Poison dart frog

Dyeing 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 subtropical and 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 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

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 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.[5][22] 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.[23][24] 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.[25] 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.[26] 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.[27]

Chemicals extracted from the skin of Epipedobates tricolor may be shown to have medicinal value. Scientists use this poison to make a painkiller.[28] 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.[29] A derivative ABT-594 developed by Abbott Laboratories, called Tebanicline got as far as Phase II trials in humans,[30] but was dropped from further development due to unacceptable incidence of gastrointestinal side effects.[31] Secretions from dendrobatids are also showing promise as muscle relaxants, heart stimulants and appetite suppressants.[32] 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 ten thousand mice. Most other dendrobatids, while colorful and toxic enough to discourage predation, pose far less risk to humans or other large animals.

Ranitomeya amazonica

Skin toxicity evolved alongside bright coloration,[33] perhaps preceding it.[3] Toxicity may have relied on a shift in diet to alkaloid-rich arthropods,[23] which likely occurred at least four times among the dendrobatids.[23] Conspicuous coloration in these frogs is further associated with diet specialization, body mass, aerobic capacity, and chemical defense.[8] Either aposematism and aerobic capacity preceded greater resource gathering, making it easier for frogs to go out and gather the ants and mites required for diet specialization, contrary to classical aposematic theory, which assumes that toxicity from diet arises before signaling. Alternatively, diet specialization preceded higher aerobic capacity, and aposematism evolved to allow dendrobatids to gather resources without predation.[8]

Conspicuousness and toxicity may be inversely related, as polymorphic poison dart frogs that are less conspicuous are more toxic than the brightest and most conspicuous species.[34] Energetic costs of producing toxins and bright color pigments lead to potential trade-offs between toxicity and bright coloration,[35] and prey with strong secondary defenses have less to gain from costly signaling. Therefore, prey populations that are more toxic are predicted to manifest less bright signals, opposing the classical view that increased conspicuousness always evolves with increased toxicity.[36]

Prey mobility could also explain the initial development of aposematic signaling. If prey have characteristics that make them more exposed to predators, such as when some dendrobatids shifted from nocturnal to diurnal behavior, then they have more reason to develop aposematism.[3] After the switch, the frogs had greater ecological opportunities, causing dietary specialization to arise. Thus, aposematism is not merely a signaling system, but a way for organisms to gain greater access to resources and increase their reproductive success.[37]

Dietary conservatism (long-term neophobia) in predators could facilitate the evolution of warning coloration, if predators avoid novel morphs for a long enough period of time.[38] Another possibility is genetic drift, the so-called gradual-change hypothesis, which could strengthen weak pre-existing aposematism.[39]

Sexual selection may have played a role in the diversification of skin color and pattern in poison frogs.[40][41][42][43] With female preferences in play, male coloration could evolve rapidly. Sexual selection is influenced by many things. The parental investment may shed some light on the evolution of coloration in relation to female choice. In Oophaga pumilio, the female provides care for the offspring for several weeks whereas the males provides care for a few days, implying a strong female preference. Sexual selection increases phenotypic variation drastically. In populations of O. pumilio that participated in sexual selection, the phenotypic polymorphism was evident.[44] The lack of sexual dimorphism in some dendrobatid populations however suggests that sexual selection is not a valid explanation.[45]

Functional trade-offs are seen in poison frog defense mechanisms relating to toxin resistance. Poison dart frogs containing epibatidine have undergone a 3 amino acid mutation on receptors of the body, allowing the frog to be resistant to its own poison. Epibatidine-producing frogs have evolved poison resistance of body receptors independently three times. This target-site insensitivity to the potent toxin epibatidine on nicotinic acetylcholine receptors provides a toxin resistance while reducing the affinity of acetylcholine binding.[46]

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.[47] 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.[48] 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.[49][50][51] Some are listed as threatened or endangered as a result.[52] 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.[53]

The poison dart frog is one that is known for its aggressive and predatory behavior. As tadpoles, the individuals a part of the Dendrobates genus are said to exhibit cannibalistic tendencies along with many other forms of predatory behavior. 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 to a much higher growth rate and lived longer lives.[54] Reasons for this behavior could be that predation and aggression was selected for and favored in order to eliminate predators as well as serving as a source of food in habitats that had low resources. This predation soon could have evolved into other forms and led to cannibalism as a means of survival that proved vital.[54] One observation has been noted in the general characteristic of Dendrobate 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.[54]  

Dendrobates are a family of species very well known for their territorial and aggressive behavior not only as tadpoles, but as adults too. Many female Dendrobates are known to defend their own territory that is native to them.[55] The areas were typically in regions that were calling sites for the males of the group.[55] The males in fact were seen wrestling with intruders of their territory in order to defend their calling sites as well as their vegetation.[55] The different ways in which Dendrobates defended their own territories included physical combat and aggression which is common in the family as well as vocalization and various behavioral displays.[55] 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 Dendrobate frog then it was seen as an attack of sorts and the resident frog would see to it that they eliminated the competition.[55] The resident frog would initially try to make its appearance seen and exert its dominance, and if that did not scare away the intruder it would hop to the intruder and hit it on its head.[55] This would immediately escalate into a full on fight where both are striking each other and grasping each other's limbs.[55] On the flip side of the picture, the females also often get into fights and display aggressive behaviors. In many occasions of observations it was seen 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.[56] After a female courts with a male, they are also very likely to exhibit aggressive behavior towards any females that approach that male.[56] Both the males and females bought their own respective sexes for each other in a fairly similar fashion.[56]

The Dendrobate family was observed and it was seen that the males of the species would typically call between the time of 6:30 and 11:30.[55] The calling would typically come from a place of elevation. The males would usually be one meter above the ground on limbs, trunks, and stems, or logs of trees so that their voice traveled further as well the fact that they could be seen.[55] The calls were signaled towards the stream as the females typically were in that area.[55] 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.[55] After the calling is heard and a female makes its way to the male, and interestingly in the few incidences that the researchers saw fertilization it was not in fact accomplished through amplexus.[55] Initiation and interaction during courtship typically were the result of active females rather than males.[56] The females stroked, climbed on, and jumped on the other in tactile courtship, and were by far the more active sex among both frogs.[56]  In the majority of cases, the men in fact chose the oviposition site and led the way and females usually followed the male to the site.[55]

The diet of the Dendrobates is one that is unique and vital in giving this family of frogs characteristics distinct from the rest. The diet of Dendrobates is what gives it the alkaloids that are found in its skin.[57] 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.[57] Both skin alkaloids and venoms and toxins are substances that can be from endogenous sources meaning that the individual themself is producing or synthesizing those substances on their own, or they can be obtained through exogenous sources like consumption.[57] Researchers have studied these characteristics and have formed the diet-toxicity hypothesis which just implies that they believe that the diet of these Dendrobates is what gives them their toxicity and skin alkaloids.[57] The data from numerous studies has indicated direct correlations from diet to skin alkaloids that act as evidence proving the diet-toxicity hypothesis .[57] The diet responsible for these characteristics consists of a majority of small and leaf-litter arthropods found in its general habitat, typically ants.[57] Their diet is typically separated into two distinct categories.[58] The first is prey that are slow-moving, large in number, and small in size.[58] This typically consists of ants, while also including mites, small beetles, and minor litter-dwelling taxa.[58] The second category of pretty are much rarer finds and are much larger in body size and have high palatability and mobility.[58] These typically consist of you orthopteroids, lepidopteran larvae, as well as spiders.[58] The natural diet of an individual Dendrobate depends on its species, prey abundance in its location, as well as many other factors. 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.

The chemical defense mechanisms of the Dendrobates family is one that has been said to be a result of endogenous means.[57] As explained before, essentially this means that its ability to defend through chemical violence and defense is a means that has come through the consumption of a particular diet. In fact when captive raised species were raised they exhibited non-detectable levels of toxins whereas the wild-caught species did as a result of their calcium and vitamin Drosophila and cricket diet. The secretion of these chemicals is released by the granular glands of the frog.[57] The discharge happens as a result of acetylcholine being released into the body which triggers the sympathetic or parasympathetic system depending on the species which leads to the chemical being secreted.[57] The chemical secreted by the Dndrobatid family of frogs is said to be incredibly unique as some species secrete alkaloids that consist of a greatly different and unique chemical structure and toxicity.[57] It has been suggested that initially the granular glands were solely responsible for the production and synthesis of the diverse toxins a part of the 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.[57]

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. However, it has been studied that in the family of Dendrobates, 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.[56] 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.[56]  Dendrobates also exhibit the parental quality hypothesis which 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.[56] Whereas in many species, the competition is prominent among the males, among the Dendrobates, the females seem to have a great deal of competition among themselves. Females will even go to destroy 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.[56]

• 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