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The Power of Poison

Poison-Arrow Frog
  The poison-arrow frog (Dendrobates tinctorius) of South America. Scientists have recently discovered that the poisons secreted by these frogs can also be used to block pain in humans. [Click for a larger image.] Photo by Amy Snyder.

What you've heard is true: It's not easy being green. Frogs face a formidable number of predators, yet their defenses are few. For most frogs, the best defense is to stay hidden. But when camouflage fails, what's a frog to do? Playing dead is a frog favorite; most predators prefer live prey. Other frogs will try to beat a hasty retreat. Shows of intimidation can also give predators pause. The tomato frog of Madagascar puffs itself up like a balloon, making itself appear too big to swallow.

For some frogs, the ace up the sleeve is poison. All frogs have poison glands in their skin. In most cases, these toxins aren't strong enough to discourage predators. (Witness the fact that frogs are a basic food for many animals, including birds, rodents, fish, foxes, and wolves--and people!) But certain frogs in Central and South America have especially strong poisons in their skin. These frogs are called poison dart frogs, because native people used the poison from their skins on the tips of hunting arrows and blowdarts. There are about 170 different kinds of poison dart frogs. The most poisonous is the golden poison frog, each of which contains enough poison to kill eight people.

Toads make use of poison, too. Behind their eyes they have a pair of poison glands, called parotoid glands. When the toad is threatened, a milky poisonous fluid oozes from the glands. The poison is stronger in some toads than others, but even in its mildest form it causes a burning sensation if it gets in the eyes or mouth of a predator. The stronger poisons can also cause muscle spasms, heartbeat irregularities, and breathing trouble. As terrifying as this may sound, it doesn't mean that you can't handle a toad. Just wash your hands afterwards, and be careful not to touch your eyes or mouth before doing so.

Calling Frog
An African reed frog (Hyperolius bayoni) from the Kakmega forest in Kenya, seen here calling with inflated vocal sacks. [Click for a larger image.] Photo by Dante Fenolio.  
RealVideo Exploratorium frogkeeper Ned McAllister talks about the variety and meaning of frog calls. [Need Help?]

Croaks & Courtship
Frogs are formidable musicians. Visit a pond on a spring night and you'll hear a chorus of croaks, twitters, chirps, and trills. Generally, it is the male frogs who serenade, though there are exceptions: The South African clawed frog indicates her interest in a suitor with loud clicks.

Frogs make their calls with the help of one or two pouches of skin called vocal sacs. Sound is produced when air rushes over the vocal chords on its way from the lungs into the vocal sacs. The vocal sacs work like echo chambers to amplify the sound. Some frog calls are so loud, they can be heard up to a mile away.

With any luck, the females are listening. Frogs' ears, called tympanum, look like two flat circles behind the eyes. (In some frogs, the tympanum are very small and difficult to see.) The size of the tympanum and the distance between them determine which sounds a frog will hear best. In this way, frogs ears are "tuned" to hear the calls of their own species. This helps the females identify and locate the males of her species--no small feat in a noisy pond packed with all kinds of frogs and toads singing their hearts out.


Frogs coupling
  Amplexus between two African reed frogs. [Click for a larger image.] Photo by Dante Fenolio.  
The culmination of this clamor is amplexus, when the male frog climbs onto female's back, grasping her with his front legs. As the female lays her eggs, usually into water, the male releases sperm that fertilizes them. There may be anywhere from one to hundreds or even thousands of soft, jelly-covered eggs. Of these, few if any will survive the difficult trip to adulthood.
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