POISON FROGS: Dendrobatidae

PHYSICAL CHARACTERISTICS

Poison frogs are known for the very poisonous skin that many of them have. Actually, all frogs have poison glands, or small groups of cells, that ooze poison. In most species, the poison is very mild and, at most, only serves to make the frog taste bad. In some of the species in this family, however, the poison is far more potent and may even be deadly. The skin of the golden dart-poison frog, for instance, contains an especially dangerous poison, or toxin. Even the tiniest of droplets of this toxin in a predator's bloodstream can be fatal. Not all of the poison frog species are equally toxic, and some have no more poison than most other frogs in the world. Even within one species, different individuals may have different levels of toxin.

Scientists think that the especially toxic poison frogs may not actually make their toxins themselves, but instead get them by eating poisonous insects that, in turn, get their toxins by eating poisonous plants. The insects can eat the poisonous plants, and the frogs can eat the poisonous insects without having any health problems. When these especially toxic poison frogs are taken out of their natural habitat, placed in an aquarium, and fed non-poisonous food, the frogs eventually lose their high levels of toxins. The word toxicity (tox-ISS-ih-tee) means the level of toxins.

The toxic poison frogs are very colorful. These bright shades, called warning colors, caution predators against eating the frogs. The typical strawberry poison frog, for example, stands out with a bold red body and vivid blue legs. The Brazil nut poison frog has a black body with white to cream-colored spots and blotches on its back and red patches on its front and hind legs. People often describe these lovely little frogs as "jewels."

Those poison frogs that do not have especially toxic skin typically have much more drab colors, such as browns, tans, and olive greens. Often, these colors are in patterns that blend in with the frogs' surroundings. Rather than alerting predators to their presence, their colors camouflage them from predators. Stephen's rocket frog and the Trinidad poison frog are examples. Both are brown with darker brown stripes on the sides of the body and have dark banded legs.

In some species, like the strawberry poison frog, individuals may come in different colors. Those that live in one part of Panama are red and blue as described above, but those that live in other parts of the country may be green, yellow, or orange and may have a variety of patterns on their backs, including stripes or spots. Trinidad poison frogs are an example of a species with different-looking males and females. Both the males and the females have brown backs, but the males have gray throats with a black collar, while the females have the black collar but have bright yellow throats.

A CUP OF WATER

The tadpoles of some species of poison frogs can survive in even the tiniest pools of water. Tadpoles of the very small Stephen's rocket frog and the Brazil nut frog are some of the most amazing. The female Stephen's rocket frog lays her three to six eggs in the water within cup-shaped leaves that lie on the forest floor. The male stands guard, but does not carry the tadpoles to a bigger pool of water. The eggs hatch into tadpoles in the small puddle inside the leaf, and the tadpoles remain there until they turn into froglets. In the Brazil nut frog, the males carry the eggs to water, but that water is the tiny puddle that forms inside the empty shell of a Brazil nut.

Perhaps the most unusual species in its appearance is the imitating poison frog. This small frog takes on a whole new look depending on its neighbors. If it lives near a Zimmermann's poison frog, the imitating frog looks like that species with its black-spotted yellow back, black-spotted blue legs, and blue belly. If it lives in the same area as the Amazon or Amazonian poison frog, which is an orange or yellow frog with long black stripes or spots, the imitating poison frog has that pattern. In addition, when the imitating frog shares a habitat with the red-headed poison frog, which is also known as the crowned poison frog, it has the half-orange or -red, half-black body of that species. The imitating frog is the only frog or amphibian known to copy, or mimic, the appearance of another amphibian. The imitating frog and all three of the species it mimics are highly toxic, but they are not close relatives of one another.

Regardless of the types of toxins in their skin or their colors, all poison frogs have a few things in common. They have powerful, although short, hind legs for leaping and, in some species, for climbing. They have thick pads of skin on the tops of their front and rear toes. The vast majority of them are also quite small. Most grow to 0.75 to 1.5 inches (1.9 to 3.8 centimeters) long from the tip of the snout to the end of the rump. The Brazilian poison frog and the blue-bellied poison frog are especially tiny. Female Brazilian poison frogs reach 0.68 to 0.8 inches (1.72 to 2.03 centimeters) long, and males are even smaller at 0.63 to 0.71 inches (1.6 to 1.8 centimeters) in length. Female blue-bellied poison frogs grow to 0.47 to 0.61 inches (1.19 to 1.55 centimeters), while the males reach just 0.47 to 0.59 inches (1.19 to 1.49 centimeters) in length. The Venezuelan skunk frog is one of the largest poison frogs. Females of this olive-green frog can grow to 2.5 inches (6.35 centimeters) long.

GEOGRAPHIC RANGE

Poison frogs live in central to southern parts of Central America, including Nicaragua, Costa Rica, and Panama. They also live throughout much of northern to central South America, as far south as Bolivia and southern Brazil. The green and black poison frog, also known as the green poison frog, is the only poison frog to have jumped from its native home in Central and South America to Hawaii. In 1932, people brought the frogs from Panama to the Hawaiian island of Oahu with the idea that the frogs would eat some introduced insects on the island. The frogs liked their new home and still live on Oahu today.

HABITAT

Most of the frogs in this family live in moist forests or rainforests that have never been cleared. Such uncut forests are called primary forests. A few can also survive in old, now-overgrown pastures. Among the poison frogs, only one species spends its life in the water. This is the Venezuelan skunk frog that lives in small streams and has fully webbed rear toes to help it swim.

DIET

Poison frogs typically eat small insects and other arthropods (AR-throe-pawds), which are animals that have jointed legs and no backbones. Some of the common arthropods in the typical poison frog's diet include mites, ants, and small spiders, flies, and/or beetles. Researchers have been particularly interested in why the especially toxic poison frogs are so poisonous. In 2004, a group of scientists announced that ants may be part of the reason for at least some of the frogs' toxicity. The ants have the same toxins as the frogs. When the frogs eat the ants, the frogs take on that poison and become toxic themselves. Although the scientists are not sure what makes the ants poisonous, they suspect that the ants eat poisonous plants that make the poisons.

BEHAVIOR AND REPRODUCTION

Almost all of the poison frogs are active during the daytime, usually at dawn and late in the afternoon, and especially when it rains. The most toxic species hop about in plain view of potential predators, but the predators normally leave them alone to avoid a dangerous mouthful of poison. The frogs that do not have dangerous toxins in their skin are typically dull-colored and blend in with their surroundings. This camouflage usually keeps them out of their predators' sights. If a predator does spot a frog, the poison frogs have strong legs to help them leap away. Some of the frogs also are very good climbers and can avoid ground-living predators by scrambling up bushes and trees. The family name, Dendrobatidae, actually comes from two words that mean tree and walker in Greek. The Brazilian poison frogs and the polka dot poison frogs are two species that spend their lives in the trees.

The Venezuelan skunk frog is unusual, in part, because it is active at night. This species has an odd but effective defense against predators. As its common name suggests, it has a very strong, skunklike odor that wards off attackers. Scientists are interested in this species, which was discovered in the 1980s, because it may help them figure out which other families of frogs are most closely related to the poison frogs. Although scientists are not sure, they think the poison frogs' nearest relatives may be the true toads in the family Bufonidae, or the leptodactylid frogs.

The males of many species set up and defend territories against other males. These territories can be important during the mating season. Studies of strawberry poison frogs, for example, have shown that the males with the best territories are the best at attracting females. In this case, the best territories are those that are larger and have more tall places where the males can call to females. Often, male poison frogs defend their territories with a certain type of call, called an encounter call, that tells other males to stay away. In some species, like Stephen's rocket frog, the encounter call is different from the call that they use to attract females. In other poison frogs, the two calls sound much alike and may even be the same. The Trinidad poison frog is different because the females instead of the males are the ones who set up and defend territories. The females cannot call, so they defend their territories by rippling their bright yellow throats while sitting up tall in a high spot of the territory. In the green poison frog, the female does not set up territories, but she will fight with other females that approach her mate.

The breeding time for poison frogs is commonly during the rainy season, which runs from about November to April, although it may be a bit longer or shorter in some areas. Males call the most early in the day; then they quiet down. If the day is rainy, however, they may start calling again later in the afternoon. Some species, like Stephen's rocket frog, may call any time of day if it is raining. The males call from land. Some call from the leaf-covered ground, others from a hole in a tree trunk, and some species from plants that grow on the sides of trees.

For many species, scientists have never seen the males and females mate. In others, like the harlequin poison frogs, they have a good deal of detail. In this species, the male calls to attract a female. When she comes toward him, he continues calling while hopping away and leading her to the mating site, which is under leaves on the forest floor. This species is one of many, including the green poison frog, the blue-bellied poison frog, and the strawberry poison frog, that lay their eggs in leaves lying on the ground. The female blue-toed rocket frog places her eggs inside rolled or folded leaves. Other species of poison frogs lay their eggs in the trees. The female Brazilian poison frog, for instance, lays her eggs in small, wet tree holes just above a water puddle inside the hole. All of the poison frogs lay their eggs out of the water, except possibly the Venezuelan skunk frog. No one has seen where this water-loving frog lays its eggs. The female may lay them in the water, or she may come out to lay the eggs on land.

Some of the poison frogs have only a few eggs at a time. The female blue-bellied poison frog usually lays just two eggs, the female Brazilian poison frog lays two or three, and the strawberry poison frog lays two to six in a clutch. Many additional species also have small clutches. Other frogs in this family, including the blue-toed rocket and phantasmal poison frog, have larger clutches. The female blue-toed rocket usually lays about nineteen eggs, while the female phantasmal poison frog lays between fifteen and forty eggs in a single clutch.

In most other types of frogs, both adults leave after the eggs are laid. In poison frogs, however, either the male or the female stays behind with the young until they hatch into tadpoles. Occasionally, both adults stay with the eggs. In the green poison frog, for example, the male continues to check on the eggs during the two weeks it takes them to hatch. During this time, he turns the eggs, adds water to them to keep them moist, and removes any fungus that may have started to grow on them. The male is also the caregiver in the blue-bellied poison frogs, the Amazonian poison frogs, the phantasmal poison frogs, the Trinidad poison frogs, and others. The harlequin poison frog is one of several species in which the female stays with the eggs. In a few species, including the strawberry poison frog and the Brazilian poison frog, the adults share the job.

Part of the care includes carrying the tadpoles to water where they will continue to grow and develop into froglets. In most cases, the adult sits in the middle of the hatching eggs, and the tadpoles squirm onto the adult's back. The adult then moves over to water, sometimes spending quite some time searching for the perfect spot, and drops off the tadpoles. In some species, the adult carries only one tadpole at a time and has to make a few trips from the nest to the water before he or she has moved the entire family. Once the adult has delivered all of the tadpoles to the water, the tadpoles are on their own. Among green poison frogs, one male may mate with and have young by more than one female during one breeding season. Since he is the caregiver for the eggs, he has to watch over several nests at once. Sometimes, he is not successful, and some of the tadpoles die before he can get all of the young to water.

In the strawberry poison frog, the male cares for the eggs by keeping them moist and clean, but the female takes over when the eggs are ready to hatch. She carries one tadpole at a time to plants that have puddles of water laying at the base of their leaves or filling a cup that forms from their overlapping leaves. These small puddles do not contain much, if anything, for the tadpoles to eat, so the female comes back to her growing young every five days or so to feed them. The food she leaves is additional eggs that she lays. The eggs are infertile (in-FER-tul), which means that they will never develop into young. The tadpoles eat the infertile eggs until they grow and mature into froglets.

POISON FROGS AND PEOPLE

Many of the colorful poison frogs are common in the pet trade. They have also become popular attractions in exhibits at aquariums and zoos. Some local people in their native lands have dipped darts into the skin toxins of some of the especially poisonous species. Using blowguns, they shoot the darts at small animals they were hunting. The darts would kill the animals. Scientists, on the other hand, have become interested in these frogs' powerful toxins for the possible development of new pain-relieving drugs.

CONSERVATION STATUS

The World Conservation Union (IUCN) lists seventy-nine species—thirty-eight percent of all of the species in the family—as being at some risk. It also considers ninety-three others—another forty-five percent—as Data Deficient, which means that too little information is available to make a judgment about the threat of extinction.

Of the seventy-nine at-risk species, the IUCN lists nineteen as Critically Endangered and facing an extremely high risk of extinction in the wild. Several of these species, including the skunk frog and the Bloody Bay poison frog, have had huge drops in number during recent years. In just three generations, which would include the grandparents, parents, and children, the number of Bloody Bay poison frogs has plunged by eighty percent. In other words, for every one hundred individuals in the grandparents' generation, this species has only twenty individuals left in the children's generation. The number of skunk frogs also dropped by eighty percent in just a ten-year period. In the skunk frog's case, the decline is probably due to a loss of their habitat as people have built roads, farms, and ranches. In addition, a dry spell has lowered the level of water where the frogs mate and may have hurt the young. In the case of the Bloody Bay frog, scientists are not certain, but they think infection with a type of fungus may have been to blame for the frogs' disappearance. This fungus, called chytrid (KIT-rid) fungus, has killed members of many different frog species worldwide and may be hurting some of the other poison frogs, too.

Many of the Critically Endangered poison frogs are very rare and live in small areas that are being destroyed or are under other threats from human activities, like the clearing of land through logging or the use of farm pesticides that are dangerous to frogs, including their eggs and tadpoles.

Besides the nineteen Critically Endangered species, thirty are Endangered and facing a very high risk of extinction in the wild; sixteen are Vulnerable and facing a high risk of extinction in the wild; and fourteen are Near Threatened and at risk of becoming threatened with extinction in the future. Some, like the splendid poison frog, were once common but are now extremely rare and may even be extinct. The IUCN currently lists the splendid poison frog as Endangered, noting its popularity in the pet trade and the loss of its habitat as likely causes for its much lower numbers. Scientists are unsure whether any of the frogs still live in the wild.

GOLDEN DART-POISON FROG (Phyllobates terribilis): SPECIES ACCOUNTS

Physical characteristics: Golden dart-poison frogs, also known as golden poison frogs, come in different colors. In one, called the golden phase, the frogs have a yellow to orange yellow back and head and large dark eyes. Their legs may be yellow, orange yellow, or slightly greenish. In the mint green phase, the frogs' backs and legs are a very light greenish white color. The undersides of golden or mint phase golden frogs may have a few dark marks here and there, especially on the legs where they meet the body. Their backs are often quite smooth and shiny, but their back legs may have small bumps on them. Sometimes, their backs are also covered with tiny bumps. They have strong back legs for leaping and thin front legs. The head is short and becomes narrower toward the tip of the snout, giving it a triangular shape. They have a wide mouth that crosses the snout and reaches around the sides of the head, past each eye to a spot below the eardrum, which can be seen as a round area behind and below each eye. Females are usually a bit larger than males, but only barely. Females usually are 1.6 to 1.85 inches (4 to 4.7 centimeters) long from snout to rump, while males are usually 1.5 to 1.8 inches (3.8 to 4.5 centimeters) in length.

Geographic range: The golden dart-poison frog lives in Cauca, Colombia, which is in northwestern South America.

Habitat: It is found on land in lowland rainforests along the west coast of Colombia up to 656 feet (200 meters) above sea level. The tadpoles hatch in freshwater, but the exact kind of water body is unknown.

Diet: Adults eat various small arthropods, including insects.

Behavior and reproduction: Rather than climbing into trees like some of the other poison frogs do, the golden dart-poison frog stays on the ground. It is active during the daytime, when it hops about in plain view. It is one of the most poisonous animals on Earth. The poison in its skin is so powerful that even a tiny amount in another animal's bloodstream, including that of a human being, is enough to cause death. Called batrachotoxin (buh-TRAK-oh-tox-in), the poison attacks the nervous system.

Golden dart-poison frogs and people: This frog can be seen in the pet trade, but it is not common. Some local people use the poison in the frog's skin to make deadly darts. The golden dart-poison frog is one of only three species of frogs—the other two are closely related species—that are used to make the darts.

Conservation status: The World Conservation Union (IUCN) considers the golden dart-poison frog to be Endangered, which means that it faces a very high risk of extinction in the wild. Although the frog is common where it lives, it lives only in a tiny area and only in places where the forest has never been cut. Conservationists are not sure whether it could survive if the trees were ever removed and are concerned because forests near the frog are already falling as people remove trees for lumber and to make way for farms or buildings. Conservationists also think that the spray poisons farmers use to kill crop-eating insects could hurt the frogs. So far, the frog's habitat is not protected from logging, but it is now illegal to capture the frogs from the wild. ∎

PHANTASMAL POISON FROG (Epipedobates tricolor): SPECIES ACCOUNTS

Physical characteristics: Phantasmal poison frogs are dark brown or brick red with three cream-colored, yellow, or light green stripes running from the head to the rump. The center stripe widens out at the front of the head to cover the whole snout, and the two side stripes may also come far enough forward to blend into this snout blotch. Sometimes, the stripes are broken into dotted lines or blotches. The green, yellow, or cream color also appears on the front and back legs in spots. The frog has long hind legs for leaping and fairly long but thin front legs. It has two large eyes on its head, which slopes toward the front. Often the frogs have a short green, yellow, or cream-colored line under each eye. The underside, including the belly and throat, has numerous green or cream blotches that sometimes almost completely color the underside. Females and males are nearly the same size. As adults, females usually reach about 0.8 to 1.1 inch (2.1 to 2.7 centimeters) long from snout to rump. Males typically reach slightly less than an inch (2.5 centimeters).

Geographic range: Phantasmal poison frogs live in southwestern Ecuador and northwestern Peru to the west of the Andes Mountains.

Habitat: Adults live on land in mountain valleys. Although they survive in wet or dry areas, they usually remain near streams. Tadpoles develop in streams or small pools of water.

Diet: Adult phantasmal poison frogs eat various small arthropods, including insects.

Behavior and reproduction: Scientists know little about this species outside of its breeding behavior. Mating occurs on land, as it does in the other poison frog species, but phantasmal poison frogs mate differently. While the male mates by climbing onto the back of a female, he does not hold onto her near her front or back legs, as nearly all other frogs do. Instead, he grips her with his front legs around her head. From this awkward-looking position, the female lays fifteen to forty eggs. Afterward, the female leaves, but the male stays behind with the eggs and watches over them. As each egg hatches into a tadpole, the tadpole scrambles up the male's leg and onto his back, and he carries the tadpole to a nearby stream or pool. The tadpole swims off, and the male returns to the hatching eggs to pick up the next tadpole. He continues until he has carried all the young to the water. The tadpoles develop into froglets in the water.

Phantasmal poison frogs and people: The poison in this frog's skin, while very dangerous, has helped scientists to design effective painkillers for human patients. By studying the frog's poison, which is two hundred times more powerful than the drug morphine (MORE-feen), scientists have made drugs that work in the same way that the poison does, but are not unsafe.

Conservation status: The World Conservation Union considers the phantasmal poison frog to be Endangered, which means that it faces a very high risk of extinction in the wild. This species lives in only seven spots on mountains in part of Ecuador, but it once lived over a larger area. In the remaining populations, the number of adult frogs is continuing to drop. Its small habitat is at risk because the land is being developed for farming, and because farming chemicals are polluting the water in the frogs' habitat. Conservationists are not sure, but they think that the frogs might also be at risk from people who collect them for the pet trade or from infection with chytrid fungus. This fungus has killed many different kinds of frogs around the world. ∎

BLUE-TOED ROCKET FROG (Colostethus caeruleodactylus): SPECIES ACCOUNTS

Physical characteristics: The blue-toed rocket frogs are named for their blue toes. During the breeding season, the males have blue front toes and blue pads on their back toes. Females have blue pads on both front and back toes during the breeding season, but their front toes are not all blue, as they are in males. Besides the blue toes and/or toe pads, the frogs have brown backs and heads and white chins and bellies. They have long hind legs and short front legs. The head narrows to a rather triangular-shaped snout. Females may be just a bit bigger than the males. Adult females usually grow to 0.59 to 0.67 inches (1.5 to 1.7 centimeters) from snout to rump, while males typically reach no more than 0.63 inches (1.6 centimeters).

Geographic range: Blue-toed rocket frogs have only been found in one place: about 25 miles (40 kilometers) south of Manaus, Amazonas, Brazil.

Habitat: The frogs live in a small piece of tropical rainforest valley that is flooded with water during the rainy season. The floods rush water into rivers of the valley, causing the rivers to overflow into small streams and create deep pools in the streams. The frogs live among the dead leaves that cover the slopes of forest floor above the streams. Their tadpoles develop in the deep stream pools.

Diet: Adult blue-toed rocket frogs eat various small arthropods, including insects.

Behavior and reproduction: Blue-toed rocket frogs live on land. The males set up territories that are about one thousand square feet (ten square meters). To keep others away, a male will give a short, loud call. Males and females mate on land during the rainy season, which lasts from January through April. Each female lays her eggs on the forest floor, hiding them away in folded or rolled leaves. The male then stays with his eggs, usually about nineteen in each clutch, even after they hatch into tadpoles. When the rainy season ends and the ground begins to dry up, the male carries all of the tadpoles to deep pools in the streams, where the tadpoles continue to grow.

Blue-toed rocket frogs and people: Very few people have ever seen this frog.

Conservation status: Too little information is available about this frog to make a judgment about the threat of extinction, so the World Conservation Union (IUCN) lists it as Data Deficient. Scientists only recently discovered this frog, naming it in 2001. They found it in just one spot, where it was very common, and have not yet found it anywhere else. The scientists believe that the logging of the forest would cause it to become extinct. The frogs' forest is on private property and therefore not protected from logging. ∎

FOR MORE INFORMATION

Books:

Badger, David. Frogs. Stillwater, MN: Voyageur Press, 2000.

Fridell, Ron. The Search for Poison-Dart Frogs. New York: Franklin Watts, 2001.

Halliday, Tim, and Kraig Adler, eds. The Encyclopedia of Reptiles and Amphibians (Smithsonian Handbooks). New York: Facts on File, 1991.

Heselhaus, Ralf. Poison-Arrow Frogs: Their Natural History and Care in Captivity. London: Blandford, 1992.

Mattison, Chris. Frogs and Toads of the World. New York: Facts on File Publications, 1987.

Miller, Sara Swan. Frogs and Toads: The Leggy Leapers. New York: Franklin Watts, 2000.

Patent, Dorothy Hinshaw. Frogs, Toads, Salamanders, and How They Reproduce. New York: Holiday House, 1975.

Showler, Dave. Frogs and Toads: A Golden Guide. New York: St. Martin's Press, 2004.

Walls, Jerry G. Poison Frogs of the Family Dendrobatidae: Jewels of the Rainforest. Neptune City, NJ: TFH Publications, 1994.

Periodicals:

Jenkins, Jeanette. "The Poison Dart Frog." Science News, February-March 2002 (vol. 7): page 10.

Milius, Susan. "Toxin Takeout: Frogs Borrow Poison for Skin From Ants." Science News, May 8, 2004 (vol. 165): page 291.

"Strawberry Poison Dart Frog Born at Bristol Zoo Gardens." Current Science, a Weekly Reader publication, October 10, 2003 (volume 89): page 14.

Web sites:

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"Dendrobates. WhoZoo.http://www.whozoo.org/Intro98/jenntrev/jendex3.html (accessed on April 14, 2005).

"Frog Chemist Creates a Deadlier Poison." Science News for Kids. http://www.sciencenewsforkids.org-articles-20030910-Note3.asp (accessed on April 14, 2005).

"Frogs Get Poison from Ants." Science News for Kids. http://www.sciencenewsforkids.org-articles-20040512-Note3.asp (accessed on April 14, 2005).

"Phyllobates terribilis 'Mint'." Frog of the Month, Arachnokulture.http://www.pumilio.com/frogofthemonth/december2001.htm (accessed on April 14, 2005).

"Poison Dart Frog (Dendrobates pumilio)." Basic Science and Remote Sensing Initiative, Michigan State University. http://www.bsrsi.msu.edu/rfrc/tour/dendrobates.html (accessed on April 14, 2005).

"Poison Dart Frog (Dendrobates pumilio)." Woodland Park Zoo.http://www.zoo.org/educate/fact_sheets/psn_frog/psn_frog.htm (accessed on April 14, 2005).

"Poison Dart Frogs." Smithsonian National Zoological Park.http://nationalzoo.si.edu/Animals/Amazonia/Facts/fact-poisondartfrog.cfm (accessed on April 14, 2005).

"Poison Dart Frogs, Mantellas, etc." All About Frogs.http://allaboutfrogs.org/info/species/poison.html (accessed on April 14, 2005).