Strawberry Poison-Dart Frog
Willem is a wildlife artist based in South Africa. He says "My aim is simply to express the beauty and wonder that is in Nature, and to heighten people's appreciation of plants, animals and the wilderness. Not everything I paint is African! Though I've never been there, I'm also fascinated by Asia and I've done paintings of Asian rhinos and birds as well. I may in future do some of European, Australian and American species too. I'm fascinated by wild things from all over the world! I mainly paint in watercolours. . . but actually many media including 'digital' paintings with the computer!"
This is a coloured pencil drawing of a Strawberry Poison-Dart Frog, Oophaga (=Dendrobates) pumilio, that I made for kids who were taking art classes with me. This is again one of the species I’m illustrating that is not South-African. Poison-dart, or poison-arrow, frogs occur in South and Central America. They are famous for being among the most toxic of all creatures. The natives of the Amazon use some species – actually only four have been recorded – for poisoning the tips of their arrows, which greatly facilitates hunting. It is safe to eat meat from an animal that has been killed with a poison arrow; I can’t get detailed info on this, but perhaps the toxin denatures when the meat is roasted, or perhaps the dosage in ingested meat is too low to affect a human.
The strawberry poison-dart frog is not one of those traditionally used for arrow poison, but a related species. Like them it is quite tiny, adults being 17.5-22 mm (0.7"-0.9") in snout-to-vent length. It occurs in Central America from Nicaragua to Panama. Like the Painted Reed Frog it comes in a great many different colour schemes, most of which would seem to belong to entirely different species. The one illustrated here, with an orange-red body and blue legs, is a common one; it is called the 'blue jeans morph'. Others are all-red, others blue with black speckles, others yellow, greenish or white with speckles, blotches or mottling. All in all there are about 30 different colour schemes or morphs of the species. The genus Oophaga seems to be fairly recently evolved, as far as frogs go, dating from around the formation of the Panama land bridge 5 million years ago. There are nine recognized species. The genus is sometimes included in Dendrobates, but this group has mostly been broken up into a few different genera now. Poison-dart frogs are generally brightly coloured, this acting as a warning to predators of their toxicity. A few species though are not very toxic, and more cryptically coloured.
A very interesting hypothesis about the many colour schemes of the poison-dart frogs is that these exist to confuse predators! Once predators learn that some frogs are bad to eat, they will avoid pretty much any frog with a colour pattern different from what they know to be edible, whether that frog is poisonous or not. This means even frogs not quite as toxic as others can gain protection from looking toxic or just strange. It would seem that the poison-dart frogs, and the strawberry poison-dart frog in particular, are evolving new colour schemes very rapidly; the great variety in this species seems to have arisen over the past 6000 years, actually a very short period in evolutionary terms.
Another driver of colour diversification is sexual selection. The females choose their mates based on many qualities, but colouration is probably one of them, the frogs showing signs of being able to see in colour. In different regions females may have different 'tastes' in colour fashion, this too driving differentiation.
The scientific name 'Oophaga', which is the genus this species is sometimes classified in, means 'Egg-eating'. It refers not to the adult frog, but to its tadpoles! So how could tiny frog tadpoles be egg-eating? Let's look at the entire breeding biology of this little froggy.
The rainforests these frogs live in experience moister and drier periods; the frogs breed during the rainy season. The high-pitched calls of the male act to proclaim their territories as well as to attract females. A female will approach a male she finds alluring, and mate with him. Unlike other frogs where the male will clasp the female around the waist, a position called amplexus, strawberry poison-dart frogs press their cloacae together. The female lays her eggs and the male fertilizes them immediately this way. The clutch of 3-17 eggs are laid in the leaf litter and the male has the job of guarding them. He also keeps them moist by emptying his bladder over them! When the tadpoles hatch, they are of course in a bit of a fix, being on land and not in the water. Not to worry! The female comes and wriggles each tadpole onto her back and carries it to a special little pool.
The female frog can choose any cavity that will hold water long enough, but often there are tailor-made little pools occurring in plants called bromeliads, of which the pineapple is an example (but not a pool-harbouring one). Rainforest bromeliads are mostly adapted to grow on trees. Up there the bromeliads grow without the benefit of soil, their root systems mostly just anchoring them to the twigs and branches. To actually get nourishment, they've adapted their leaves to grow with tightly overlapping bases to form bowls that will catch and hold rainwater. Also, leaves and other bits of organic matter will drop into these bowls from time to time. The bromeliads can absorb the water and dissolved nutrients, but many animals also make use of these little pools up in the canopy! And so too the poison-dart frogs.
The female deposits each tadpole in its own little pool. These are too small to contain dangerous predatory creatures like fish or dragonfly nymphs … but they are also too small to contain any sort of food supply that will sustain the tadpole through its growth and metamorphosis into a frog. This is where the egg-eating comes in! There are no eggs to begin with in the bromeliad ponds. They are supplied by the mother frog! She periodically comes back to each tadpole's pool, and deposits an unfertilized egg into it. And the tadpole eats the egg! This is metabolically costly to the female, but ensures rapid growth for each tadpole. This means that this little frog is caring for and providing food for its offspring from its own body tissues, much like mammals do!
After six to eight weeks the tadpoles metamorphose into tiny frogs, just 10 mm/0.4" in length. They crawl from the bromeliad pools and climb down to the ground; from then on they will feed themselves, catching tiny insects in the leaf litter. They specialize in eating ants and some kinds of beetle that have toxic juices; they extract and concentrate these, with no harm to themselves, in their skins, so becoming poisonous as protection against predators (but at least one snake has developed an immunity to the frog-poison). When the frogs don't have these beetles and ants to eat, they don't become poisonous either! This makes them safe to keep in captivity. But I don't think it's right to keep them as pets! They don't breed well in captivity, the egg-eating tadpoles being very difficult to feed properly. Because of this, many frogs are captured in the wild to feed the trade; not only do many frogs die as a result of this, but there is also a lot of disturbance and destruction of their habitat, and while this species is still numerous, this is a drain on their numbers in the wild.