Poison dart frog

Dyeing poison dart frog (Dendrobates tinctorius)

Most species of poison dart frogs are small, sometimes less than 1.5 cm (0.59 in) in adult length, although a few grow up to 6 cm (2.4 in) in length. They weigh 1 oz. on average.[7] 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. For example, frogs of the genus Dendrobates have high levels of alkaloids, whereas Colostethus species are cryptically colored and are not toxic.[4]

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.[3][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, in some species fed by unfertilized eggs laid at regular intervals by the mother.[9]

Poison dart frogs are endemic to humid, tropical environments of Central and South America.[5] These frogs are generally found in tropical rainforests, including in Bolivia, Costa Rica, Brazil, Colombia, Ecuador, Venezuela, Suriname, French Guiana, Peru, Panama, Guyana, Nicaragua, and Hawaii (introduced).[5][10]

Natural habitats include moist, lowland forests (subtropical and tropical), high-altitude shrubland (subtropical and tropical), moist montanes and rivers (subtropical and tropical), 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 m (33 ft) from the ground.[11]

Ranitomeya imitator's developmental life stages

Many species of poison dart frogs are dedicated parents. Many poison dart frogs in the genera Oophaga and Ranitomeya 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 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; the female lays a cluster of eggs and a male fertilizes them afterward, in the same manner as most fish. 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 organisms 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] The family Dendrobatidae currently contains 16 genera, with about 200 species.[14][15]

The skin of the phantasmal poison frog contains epibatidine

The chemical defense mechanisms of the Dendrobates family are the result of exogenous means.[22] Essentially this means that their ability to defend has come through the consumption of a particular diet. In fact, captive raised species exhibited non-detectable levels of toxins whereas the wild-caught species did as a result of their calcium and vitamin-rich Drosophila and cricket diet. The secretion of these chemicals is released by the granular glands of the frog.[22] The discharge happens as a result of acetylcholine being released into the body which triggers the sympathetic or parasympathetic system depending on the species.[22] The chemicals secreted by the Dendrobatid family of frogs are unique alkaloids that consist of greatly different and unique chemical structures and toxicity.[22] It has been suggested that initially the granular glands were solely responsible for the production and synthesis of the diverse toxins in different species of Dendrobates, then through evolution the glands were adapted to also be responsible for storage, and then finally even the secretion of them upon triggering.[22]

Many poison dart frogs secrete lipophilic alkaloid toxins such as allopumiliotoxin 267A, batrachotoxin, epibatidine, histrionicotoxin, and pumiliotoxin 251D through their skin. Alkaloids in the skin glands of poison dart 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 dart frogs.[5][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 diet-toxicity hypothesis.[24][25] Because of this, captive-bred animals do not possess significant levels of toxins as they are reared on diets that do not contain the alkaloids sequestered by wild populations. In fact, new studies suggest that the maternal frogs of some species lay unfertilized eggs, which are laced with trace amounts of alkaloids, to feed the tadpoles.[26] This behavior shows that the poisons are introduced from a very young age. Nonetheless, the captive-bred frogs retain the ability to accumulate alkaloids when they are once again provided an alkaloidal diet.[27] Despite the toxins used by some poison dart frogs, some predators have developed the ability to withstand them. One is the snake Erythrolamprus epinephalus, which has developed immunity to the poison.[28]

Chemicals extracted from the skin of Epipedobates tricolor may have medicinal value. Scientists use this poison to make a painkiller.[29] One such chemical is a painkiller 200 times as potent as morphine, called epibatidine; however, the therapeutic dose is very close to the fatal dose.[30] A derivative, ABT-594, developed by Abbott Laboratories, was named as Tebanicline and got as far as Phase II trials in humans,[31] but was dropped from further development due to dangerous gastrointestinal side effects.[32] Secretions from dendrobatids are also showing promise as muscle relaxants, heart stimulants and appetite suppressants.[33] The most poisonous of these frogs, the golden poison frog (Phyllobates terribilis), has enough toxin on average to kill ten to twenty men or about twenty thousand mice.[34] Most other dendrobatids, while colorful and toxic enough to discourage predation, pose far less risk to humans or other large animals.

Ranitomeya amazonica

Captive female D. auratus.

All species of poison dart frogs are Neotropical in origin. Wild-caught specimens can maintain toxicity for some time (this can be obtained through a form of bioaccumulation), so appropriate care should be taken when handling them.[49] 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.[50] 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, chytrid diseases, and collection for the pet trade.[51][52][53] Some are listed as threatened or endangered as a result.[54] Zoos have tried to counteract this disease by treating captive frogs with an antifungal agent that is used to cure athlete's foot in humans.[55]

The poison dart frog is known for its aggressive and predatory behavior. As tadpoles, the individuals of the genus Dendrobates are said to exhibit some unique cannibalistic tendencies, along with many other forms of predatory behavior.[56] Dendrobates tadpoles that either consumed three or more conspecific tadpoles and/or relatively large larvae of a specific species of mosquito by the name of Trichoprosopon digitatum common in their environment led them to having a much higher growth rate and typically lived much longer lives.[56] Reasons for this behavior could be that predation and aggression was selected for and favored for a few reasons. One reason is to eliminate predators, and the second reason is that it serves as a source of food in habitats that were low in resources. This predation could have evolved over time and led to cannibalism as another form of predatory behavior that had benefitted individuals survival fitness.[56] However, one observation has been noted in the general characteristic of Dendrobates tadpoles including D. arboreus, D. granuliferus, D. lehmanni, D. occultator, D. pumilio, D. speciosus, and many other Dendrobates species is that they have reduced mouth parts as young tadpoles which limits their consumption typically to unfertilized eggs only.[56] Thus, it can be assumed that the cannibalistic tendencies of Dendrobates is limited to their lifetime as a tadpole and does not cross over into their adult life.

The Dendrobatidae are a family of species very well known for their territorial and aggressive behavior not only as tadpoles, but as adults too. These aggression behaviors were not only limited to males, as many female Dendrobatidae also are known to defend their own native territory very aggressively.[57] These areas that were being defended aggressively by Dendrobatidae were typically found to be regions that served as male calling sites.[57] In fact, the males were seen wrestling with intruders of their territory in order to defend their calling sites as well as their vegetation.[57] The different ways in which Dendrobatidae defended their own territories included physical combat and aggression. While vocalization and various behavioral displays served as a way of exhibiting one's strength or fitness, territorial disputes and fights led to physical combat and aggression at times too.[57] Physical violence and aggression is particularly common at times of calling. If it was seen that an intruder was making calls in the territory of a Dendrobatidae frog then the resident frog would attempt to eliminate the competition to claim the territory and the females in it to himself.[57] The resident frog would initially try to make its presence known by the means of vocalization and various behavioral displays as a way to exert dominance.[57] If this did not scare away the intruder, then the resident frog would move towards the intruder and strike them.[57] This encounter would immediately escalate into a full on fight where both are striking each other and grasping each other's limbs.[57] Similarly, the females also often would get into fights and display aggressive behaviors in disputes over territory or a mating conflict. It was also observed that females who were going after the same male, after hearing their call, would chase each other down and wrestle to fight for the male.[58] After a female courts with a male, they are also very likely to exhibit aggressive behavior towards any females that approach that male.[58] Both the males and females bought their own respective sexes for each other in a fairly similar fashion.[58]

Observations of the Dendrobatidae family suggest that males of the species would typically make their mating call in morning between the times of 6:30 am to 11:30 am.[57] The calling would typically come from a place of elevation from various pieces of nature. The males would usually be on average one meter above the ground on limbs, trunks, and stems, or logs of trees so that their voice traveled further as well as the fact that it would help themselves be seen.[57] The calls were signaled towards the stream as the females typically were in that area.[57] Each male typically had their own region in which they only made calls from, and typically an individual would repeat their calls from the same spot during a mating season.[57] After the call is received, the female makes its way to the male, and fertilization occurred in a few instances.[57] This observed fertilization was not accomplished through amplexus.[57] Initiation and interaction during courtship typically were the result of active females rather than males.[58] The females stroked, climbed on, and jumped on the other in tactile courtship, and were by far the more active sex.[58] The duration of courtship in poison frogs is long and females could even reject males even after an entire day of active following which was found to occur in a few instances.[59] In the majority of cases, the males in fact chose the oviposition site and led the way and females usually followed the male to the site.[57] In some Dendrobatidae species, such as strawberry poison frog, visual cues under high light intensity were also used to identify individuals from the same population.[59] Different species use different cues to identify individuals from their same population during the time of mating and courtship.

The diet of the Dendrobatidae is unique and vital in giving this family of frogs characteristics that are distinct from the rest. The diet of Dendrobatidae is what gives them the alkaloids that are found in their skin.[22] This unique skin color and pigmentation and ability to transform color is an distinct feature that finds its basis in its diet. The diet is also what allows this family of frogs to have a defense mechanism such as poison, which is why it's commonly referred to as the poison dart frog.[22] Both skin alkaloids and venoms and toxins are substances that can be from endogenous sources.[22] Endogenous sources essentially means that the individual themselves is producing or synthesizing those substances on their own, or they can be obtained through exogenous sources like consumption.[22] Researchers have studied these characteristics and have formed the diet-toxicity hypothesis which just implies that they believe that the diet of these Dendrobatidae is what gives them their toxicity and skin alkaloids.[22] The data from numerous studies has led to the conclusions that there are direct correlations present between the frog's diet and its skin alkaloids that act as evidence to prove the diet-toxicity hypothesis.[22] The diet that is responsible for these characteristics consists primarily of small and leaf-litter arthropods found in its general habitat, typically ants.[22] Their diet, however, is typically separated into two distinct categories.[60] The first is the primary portion of Dendrobatidae's diet which include prey that are slow-moving, large in number, and small in size.[60] This typically consists of ants, while also including mites, small beetles, and minor litter-dwelling taxa.[60] The second category of prey are much rarer finds and are much larger in body size, and they tend to have high palatability and mobility.[60] These typically consist of the orthopteroids, lepidopteran larvae, and spiders.[60] The natural diet of an individual dendrobatid depends on its species, prey abundance in its location, as well as many other factors. The available species present in the individual's enivornment can influence its diet vastly. Frogs of one species can specialize in preying on one particular prey, whereas another can focus on a different type of prey. The preference is one that depends on various factors that are calculated and decided upon as the most beneficial for one's own fitness.

Typically in many species the larger portion of parental investment falls on the shoulders of the female sex, whereas the male sex has a much smaller portion.[58] However, it has been studied that in the family of Dendrobatidae, many of the species exhibit sex role reversal in which the females are competing for a limited number of males and the males are the choosers and their parental investment is much larger than the females.[58] This theory also says that the female will typically produce eggs at an exceedingly fast rate that the males cannot possibly take full care of them which then leads to some of the males becoming unreceptive.[58] Dendrobatidae also exhibit the parental quality hypothesis. This is where the females mating with the males try to ensure that their male mates with as few individuals as possible so that their number of offspring is limited, and thus each individual offspring receives a larger portion of care, attention, and resources.[58] However, this creates an interesting dynamic of balance as there is a limited number of males available, and with many females competing for a limited number of males for courtship this makes it difficult to limit the number of individuals a male mates with. Whereas in many species, the competition is flipped in that the competition is prominent among the males, among the Dendrobatidae it is the opposite as the females seem to have a great deal of competition among themselves for males. Females will even take the drastic measures and resort to the destroying of other female's eggs in order to make sure that the male they mated with is receptive and that it scares the male from mating with other females.[58]

• Mantella – Malagasy poison frogs
• Poisonous amphibians

Wikispecies has information related to Dendrobatidae.

Wikimedia Commons has media related to Dendrobatidae.

• Dendrobates.org – ecology, evolution and conservation of poison frogs
• Dartfrog.tk
• Poison Arrow Frog Facts
• Frognet – mailing list for dendrobatid hobbyists
• Some Pictures
• "Poison dart frog" at the Encyclopedia of Life
• Frog Poison – Histrionicotoxin at The Periodic Table of Videos