Sierra de Alamos, Sonora, in March... |
...and September, after the summer rains |
Some tropical amphibians are subject to little seasonal variation in their environment and have abundant free water, a usual prerequisite for breeding, and apparently breed year-around. In sharp contrast are species adapted to survive in extremely 'd environments; some of these must limit their reproductive efforts to a few brief hours each year. Within and between these extremes lies a vast array of breeding patterns, all of which provide that vital step in perpetuating the species. The Mexican leaf-frog, Agalychnis dacnicolor, evolved from a group with a breeding pattern specialized for life in tropical forests and it lacks many of the specializations essential for desert habitation. This species, therefore, has not penetrated the interior of the Sonoran Desert.
male Mexican leaf-frog calling from the ground |
a pair of leaf-frogs resting while in amplexus. |
In the Hylidae - a large and diverse family of some 450 living species of frogs distributed on many islands and all continents except Antarctica -a number of species are somewhat unconventional in their reproductive habits. Some deposit their eggs in tiny pools of water formed in tree holes or in the axils of leaves. Others carry their eggs and young in a brood pouch, and a few prepare remarkable mud or foam nests for their progeny. And still others typically affix their eggs to the leaves of shrubs and trees. These are appropriately called leaf-frogs and comprise a distinct New World subfamily, the Phyllomedusinae.
The Mexican leaf-frog is a handsome amphibian. It can rapidly change colors from brown to intense green with gold spots dorsally, has cream-colored underparts, and has golden eyes reticulated with black. Individuals are about sixty-five to one hundred millimeters (two and onehalf to four inches) in body length. They are abundant in the semi-arid subtropical forest of the Mexican lowlands along the Pacific coast, ranging from the Isthmus of Tehuantepec northward to southern Sonora. Although this leaf-frog is sometimes classified in a genus of its own, Pachymedusa, I believe the breeding habits it shares with other species place it in the genus Agalychnis.
I was fortunate in having the opportunity to observe Mexican leaf-frogs during the summer of 1969 near Alamos, Sonora. There on the fringe of the Sonoran Desert near the northern limit of this species, its reproductive success must surely fluctuate widely from year to year. Although the summer rains usually begin near the end of June, that month and the first week of July remained unbearably hot and dry. Each afternoon clouds built steadily over the nearby Sierra de Alamos, and finally on July 7th the sky and earth exploded simultaneously. Rain streamed down the hillsides faster than the starved ground and thirsting plants could catch it; the dry ponds filled rapidly and soon overflowed. A multitude of animals entombed in the parched ground were suddenly freed, ready to begin a new cycle. At dusk a few voices rang out at every pond; by nine o'clock the air reverberated with quacks, squawks, trills and honks. Male frogs and toads eagerly calling to mate soon became a striking feature of the now dynamic landscape.
Aside from some the more vociferous treefrogs and toads calling from the ground, less conspicuous but distinct squawks sounded from almost every bush and tree along the shore of a small pond nestled in the forest. Failing to locate the male that called from a branch overhead, I slowly raised a microphone into the leaves to record his voice. Cold, sticky fingers greeted mine. After flashing on the light, I found a contented-looking, bright green frog perched on the microphone and was immediately captivated by the animal's deliberate actions and calm disposition. This was my first contact with a live Mexican leaf-frog.
Approximately twenty males were calling, yet two rarely called concurrently. Most were perched on limbs one to three feet above the ground; others called from the ground or from heights up to ten feet. The characteristic croak can be described as a rubbery squawk usually sounded once, sometimes twice in rapid succession. This vocalization is often followed by a vibratory purr, which is sometimes uttered alone when one male jumps upon another as they frequently do. Occasionally males persistently pursued and clasped other males, eliciting an emphatically repeated "yip!" from the less aggressive animal.
In contrast to the immediate breeding activity of frogs and toads well adapted to desert conditions, there was a conspicuous absence of female leaf-frogs. After much prowling about through the dense vegetation along the pond, I located a solitary female crawling over an entanglement of vines near the ground. The chorus of calling males had apparently lured her from a retreat in the forest. A male calling in the vicinity noticed her, rather awkwardly crawled onto her back, and seized her by burying his first two fingers in her axillae (or armpits) and neatly curling his other fingers over her upper arm. She was remarkably larger than he, as female frogs generally are larger than males, and his presence burdened her little. The pair was quite inactive most of the night, often resting for hours at a time in one place on the ground or the branch of a tree. Such pauses are probably necessary to assure that the female is physiologically ready to release her eggs. The male remained silent when the female was stationary but croaked when she moved about.
