Scientifi c name: Dendrobates pumilio Scientifi c classifi cation:
Phylum: Chordata Class: Amphibia Order: Anura
Family: Dendrobatidae
What does it look like? Th is is a tiny, jewel-like frog measuring only 25 mm from the tip of its snout to its rear end. Th e body of this minute amphibian is a fabulous red, while its limbs and small marks on it back are a shimmering blue. Th e eyes are large and dark, and beneath the head there is an infl atable throat sac.
Where does it live? Th is species of poison-arrow frog inhabits the perpetually wet rain forests of Costa Rica in Central America. It can be found on the forest fl oor and also up in the trees, where it breeds.
Parenting in a Poisonous Amphibian
A red-and-blue poison-arrow frog hops and scrambles along the rain forest fl oor, providing a lively splash of color in the brown carpet of decaying vegetation. It reaches the base of a tree and surmounts a small mound of moss. Here, it perches for a few moments and emits a shrill call
by infl ating its throat sac. Another fl ash of color in the undergrowth nearby reveals this frog’s amorous intentions. He is a male, and he has spotted a female that he is trying to impress with his calling. His voice will not be enough to impress the female, and he gives chase with the hope of catching her and engaging in some romantic wrestling. Impressed by the calling, chasing, and wrestling abilities of this suitor, she lays 2–16 eggs on the ground in a secluded spot. Th e male, taking his cue, fertilizes all the eggs with his sperm and moistens them with fl uids from his skin.
Th e frogs are model parents, and they guard their eggs religiously until they hatch. Soon, the waiting is over and young tadpoles break free from their eggs and instinctively wriggle upon to the back of their parents, who must now take them to water. Th e journey may take several hours, so the tadpoles are securely glued to the parents with sticky mucus that also keeps them moist.
Ponds and streams in the rain forest are crawling with all sorts of animals that would make short work of a tiny tadpole; therefore, the devoted parents head for the tree tops. Small pads on their feet aff ord them an excellent grip, and before long, they are in amongst the branches. Trees are not the sort of places you would expect to fi nd good amphibian breeding spots, but lo and behold, growing on the branches are epiphytic plants, like bromeliads (related to pineapples), which, at their center have a bowl that collects condensation and rainwater. Th e parents deposit up to three of their young in every suitable pool they fi nd. In the pool, there will be various insect larvae and other invertebrates that the tadpoles will feed on. To supplement the diet of her young, the female frog will also deposit unfertilized eggs in the bromeliad bowls, which will be greedily eaten by the tadpoles. In this sheltered microhabitat, the young develop rapidly and eventually metamorphose into adult frogs.
After the careful nurturing provided by their parents, the froglets must fend for themselves.
Th ey descend to the forest fl oor and selectively hunt ants and other small animals among the leaf litter. Trace quantities of toxins in the frog’s diet are assimilated by the frog into its own defensive systems. Th e chemicals are stored in skin glands, and should any animal eat or even lick one of these frogs, it will be dead in no time at all. Th e amazing colors of the frog are not to brighten up the drab forest fl oor, but are a warning for other animals to give it a wide berth.
Red-and-Blue Poison-Arrow Frog—A female red- and-blue poison-arrow frog taking the tadpole on her back to a bromeliad nursery. (Mike Shanahan)
Red-and-Blue Poison-Arrow Frog—An adult speci- men of this tiny frog, photographed on the forest floor in Costa Rica. (Marcus Bartelds)
LOOKING OUT FOR THE NEXT GENERATION 167
• Th ere are approximately 150 species of poison-arrow frog, but their small size and habitat means that new species are being found all the time. Th ey are only found in South and Central America. Th e red-and-blue poison-arrow frog is one of the more northerly species, but poison-arrow frogs as a whole are found all the way down to southern Brazil. Of the 150 or so species in the family, only around half are brightly colored like the species mentioned here.
• It is quite common for small amphibians in the forest to use bromeliads and other epiphytic plants as breeding pools. In a large patch of forest, there will be thousands of these pools, adding another level of complexity to the already bewildering array of microhabitats that can be found in pristine equatorial forests.
• A large brood of red-and-blue poison-arrow frog tadpoles may require repeated trips to the forest fl oor by their parents to transport them to their nursery pools.
• Th e poison-arrow frogs, although remarkable for the lengths they go to rear their young, are also amazing for the toxicity of the substances produced by their skin glands. Th e red-and-blue poison-arrow frog is one of the less toxic species, but its relative, Phyllobates terribilis, contains enough poison to kill 100 fully grown men.
Th e sheer potency of this poison in such a tiny animal is astonishing, but in the lushness of the rain forests, where danger lurks everywhere, it makes sense to be well defended.
• When these frogs are maintained in captivity and fed on small fl ies and crickets, they gradually lose their toxicity, suggesting that their poisonous secretions are derived from the foods they eat, such as ants and beetles. It is thought that the same toxic compounds in the insects are taken in the by the frog and concentrated in the skin glands.
• Advertising the production of a toxin with bright colors is common in the animal world. Such colorful displays are known as aposematism. It makes no sense for a poisonous animal to be drab and instantly forgettable. Bright, bold colors ensure that a predator with no experience will remember an animal that makes it sick, thus ensuring it will give the same species a wide berth in future encounters.
• American Indians use the toxins secreted by these frogs to coat the points of their blow darts. Some of the more virulent species are held down with a stick, and the toxin is simply wiped on to the dart. Other, less-dangerous species are not so lucky.
Th ey are caught, impaled on sticks, and gently toasted over a fi re to yield their poison, which is then concentrated and smeared over the dart points. Today, these rapidly acting nerve toxins are used for hunting game in the forest, but they were probably once used in tribal warfare as well.
• Although these frogs are very poisonous, there are some animals that appear to be immune to the toxins produced by these frogs. A snake, Leimadophis epinephelus, is a specialist predator of these frogs and is an example of the constant evolutionary arms race that takes places between all predators and prey.
Further Reading: Prohl, H., and Hodl, W. Parental investment, potential reproductive rates, and mating system in the strawberry dart-poison frog, Dendrobates pumilio. Behavioral Ecology and Sociobiology 46, (1999) 215–20; Saporito, R. A., Garraffo, H. M., Donnelly, M. A., Edwards, A. L., Longino, J. T., and Daly, J. W. Formicine ants: An arthropod source for the pumiliotoxin alkaloids of dendrobatid poison frogs. Proceedings of the National Academy of Sciences USA 101, (2004) 8045–50.