Pacific Tree Frog – Anura Hylidae Hyla regilla


Identification & Description:
The Pacific Treefrog is the most commonly heard frog on the west coast. The color of the individual can vary. They are most often a green or a gray, but may also be brown or black. Within the same individual it may change colors within only a few minutes from one color to another. They usually have some dorsal markings as shown in the upper right photo, but the photo to the right shows that there are exceptions. One thing that goes without exception, however, is the dark band running through the eye towards the front limb and the white underbelly. They are a small treefrog attaining lengths up to 2 inches. The voice of this frog is known around the world – the “ribbit” that Hollywood uses in all of its films as the “standard” frog call is actually the call of the Pacific Treefrog!

Taxonomic Classification:
: Animalia
Phylum: Craniata
Class: Amphibia
Order: Anura
Family: Hylidae
Genus: Hyla
Species: Hyla regilla

Hyla regilla also known as the Pacific tree frog resides along the Pacific Coast of North America as far south as Baja California and as far north as British Columbia. They range from the coastal waters to the top of Mt. Whitney (approximately 10,000 ft.), from the moist climate of the Pacific Northwest to the dry, arid climate of the Mojave Desert. The taxonomy and nomenclature is presently up for debate, specifically whether or not the species belongs in the family Hyla or Pseudacris based on phylogeny and the physical attributes of the creature. Morphologically, the Pacific tree frog is a relatively small creature that has only one distinguishing, defining, characteristic – a black stripe on either side of the frog’s face. This stripe is evident on any individual within the genus despite its ability to change coloration relatively rapidly under various conditions. The Pacific tree frog is particularly successful in the region within which it resides because of its distinct mating call and innate and intrinsic ability to breed and lay eggs in almost any body of water. It has been written that, in the past before the spread of urbanization, after the first major rains of winter in Central California, “every roadside ditch, every pond, every puddle, every little spot of water [had] attracted to itself a Tree-toad chorister”.

The taxonomy and nomenclature of the Pacific tree frog is currently subject to debate; the issue revolves around the two genus’ Hyla and Pseudacris. The genus Hyla, meaning “tree,”, is comprised of arboreal frogs who have large disks at the ends of their fingers and toes, have long, oval, white testes and breed in cold weather. The genus Pseudacris, meaning “chorus,” is comprised of terrestrial frogs who have small disks on the ends of their fingers and toes, have dark spherical testes and breed in cold weather. Based on physical and behavioral characteristics it is difficult to determine which genus the Pacific tree frog belongs in because individuals within the genus carry some part of all of the aforementioned features. This discrepancy over nomenclature with respect to the physiological structure and actual activities of the frog has led some scientists to investigate the accuracy of the title. Hedges wrote a scientific investigation of the evolution of H. regilla using DNA, phylogenetic trees, and allozymes to determine whether or not the tree frog belongs to the genus Hyla or Pseudacris. Hedges concluded that the frog belongs in the family Pseudacris based on the fact that its DNA is closer to that genus than to the genus Hyla. DaSilva using anatomical and morphological characteristics and a consensus of three phylogenetic trees to conclude that the frog belongs in the genus Hyla. The issue remains up to debate because both scientists have compelling arguments based on hard scientific data, however this paper will refer to the species as Hyla regilla because it is most commonly referred to as such.

H. regilla is the smallest amphibian on the west coast, growing between ¾ of an inch to two inches long (one to five centimeters) with the female being decidedly larger than the male. The Pacific tree frog, along with all frogs, is an ectotherm, which means that the temperature of its body is dependent on the temperature of its surroundings . It has long legs with its tibia equal in length to its femur, and the total leg length equaling half the length of the frog. H. regilla has a distinct ear in that it is less than half the diameter of the eye. It has long fingers without webbing and toes that are webbed at half their length and “adhesive disks on the tip of the claw-like toes”. Generally speaking, tree frogs must have proportionate pads on their toes in relation to their body size in order to climb; however there is little correlation between the actual size of the pad and the amount of climbing that a frog might carry out. Although it has the ability to climb trees and other surfaces, rarely does it travel more than a couple feet above the ground.

In morphological terms, the most distinguishing characteristic of H. regilla that is carried by all individuals within the genus, regardless of body color at the time of examination, is a narrow black band or stripe that extends from the creatures nose laterally across both sides of its face, parallel above the sides of its jaw, through the eye to the base of its ear around the top of its head; this marking looks somewhat like a mask that a fictional villain might wear. H. regilla come in many different colors: green, brown, gray and sometimes red; color variation ranges greatly such that often the Pacific tree frog is mistaken for other species, among novices. It has the ability to change color extremely rapidly (under ten minutes) depending on its location, the amount of light in the area, or whether or not it has been perturbed.
Partially attributable to the success of the Pacific tree frog is its quintessential frog croak. H. regilla can be heard up to half a mile away on a quiet night. It has been observed sitting “with the vocal pouch partially distended, the small bag palpitating with the breathing movements,” an action that might be interpreted to mean that H. regilla understands the significance and power of its call and is eager to put it to use. Defining or articulating in human language exactly what this call sounds like is difficult, but it has been expressed as “ribbet, ribbet” or “shirk-it, shirk-it”. H. regilla’s call is so distinctive that “ever since the days of the first talking pictures, Hollywood movie-makers have used recordings of the voice of the Pacific Tree Frog as sound for scenes”. In the past, if one lived in a rural area off of the Pacific Coast, the call of H. regilla was abundant and sometimes overwhelming, but always endearing.

It is with this lovely sound that the male H. regilla calls out to any willing female during mating season. When exactly this mating season is varies from source to source, except that it usually happens after the rains have come, from early winter to early spring. The breeding time also seems to be dependant on the latitude at which the individual resides, earlier for lower latitudes and later for higher latitudes. H. regilla is dependant on the rains for mating because it relies on rain puddles or small water reserves to lay its eggs. After a significant amount of rain has fallen, the male H. regilla gather in small groups near a puddle. There they croak at the top of their lungs a small number of them can produce a noise so great that it sounds as though made by thousands, calling out to females in the area. “When a number of [H. regilla ] are in a marsh or pool together, their notes tend to be given in unison, so that there is a continued series of notes, every alternate note being slightly stressed” .

An interesting secondary sexual feature, and an example of the parallel evolution of diverse species, is that all males within the genus’ Hyla, Leptodactylus, and Rana possess when mating a slight extension or expansion of the external vocal pouches on each side of the lower jaw. Why this sexual dimorphism occurs is unknown, but one can hypothesize that it is related to the male’s distinctive and desirable mating call. When the much larger female, which is attributed to the eggs that she is carrying inside her, descends to the puddle or shallow pond, the male clasps himself to the female from behind by grasping her hind legs with his front legs; the pair may wait an extended period of time, possibly a few hours, before any egg laying commences. By grasping the female in this manner, the male is assisting in the extrusion of the eggs from the female’s body and fertilizing them as they are laid into the puddle or pond,. The eggs, at approximately one millimeter in diameter are laid in small, loose masses of five to 70 (the greatest range ascertained), which are then attached to small twigs, grasses or leaves in the shallow water . After the mating ritual is complete both the male and female leave the water, the female leaving much sooner, abandoning the eggs completely, to the surrounding area searching for coolness and moisture . However, there is no intraspecies competition for breeding space, and other males and females stay in the area to use the shallow water and mate throughout the season.
It takes approximately three to seven days depending on the temperature of the surrounding environment for the eggs to form into tadpoles. After that, the new tadpoles wriggle out of the jelly mass but depend on that reserve of food they carry over from the egg for the next day or two. Then, the small tadpoles begin to swim around the shallow water and feed on whatever is available, be it minute decaying material on the puddle’s bottom or living green strands of algae floating in the water,. It takes a H. regilla tadpole approximately three months to metamorphose into a froglet. The age that H. regilla reaches sexual maturity is disputed, be it one year or two. Nussbaum argues that individuals within the group of H. regilla that he observed were sexually mature within one year and were seen as members of the breeding chorus the following season. Pickwell argues that young H. regilla return to water after the first winter rains of the following season but do not necessarily mate until the following year, thus are sexually mature at two years of age. Both sources are fairly outdated and conclusions are based on specific populations being observed at two different times and places. Nonetheless, H. regilla’s gestation period is relatively short and the species adaptability to its environment is evolved and refined. Therefore withholding a major food shortage, drastic climate change, or environmental contamination, hypothetically H. regilla should proliferate indefinitely.

A very interesting phenomenon, with respects to the behavioral studies of H. regilla tadpoles, was observed and described by the authors Goin, Goin & Zug. In a pool of water there were 180 H. regilla tadpoles with approximately ¾ of the group situated with the dark dorsal surfaces of their backs on their tails towards the sun. The tadpoles do this such that their dark dorsal surfaces acquire maximum exposure to the sun’s rays, which act like solar panels and heat up the water around them to such a degree that there is a temperature difference in the area where the tadpoles are in comparison to other parts of the pond. When the water around the tadpoles heats up, so do their metabolism, thus speeding up the rate of metamorphosis. This is an example of a positively beneficial environmental change due to the incidence of a large number of individuals in one area.

Habitat / Distribution:
H. regilla, an extremely prolific frog, is found continuously along the western coast of North America. The genus Hyla, along with Rana and Bufo are thought to be by far the most pervasive and thriving genera of amphibians in the world. The species’ habitat extends from along the Pacific Coast of North America to western Montana and eastern Nevada and longitudinally from British Columbia, Canada to Baja California, Mexico. It is also found on the Cerros and Santa Cruz Islands, west of California, as well as other islands spotted up and down the western coast of North America. It is possible that the frog arrived from the mainland via a log or ship; this is known as sweepstakes dispersal method (which means that by chance, a species travels to a destination which will be conducive to that species habitation). It is known that the Pacific tree frog was introduced (although it is unclear of the exact source of introduction) at Eutsuk Lake, B.C. and throughout California, particularly California City and Soda Springs . H. regilla can be regarded as a eurytopic species although it is not considered cosmopolitan because it is endemic to only the west coast of Northern California. Although continuously distributed throughout the west coast of North America, it is endemic due to the natural topographic boundaries of the Pacific Ocean, the Sierra Nevada mountain range in California, the Cascades mountain range in Oregon, Washington and British Columbia and the deserts of Southern California and Northern Mexico.

The H. regilla’s success can be attributed to its distinct and highly audible mating call and its ability to breed in a variety of climatic conditions, providing the frog has a place to convene and mate. Usually a small amount of water in the form of a pond or pool, either temporary (after a rain) or permanent, will suffice. This is both positively and negatively beneficial in that any reserve of water will do; however if the water dries up before the eggs have had sufficient time for gestation, then the year’s reproductive efforts are lost. Positively speaking, this ability to mate and lay eggs almost anywhere has allowed the H. regilla to conquer territory from sea level to approximately 11, 600 feet . Easily adaptable mating conditions also have allowed for H. regilla to extend its habitat into the deserts of Southern California, where although originally introduced, have sustained their establishment .

The Pacific tree frog is not as successful in hot and dry climates as it is in cool, moist areas. When the climate becomes too warm and arid, H. regilla becomes nocturnal, inhabiting low-lying ground cover, rock crevices, fallen tree trunks, the burrows of small animals or other sheltered alcoves. H. regilla can survive in a variety of habitats: grassland, chaparral, woodland, forest, desert oases and farmland. If a group of H. regillas congregates in one area, the temperature and subsequently the humidity in the air will increase; thus, to reduce the threat of dehydration the frogs will collectively begin to lose less water from their bodies. This behavioral adaptation also corresponds to the environment in which H. regilla, and all frogs, live; when there is an increase in temperature or humidity due to climate change, frogs will loose less water from their porous skin, preventing dehydration and death. As terrestrial amphibians, they reside on the ground, under vegetation, around moist places of all sorts and will eat almost anything: small beetles, spiders, ants, leafhoppers, and isopods. In the Great Basin of Washington and Oregon, H. regilla can be found living along the streams thriving in disparate conditions. Although H. regilla can survive and thrive under a multitude of conditions, ideally the Pacific Tree Frog prefers to exist searching for food, which can be anything smaller than itself, on cool ground in the low, moist foliage of more temperate biomes.