BREEDING BIOLOGY
OF THE MEXICAN LEAF-FROG
page 3 of 3
by T. A. Wiewandt
reprinted with permission of the author from Fauna magazine, March/April, 1971
It is significant that oviposition with only one male per female was equally frequent to cases where two or more males fertilized one egg mass. The ratio of two males per female is probably more common than three or more males per individual female. Dr. Pyburn also noted that two females were each clasped by more than one male in the several matings he observed in Nayarit. Polyandry, it seems, is a well established behavioral pattern in Mexican leaf-frogs and may, as a consequence, enhance the genetic variability within such populations.
The freshly deposited egg is encased in a crystal clear spherical membrane, 4.1 millimeters in diameter, holding a pale green vitellus (the egg cell proper includingthe yolk) that is 2.6 millimeters across. 

Between the outer casing and the vitellus is an inner membranous envelope and a thick layer of gel. As the embryo begins to develop, the animal pole fades to a tan color, grading evenly into the pure green of the vegetal pole. 
   The eggs that were once uniformly spaced slip to the periphery of the gelatinous egg mass by the third day. Although this movement may be incidental it would assure a sufficient oxygen supply and facilitate evaporative cooling for the rapidly developing embryos. Each somewhat swollen capsule now contains a twitching embryo suspended in watery in what embryologists term "embryonic stage 18."
By the fourth day (stages 19 and 20), a tadpole-like body is formed that bears conspicuous, heavily pigmented eyes, two pairs of external gills and many dermal melanophores or dark pigment cells. 

The confined, but active, larva looks most spectacular during its fifth day (stages 21-22), just before hatching. 
Imagine a smoothly curled form, tapered and thinned to transparency, within an ultra-clear sphere of confinement. Its body is of subtle shades of brown, yet still with a potbelly of brilliant green, and mostly decorated with a sharply contrasting pattern of dark pigment cells. With a spasmodic twitch this micro-sea of tranquility is sent surging, as the black bulging eyes appear to be watching its own long, delicate gills swaying to and fro.

How do leaf-frog eggs deposited in shrubs and trees withstand extreme temperatures of the summer day without dehydrating? Because late afternoon showers are frequent during the rainy season, the soil and dense vegetation are often sprinkled and the relative humidity becomes quite high during these weeks as a consequence. Leaf-frogs usually lay their eggs in places that are shaded from direct sunlight and evaporation cools the eggs appreciably. When I measured the temperatures of such egg masses, I found they remained 5º to IOºC (about 9º to 18ºF) below the highest temperatures in the adjacent shade. It is likely, too, that the gelatinous matrix of the egg mass resorbs dew and rain water. And, incidentally, while studying Mexican leaf-frogs in Sonora I saw no signs of predation upon their eggs.

Apparently most of the eggs hatch during nighttime and early morning hours. The young tadpoles, eight to ten millimeters (up to three-eighths of an inch) in length, fall directly into the pond or to the ground, depending upon where the eggs were laid. Four-fifths of the masses were situated over land and almost all of them were within four feet of the water's edge. All the leaf-frogs had deposited their eggs on trees, shrubs, or vines that grew along a bank that dropped sharply to the pond at a thirty to forty-degree incline. Thus many tadpoles that fell to the ground could easily make their way to the water by flipping down the bank like tiddlywinks, spanning some three or four inches with each flip. They were able to clear hurdles of about an inch in height above the terrain.

By watching newly hatched tadpoles flipping on damp soil in the shade, at air temperatures between 24º and 26ºC (75º to 79ºF), I discovered that they lose their vigor and expire toward the end of two hours' activity if they fail to reach the water by then. Subsequent laboratory observations suggest, however, that rain or dew can prolong this critical period. I also learned that tadpoles living in pond water for only four days, and by then thirteen to fourteen millimeters (about one-half inch) in length, can survive on damp soil for as long as new hatchlings can. But having developed changes in body structure, proportions, and/or musculature, the four-day-old tadpole is no longer able to flip effectively. I put a mass of 350 eggs in a terrarium that simulated natural conditions, but with the ground sloping at an incline of fifteen degrees. Approximately half of the newly hatched tadpoles successfully flipped to the water two feet away. Despite the fact that man e are deposited over land, it seems a substantial number of tadpoles do survive the journey to the pond when conditions are favorable.

If this population of leaf-frogs is to persist, many tadpoles must also endure the rigorous environment of the pond. Once swimming freely the tadpoles feed ravenously upon algae and decaying organic matter.

Their outer "lips" are bordered by fleshy papillae and bear five rows of epidermal teeth for rasping food. Interior to these and surrounding the mouth opening is a black, horny beak the tadpole uses for scraping and chopping its food.
 But the pond was teeming with predaceous insects and the hazards of drought were realized by mid-August.

Although other species better adapted to arid conditions had escaped by quickly transforming from aquatic to terrestrial creatures, the leaf-frog tadpoles had not undergone metamorphosis, which occurs approximately five weeks after they hatch and have attained an overall length of some fifty millimeters (almost two inches). As the pond shrank their numbers were soon confined to a small puddle less than two inches deep, and the tadpoles became easy prey for a local tiger heron (Heterocnus mexicanus). Possibly a few had escaped the dwindling, confining habitat by metamorphosing into froglets before the sun cooked the shallow water to a high of 41.8ºC (about 107ºF) on August 21st. A number of tadpoles were dead that afternoon. The following day there was a trace of water in trackways left by mud turtles (Kinosternon sonoriense) and the temperature soared to 45ºC (113ºF), proving lethal to all the remaining tadpoles. The pond dried completely within twenty-four hours only to be refilled by an enormous downpour on the following day. Spadefoot and narrow-mouth toads (Scaphiopus couchi and Gastrophryne olivacea) commenced calling and breeding at once, but the Mexican leaf-frogs were silent, as they undoubtedly remained until next July.

REFERENCES

Duellman, William E. 1968. The genera of phyllomedusine frogs (Anura: Hylidae). University of Kansas Publications, Museum of Natural History, 18 (1): 3-10.

Duellman, William E. 1970. Hylid Frogs of Middle America, 2 vols. Lawrence: Museum of Natural History, University of Kansas (Monograph no. 1).

Gentry, H. S. 1942. Rio Mayo Plants. Washington, D.C.: Carnegie Institution of Washington.

Gosner, Kenneth L. 1960. A simplified table for staging anuran embryos and larvae with notes on identification. Herpetologica, 16: 183-190.

Pyburn, William F. 1963. Observations on the life history of the tree-frog, Phyllomedusa callidryas (Cope). Texas Journal of Science, 15: 155-170.

Pyburn, William F. 1965. Breeding behavior of the leaf-frog, Phyllomedusa callidryas in southern Veracruz. Yearbook, American Philosophical Society, 1964, pp. 291-294.

Pyburn, William F. 1970. Breeding behavior of the leaf-frogs Phyllomedusa callidryas and Phyllomedusa dacnicolor in Mexico. Copeia, (2):209-218.