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NATURE HAPPENINGS - May 2003

Tigers in the Tank
by Damian Fagan

The full moon has risen over the snow-capped peaks of the La Sals, casting nighttime shadows as we walk across the open desert. Though the light is bright enough to see by, we bring flashlights for our foray. Tonight’s search - to see if there is a tiger in this tank.

We don’t seek the feline variety, rather Ambystoma tigrinum - the Tiger Salamander. The adult salamanders are black or sooty colored with a banded or mottled tiger-stripe like pattern, hence, their common and species name. The genus name, Ambystoma, means “two mouths” in reference to the sharp, “eyeteeth” of the juveniles and the bicuspid teeth of the adults.

Some might consider the predatory nature of the salamander sufficient to associate this amphibian with its mammalian counterpart. Adult salamanders feed on earthworms, insects, mice and other amphibians. And in the juvenile stage, with their sharp pincer teeth, they can be just as carnivorous.

We reach the stock pond at the base of a sandstone projection. Winter and spring runoff have filled this pool to overflowing. Though dry last year, the rains have replenished this habitat for a number of aquatic species. Even if we didn’t know where the pond was, we could have easily followed the sounds of chorusing red-spotted toads, their trilling calls leading us to this oasis. Our lights illuminate numerous male toads, and their single vocal sacs extended helping to resonate their bleating mate-attracting calls.

Though the pond is full, we search the shallows along the edge for our desert tigers. We don’t expect to see adults - that would be a real treat, rather we search for the aquatic larvae nicknamed mud puppies. Seeing this underwater juvenile stage is a strange sight. Resembling huge tadpoles, the larvae have large heads, four small spindly legs, and three pairs of external gills that resemble bird feathers. The long, laterally flattened tail helps propel the larvae through the water as they search for amphibians, aquatic invertebrates, and smaller salamanders. Of course, these aquatic tigers may also be predated upon by fish, but not in this tank. Here they are the kings of the pond.

Interestingly, some of these juvenile individuals remain as permanent larvae, and never mature into adults. Called an axolotl, it is not well understood why these larvae never metamorphose into adults. In some artificial habitats, such as this stock pond that doesn’t normally dry out, these larvae take advantage of the permanent food resources and grow sex organs and breed in the pools. If the pond does dry up, like last year, the larvae may change into adults and either move to another pool or burrow into the wet mud.

As we cast our beams across the water, we locate several of these aquatic larvae. They propel themselves along with their long laterally-flattened tails in a back-and-forth motion called sculling. Their small legs are almost useless, but they do help maintain a bearing as the juveniles move through the water. Leery of our presence, the salamanders glide away into the darker recesses of the pond.

As the nighttime temperature drops, we pull on sweatshirts and continue our vigil. The toads have not grown quite, although they temporarily stop chorusing when we pass by them. We have to be careful where we step, for often the toads bump against our feet. We feel fortunate to observe this desert phenomenon, made more amazing by the nature of the habitat - a pond of water in the desert.

Having enjoyed our amphibious serenade and the watery parade of salamanders we gather our packs and head back to our vehicle. I am reminded of a Madison Avenue gasoline advertizement from my youth, something to do with putting “a tiger in your tank.” The inference was for additional automotive power, but tonight that phrase takes on a whole new meaning.

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