Courtesy of El Sol de Osa The Osa Peninsula's Newspaper
Osa Safari: Jewels of the Forest
by Mike Boston of Osa Aventura
Many animals, from birds to insects, could justly tender a claim to the accolade of ‘jewels of the rainforest’, but none perhaps, with the deservedness of the dendrobatids. These tiny, often colorful frogs, bedecked in greens, reds, blues and yellows, are among the most striking and gaudy denizens of our rainforests. They are as familiar to us as the poison-dart frogs (family: Dendrobatidae), found throughout the rainforests of the New World.
Of the 170 species of Dendrobatidae thus far described, only 65 are gaudily colored and produce potent skin toxins – these are the true “poison-dart frogs”. The remaining species are more somberly colored in hues of browns and creams and lack potent skin toxins. This fact confirms, incidentally, that the true poison-dart frogs use bright coloration to advertise their toxicity to potential predators. I shall, therefore, refer to the family as whole as the “dendrobatids”.
The name “poison-dart frog” is derived from the dependence of native Indians in the Choco region of western Colombia upon the toxic secretion of these frogs to anoint their blow-gun darts. Only two or three species of dendrobatids were used in this practice, themost notable being Phyllobates terribilis, the golden poison-dart frog. This striking little frog, entirely bright golden yellow in color, secrets from its skin some of the most potent toxins (batrachotoxins) known man. One frog, it has been estimated, can produce enough toxin to kill 20,000 mice - or 10 people
This hyper-toxicity is the exception to the rule, however. Although all of the colorful dendrobatids do secrete an elaborate cocktail of toxins from their skins, most pose little threat to us. These toxins are designed to primarily to make the frogs unpalatable to predators - when you think of it, this advantage is lost if a naive predator dies from its first encounter with one of these frogs; it will have learned nothing! Like many butterflies (the monarch for example), these colorful dendrobatids advertise their unpalatability to predators through gaudy, easy to remember, coloration. In consequence, these engaging little frogs go about their business on the forest floor with an air of nonchalance, confident in their invulnerability, and make only token efforts to flee when they are encountered.
The skin toxins of dendrobatids (and, indeed, other frogs) have been the subject of a considerable amount of research in recent years – research in which I, myself, have participated with Queen’s University of Belfast. Many components of the cocktail of toxic secretions these frogs produce have been found to have useful medical applications, anticancer and antibacterial compounds among them. However, one compound isolated from Epipedobates tricolor, the phantasmal poison-dart frog from Ecuador, has caused a sensation in the pharmaceutical world. The compound, called Epibatidine, is found to be 200 times more effective as a painkiller than morphine – the most potent analgesic hitherto known. Unfortunately, Epibatidine is also toxic, but research is under way to manipulate its chemical structure to remove the toxic component while retaining its incredible analgesic properties.
So much for the toxicity of the dendrobatids, what about their behavior and ecology? Well, unusually for frogs, dendrobatids are diurnal in habit. They forage by day among the leaf litter of the forest floor for their insect prey. A major component of the diet of the colorful dendrobatids is ants, from which, it is assumed, they derive their toxins – an assumption based on the fact that captive dendrobatids, fed on non-ant prey, soon loose their toxicity. Female dendrobatids lay only a few eggs, from four to twelve, in moist locations amongst the leaf litter. The eggs are guarded vigilantly by either the male or the female (depending on the species) until they hatch, about two weeks later. The guardian will, during this period, moisten the eggs frequently, either with their urine or moisture carried on their skin from a nearby water source.
The guardian (male or female depending on the species) will then carry the tadpoles on its back and deliver them to a small water reservoir, in a tree hollow or a bromeliad, high among the trees – a characteristic peculiar to the Dendrobatidae. There the tadpoles feed on organic detritus or insect larvae (again, depending on the species) until they metamorphose, two to tree months later. Some species of dendrobatids, where the female is the guardian, periodically lay unfertilized eggs in the water reservoirs for their tadpoles to feed on. This degree of parental-care is quite unusual among frogs!
Costa Rica is home to eight species of dendrobatids, five of which occur in the Southwest Pacific region and the Osa. And two of these species are endemic to the Southwest Pacific region: Dendrobates granuliferus, the granular poison-dart frog, and Phyllobates vitttatus, the golfodulcean poison-dart frog.
Our largest dendrobatid, Dendrobates auratus, the green and black poison-dart frog, is also the most widely distributed. It is found from Southern Nicaragua through to Southern Panama, and occurs on both versants of Costa Rica. Dendrobates pumilio, the strawberry poison-dart frog or blue-jeans frog, is common along the Atlantic versants of southern Nicaragua, Costa Rica and Northern Panama. Its red body and blue legs, typical of the Costa Rican populations of this species, closely resembles the color form of the granular poison-dart frog of the Osa and adjacent areas. Similarly, Phyllobates lugubris, the lovely poison-dart frog of the Atlantic versant of Costa Rica and Panama, is replaced by its sibling species, the golfodulcean poison-dart frog endemic to the Osa area.
The remaining three species of dendrobatid found in Costa Rica belong to genus Colostethis, drab-colored members of the Dendrobatidae. They are called rocket-frogs from their habit of launching themselves, head long into streams when disturbed. Rocket frogs are among the most common dendrobatids in our forests. The high-pitched “peet – peet – peet” call of Colostethus talamancae is a common accompaniment to anyone hiking through Corcovado.
Dendrobatids, especially the colorful poison-dart frogs, are in high demand among hobbyists in Europe and North America. This is a two-edged sword, however. On the one hand their high commercial value has put a strain on wild populations from over-collecting. But on the other hand, these hobbyists are successfully breeding many species of dendrobatid in captivity, thus maintaining captive populations and preserving them from their main threat, habitat loss. Amphibians are on the decline world wide for reasons that are not entirely clear, but go beyond habitat loss. Global warming, ozone depletion, desertification, insecticides, air pollution and even fungal diseases spread by herpetologists have all been proffered as contributing factors. So concerned are conservationists that The International Union for Conservation of Nature (IUCN) has formed a special ‘Declining Amphibian Populations Taskforce’ to investigate the problem.
Surely our majestic rainforests would be all the duller were it not for these jewels to adorn their splendor!
Mike Boston is a biologist, wilderness expedition guide, and the president of Osa Aventura. You can contact him at at firstname.lastname@example.org