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Idaho Giant Salamander



Giants come in all shapes and sizes. In the amphibian world, the gargantuan salamanders Andrias japonicus and A. davidianus from Japan and China, respectively, top the charts. At lengths of nearly 5 feet and weighing in at around 55 pounds, their title as the world’s largest amphibians is uncontested. There is another species in the same family, Cryptobranchidae, which occurs in the eastern United States. The hellbender (Cryptobranchus alleganiensis) is smaller, only reaching lengths of around 2.5 feet. Hellbenders look similar to their Asiatic cousins, and they can be found in similar habitats: clear-flowing mountain streams and rivers. Cool, highly oxygenated water is necessary for all of these aquatic salamanders, because they breathe primarily through their skin, and cold water holds more oxygen than warm water. Other salamander species living in the southeast are equally impressive in size. The aquatic, eel-like Amphiumas and sirens, for example, can reach lengths of approximately 4 feet and 3 feet, respectively. All of these salamanders can reach impressive sizes. But not every giant is giant!

Idaho giant salamander

John Cossel Jr.

Idaho giant salamanders (Dicamptodon aterrimus), like this terrestrial adult, reach approximately 9 inches long and weigh up to 2.5 ounces.

The smallest of the amphibian “giants” are in the family Dicamptodontidae, found in the Pacific Northwest. Their maximum size of up to 14 inches and a little over 2 ounces does not seem comparatively big. However, they hold the record as the largest terrestrial salamanders alive today. There are four species in this family, and three of them (Dicamptodon ensatus, D. copei and D. tenebrosus) range in distribution from California to Washington. One species of these diminutive giants lives almost exclusively in Idaho, and it is appropriately named the Idaho giant salamander (Dicamptodon aterrimus). Idaho giant salamanders are perhaps a little shorter and stouter than their cousins reaching total lengths of around 9 inches and weighing in at around 2.5 ounces.

Idaho Giants

The range of Dicamptodon aterrimus in Idaho is primarily in the north-central portion of the state with a disjunct population to the south associated with the South Fork of the Salmon River and its tributaries. Recent populations have been discovered in extreme western Montana, and eastern Washington may harbor its own populations just waiting to be discovered. Anyone encountering an Idaho giant salamander in eastern Washington should document the find with a photograph and record locality information to share with conservation biologists.

In certain areas and in the right habitat, Idaho giant salamanders can be locally abundant. However, many people have never seen one. These salamanders are secretive and largely active at night. Another reason they are seldom encountered is that many populations exhibit a high degree of paedomorphism. Paedomorphism translates into “child form.” It is when larvae fail to go through metamorphosis yet become sexually mature. Given that they remain aquatic, paedomorphs are not frequently seen. Most people do not spend much time wading in cold streams looking for secretive salamanders.

Idaho giant salamanders are found in moist, coniferous forests and associated streams, and occasionally in rivers and lakes. During warmer seasons, active terrestrial adults may be found at night, especially after rain showers. Terrestrial adults can also be found occasionally in streams along with paedomorphs and larvae. Little is known about their specific habitat needs, and ongoing research is evaluating how these salamanders use stream habitats in relation to their availability. Initial results from a population in northern Idaho suggest that pools and pockets of still water are used more frequently than rapids. Swift or channelized sections of stream do not seem as “giant friendly” as do the slower-moving waters along stream edges, behind boulders or in shallow riffles. There can be too much of a good thing. Streams that are too slow and too warm, such as those found in meadows and lowlands, are less likely to host populations of giants, particularly if the stream bottom is mud instead of gravel and cobble. The nooks and crannies created by the rocks are important, because they provide hiding places.

Robust Appeal

Idaho giant salamanders are very robust, and their shape resembles a tiger salamander on steroids. Even the eyes of giants are bigger than those of tiger salamanders. The color of D. aterrimus is somewhat variable and differs between terrestrial and paedomorphic adults. Terrestrial adults have ground colors ranging from gray to brown with beautiful copper or bronze reticulations. These dorsal colors fade along the sides, transitioning to a much lighter coloration ventrally. Paedomorphic individuals usually retain the slate gray or dark brown color of larvae, and they can occasionally have a faint caramel-colored pattern. The ventral surface of paedomorphs and larvae is a light cream or white, especially in younger, small individuals. Paedomorphs can grow as large as or larger than their terrestrial counter parts, yet they retain their gills and larval head shape. They also keep their tail fin, which helps them swim through swift streams. Low-profile eyes and flattened heads likely assist in their burrowing habits, as do the tough, black keratinized pads at the ends of their digits. True to their secretive nature, they spend much time under rocks and logs and in aquatic burrows. When an Idaho giant salamander is discovered in a stream, all of these features allow them to rapidly swim away and then quickly disappear by scrambling under the streambed’s cobbles and boulders.

Predators and Prey

Idaho giant salamanders share their cold mountain streams with the usual host of invertebrates, such as caddisfly, stonefly and mayfly larvae, and even crayfish, all of which the salamanders eat. They will also eat other stream-dwelling amphibians. The Rocky Mountain tailed frog (Ascaphus montanus) is commonly found in streams occupied by giants, and their tadpoles are eaten by larger-sized giant salamanders. Terrestrial garter snakes (Thamnophis elegans) share these waters, and they, too, prey on tailed frogs. Garter snakes and the various salmonid fish, such as bull trout and steelhead, are likely predators of Idaho giants, as are water shrews and other mammals. There is speculation that Idaho giants are cannibalistic, but we have encountered individuals of various sizes together in the field and we have housed them together in captivity without mortalities. However, they do seem territorial, and I have observed aggression and biting in captive individuals. I’ve also seen missing limbs, gills and chunks of tail fin, but whether the missing body parts were a result of fish or other Idaho giants is unknown.

Growth and Reproduction

Not only is moving around in cold mountain streams challenging, the frigid water also makes rapid growth difficult. We have observed annual growth ranging from 1⁄4 to 1⁄3 of an inch, with the rate slowing as the animals mature. On the positive side, slow growth may mean long-lived. A colleague, David Pilliod, examined thin cross-sections of bones and found circular growth rings (similar to tree rings) suggesting up to eight distinct growth phases. These periods of accelerated and arrested growth likely represent annual cycles, so Idaho giant salamanders living more than 10 years may be possible. It is unknown how long they live in the wild. The age at which Idaho giant salamanders become sexually mature is unknown, but is likely in the third year or later.

Once sexually mature, both terrestrial and paedomorphic forms of Idaho giant salamanders mate in streams. As far as this author knows, only one nest for this species has ever been found. The eggs were deposited on the bottom side of a submerged stone. The clutch size is estimated to range from 135 to 200 eggs. Like other amphibian eggs, there is a jelly coat, but under the transparent jelly, the eggs are unpigmented and white. Females remain with the eggs, presumably guarding them from predators. Observations on closely related species of giant salamanders suggest that upon hatching, the larvae absorb their yolk before they begin feeding on small aquatic invertebrates. Tiny first-year larvae seem particularly abundant in the seeps and headwater springs that feed into streams.

Captive Giants

Before studying Idaho giant salamanders in the field, my research collaborators and I wanted to make sure that our research techniques would not harm the salamanders. We were faced with the challenge of housing a species not regularly kept in captivity. We kept the giants we collected cool during transport in a beverage cooler. At the lab, some of them were introduced to their vivarium. The remaining salamanders have been maintained in plastic storage tubs in a refrigerator at approximately 48 to 50 degrees Fahrenheit. Adequate oxygen levels are maintained using an air pump and bubble stone. This setup has worked well for four years. Partial water changes are conducted every two to four weeks.

Because so few people ever see Idaho giant salamanders, my students and I wanted to put them on public display. This motivated me to create a more naturalistic housing. I decided that a 60-gallon aquarium with water about 8 to 10 inches deep would work well. Although the individuals I had were larvae, I thought they might use a land area if they metamorphosed. I carved a shape resembling an overhanging bank out of a large 3-foot-long-by-3-foot-wide block of Styrofoam. I included a scooped-out portion on the “land” surface of the foam to hold the root mass of a plant. To ensure proper drainage, I made a hole in the bottom of the bowl that would allow excess water to drain to the “stream” water below.

To disguise the Styrofoam I smeared a thick layer of brown silicone over the surface that would be visible to viewers. While it was still wet, I pressed sand and dirt into the silicone. This created a natural-looking color and texture. I used sand and gravel from a nearby stream for the substrate and included granite river rocks of various sizes. The rocks are typical of stream environments and along with a small hollow branch, provide hiding places. An old root mass that I sawed in half was a perfect way to finish off the look of a stream bank and provide a way to hide part of the plumbing.

The habitat is designed to resemble a cut bank, with the slightly flowing water found at the edges of pools. Flowing water is created using two external canister filters. These serve the dual purpose of cleaning and circulating the water. After being cleaned, the filtered water passes through a chiller before returning to the tank. The aquarium chiller (a 1⁄6 HP Aqua Logic) is used to create water conditions mimicking their cold mountain streams, with temperatures around 55 degrees. A wire-screen lid keeps salamanders and crickets in the terrarium, and Plexiglas over a portion of the top helps maintain humidity. Compact fluorescent bulbs (full-spectrum lighting, either 6,500 or 10,000 Kelvins) keep the ivy, moss and aquatic vegetation green and healthy. We replace the water that evaporates with distilled water. We also use distilled water (or vinegar for hard-water deposits) to clean the glass. Partial water changes and filter media changes are made periodically to minimize waste buildup.

Easy to Feed

Fortunately, Idaho giant salamanders are not picky eaters and thrive on a diet of one to three earthworms given once or twice a week without mineral or vitamin supplements. However, crickets are occasionally used to supplement their diet and to provide food for the two terrestrial adults that now live in the vivarium. It is very entertaining to watch a lurking giant lunge from its hiding place to chomp a hapless worm. These salamanders soon acclimate to captivity and will even take worms from fingers or forceps. Dicamptodon aterrimus is rarely encountered in captivity or in the wild, but for the lucky few that come face to face with a “giant,” it is sure to be a memorable experience.

Preserving Wild Giants

The IUCN lists the conservation status of Idaho giant salamanders as “Least Concern,” however, state wildlife managers have classified the species as vulnerable. Researchers failed to find them in some of streams where they were historically present. In addition, the use of the pesticide dichlorodiphenyltrichloroethane (a.k.a. DDT), extensive logging and a number of other practices have raised concerns over the status of this species. This species is the least studied of all the Dicamptodon genus and basic information is lacking. My students and I are collaborating with Joel Sauder of the Idaho Department of Fish and Game to determine the habitat needs, distribution and relative abundance of this species. We aim to increase our knowledge, so that we can confidently comment on their status. We also want to provide land managers with data to facilitate informed management decisions. It is very rewarding to be part of the effort striving to ensure there will always be “giants” in the Idaho woods. The Northwest Nazarene University Herpetology Lab hopes to breed this species in captivity. But, as of yet, there are no known captive-bred Idaho giant salamanders available to hobbyists.

Finding Giants

Finding and photographing an Idaho giant salamander in its natural habitat is very rewarding. The Idaho Department of Fish and Game reminds people that many species of native amphibians and reptiles do not do well in captivity (without special care, of course). However, law allows the holder of a valid Idaho hunting license to collect and possess four individuals of protected, nongame wildlife species (amphibians and reptiles). It is important that the laws and regulations pertaining to the transport and holding of this species in locales other than Idaho are reviewed. It is vital not to place undue pressure on this species by collecting them irresponsibly. Over collection has caused the decline of a number of species, and it would be a shame to see this become true for giants. Similar husbandry will likely work for the other Dicamptodon species, but review the laws and regulations of each state carefully before collecting. Every effort should be made to produce captive-bred offspring so that natural populations will be unharmed, and people will be able to enjoy this species in captivity and in the wild.

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