Poison dart frogs are endemic to humid, tropical environments of Central and Latin America (South America). These frogs are generally found in tropical rainforests, including in Bolivia, Costa Rica, Brazil, Colombia, Ecuador, Venezuela, Guyana, Suriname, French Guiana, Peru, Panama and Nicaragua.
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 savannah 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. 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 back 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 in. 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.
Some poison dart frogs species include a number of con-specific colour morphs that emerged as early as 6,000 years ago. Therefore, species such as Dendrobates tinctorius can include colour pattern morphs that can be interbred (colours are under polygenic control, while the actual patterns are probably controlled by a single locus). Differing coloration has historically misidentified single species as separate, and there is still controversy among taxonomists over classification.
Many poison dart frogs secrete lipophilic alkaloid toxins through their skin. Alkaloids in the skin glands of poison frogs serve as a chemical defence 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. 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. This is known as the dietary hypothesis. Because of this, captive-bred animals do not contain significant levels of toxins. Despite the toxins used by some poison dart frogs, there are some predators that have developed the ability to withstand them, including the Amazon ground snake.
Chemicals extracted from the skin of Epipedobates tricolor may be shown to have medicinal value. One such chemical is a painkiller 200 times as potent as morphine, called epibatidine, that has unfortunately demonstrated unacceptable gastrointestinal side effects in humans. Secretions from dendrobatids are also showing promise as muscle relaxants, heart stimulants and appetite suppressants.
The most poisonous of these frogs, the Golden Poison Frog (Phyllobates terribilis), has enough toxin on average to kill10 to 20 men or about 10,000 mice. Most other dendrobatids, while colourful and toxic enough to discourage predation, pose far less risk to humans or other large animals.
Like many frog families, dendrobatids have also been affected by the worldwide decline in amphibian populations. Habitat loss (due to logging and farming) and predation by introduced species are among the more common causes, but the cutaneous chytridiomycosis has struck dart frogs the hardest in the past 25 years. 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.
(Source: Wikipedia – The Free Online Encyclopedia)

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