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 (Noble, 1931).  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 (Pickwell, 1947; Nussbaum, 1983).  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, (Pickwell, 1947).  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 (Oliver, 1955; Dickerson, 1969).  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 (Nussbaum, 1983; Goin, 1978).  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 (Pickwell, 1947).  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, (Pickwell, 1931).  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 (1983) 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 (1947) 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 (1978).  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.    

 

Hyla regilla's succession of successful reproduction - Croaking, Mating, Fertilized Eggs & Froglets
(Photo 1: Dr. Robert C. Drewes, 1999. California Academy of Sciences)	(Photo 2: Joyce Gross, 1998. U.C. Berkeley)	(Photo 3: Joyce Gross, 1998. U.C. Berkeley)	(Photo 4: Joyce Gross, 1998. U.C. Berkeley)

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 (Porter, 1972).  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 (Stebbins, 1985).  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 (Dickerson, 1969) (Figure 1).  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 (Stebbins, 1985).  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 (Nussbaum, 1983).  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 (Stebbins, 1985).  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 (Stebbins, 1985).  

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 (Nussbaum, 1983).  H. regilla  can survive in a variety of habitats: grassland, chaparral, woodland, forest, desert oases and farmland (Stebbins, 1985).   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 (Goin, 1978).  This behavioral adaptation also corresponds to the environment in which H. regilla, and all frogs, live (Figure 2); 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 (Dickerson, 1969; Nussbaum, 1983).   In the Great Basin of Washington and Oregon, H. regilla  can be found living along the streams thriving in disparate conditions (Dickerson, 1969). 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 (Nussbaum, 1983). 

 

Figure 1: Distribution of Hyla regilla	Figure 2:  Chart shows the proportion of water loss to an increase in temperature brought on by a group of frogs congregating in a small area.  As the temperature increases due to the latter circumstances, Hyla regilla, due to an intrinsic behavioral adaptation, experiences an decrease in body water loss, thus preventing dehydration.
(Stebbins, 1995)	(Goin, 1978).

Evolution:
 

Millions of year ago, amphibians, in some form or another, were the first creatures to make their way onto dry land from water.  At one point in time, they were the dominant species on the planet with their numbers in the billions.  Abstractly, this means that amphibians are the evolutionary parents of mammals and therefore humans.  Thus, with acceptance of this theory, the value of this class assumes increasing importance.  Although amphibians must be recognized for the keystone that they are, the lineage or connection between the two radically different classes of creatures is distant and ancient.

It is very difficult to determine from exactly which line of amphibians frogs descended because their soft tissue, and the moist environment of their habitat generally do not produce sufficient fossils with which to study their lineage.  It is thought that present day amphibians descended from lobe-finned bony fishes (subclass Crossopterygii).  These amphibians had lungs, a structured skeleton that was well-ossified and bones that supported limbs in which to crawl onto land out from the water (Porter, 1972).   Hyla regilla  is indirectly related to those amphibians that crawled onto land, in that there were many, many species that evolved and have since become extinct since lobe-boned bony fishes.  The genus and the species, as a highly developed part of the family Hylidae, evolved fairly recently (Green, 1991).   Fossil records document the family in the Paleocene, around 50 million years ago, and are believed to have derived from a leptodactylid stock, or a family of frogs that are poisonous (Green, 1991; Peters, 1964) (Figure 3).   There are approximately 39 genera within the family and at least 719 different species (Cannatella, 1993) (Figure 4).  Presently there are at least three genera known only as fossils (Porter, 1972).  However, these numbers are subject to change because scientists believe that the current organization of the genus characterizes a monophyletic assemblage, meaning that there is a uniform classification for the way the genes are expressing themselves, and are trying to assign the species into more significant generic categories (Green, 1991).   And there is always the possibility of uncovering new fossils that may alter the number of genera within the family or the present system of classification.

The genus Hyla  probably originated in South America where it was extremely prolific.  This was at the beginning of the Tertiary (65 million years ago) after the dinosaurs were extinct, when the continents of North and South America were adjoined and a northern group of ancestral Hyla  made their way up from South America to Mexico.  The latter group than underwent a second expansion throughout North America.  Disjunct distribution took place when a rise in sea level separated the Northern Hyla  from their Southern relatives.  By the end of the Pliocene (2-7 million years ago) a land connection was re-established and although the South American Hylas  were able to spread out into the newly accessible regions in Central America they were unable to re-colonize North America (Green, 1991).  The North American Hylas  were therefore potentially separated from the their South American relatives for approximately 65 million years.  It is at this point that the Holartic (or new world) Hylas  expanded into California (Green, 1991). 

Once the Holartic Hyla  was established in North America, the genus adapted to their environments significantly enough to warrant generic distinction (Green, 1991).  Scientists did this by separating the Hyla  into three different general categories: species of Acris, which are aquatic, Limnaoedus, which are semi-aquatic and Pseudacris,  which are terrestrial; most species that are assigned to the Holarctic Hyla  are arboreal or semi-arboreal (Green, 1991). 

This is where the discrepancy lies in determining whether or not Hyla regilla  belongs in the family Hyla  or Pseudacris.  Presently, scientists are unable to agree on a fundamental Holartic hylid phylogeny (Green, 1991).  The North American Acris  and specifically the Pseudacris  are not structurally independent from Hyla; Pseudacris  is a group within genus Hyla  that lacks webbing between its phalanges (Noble, 1931).   As mentioned in the first section of this paper, there were tests done by Hedges (1986) and DaSilva (1997) that examined the DNA of H. regilla  and attempted at assessing whether or not the frog belongs in the genus Hyla  or Pseudacris.  There seems to be an increase in the number of chromosomes shared between H. regilla  and all of the Pseudacris  species represented, compared to the amount of chromosomes shared with other species of Holartic Hyla.  Even though there is statistical evidence the hypothesis abound as to possible interpretations.  It is not how closely related two frogs are in terms of family lineage over time, it is variations in genes that become evident through research that is the determining factor in deciding to which genus frogs belong (Goin, 1978). 

Figure 3: General frog cladogram from Class (Amphibia) to Order (Anura): This cladogram shows, in general, the descent of frogs from other taxa and their distinction within the class of amphibia. (Cannatella, 1997.  http://cluster3.biosci.utexas.edu/courses/herpetology/herptree.pdf).

Figure 4: This cladogram shows how once within the order Anura (which encompasses all frog species)Hyla regilla  (within the Family Hylidae) evolved.  This cladogram also illustrates the controversy behind classifing Hyla regilla  as Hyla  or Pseudacris  (as the two genus' are so closely related).  (Cannatella, 1997.  http://cluster3.biosci.utexas.edu/courses/herpetology/amphibdivers/frogtree.pdf).

Other interesting issues - 

The Declining of Amphibians in the Wild, 

the Ozone Layer 

and the Pacific Tree Frog:

(based on Kathryn Phillips' Tracking the Vanishing Frogs, 1994)

During the summer of 1990 there was a meeting in Irvine, California of a large group of herpetologists.  There they discussed a phenomena that they could not explain, a decrease in the numbers of amphibians in the wild.  This was something that they had all individually noticed, but had not brought to the attention of the collective group until this meeting, for fear of personal persecution due to competitive nature of science.   Nonetheless, the sharp reduction in numbers of amphibians in the wild is an issue that is currently in the forefront of herpetological science because it is yet to be fully understood.

Many theories have been offered throughout the scientific community as to why this decrease might be occurring:  habitat destruction, urbanization, disease, acid rain, the introduction of non-native species.  All of the latter theories have been theoretically tossed-around to no avail.  Mark Hayes an independent herpetologist who worked for an environmental consulting firm, from Davis, California, along with a few other scientists at the meeting suggested the possibility that the excessive amounts of the suns’ UV rays coming through the thinning of the stratospheric ozone and was somehow negatively effecting amphibian life, specifically frogs.  The ozone is thinning due to an increase chlorofluorocarbons (CFC’s); this has been happening since the industrial revolution.   Presently there is legislation enacted by most major world powers to halt CFC production, except that it takes decades for the damage done to the ozone layer to diminish. 

One of the problems with having a hole in the ozone is an increase in the amount of UV-B rays that hit the Earth’s surface.  This type of UV radiation is extremely detrimental to the health of all living things: causing cancer, cataracts in humans or a reduction in the vitality of the immune system, all with the potential to be deadly.   Because frogs are ectotherms, they often lay in the sun for extensive periods of time to warm up or to simply enjoy the sun’s heat.   Hayes, after reading about the effects of UV rays and along with his knowledge of frog behavior built a hypothesis that the loss of amphibians in the wild was correlated to the thinning of the ozone.  Together with Andrew Blaustein, a professor at Oregon State University, Hayes developed a proposal that entailed determining the effects of UV-B radiation on frog reproduction. 

Money was attained though a grant from the National Science Foundation and the wheels were set in motion.  But like any meeting-of-the-minds, there were personal issues.  Hayes was fired from the project and Blaustein hired John Hays, a molecular geneticist who had done UV work on plants.

Hays set up an experiment that tested a theory involving the development of frog eggs and UV-B radiation.  This was tested by placing some eggs from three different species of frog, the Cascade frog, the Western toad and the Pacific tree frog, into one of two kinds of boxes, screened or unscreened against UV-B in the mountains of Oregon.  The theory was that those eggs shielded would survive and those eggs not shielded, would not; this based on a previous experiment that proved that frogs could detect UV-B light.  The experiment yielded the following findings: approximately one-fourth of the Cascade frogs and Western toads died when exposed to UV-B radiation, virtually no Pacific tree frogs died under the same conditions.  These findings are consistent with prior research that had been conducted by Hays’s lab, which was that Pacific tree frog cells have a greater ability to repair damage from UV-B radiation than either the Cascade frog or Western toad (Phillips, 1994).  This means that UV-B radiation that hits the Earth every day is destroying the eggs of frogs and is possibly and indicator of greater damaging effects to other forms of life.

These scientific findings became a testament to the rapidly declining populations of amphibians in this world.  It is not just the loss of a few small frogs but also the implications of a greater loss of life due to the fact that we all live on the same planet under these same conditions.  Published in the Proceedings of the National Academy of Science, the research vaulted Blaustein and Hays into a realm of expertise on the subject such that they are known and referred to worldwide (Phillips, 1994).

 Bibliography:
 

The Columbia Electronic Encyclopedia. (October 9, 2001).  Tree Frog. [Online]. 
               Available:  http://www.biodiversity.org.uk/ibs/globalsearcher/

Dickerson, Mary C.  1969.  The Frog Book.  Toronto, Ontario.  General Publishing
             Company, Ltd.  

Green, David M., Stanley K. Sessions, editors.  1991.  Amphibian Cytogenetics and Evolution. 
             San Diego, CA.  Academic Press.

Grinnell, Joseph and Tracy Irwin Storer.  1924.  Animal Life in the Yosemite.  Berkeley, CA.  
             University of Berkeley Press.    

Goin, Coleman J., Olive B. Goin & George R. Zug.  1978.  Introduction to Herpetology.
             San Francisco, CA.  W. H. Freeman and Company

Noble, G, Kingsley.  1931.  The Biology of Amphibia.  New York, NY.  McGraw-Hill Book
            Company, Inc. 

Nussbaum, Ronald A., Edumund D. Brodie, Jr. and Robert M. Storm.  1983. 
            Amphibians and Reptiles of the Pacific Northwest.  Idaho.  The University Press of Idaho.  

Oliver, James A.  1955.  The Natural History of North American Amphibians and
             Reptiles.  Princeton, NJ.  D. Van Nostrand Company, Inc.  

Peters, James A.  1964.  Dictionary of Herpetology.  New York, NY.  Hafner Publishing
            Company. 

Phillips, Kathryn.  1994.  Tracking the Vanishing Frogs.  New York, NY.  St. Martin’s Press.

Pickwell, Gayle.  1931.  Western Nature Study: Frogs, Toads, Salamanders.  San Jose,
             CA.  San Jose State College Press.  

Pickwell, Gayle.  1947.  Amphibians and reptiles of the Pacific States.  Stanford, CA. 
             Stanford University Press.    

Porter, Kenneth R.  1972.  Herpetology.  Philadelphia, PA.  W.B. Saunders Company.

Stebbins, Robert C.  1985.  A Field Guide to Western Reptiles and Amphibians.  Boston, MA.  
             Houghton Mifflin Company.

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