A brief life history of the Tailed Frog - Ascaphus truei

Lawrence Getubig

The Tailed Frog - Ascaphus Truei: why they are called such.

The Tailed Frog, scientifically known as Ascaphus truei, earns its common namesake for the short tail structure that male A. truei posses. First postulated by G. K.Noble and J. R. Slater (independently) that the tail may be an intromittant organ, it was later confirmed by studies done by field researchers Noble and Putnam (1931). This was and still is a striking discovery, since it turns out that A. truei belong to an exclusive frog group that practices an internal method of fertilization.

This novel structure and behavior is lost in the scientific name. The genus title 'Ascaphus' stems from the Latin meaning of "no digging" or "no shovel". This describes the fact that A. truei do not exhibit digging behavior. The species title 'truei' attempts to honor F.W. True, a former curator of the U.S. National Museum, Biology Department.

The adult A. truei : How to recognize them.

A. truei color descriptions have been highly variable. Adults may be greenish to brownish in color. They may even assume a tannish to reddish hue. They are adorned with indistinct dark blotches. They may posses elliptical marks behind each eye. They often have a light triangular bar of coloration between the eyes and snout. These frogs are known to grow between 2.2 centimeters to 5 centimeters in adult length from snout to vent.

A more reliable means of identifying these frogs is to look at their eyes; they have vertical pupils similar to that of a cat or western rattlesnake. They have no visible tympanum. They emit no voice since they posses no vocal apparatus. It is no surprise that their lung volume is unusually small. Incidentally, since their lungs are very small, cutaneous respiration is responsible for a bulk of their oxygen supply. Hence these frogs have been observed not to surface for prolonged periods of time. A conclusive identification clue to recognizing these creatures though, of course, is to look at a male specimen. The males have the distinct intromittant organ protruding conspicuously at the posterior end.

A. truei exhibit sexual dimorphism; the males are smaller than the females. The males also have noticeably more muscular forelegs than do females. During breeding, subtle secondary sexual features are expressed. Males develop black, horny nuptial pads in the insides of their forelegs, aiding in gripping the female during amplexus. Spines located inside the cloaca opening at the tip of the tail become longer. These help maintain the tail's structural rigidity in the female cloaca. It has been likened to the spines of the hemipenis of snakes. Females may have seasonal spines that are located on the dorsal region of the thighs. Noble and Putnam postulated that these spines help the male secure a grip during copulation.

Habitats: where and when to look for them.

A. truei is adept in living in cold swift water temperatures. In the laboratory, Noble and Putnam noted that these frogs were able to tolerate temperatures that they subjected them to; namely 5.7 degrees centigrade to 10.7 degrees centigrade. A. truei were found to be most active in the higher 10 degree centigrade range. In the wild, samples of frogs have been taken where waters were at a temperature of 16 degrees C. A. truei, regardless of developmental stage, prefer shallow, cold, clear-water, shaded streams, typically found in permanent forests. These streams need to posses a bed that contains rocks that adults can hide under, and flat, protruding, moss-free stones that tadpoles can cling onto with their adhesive organ. Adults tend to be highly aquatic, though they have been seen to venture into land during the night when the humidity and moisture content of the forest is high. A. truei are rarely seen during the day. They are mostly nocturnal in behavior. Because of this fact, capturing them is easy during the day when they are sluggish. One merely has to overturn large rocks and cobbles to see if any adults are hiding. During this time, the most common reason that A.truei adults escape is because they tend to be immobile and instead float with the water, looking very much like stream bed debris. On the other hand, the adults do try to escape by non passive means during the night; they are known to jump and climb rocks and vegetation for evasion during this time.

Like many anurans, these creatures are sensitive to logging. Reasons for this sensitivity include increased temperatures of water due to loss of shade, and massive siltation of the stream waters. As mentioned before, these frogs have a greatly reduced set of lungs, and clearly rely on cutaneous respiration. Increased temperatures, even by a handful of degrees, greatly reduces the oxygen content of the waters. Hence A. truei are never found in open, sunny creeks or heavily silted water.

A survey was done by Corkran and Thoms regarding where A. truei and its various stages of development may be found. It appears that eggs, hatchlings, tadpoles, juveniles and adults may alll be found in medium streams. The oddity of it all though is that in large streams, only tadpoles were found. Adults and juveniles were found in smaller streams or on land (night). Though not observed by Corkran and Thoms, Noble and Putnam observed that tadpoles sometimes climb out of the water and elevate themselves as far as 20cm away from the stream.

The physiographic provinces in Oregon that these species may found are in the Oregon Coast Ranges, Klamath Mountains, west and east slopes of the Cascades and the Blue Mountains.

Their status in the food chain: A. truei food and A. truei as food

A typical menu for an A. truei adult would consist of choice snails and slugs, various delectable slow moving invertebrates such as spiders and various arthropods. Although adults are carnivores, they are not known to practice cannibalism against their tadpoles.

On the other hand, A. truei tends to be a favorite menu item for many animals. Known predators of this frog are snakes. Thamnophis elegans and Thamnophis sirtalis ( common garter snake ) have been seen preying on adults and larvae. Other amphibians such as Dicamptodon ensatus and predatory freshwater fish, such as trout, are also known to have an apetite for these frogs. The presence of trout in less shallow streams has been postulated to be a factor for the absence of A Truei in such depths.

Known parasites of A. truei are Metacercaria (flukes). These organisms manifest themselves as cysts under the skin usually in the webbing of the digits.

Breeding Habits:

A. truei are known to breed during late spring to early fall. Their breeding seasons variate, differing from population to population. Since they have no voice, and males do not exhibit bright coloring, it may be cautiously inferred that these creatures are not vocally or visually cued. Indeed, females do not search out for males. Instead, when the breeding season arrives and females emerge as the ice thaws out from early spring, males can be seen with their tails in the "erect" position (Noble), flipped under the abdomen facing in an anterior direction, searching for females. This behavior is always under water and during the night.

The goal of the male is to amplex with the female. To do this, the male sometimes first throws the female onto her back. A female thrown on her back seems to render her in a state of tonic immobility; in other words, the female does nothing in terms of resistance. A.truei amplex with the pelvic grip, though other forms have been tried by males, with limited success. Pelvic amplexus is one where the male faces the dorsal - anterior side of the female and grasps her "torso" with his forelegs, just below the "armpits" of her forelegs.

Occasionally, males may even interconnect their foreleg digits to secure a firm hold on the female. As a notional curiosity, males are not particular about who they amplex with. Males grab females as well as males in their search of a mate. If a male happened to be clasped, whether or not it resisted, it was soon released after the attempted amplexus. Upon a successful amplexus, males proceed to insert their tail into the female cloaca. Females respond by first straightening their hindlegs, and then forming a triangular "basket" with these legs around the tail, supposedly to block water currents.

The unique internal fertilization mode employed by A.truei is attributed as an adaptation to a swift water environment. Internal fertilization ensures that spermatozoa are not wasted in the strong currents, hence through evolutionary selection, this behavior and tail structure have been assumed. This concept is widely accepted by many. There is another alternative hypotheses circulating that attempts to explain A. truei's modus vivendi; it theorizes that internal fertilization in these frogs may have evolved in accord with the idea that males compete in carrying on their progeny, thus internal fertilization guarantees that the male's genome perseveres. One flaw of this hypothesis is that males do not invest time in guarding the female, after insemination, from other males who may copulate with her. In addition, the female's oviducts are spacious enough that it can accommodate more than one insemination.

The male A. truei tail: Physiology

The tail has muscles that is connected to the pelvic bone. These muscles enable an up and down movement. The tail contains a prepelvic tendon bone with marrow cavities (Villiers). This bone is fused synchrondrotically to the pelvic girdle. Further exploration of the structure of the tail reveals that it is comprised of a mass of spongiose fibres with the interstices of the fibres capable of being filled with blood. In a dissection study by Villiers, he reported 2 strands of erectile tissue ventral to the cloaca that give support during copulation. Noble and Putnam later described the tail as containing 2 strengthened vascular pads homologous to the corpora cavernosa of the penis of higher vertebrates. At an external glance, when erect, the tail is pointed down and is bulbous in shape and darkened with blood in color. The tail terminates with the cloacal opening of which transitory spines exist.

Young A. truei: the eggs, the hatchling and the tadpole juvenile

Young hatchlings posses adhesive organs located at the rim of their cup shaped mouth. Hence, at still an early age, A. truei are already fit to live in a turbulent water environment. Noble describes the shape of the adhesive organ rim as being 'A' shaped. The organ is comprised of hypertrophied secretory cells and ciliate cells.

The hatchlings soon turn into dark tadpoles. Tadpoles vary in size from .7 to 2.5 centimeters. They retain their adhesive organ, and may be identified by a white spot on the tip of their tails. Tadpoles may contain 2 rows of rasping teeth plates in front and 8 to 13 towards the bottom. Some tadpoles are known to climb the stream banks at distances of up to 20cm. Noble postulated that perhaps the detritus that they eat are numerous in these areas, and that these can be taken in through the nostrils, leaving the mouth to perform as a sucker to adhere to the mossy bed.

A.truei tadpoles do not grow their hindlegs until their final year as a tadpole. They can remain as a tadpole for as long as 2 to 5 years. This usually indicates that food supply is ample in the streams they live in and that duration of terrestrial environment availability is short. Upon growing their hindlegs, it takes them 1 year before they are fully metamorphosed as a juvenile. Once they become adults, they may still remain in the water most of the time.

Though they coexist with other hatchlings and tadpoles, they no longer compete for the same food source as the hatchlings and tadpoles do. Like all adult anurans known, the Tailed frog is now a carnivore.

References:

C. Corkran and C. Thoms 1996. Amphibians of Oregon, Washington and British Columbia. Lone Pine Publishing. 1996.

Metter, D. E. 1964. A morphological and ecological camparison of two populations of the tailed frog Ascaphus truei. Stejneger. Copeia 1964(1) : 181-195. 1967.

Variation in the ribbed frog Ascaphus truei Stejneger, Copeia 1967(3) : 634-649.

Noble, G. K. and P. G. Putnam, 1931. Observations on the life history of Ascaphus truei. Stejneger. Copeia 1931: 97-101.

Slater, J. R. 1931. The mating of Ascaphus truei Stejneger. Copeia 1931:62-63.

Villiers, C. G. S. 1933. The "tail" of the male american toad Ascaphus. Nature (London), 131 (3315):692 - 693.