SOME FROGS POISONOUS?
November 15, 2009
received the following question about frogs and toads.
Q. My friend
says that some frogs are poisonous. However, he also says that toads do
not cause warts. What is the truth?
A. Your friend
is correct on both counts. Toads have bumpy skin and some of these bumps
are glands that produce toxins, but scientists have no evidence that a
person can get warts from touching a toad, despite the common superstition.
And although many, perhaps most, frogs and toads have at least some toxins
produced by glands in their skin, no frogs inject venom through fangs
or stingers. Hence frogs are poisonous but not venomous.
produced by skin glands of frogs unquestionably serve as defense against
some of their predators by making the frogs unpalatable. The toxic-producing
glands are all over the body and legs in some species, providing protection
no matter where a predator grabs the frog. The toxic substances probably
provide an additional service to the frog in some species by discouraging
parasites from attaching themselves to the frog's skin and inhibiting
the growth of fungus or bacteria.
A pair of
paratoid glands is evident on the top of the head in some species of frogs
and toads. These large glands produce toxic secretions that can sometimes
be seen as a milky liquid if the gland is squeezed. Secretions from the
paratoid glands of some species, such as the marine toad, can be extremely
toxic and even lethal for some animals that bite them. The poisonous substance
is called bufotoxin. The effect it has on one potential predator is evident
when a dog bites a common garden toad and then begins to foam at the mouth.
skin secretion of frogs such as the common gray treefrog and the introduced
Cuban treefrog can cause extreme discomfort if it gets into the membranes
of the eyes or nose. A few South American species of frogs known as the
poison dart frogs in the family Dendrobatidae produce alkaloid toxins
on their skin that are among the deadliest poisons known. People have
been known to go into a comalike state and almost die after picking up
one of these frogs if the toxins enter even a minor cut on their hand.
Sufficient amounts of these toxins in the bloodstream can actually kill
a human being.
poisonous frog, possibly even terrestrial vertebrate, in the world is
the golden poison frog, Phyllobates terribilis, of Colombia. Among
the toxins produced by the species is batrachotoxin, a powerful alkaloid
that can kill large mammals, including people. When natives of a certain
region of Colombia rub the skin of golden poison frogs on their blowgun
darts, they produce a lethal weapon that can kill an animal almost instantly
patterns of poison dart frogs and other Dendrobatidae include brightly
colored blues, reds, and yellows, which are presumably warning colors
to other animals that might try to eat them. Many species come out in
the daytime, instead of at night like most other frogs. Poison dart frogs
are presumed to acquire their toxins by eating certain kinds of invertebrates
that produce batrachotoxins and other alkaloids. Prey items of poison
dart frogs that have been identified include a particular family of beetles,
several species of ants, and a species of millipede. The frogs apparently
are able to sequester the toxins without being harmed themselves. The
level of toxicity varies considerably among species within the family
of Dendrobatidae and among other species of frogs worldwide. Interestingly,
when poison dart frogs are kept in captivity and fed diets of crickets
or mealworms, they eventually lose their skin toxicity. But they retain
their brilliant color patterns.
and tadpoles of certain species of frogs and toads have been found to
be unpalatable to some predators, such as fish and salamanders, but whether
poisonous substances are involved is unknown for most species that have
been examined. Though the tadpoles of the golden poison frog are not poisonous,
juveniles become poisonous soon after metamorphosis, when they assume
a terrestrial diet.
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