The Neotropics, or new world tropics, includes the area of land in the western hemisphere that falls between the lines of latitude known as the Tropics of Cancer in the north and Capricorn in the south (Kricher, 1997). This fifty-degree latitude band contains all of Central America, the Southern portions of North America, the Northern portions of South America, and various Caribbean and Pacific Islands (Kricher, 1997). Within this small area of land resides one of earth's most diverse ecosystems, the tropical rainforest. The tropical rainforest ecosystem, of which 57% is found in the Neotropics, contains an estimated 50% of the earth's biodiversity (Kricher, 1997). This includes plants, animals, and other forms of life that are found nowhere else on earth. The Neotropics are home to one group of amphibians that live nowhere else, the Poison Frogs. These beautifully colored frogs have a unique role in the tropical ecosystem, display behavioral patterns that are found in no other animals, are among the most toxic creatures in existence, and have a long an continuing interest to humans for uses in hunting and medical research. This paper will attempt to give an overview of the taxonomy of the Poison Frogs, their general biology, the basis of their toxicity, and the valuable uses they fulfill for human beings.
Taxonomy
The taxonomic classification of plants and animals is very difficult and constantly changing science. The grouping of organisms depends on characteristics that separate them from other organisms. This begins rather simply with the determination of the organism as a member of the one of six Kingdoms: Archaebacteria, Eubacteria, Protista, Fungi, Plantae and Animalia (Purves et al. 1995). This increases in complexity of differences down to the species, which is an organism that is entirely different from all others. The division of organisms often depends on who is doing the organizing and thus can vary considerably. This is especially true of the Poison Frogs. All Poison Frogs belong to the taxonomic family Dendrobatidae, of which there are approximately 175 described species (Walls, 1999). Several features separate the Dendrobatidae family from other amphibian families of the order Anura including the presence of a retroarticular process of the mandible (Cannatella, 2001). This family is further broken down into four main genus groups: Dendrobates, Epipedobates, Minyobates, and Phyllobates (Walls, 1999). Sometimes, a fifth genus is included, Colustethus, although none of the described species of this genus contain the skin toxins from which the other species derive their common name of Poison Frog or Poison Dart Frog. Each genus in the Dendrobatidae family shows remarkable color variation and toxicity with the exception of the previously mentioned Colustethus. The differences between each genus are quite subtle and it is often difficult to determine which genus is being observed (Walls, 1999). Each genus is described in the following as well as characteristic species.
Dendrobates
Frogs of the genus Dendrobates tend to be on the larger end of the Poison Frog size scale, often measuring 25 to 50 mm long. They have greatly expanded fingertips, with the first finger being shorter than the second. Members of this genus tend to lack a stripe that runs from the base of the thigh to the inner arm, which is characteristic of other genus groups. This genus contains approximately twenty-six species (Walls, 1999).
Epipedobates
This genus is characterized by having a first finger of equal length to the second finger, moderately expanded fingertips, and, generally, the absence of dorso-lateral stripes that run from the eyes to the base of the thigh. These frogs rarely display metallic coloring and most have teeth (Walls, 1999). The following are species that are found within the genus Epipedobates:
Minyobates:
The members of this genus are the smallest of the Poison Frogs measuring only 15 to 21 mm in length. They have a similar hand structure to Dendrobates with the second finger smaller than the first and the large fingertips. These frogs have many patterns of coloration narrow stripes to solid red coloration (Walls, 1999). Some species in this genus include the Green Poison Frog, Minyobates viridis, and the Yellow-bellied Poison Frog, Minyobates fulguritis.
Phyllobates
There are only five species currently assigned to this genus and all have the second finger longer than the first and moderately expanded fingertips. They all have dorso-lateral stripes extending from the eyes to the base of the thigh. These species also appear metallic in color (Walls, 1999). The following species are found in the genus Phyllobates
General Biology
Range
Poison Frogs are found throughout Central and South America. Specific species are often very regionally restricted however, with certain species found only within certain regions of a country and nowhere else. For example, the Blue Poison Frog, D. azureus, is only found in two places: the Sipalinwini savannah of Surinam and along the border of Surinam and Brazil (Tesoro, 2001). The Green-and-Black Poison Frog, D. auratus, is far more cosmopolitan, however, as it is found in Costa Rica, Columbia, Nicaragua, and Panama (Tesoro, 2001). In terms of countries, Costa Rica is home to several species of poison frogs including the previously mentioned D. auratus, D. pumilio, P. lugubris, and P. vittatus (Tesoro 2001). Due to deforestation of the rainforest, the range of poison frogs has been shrinking and becoming more isolated. The fragmentation of the rainforest restricts the frogs to pockets of habitat that are geographically isolated from other areas of suitable habitat.
Habitat
Poison Frogs live in two types of habitat depending on the species. Many species are ground dwellers. These ground dwelling species reside in leaf-litter and debris on the floor of the rainforest (Walls, 1999). Other species are primarily arboreal, living on the trunks of trees and low-lying shrubs approximately three to four meters off the ground (Walls, 1999). Some species, particularly D. ventrimaculatus, are highly arboreal and live in bromeliads and other high plants (Walls, 1999).
Food and Foraging
Poison Frogs are highly diurnal and insectivorous. Frogs may forage for food at any time of the day or look for food for a few hours after dawn and a few hours before dusk, depending on the species in question (Walls, 1999). Their prey generally consists of ants, soil mites, termites, springtails, and beetle and fly larvae. The Poison Frogs are almost all active foragers, that is they move about the forest in search of prey (Taigen and Pough, 1983). This is different from many other frog species that use a sit and wait approach, eating anything that happens near them (Taigen and Pough 1983). There are many consequences of adopting an active foraging habit. On the plus side, active foragers encounter more prey and localized sources of prey, such as anthills or termite nests (Taigen and Pough, 1983). The cost of this, however, is a large use of energy that requires a consistent and adequate food supply to avoid starvation (Taigen and Pough, 1983). Moreover, animals that actively move about for prey are subject to notice by other animals, generally ones that would like to eat them. This is not a problem with the Poison Frogs, however, as they have few predators due to the toxins contained in their skin. The bright coloration displayed by most species in the Dendrobatidae family is thought to serve as a warning sign to would be predators that the frogs are toxic, although this remains in debate as one of the more toxic frogs are darker in color and less noticeable (Daly and Myers, 1967).
Reproductive Behavior
The Dendrobatidae family displays some of the most unique reproductive behavior in the Kingdom Animalia. Male Poison Frogs tend to be highly territorial and defend their areas against other males. These territories are generally three to four meters apart and contain calling sites, space for courtship, and areas for egg-laying (Prohl and Hodl, 1999; Walls, 1999). Males are often in competition for females as the female generally spends more time providing for the offspring (Prohl and Hodl, 1999). This leaves fewer mating opportunities for the females and thus they are more selective. The frogs are generally polygamous as well. As always, this is very species dependent as some male frogs invest time and care into the offspring as well.
Mating occurs at variable times of the year, depending on the species of Poison Frog. Generally, mating is initiated during periods of high humidity that occurs during the rainy season, although some species can mate all year (Walls, 1999). During this time, the male will generally assume an elevated position in a shrub or on the leaf of a plant and begin calling. This vocalization can sound like a low buzzing to a serious of continuous trills, again depending on the species (Walls, 1999). The vocalization serves two purposes: the first is to attract egg-bearing females and the second is to warn other males away from the area. Males that enter the area are wrestled, although apparently rarely injured, until they leave the area (Walls, 1999). One or several egg bearing females will enter the male's zone and approach him. This triggers an increased rate of calling in the male and a slow courtship ritual that involves the male leading the female to an area where eggs may be deposited. Such sites include moist areas of dead leaves on the ground or in the water-filled crevices of bromeliads (Walls, 1999). The female will first deposit the eggs in this area and the male will later fertilize them.
After the eggs hatch, the male frog will bend down and allow the developing tadpoles to crawl on his back. He will then transport the tadpoles, one or two at a time, to a small body of water where they will develop. The tadpoles will eat detritus, algae, larvae, and sometimes each other while they develop. A very interesting exception to this normal pattern occurs in some Dendrobates species, including D. histrionicus and D. pumilio, in which the female will lay unfertilized food eggs in the final deposition site for the tadpoles to eat (Walls, 1999). These are the only known creatures to produces eggs for food purposes of the young.
Toxicity
Poison Frogs derive their name from the fact that they produce toxins that are secreted from their skin in times of stress. These chemicals serve as a defense against predators who would eat the frogs otherwise. There are many toxins that have been isolated from the skin of the Dendrobatidae frogs. However, four main ones have sparked the most interest in the world of science. These four toxins are Batrachotoxin, Compound A, Compound B, and Epibatidine. Nearly all of them are neurotoxins, or chemicals that affect and disrupt the nervous system. Since this is the case, it is important to first review the functioning of the nervous system.
The nervous system is responsible for the coordination and continuing function of the processes of body. It carries electronic messages from the brain to other parts of the body, such as muscles, to carry out a desired action. The nervous system also carries information from the body back to the brain. The nervous system is made up of cells known as neurons. Neurons are composed of a cell body that contains the nucleus and has several thin projections known as dendrites that receive information or action potentials from other cells. At one end of the cell body is a long projection that allows the cell to pass information on to other cells, known as the axon. This axon terminates in projections known as terminal buttons that allow the neuron to pass its information to the dendrites of another neuron.
Neurons communicate with each other and with skeletal muscles through action potentials. An action potential is a serial depolarization of the membrane of a neuron. When a nerve cell is resting, the potential, or voltage, inside the cell is primarily negative. This means that there are more negatively charged ions inside the cell. Positive charges gather outside the cell wall, being attracted by the negative charges inside. An action potential from another nerve cell causes the nerve cell to open channels along the axon that allows sodium ions to come into the cell. This makes the cell potential more positive or depolarizes it. This causes more channels to open further down the axon and more sodium ions to come in. At the initial site of depolarization, the sodium channels close and potassium channels open that allow the potassium ions, which are also positively charged, to leave, thus repolarizing the membrane. This continues on until the terminal buttons where neurotransmitters are released which cause an action potential in other nerve cells or a muscle contraction, depending on what the nerve cell is connected to (Purves et al., 1995).
Now that the basic functioning of the nervous system is clearly elucidated, the four main poisons will be discussed. The first is Batrachotoxin, which was isolated was isolated from species contained in the genus Phyllobates. All five frog species of the Phyllobates genus produce Batrachotoxin, with D. terribilis producing the most at approximately 1 mg per frog (Daly, 1995). Batrachotoxin belongs to a class of chemicals that are known as lipophilic alkaloids (Daly, 1995). An alkaloid is a naturally occurring substance that contains a basic amine group. Batrachotoxin has considerable resemblance to a steroid, hence its lipophillic, or non-polar, character. Batrachotoxin enters the body generally through the mucosal membranes of the mouth when an animal attempts to eat a Phyllobates frog. Humans can be exposed if they handle the frog and have a cut on their hands or bring the toxins in contact with the mucous membranes of the mouth or nose. Once the Batrachotoxin is internalized, it blocks neuromuscular transmission irreversibly (Albuquerque et al., 1971). It is theorized that this toxin causes an increase in the permeability of the nerve axons to sodium ions (Daly, 1995). Thus, the sodium channels are, through an as yet unidentified mechanism, left open when an action potential passes through a nerve cell. Since the channels do not close, the nerve cell cannot reset and prepare for another action potential. This results in muscle contraction, paralysis, and death from either suffocation or heart failure. Batrachotoxin is very toxic with a minimum lethal dose of .0002 mg/kg body weight in mice (Daly and Myers, 1967). If this can be extrapolated to humans, a 180-pound man would be killed by .016 mg of the poison.
Compound A and B occur in frogs found in the genus Dendrobates. Also known as pumiliotoxin A and B, these poisons are also neurotoxins. These two substances are only known in nature from frog/toad skin and are alkaloid structures with an idolizidine ring (Daly, 1995). They have been shown to have effects on sodium and calcium channels, causing similar results as Batrachotoxin. The pumilotoxins are much less toxic, however, as A has a minimum lethal dose of 2.5 mg/kg body weight in mice and B has a minimum lethal dose of 1.5 mg/kg.
Epibatidine is a toxin isolated from one species of frog, the Phantasmal Poison Frog or Epipedobates tricolor. Epibatidine is also an alkaloid and consists of a complex ring structure with an aromatic side chain. Chemically very similar to nicotine, epibatidine binds to nicotinic-acetylcholine receptors in nerve cells and serves as an agonist (Khan et al., 1997). An agonist is a chemical that binds to a receptor and helps to activate a response. Epibatidine is toxic in large doses as it causes an increase in heart rate resulting in tachycardia and eventually death (Khan et al., 1997).
Although the basis of the synthesis of these and other toxins have yet to be determined, scientists have an indication that there may be a dietary source involved with at least some of them. Studies have shown that the poison frogs cease to secrete poisons when raised in captivity (Daly, 1997). No one has, as yet, determined what the source is for these frogs nor have they determined how the frogs synthesize the precursor materials into the toxic chemicals that they later secrete.
Human Uses of the Poison Frogs: Past and Present
Human beings have had a long history with the poison frogs. The Choco Native American tribe of Columbia has used the toxins secreted by Phyllobates terribilis, P. bicolor, and P. aurotaenia to poison their darts since before the arrival of the Europeans (Daly, 1997). These darts were then used in blowguns to hunt for food. This practice is still employed by the tribe today. For this reason, the Poison Frogs are commonly called Poison Dart Frogs or Dart-Poison Frogs.
The Poison Frogs have also made an impressive contribution to the understanding of the nervous system and how it functions. Batrachotoxin and epibatidine have been used to explore the method of neurotransmission in many animal systems. They are helping to unlock the secrets of the brain and the nervous system.
Epibatidine has been found to serve as a very potent analgesic and anesthetic. Research has shown the substance to be 200 times more effective than morphine at relieving pain and without the side effects of being addictive (Associated Press, 1998).
While too toxic for human use, analogs are currently being developed that may one day become the most potent painkiller known to man.
Finally, a new and increasing hobby is the collection of Dendrobatidae frogs for the purpose of pets and displaying them in terrariums. Many people enjoy the beauty of these frog species and have begun to make a concerted effort for their conservation.
Conclusion
In closing, Dendrobatid frogs are clearly some of the most interesting creatures on earth. They are beautiful, display rare behavior, synthesize chemicals found nowhere else in nature, help us to understand more of how our own bodies work, a supply chemicals that have great pharmacological importance. However, their habitats are shrinking with each clear cutting and their numbers are dwindling. Such a marvelous group of species is clearly worth trying to protect. Hopefully, the colors of the Dendrobitidae family will continue to brighten the tropical rainforests of Central and South America for centuries to come.
Works Cited
1. Albuquerque E.X., J.W. Daly, B. Witkop; "Barachotoxin: Chemistry and
Pharmacology", Science, Vol. 172, Iss. 3987, 1997, pgs. 995-1002
2. Associated Press; "Poison grog may provide powerful new pain killer",
available on the web at www.canoe.com/HealthNews, accessed 5/12/01
3. Cannatella, David; "Dendrobitidae", Availabile via the web at
www.utexas.edu/courses/zoo453/tol/salientia/dendrobatidae/dendrobatidae.html, accessed 5/12/01
4. Daly, John W.; "The Chemistry of Poisons in Amphibian Skin" Proceedings of
the National Academy of Sciences of the United States of America, Vol. 92, Iss. 1, Jan. 3, 1995, pgs. 9-13
5. Daly, John W. and Charles W. Meyers; " Toxicity of Panamanian Poison
Frogs (Dendrobates): Some Biological and Chemical Aspects", Science,
Volume 156 Issue 3777, May 19, 1967, pgs. 970-973.
6. Khan, Imran M., Tony L. Yaksh, Palmer Tyler; "Epibatidine binding sites and
activity in the spinal cord", Brain Research, Vol. 173, 1997, pgs. 269-282
7. Kricher, John; A Neotropical Companion, Princeton University Press,
Princeton New Jersey, 1997
8. Prohl, Heike and Walter Hodl; "Parental Inestment, potential reproductive
rates, and mating system in the strawberry dart-poison frog, Dendrobates
pumilio", Behavioral Ecologial Sociobiology, Vol. 46, pgs. 215-220
9. Purves, William K., Gordon H. Orians, H. Craig Heller; Life: The Science of
Biology, Sinauer Associates, Inc., Sunderland, Massachusetts, 1995
10. Taigen, Theodore L., F. Harvey Pough; "Prey Preference, Foraging Behavior,
and Metabolic Characteristics of Frogs", American Naturalist, Vol. 122, Iss. 4, 1983, pgs. 509-520
11. Tessoro, Brian; "Poison Dart Frogs", Available via the web at
http://members.home.net/breviceps/main/index.html, Accessed 5/12/01
12. Walls, Jerry G., The Guide to Owning Poison Frogs, T.F.H. Publications Inc., Neptune City, New Jersey, 1999
Next Article
Previous Article
Return to Topic Menu
It is 6:27:27 PM on Saturday, July 4, 2009. Last Update: Monday, December 9, 2002