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

FROGS ARE JUMPING!

Spring in the Pacific Northwest was made for amphibians. They flourish in cooler temperatures than reptiles, and they do best where it’s wet, so the cool, wet springs of this region are great for them. And because amphibians need moisture and are small and have many predators, most of them are active at night, so that’s the time to be out looking for them. Most amphibian hunters use a headlamp to keep their hands free, as amphibians are elusive little critters.

There is a moderate diversity of frogs in this area, some of them common. The most ubiquitous is the Pacific Chorus Frog (Pseudacris regilla), and it seems as if they can hardly wait to start breeding. Individuals are heard calling during warmer winter days, and the choruses really start up by February. Their ribbit call is known to all because so many movies are made in Hollywood, where the species is common. The call is first heard individually and then swells into a chorus, as more and more males head for the wetlands in which they breed.

The Western Toad (Anaxyrus boreas [formerly Bufo boreas]) was also one of the most commonly seen frogs in the Northwest, but this is one of the species that has declined greatly in recent years, as part of the Great Amphibian Problem. We don’t see as many of them as we used to, but they are still widespread. If you can’t find an adult, you may see masses of little black tadpoles swimming together in a mountain lake or a shore lined with toadlets that have just undergone metamorphosis in late summer.

The Red-legged Frog (Rana aurora) is the common native frog that looks like a classical frog – a long-legged jumper with smooth skin. It is most common in wooded areas with abundant ponds, where you may find its big floating mass of around a thousand eggs in spring. You have to have one in the hand to see the conspicuous red colors on the underside of the legs and body. In the mountains and east of the Cascades you can find two related species, the Cascades Frog (Rana cascadae) and Columbia Spotted Frog (Rana luteiventris).

The Bullfrog (Lithobates catesbeianus [formerly Rana catesbeiana]) is one of the villains in the amphibian story. Brought from eastern North America for culinary and sporting reasons, this species seems to flourish almost anywhere it is introduced. Unfortunately, it does so at the expense of other amphibians and other aquatic creatures. A virtual eating machine, a Bullfrog will take in anything that fits in its capacious mouth, and it has been implicated in the decline of some other frogs that share its aquatic habitat.

One of the most interesting things about Northwestern frogs is their silence. Anyone coming from the East is used to frog choruses with multiple species calling, one of the real thrills of a rainy spring evening in Massachusetts or Florida or Missouri. But our frogs don’t say much. Although some toads have ear-splitting trilled calls, ours makes nothing more than chirps. Red-legged Frogs call from under water and can be heard only at very close range. In fact, our only noisy native frog is the chorus frog. Of course the introduced Bullfrog has added much to our soundscape, its loud jug-o-rum resounding from warm-water lakes all over the region.

The Tailed Frog (Ascaphus truei) is another silent species - it doesn’t even have ears. It lives in rushing streams, where almost any sound would be obscured by the sound of the water. Unlike many other frogs, the males don’t attract females by their calls, but instead go looking for them. They also can’t fertilize the eggs in the standard way by depositing sperm on them, as the current would wash the sperm away. Thus the males have evolved an extension, but it’s not a tail!

There is still much controversy about what has caused so many frog species all over the world to become rare or, in some cases, extinct. A special kind of fungus called a chytrid (Batrachochytrium dendrobatidis) is heavily involved, as infestations of this fungus can kill off all the individuals of some species in an area. Fortunately, our Northwest amphibians have not declined as thoroughly as have populations of many tropical montane species.

Dennis Paulson

IT’S SALAMANDER SEASON!

Spring has sprung, or at least it’s about to spring, and it’s the time of year when amphibian activity becomes most noticeable.

Washington supports populations of 14 species of salamanders and 11 frogs. Amphibians like wet climates, and we’ve got that. Salamanders are diverse in temperate latitudes, frogs a bit less so, but as usual, the tropics are the home of the greatest diversity (compare Costa Rica’s 43 salamanders and 133 frogs!).

Our rainy winter provides the water to allow frog and salamander breeding, and as soon as it warms up enough in spring, off they go. This happens in late February or March for a variety of species.

Northwestern Salamanders (Ambystoma gracile) live underground for much of the year but come to the surface to feed in spring and during fall rains. They are rarely seen but could be observable at night at breeding ponds, when the adults come to the water to breed. Males court females, then when both are ready, the male deposits a spermatophore which the female picks up in her cloaca. The sperms within fertilize her eggs as she lays a big clutch of around 100 eggs in a jelly envelope the size of an orange, usually wrapped around some underwater object. The larvae stay in the water for a year or more, then undergo metamorphosis to the terrestrial stage and breed in the next spring.

Meanwhile, Long-toed Salamanders (Ambystoma macrodactylum) are migrating to other ponds, often smaller ones. This species is more common and wide-ranging, occurring in drier parts of the Northwest and well up in the mountains as well as the wet western lowlands. Breeding occurs in the spring, but females lay much smaller egg masses, averaging only a dozen or so eggs. Being a smaller species, metamorphosis and maturation are quicker than in the Northwestern.

Rough-skinned Newts (Taricha granulosa) are also stirring from their winter retreats and heading for the water. Like the others, they migrate mostly on rainy evenings to their breeding lakes. Courtship is prolonged, even lasting a day or more, but usually ending with spermatophore pickup, fertilization, and egg-laying. Eggs are laid singly in aquatic vegetation. The larvae metamorphose and leave the water in fall and disperse away from the water, to which they return after 4 or 5 years. In this species, many breeding adults remain aquatic through the summer, developing smoother skin and tail crests that aid in swimming.

By contrast, two of the common salamanders of the Northwest are terrestrial, with no aquatic stage. They are lungless salamanders, breathing through their skin! Western Red-backed Salamander (Plethodon vehiculum) and Ensatina (Ensatina eschscholtzii) live in forests, where they can be found in spring by turning over logs (be sure to replace divots!). They breed during winter, laying their 10 eggs in a cluster in such sheltered places, and the female guards them from invertebrate predators until they hatch.

Most of the other salamanders of the Pacific Northwest are locally distributed and uncommon, but they are worthy of attention by any naturalist fascinated by creepy-crawly critters. There is a good reference book: Amphibians of the Pacific Northwest, by L. L. C. Jones, W. P. Leonard, and D. H. Olson, eds., Seattle Audubon Society, 2006.

Dennis Paulson