Students hard at work on the "Mangrove Challenge"! (Sierpe River, SW Costa Rica)
Poison dart frogs (poison arrow frogs) are found in the Neotropics and are usually known because of their amazing colors. This warning coloration, also known as aposematic coloration, sends out a message to their predators that they are harmful (Kricher, 1997). In the family Dendrobatidae there are five genus groups. These groups are Epipedobates, Phyllobates, Minyobates, Colustethus (not always included), and Dendrobates. All of these groups are poisonous to some degree except for Colustethus, which is sometimes referred to as the false poison frog (Melancon, accessed 2002). There are 30 species in the genus Dendrobates that contain toxic alkaloids that can be released into the environment through secretions (Kricher, 1997). These tiny frogs, even though poisonous exhibit excellent parental care, which is discussed below. If the global society is not careful there may be a chance we could lose this 'jewel of the rainforest'.
Dendrobates pumilio, otherwise known as the strawberry poison dart frog, has a range from Nicaragua to Panama on the Atlantic coast (Prohl and Hodl, 1999). This particular frog can usually be found in the lowland forests and in fruit plantations. Within Costa Rica the colors of the strawberry poison dart frog varies. The main body color can be between an orange and a red color while the hind legs can be red, black or blue (Prohl and Hodl, 1999). The breeding season for the strawberry poison dart frog is lengthened due to the tropics providing an environment with water and food that will allow females to produce clutches with an average size of 4.6 eggs (Prohl and Hodl, 1999). Most activity of D. pumilio takes place during the day when their predators can see them and take heed not to approach. In contrast, if the frogs were to be nocturnal their predators could not tell that they have bold brightly colored patterns (Kricher 1997).
These tiny amphibians, measuring only 1/2 inch to 2 inches (Babel, 1998), pack a powerful punch when it comes to defense. Poison dart frogs are known for their toxicity, but only a few are actually harmful to humans. It has been reported that approximately 500 alkaloids have been found to exist on frog and toad skin samples. These 500 alkaloids can be categorized into 20 different structural classes. The most famous classes ar Batrochotoxins, Pumiliotoxins, Histrionicotoxins, and Epibatidine. Within the family Dendrobatidae over 100 different types of toxins have been detected belonging to the class Pumiliotoxins. This class of toxins will disrupt the transport of calcium ions in calcium and sodium dependent processes within nerve and skeletal muscles. Pumiliotoxins are 100 to 1000 times less toxic than the toxins in the Batrochotoxin class (Davidson College, 2000). Batrochotoxins are found in the poison dart frog genus Phyllobates, most notably in Phyllobates terriblis. In this particular genus Batrochotoxins range from barely detectable to 1.9mg in P. terriblis. It has been recorded that it only takes 0.5 micrograms to cause death in a 20-gram mouse. With only 0.5 micrograms being a lethal dose on P. terrriblis has enough toxins to cause mortality in 20,000 mice. (Davidson College, 2000). Compared to P. terriblis, D. pumilio is close to harmless, at least for humans.
All the toxins that are found within a frog are located in cutaneous granular glands. When a frog is threatened it will release the toxins to the skin surface acting as a defense mechanism. Along with evolving toxic secretions, poison dart frogs have adapted to being immune to their own toxins (Davidson College, 2000).
The toxins that are found within dart frogs are obtained through diet. It appears that the organisms that the dart frogs consume contain these alkaloids that have been previously obtained from some tropical plants. Poison dart frogs will eat a variety of arthropods, such as beetles, millipedes, ants and flies. When these frogs are kept in captivity for research or for captive breeding it has been found that when these captive frogs are fed laboratory fruit flies the frogs will lose their toxicity over time. This explains that the frogs do indeed obtain some of the toxins through their natural diet (Davidson College, 2000; Kricher, 1997).
In the past poison dart frogs were used by the Choco Indians to provide the toxins used in their blow dart guns, hence the name of these frogs. The natives obtained the toxins through two methods. One, the rubbed the dart on the back of the frog to induce a threatened state in the frog. The frog would then secrete toxins which would be transferred onto the dart (frog not injured) (Davidson College, 2000). Two, the dart would be plunged into the frog's mouth and through the back of the head to transfer the toxins to the dart (frog is killed in this process) (Melancon, accessed 2002).
Even though poison dart frogs are harmful to their predators, the level of parental care is amazing. These tiny creatures show just how gentle they can be when it comes to their offspring. D. pumilio has one of the most extensive parental care systems within the amphibian world. It is a system that involves both the male and the female in the care of offspring from the time of eggs to after the tadpole stage.
It all begins with courtship. The females will settle in areas that contain a high number of tadpole rearing sites. Some of the common sites are bromeliad leaves or any other water filled cavities up in the trees that would be suitable for tadpoles. After the female core area has been determined males will come in and settle in around this core area and find calling sites. The males will start calling the females, which in turn attracts the females to the males. Females will visit several males and then resettle in a pattern that will be similar to the male pattern. In a study completed by Prohl and Berke (2000) found that the female home ranges were much larger than the male home ranges. This seems to be due to the idea that female ranges are formed by where the tadpole rearing sites are located. Since only one water filled cavity can contain only one tadpole, several sites will be needed. On the other hand, male ranges include their calling sites. Males tend to use the same calling sites repeatedly therefore limiting their home range. If a male finds an excellent calling site that is located within a high density of female dart frogs the male will defend that territory. As would be expected there is male fighting and possible territory takeovers (Prohl and Berke, 2001).
Once mating occurs in D. pumilio, the eggs are laid on leaves located on the ground (Woodland Park Zoo, accessed 2002; Prohl and Hodl, 1999). The leaves provide a humid environment that allows for the eggs to develop (Woodland Park Zoo, accessed 2002). The male's key role in the offspring's development occurs at the point after the eggs have been fertilized. His primary role is to keep the eggs hydrated until they hatch (Prohl and Hodl, 1999). Once the tadpoles have hatched the female will hunch down and allow the tadpoles, one at a time, to swim up on the back of the female. The female then transports them to a tadpole-rearing site high above the ground (Prohl and Hodl, 1999). It has been speculated by Allen Young (1979) that there may have been a selective pressure that resulted in D. pumilio transporting the tadpoles to the trees instead of leaving them somewhere hidden in the leaf litter. His guess was a high mortality rate occurred when and if these frogs left their eggs in the leaf litter prior to the selective pressure. In the tadpole rearing sites, which contain stagnant rainwater, fungi, algae, and other tiny insect larvae are available to the tadpoles for food (Young, 1979). In addition to the food already contained in the rearing site, the female will come back to the tadpole and deposits an unfertilized egg for the tadpole to eat (Prohl and Hodl, 1999).
Once D. pumilio finds a tree that has excellent tadpole-rearing sites many tadpoles may be brought to the same location. All of the tadpoles need their own water filled cavity, as mentioned before, due to the carnivorous habits of the tadpoles. If two tadpoles are placed in the same rearing site, the larger tadpole will consume the smaller, thereby reducing the number of surviving offspring of D. pumilio. So, one clutch is never deposited into a single water filled cavity (Babel, 1998).
In a study completed by Prohl and Hodl (1999) the number of hours that the male and female commit to parental care was calculated. After careful observation it was determined that the male only spends approximately 5 hours taking care of his offspring, this includes coming to the oviposition site to moisten the eggs. In contrast the amount of time spent by the male the female's contribution is much greater. The D. pumilio female appeared to be spending a total of 8 days to attend her offspring. So, even though both parents participate in parental care the female does much more work than the male (Prohl and Hodl, 1999). A problem arises for the males because the female is spending so much time attending her offspring. Due to the female taking time to attend her offspring that decreases the amount of time that she has to mate. In Prohl and Hodl's research they consider the time not able to mate as 'time-out'. They discovered that the female's 'time-out' was much higher then the male's 'time-out' in D. pumilio. This means that the female is spending her time taking care of her offspring to increase the rate of survival instead of spending her time mating to increase the number of offspring. An additional problem due to the level of maternal care is that the number of females that are available to mate decreases, so there is a higher level of male competition to mate with those females that are not attending to offspring (Prohl and Berke, 2001).
Costa Rica is known for its large amount of biodiversity. This country comprises 0.01% of the global territory while housing almost 4% of all living species. Costa Rica has approximately 6% of the entire amount of biodiversity. (INbio, 2002). Costa Rica was once one of the most deforested countries in the world. After the 1950's, land use began a transformation into pastures and farmlands, which resulted in a decrease in forested land. During the 1980's, Structural Adjustment Programs (SAPs) were introduced by the World Bank, which reduced the economic benefit Costa Rica was receiving from agro-export production. In addition to the SAPs, the Costa Rican government put policies into place that created special conservation areas and promoted reforestation and forest management (World Bank, 2000). Costa Rica is able to promote reforestation by providing incentives that benefits small and large landowners, such as tax credits, direct payments, and subsidized loans. By developing a National System of Protected Areas, passing legislation, and taking on the 'polluters pay' principle Costa Rica has been able to protect their biodiversity and has become a role model for other countries to do the same (World Bank, 2000).
Some of the conservation strategies that Costa Rica has adopted are the National System of Conservation Areas (SINAC), mentioned above, and the National Biodiversity Institute (INbio). SINAC has been able to set aside clearly defined protected areas that account for approximately 25% of the national territory, which includes national parks, national forests, and wildlife refuges. These areas have been an attraction for tourists and contribute to the country's economy. INbio is involved in the area of bioprospecting and promotes awareness of the value of biodiversity, which promotes the value of conservation to improve the quality of life. Bioprospecting tries to increase the value that is placed upon the species that exist in certain areas. INbio documents what biodiversity exists in Costa Rica, where the biodiversity can be found, and what type of sustainable activities the country can utilize with the high amount of biodiversity. In Costa Rica bioprospecting is done in collaboration with local and international research centers, universities, and the business sector. It has been determined that the principal markets for bioprospecting are agricultural sectors, biotechnoloical sectors, and pharmaceutical sectors (Sittenfeld, Espinoza, Munoz, and Zamora, accessed 2002). INbio uses specific access permits (World Bank, 2000) to collect plants, insect, microorganisms and fragrances to develop extracts that are tested in local and international laboratories. The extracts are tested for use in the three sectors mentioned above: agricultural industry, biotechnological industry, and the pharmacological industry. The labs also test a portion of the extracts to see what effect they have upon microorganisms that cause human diseases (INbio, 2002).
The poison dart frog is one tropical species that is a bioprospect. The toxins that the frogs produce allow researchers to test and understand the physiological functioning of muscle contraction and ion transfer between muscle fibers since most toxins interfere with muscle functioning (Davidson College, 2000). Another interesting fact is that the toxins from the genus Dendrobates are chemically similar to the human adrenal gland secretions (Woodland Zoo, accessed 2002). This knowledge may be fundamental in bioprospect research in hopes that it could help out humans in one way or another. There are other compounds that can be found within the frog toxins and possibly other toxin characteristics that may prove useful for medicinal purposes (Davidson College, 2000).
At present D. pumilio is not endangered and is a common occurance in Costa Rica. However, there are several threats that could affect the population numbers of the strawberry poison dart frog. One, future deforestation could harm the poison dart frogs due to the fact that the frog's habitat would be decreasing. D. pumilio relies upon leaf litter as well as the trees themselves for reproductive purposes. If deforestation resumed in Costa Rica, there may be a chance that these frogs would decline in numbers from a decrease in reproductive success. Also, associated with deforestation may be global climate change, which could also harm D. pumilio populations. Again, the fact that the tropics are continually warm and water and food is consistently available to the strawberry poison dart frog breeding can take place virtually all year long as long as the necessary resources remain. If the climate would change, then the reproductive strategy of the frogs would either have to change or they would perish in a short time. If climate change were to occur at a large scale then the population numbers would most likely decline. Climate change may not be the root of extinction for the frogs, but the number would most likely drop until the frogs adapted to the new environment.
A new threat that is taking the spot light in recent research is that of emerging infections that are taking over some frog populations. One such amphibian disease is known as chytridiomycosis. Chytrids are fungi that are found in aquatic habitats and are responsible for the degradation of cellulose, chitin, and keratin. Characteristics of this fungus are gross lesions, sloughing off of skin, skin ulceration, and hemorrhages of the skin, muscles, or eyes. To be able to diagnosis this disease a tissue sample needs to be obtained and intercellular flask-shaped sporangia need to be identified within the epidermis for the frog to test positive for chytridiomycosis (Daszak, Berger, and Cunningham, 1999). Through laboratory tests it has been determined that this fungus fulfills Koch's postulates and this is a lethal disease for frogs. It is believed that chytrids are inhibiting the cutaneous respiration and osmoregulation and/or the frog is absorbing a toxin from the fungus. In either case the fungus has become a pathogen and has been observed to be in several frog populations. It is not known where this fungus has originated.
Some researchers have speculated three different avenues for the fungus to infect frog population. One, the fungus is endemic to the area and the frog population declines from the fungus is just being recognized. Two, the fungus is endemic to the area and has just recently become harmful to the frogs. Three, it has been recently introduced into the area and its affect on frog populations are now being observed. Many believe that since the mortality rates are high and the fungus is spread across several species that it appears to carry the characteristics of an introduced pathogen. Scientists are not positive if the disease has been introduced and if it has no one is sure of the origin. The emergent infectious amphibian disease may also be appearing as a result of other stresses that are occurring within the environment, such as UV radiation, climate change, and chemical pollution. These stresses that have been placed upon the amphibian world may have opened the doors to an opportunistic pathogen (Daszak, Berger, and Cunningham, 1999). Another possibility is that these stresses are enabling the affects of the fungus to be magnified to the level where the mortality rate is making researchers take a second look at what is happening (Morell, 1999).
Research has been focusing on the characteristics of this fungus to be able to gain as much knowledge as possible to understand how this pathogen works (Daszak, Berger, and Cunningham, 1999). Also, information on the location of the fungus and the rate at which it spreads is being recorded (Morell, 1999). This research is being collected in hopes for the ability to form a management plan to help reduce the mortality rate of frogs. Amphibians make up a large percentage of the biomass and their decline may have a significant impact upon other species in the ecosystem (Daszak, Berger, and Cunningham, 1999).
Poison dart frogs are amazing creatures with their brilliant colors, their toxic secretions, and their high level of parental care. It is a shame that they are being threatened when their toxins could be used for medicinal purposes. Information about the importance of tropical species, and for that matter temperate species, needs to be spread to increase awareness and hopefully increase conservation efforts. Costa Rica has started off with great programs for protection of species and for bioprospecting, but to make any type of impact other countries need to follow suit and use Costa Rica as a role model.
1. Babel, Sarah. 1998. Dendrobates pumilio: strawberry poison dart frog. University of Michigan. Available via the web at animaldiversity.ummz.umich.edu/accoutns/dendrobates/d.pumilio$narrative.html. Accessed 3-7-2002.
2. Davidson College. 2000. Animal Physiology: poison dart frogs. Available via the web at
www.bio.davidson.edu/Courses/anphys/2000/Todd/toxin.htm. Accessed 3-8-2002.
3. Daszak, Peter, Lee Berger, Andrew A. Cunningham , Alex D. Hyatt, D. Earl Green, Rick Speare. 1999. Emerging Infectious Diseases and Amphibian Population Declines. Centers for Disease Control. 5(6).
4. INbio. 2002. National Biodiveristy Institute. Available via the web at www.inbio.ac.cr. Accessed 3-8-2002.
5. Kricher, John. 1997. A Neotropical Companion. Princeton University Press. Princeton New Jersey.
6. Melancon, Robb. Species: Jewels of the rainforest. Available via the web at www.dartfrog.com/species/dfrog_jewel.htm. Accessed 3-8-2002.
7. Morell, Virginia. Apr 1999. FROG DECLINES: Are Pathogens Felling Frogs? V. 284, n 5415, p. 728 -731.
8. Prohl, Heiki and Olaf Berke. 2001. Spatial distributions of male and female strawberry poison frogs and their relation to female reproductive resources. Oecologia. v.129 p. 534-542.
9. Prohl, Heiki and Walter Hodl. 1999. Parental investment, potential reproductive rates, and mating system in the strawberry dart-poison frog, Dendrobates pumilio. Bhav Ecol Sociobiol. v.46 p.215-220.
10. Sittenfeld, Ana, Ana Mercedes Espinoza, Miguel Munoz, and Alejandro Zamora. Costa Rica: challenges and opportunities in Biotechnology and Biodiversity. Available via the web at www.cgiar.org/biotech/rep0100/Sittenfe.pdf. Accessed 3-8-2002.
11. Woodland Park Zoo. Poison Dart Frog: Dendrobates sp. Available via the wab at
www.zoo.org/educate/fact_sheets/psn_frog/psn_frog.htm. Accessed 3-7-2002.
12. World Bank. 2000. Country Case study: Costa Rica forest strategy and the evolution of land use. Available via the web at http://wbln0018.worldbank.org/oed/oeddoclib.nsf/View+to+Link+WebPages/A25EFCF3220878D585256970007AC9EE?OpenDocument. Accessed 3-8-2002.
13. Young, Allen M. 1979. Arboreal Movement and Tadpole-carrying Behavior of Dendrobates pumilio
Schmidt (Dendrobatidae) in Northeastern Costa Rica. Biotropica. v.11 n.3 p.238-239.
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