Monday, March 22, 2010

AMPHIBIAN DEFENSES


One reason that amphibians in general are in trouble (well, with the exception of Bullfrogs and Cane Toads) is that they have very porous skin, so they are more likely to pick up environmental contaminants than are other vertebrates. The chytrid fungus that is killing so many of them attacks their skin, perhaps interfering with dermal respiration or water balance.

Thin-skinned and small, it would seem that amphibians would be almost defenseless against the whole array of predators with which they share their world. But they’re not!

In part because of their sensitivity to drying out, most amphibians are nocturnal. But this also is a first line of defense against diurnal predators, among which birds are by far the most important. There are many hawks that relish amphibians, and in the tropics members of many different bird groups eat frogs. Of course, by being nocturnal, the frogs are subject to owl predation; you can’t win them all!

Many species, even though active at night, spend the day at least partially exposed, and they are usually very well camouflaged, green for the leaf-sitting frogs and brown for those on the forest floor.

Frogs are also escape artists. The great leap forward that a frog can take with its long legs allows it to capture prey very effectively, but the leap is probably even more important for predator avoidance. A Wood Frog (Rana sylvatica) at the edge of a pond heads for the water at the least disturbance, arcing through the air on the way there. Plunging into the water, it immediately makes for the bottom sediments, where it disappears.

Nocturnal life style, camouflage colors, and escape locomotion notwithstanding, amphibians are still captured. For many species, the next line of defense is provided by their skin toxins. Many of them secrete chemicals in the skin that are poisonous to other vertebrates. Sometimes these are concentrated in particular glands, often around the head (predators often grab their prey by the head to dispatch it most quickly). Note the big toxin-secreting parotid glands behind the eyes, presented to the potential predator by this Northwestern Salamander (Ambystoma gracile).


Many of these amphibians are nocturnal and cryptically colored. But others are either active in the daytime or can be found where they spend the day only partially hidden. Toad tadpoles are toxic, and instead of scattering out and hiding as do the tads of other frogs, they congregate into big, black, wiggly masses, and a predator that captures one and then spits out the bitter-tasting morsel can easily learn to avoid such groups.

These include many species of poisonous frogs, for example the Strawberry Poison-dart Frog (Dendrobates pumilio). Many of these species are conspicuously, even garishly, colored. This is warning coloration (also called aposematic coloration), and predators that learn not to bother with this prey presumably leave more descendants.

Rough-skinned Newts (Taricha granulosa) of the Pacific Northwest are cryptic brown when viewed from above, but when disturbed, they display their bright orange undersurface as a warning coloration. These newts are extremely poisonous and have killed people who swallowed them (just on a dare, can you believe it?). The tetrodotoxins in their skin are poisonous to just about anything that tries to eat them and in fact are among the deadliest poisons known to have evolved.

But some populations of Common Garter Snakes (Thamnophis sirtalis) have evolved resistance to this toxin and can eat the newts with no ill effects! This “evolutionary arms race” has been won by the snakes in this case, but not in all populations.

Dennis Paulson

2 comments:

Michaela said...

I wonder why some organisms' "warning coloration" is red (for example) while others are bright orange or another color. I imagine it depends on both the color-producing ability of the species (based on diet, etc) and the visual capabilities of their predators. I know poisonous frogs come in all sorts of vivid colors - it would be interesting to see whether different colors communicate specific information beyond just "I taste bad, leave me alone!"

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