There are many charismatic animals in the park: red foxes dart across the meadows and forests, hawks and vultures circle in the skies overhead, beavers construct dams in the marshes, while massive longnose gar lurk beneath the murky water. Amphibians, though, are present in more subtle ways: frogs disappear with a splash before a glimpse can be caught; toads freeze in place when approached, seemingly melting into the forest floor; and salamanders silently crawl through the leaves on dark, rainy nights. Despite their seeming subtlety, amphibians fill vital niches in park ecosystems and are important in maintaining the health of park forests.
Amphibians are vertebrates, much like fishes, birds and mammals. From an evolutionary perspective, amphibians fall between fully aquatic fish and terrestrial birds, reptiles, and mammals. Amphibians first appeared in the fossil record over 300 million years ago and were the first vertebrates to live predominately on land. At the time amphibians had an enormous presence in terrestrial ecosystems. They expanded into a myriad of bizarre forms. Today's amphibian diversity is a shadow of their abundance during the Paleozoic era. Nevertheless, there have been over three thousand species described so far, with new species constantly being discovered. Eastern North America is home to over 150 different species (primarily salamanders) with more than twenty species occurring in the park. Taxonomically, amphibians are separated into three distinct orders: Anura (the frogs and toads), Caudata (the salamanders), and Gymnophiona (the caecilians, which will not be mentioned further).
Amphibians possess many unique anatomical features, given their relationship to other vertebrates. For example, most adult amphibians have a heart with only three chambers: two atria and a single ventricle. Freshly oxygenated blood from the “lungs” can be diluted with deoxygenated blood from the body in that single ventricle. Mammals, birds, and most reptiles have hearts with two ventricles that keep blood from the lungs and blood from the body separated.
Amphibians have evolved several different means of obtaining oxygen in response to various environmental pressures. Most adult amphibians, including frogs, toads, and many salamanders, possess a pair of lungs similar to those of higher vertebrates. However, most amphibian larvae—as well as several species of adult salamanders—possess a pair of gills, an array of vascularized, feathery projections through which oxygen diffuses from the water directly into the amphibian's blood, much like fish. One family of salamanders (plethodontidae) has, over the course of their evolution, lost both lungs and gills. Instead, oxygen diffuses directly from the air into their blood via their moist skin. The current hypothesis as to why this occurred, is that it originally evolved as an adaptation for life in mountain streams, where being buoyant would cause the salamanders to be swept away by the current.
Perhaps the most important feature of amphibian anatomy is the degree to which it changes between juvenile and adult phases. This is what gives amphibians their name: “amphibian” from “amphi” and “bios,” or two lives. Most amphibians lay large numbers of gelatinous eggs in a body of water, with the developing embryos easily visible through the translucent coating. After a variable period of time, the eggs "hatch" (the gelatinous coating dissolves), and free swimming larvae are released into the water. These larvae, often called tadpoles, possess a long, finned tail for swimming; a pair of gills for respiration; initially lack limbs; and, in the case of frogs and toads, possess a completely different digestive system than their adult counterparts. Hormones, genetics, ad environmental cues guide the transformation from larvae to adult.
After a period of development that ranges from weeks to years, the fully metamorphosed larvae leave the water to begin the second part of their life on land. However, there are many exceptions to this process. One genus of salamanders and an entire family of frogs omit the aquatic larval stage. Instead, they lay their eggs in damp, sheltered places on land where the developing embryos complete their metamorphosis before hatching as fully formed adults. At the other end of the spectrum, many amphibians never complete their development and live out their entire lives as permanent larvae, a condition known as neotony. An entire suborder of salamanders, plus several distinct families, are exclusively neotonic, and many other families have individual genera or species that are so. Even more curious are species of salamanders which, in response to environmental conditions, are capable of suppressing their metamorphosis. These salamanders are capable of breeding and producing offspring while neotonic, but their offspring are still capable of metamorphosing. It is believed that the developing larvae respond to cues such as the presence of fish predators or drought conditions, to accelerate or suppress their transformation.
Frogs and Toads
Thirteen species of frogs and toads, divided among four families, make their home in a diverse array of habitats in the park. They are active from early spring to late fall.
The family Pelobatidae is represented in the park by a single species; the Eastern spadefoot Toad. This toad is notable for the presence of a sharp spade on each of their back feet, which aids them in burrowing into loose sandy soil. They are the only species of toad in the park with vertically oriented pupils in their eyes, making them easily distinguishable from other toads.
The family Bufonidae contains the “true toads,” represented in the park by two closely related species, the American and Fowler's toads. Unlike frogs, toads have dry warty skin that is better at maintaining moisture, allowing toads to live in drier upland areas further away from water. Although they lack the powerful legs of frogs, toads are capable of quick movement through a series of short hops. When they feel threatened toads typically will freeze in place, allowing their natural camouflage to hide them; this behavior makes them relatively easy to approach and observe.
The family Hylidae contains the tree frogs and their relatives, the cricket and chorus frogs. Four species of this family can be found in the park. Staring in the early spring, and lasting into the summer, the calls of these frogs create a continuous din that echoes throughout the nights. These frogs tend to hide in dense vegetation near water, and are well camouflaged. You are significantly more likely to hear these frogs calling than you are to ever see them.
The family Ranidae consists of the “true frogs.” In this park it is represented by six species, including many familiar species, such as the northern and southern leopard frogs, and the bullfrogs. These large frogs are voracious predators that spend much of their time near the edge of shallow water, and will not hesitate to dive in at the first sign of danger. The meat of these frogs is a fairly popular food source, and bullfrogs are farmed commercially for their meat.
In addition to frogs and toads, the George Washington Memorial Parkway is home to ten species of salamanders, distributed among four families. They become active during the early spring, but will disappear during the summer. They will emerge again during the fall, but will vanish yet again when the first frost comes. Rarely, they may hunt during warm spells in the winter.
The family Ambystomidae is represented in the park by the Jefferson's and spotted salamanders. This family is frequently referred to as the "mole salamanders" due to their habitation of underground retreats dug by other animals. These salamanders spend much of their adult life underground, but will emerge in large numbers to breed in quiet pools during the spring. Famous members of this family include the brilliantly marked tiger salamander, and the neotonic axolotl.
The family Salamandridae is represented here by a single species, the Eastern newt. This is primarily an old world family of salamanders consisting of the newts and “true salamanders,” with many species across Eurasia and North Africa, but only six in North America. The Eastern newt is notable among park salamanders because it undergoes three distinct life phases: an aquatic larvae, a brilliant orange terrestrial eft phase, and an aquatic mature phase. Many predators avoid the efts due to their toxic skin secretions.
The family Sirenidae, while not documented in the park, does inhabit the Washington, DC region, and is represented by a single species. These large, nocturnal, eel-like salamanders live out their lives as permanent larvae that inhabit shallow, weed-choked stretches of standing or slow moving water.
The family Plethodontidae is represented by seven species, spread amongst the dusky, red, brook, and mountain salamander generas. This family of small salamanders is known as the “lungless salamanders,” due to the absence of lungs. These are salamanders typical of woodland areas. They seek shelter under rocks, rotting logs, loose tree bark, or in animal burrows during the day. They emerge at night to hunt for insects and small invertebrates on the forest floor.