Part 11

The distribution of species of Ptychohyla reflects the distribution of cloud forest in southern Mexico and northern Central America. The frogs are restricted to mountainous areas, usually at elevations higher than 1000 meters above sea level. Ptychohyla does not range to great heights in the mountains, where west of the Isthmus of Tehuantepec the mountain streams are inhabited by frogs of the Hyla bistincta group, and in Chiapas and Guatemala by species of Plectrohyla.

Frogs of the Ptychohyla euthysanota group have a greater combined geographic range than the species comprising the Ptychohyla schmidtorum group (Fig. 7). No two species in the same group are sympatric, but members of different groups are sympatric in at least parts of their ranges. Apparently P. leonhardschultzei ranges around the southern edge of the Mexican Highlands, where the species occurs on both Atlantic and Pacific slopes; as can be seen from the distribution map, there are many gaps in the known range of this species. The range of P. euthysanota euthysanota is along the Pacific slopes of the Sierra Madre in Chiapas, Guatemala, and El Salvador, whereas that of P. euthysanota macrotympanum is along the southern interior slopes of the Central Highlands of Chiapas and the Sierra de Cuchumatanes in Guatemala. Ptychohyla spinipollex occurs on the wet Atlantic slopes of the Guatemalan and Honduranean Highlands; the range of the species in Honduras is poorly known.

The frogs of the Ptychohyla schmidtorum group have more restricted geographic ranges than members of the former group. Ptychohyla schmidtorum schmidtorum occurs on the Pacific slopes of the Sierra Madre in Chiapas and Guatemala, where it occurs with P. euthysanota euthysanota; P. schmidtorum chamulae is known from only two localities on the Atlantic slopes of the Central Highlands of Chiapas, where it occurs close to, but as now known not with, P. euthysanota macrotympanum. On the Atlantic slopes of the Sierra Madre Oriental in northern Oaxaca P. ignicolor occurs with P. leonhardschultzei.

In the Sierra de los Tuxtlas in southern Veracruz and in the cloud forests along the eastern slopes of the Sierra Madre Oriental northward to Nuevo Leon, Hyla miotympanum seems to be the ecological replacement of Ptychohyla. On the Pacific slopes north of Guerrero, Mexico, humid forests in which there are cascading mountain streams are absent; consequently, no Ptychohyla are known from that region. In the mountains of El Salvador Ptychohyla euthysanota euthysanota occurs sympatrically with another small stream-breeding hylid, Hyla salvadorensis. To the south of Honduras the highlands diminish into the lowlands of Nicaragua, where habitat suitable for Ptychohyla apparently does not exist. In the mountains of Costa Rica and Panama, the habitats occupied by Ptychohyla in northern Central America are filled by a variety of stream-breeding Hyla, such as Hyla legleri, H. rivularis, H. rufioculis, H. alleei, and H. uranochroa.

Although members of the genus Ptychohyla occur in the southern part of the Mexican Highlands to the west of the Isthmus of Tehuantepec, the greater distribution and differentiation in the genus is in the Chiapan-Guatemalan Highlands. In this respect Ptychohyla is a counterpart of Plectrohyla.

Habitat Preference

Frogs of the genus Ptychohyla are ecologically associated with mountain streams at elevations between 650 and 2200 meters; in the geographic region where these frogs occur the vegetation between those elevations consists of cloud forest or pine-oak forest. In some places the frogs have been found in a mixture of oak and semi-deciduous scrub forest. At Vista Hermosa, Oaxaca, P. leonhardschultzei and P. ignicolor were found in cloud forest, whereas at Agua del Obispo, Guerrero, the former species was found in pine-oak forest. Ptychohyla schmidtorum is known only from cloud forest; P. euthysanota euthysanota and P. spinipollex generally are found in cloud forest, but in some places they live in pine-oak forest. Ptychohyla euthysanota macrotympanum has been found in pine-oak forest and in a mixture of oak and semi-deciduous scrub forest. With the possible exception of the members of the Ptychohyla schmidtorum group, which has been found only in cloud forest, it seems as though the type of vegetation is not the controlling factor in the ecological distribution of these frogs.

Ptychohyla has been found only where there are clear, cascading streams overhung by vegetation, on which adults and young perch at night, or even by day. The presence of these streams, in which the tadpoles live, seems to be an important factor in the distribution of Ptychohyla. As has been shown previously, the tadpoles of Ptychohyla are adapted for existence in torrential streams, where the water is cool, and the amount of oxygen is high. Clearly these tadpoles are unsuited for life in ponds or sluggish streams in the lowlands, where the temperature of the water is high, a layer of silt on the bottom is deep, and the amount of oxygen is low. The tadpoles cling to rocks on the bottom of the streams; there they move slowly across the rocks, apparently feeding on the thin covering of algae. Tadpoles were not observed on rocks having a thick covering of algae or moss. The tadpoles were observed to swim against the current in torrential streams, in which no fishes were found. Therefore, it seems as though the presence of the stream-habitat for the tadpoles is a significant factor in the ecological distribution of the species of Ptychohyla.

Interspecific Competition

At localities where two species of Ptychohyla occur sympatrically (P. ignicolor and P. leonhardschultzei at Vista Hermosa, Oaxaca, and P. euthysanota euthysanota and P. schmidtorum schmidtorum at Finca La Paz, Depto. San Marcos, Guatemala) effort was made to determine what, if any, ecological interspecific relationships existed. Although adults of the sympatric species were found on adjacent leaves or branches of bushes overhanging the streams at both localities, segregation at the time of breeding seems to be maintained by the notably different breeding calls in sympatric species (see discussion of breeding calls). Thus, as has been shown by Blair (1956), Fouquette (1960), and others working on a variety of pond-breeding frogs and toads, the breeding call in Ptychohyla acts as an important reproductive isolating mechanism.

At no locality were Ptychohyla and associated species of hylids found so abundantly as were species of pond-breeding hylids in the lowlands. Apparently reproductive activity is not concentrated in a short breeding season, and it is highly doubtful if the populations of these frogs are as large as those of the lowland pond-breeders. The continual humid conditions and abundance of insect food throughout the year in the cloud forest are perhaps indicative of little interspecific competition among adults of Ptychohyla and other sympatric hylids.

At Finca La Paz, Guatemala, tadpoles of two species of Ptychohyla were ecologically segregated. The tadpoles of P. euthysanota euthysanota were found in riffles in the streams, whereas those of P. schmidtorum schmidtorum were found in slower water, chiefly in small pools in the streams. At Vista Hermosa, Oaxaca, tadpoles of P. leonhardschultzei were found in riffles, and tadpoles of the sympatric P. ignicolor were found in a small pool in a stream. Similar ecological relationships were observed for several species of Costa Rican hylids. Throughout the range of Ptychohyla east of the Isthmus of Tehuantepec, members of the genus occur with species of Plectrohyla, all of which are larger than Ptychohyla, and all of which have tadpoles that live in torrential streams. Tadpoles of Ptychohyla spinipollex have been found in streams inhabited by the tadpoles of Plectrohyla guatemalensis and P. quecchi; tadpoles of Ptychohyla euthysanota euthysanota and P. schmidtorum schmidtorum were found in streams inhabited by tadpoles of Plectrohyla guatemalensis, P. matudai, and P. sagorum. In some streams great numbers of tadpoles occur. The habitat is rather restricted, and the food supply is limited. Consequently, interspecific competition among the various species of hylids whose tadpoles live in the torrential streams probably is highest during the larval stage. Unfortunately, this aspect of salientian population ecology has received no intensive study.

Reproduction and Development

Since the cloud forests inhabited by Ptychohyla are daily bathed in clouds and have a fairly evenly distributed rainfall throughout the year, the frogs living in these forests are active throughout the year. At least some of the species evidently have a long breeding season, for I found calling males of P. leonhardschultzei in February, March, and August, and found tadpoles in February, March, June, and August. Tadpoles of the various species have been obtained throughout much of the year, as follows: P. euthysanota euthysanota, February, March, May, and July; P. euthysanota macrotympanum, March, June, and August; P. spinipollex, February, March, April, June, July, and August; P. schmidtorum schmidtorum, March, May, June, July, and August; P. schmidtorum chamulae, June and August; P. ignicolor, June. I suspect that this temporal distribution more accurately reflects the seasonal activities of collectors than of the frogs.

Calling frogs usually are on vegetation adjacent to or overhanging streams; some calling males of P. spinipollex were on rocks in or by streams. Clasping pairs of P. euthysanota and P. schmidtorum were observed on vegetation by streams. Despite intensive search, no eggs were found. It is doubtful if Ptychohyla deposit eggs on vegetation overhanging streams, as do centrolenids and Phyllomedusa, for egg-clutches of these frogs are easily found. Possibly the eggs are laid separately on vegetation above the stream, in which case they could be overlooked easily. In streams where Ptychohyla and other hylid tadpoles occur, empty egg capsules have been found on the lee sides of rocks, but there is no way to determine which species laid the eggs.

Numbers of eggs were counted in gravid females; the largest eggs have diameters ranging from 2.5 to 3.0 mm. The smaller species, comprising the Ptychohyla schmidtorum group, have fewer eggs than do the larger species. Numbers of eggs found in females of the various species are: P. euthysanota euthysanota, 108; P. euthysanota macrotympanum, 136, 160; P. leonhardschultzei, 141; P. spinipollex, 119, 134, 143; P. schmidtorum schmidtorum, 59, 61, 90; P. schmidtorum chamulae, 60, 71, 89.

Duration of the larval stage is unknown. Metamorphosing young have been found from May through August. From two to six completely metamorphosed young are available for each of the species, except for P. ignicolor of which none is available. The smallest young frog is a P. euthysanota having a snout-vent length of 14.2 mm.; the largest young frog is a P. schmidtorum schmidtorum having a snout-vent length of 17.0 mm.

PHYLOGENY OF PTYCHOHYLA

The preceding data on morphology, life histories, and behavior form the basis for the following interpretation of the phylogeny of Ptychohyla. Additional data are needed to support some of the ideas discussed below; many of the data that are available for Ptychohyla are lacking for other, possibly related, hylids. The family Hylidae is composed of several hundred species, and most of the species are poorly known. Consequently, any attempt to place Ptychohyla in the over-all scheme of hylid phylogeny would be premature at this time. But, as between the five species of two species-groups here recognized as constituting the genus Ptychohyla, some estimate of relationships can be made. First, it is necessary to determine the validity of the genus itself.

Ptychohyla as a Natural Assemblage

As stated in the diagnosis of the genus, the only character that sets this group of species apart from other hylids is the presence of ventrolateral glands in the breeding males. To many systematists the thought of being able to identify to genus only breeding males is sufficiently disturbing to cause them to view with disfavor the recognition of the genus. Nonetheless, the question is raised: Do the five species herein placed in the genus Ptychohyla constitute a natural assemblage? If the genus is considered to be more than a category of convenience, that is to say, a group of related species having a common origin, the primary problem is to determine whether or not the five species form a phylogenetic unit.

The species of Ptychohyla are divided into two groups on the basis of external morphology, breeding calls, and tadpoles. The Ptychohyla euthysanota group seems to be a natural group composed of three species, all of which are more closely related to one another than to any other hylid. Likewise, the species comprising the Ptychohyla schmidtorum group seem to represent a natural unit. If the presence of ventrolateral glands in breeding males is ignored, a student of salientian systematics might derive the Ptychohyla euthysanota group from a hylid stock containing Hyla miotympanum and Hyla mixomaculata. Likewise, Ptychohyla schmidtorum could be placed with Hyla uranochroa and related species in Costa Rica. Nonetheless, the fact remains that all of the species assigned to the genus Ptychohyla have ventrolateral glands in the breeding males; furthermore, ventrolateral glands are unknown in other hylids. If the P. schmidtorum group and the P. euthysanota group each evolved from separate hylid stocks, then the ventrolateral glands must have developed independently in both groups. That ventrolateral glands developed independently in five species of frogs in southern Mexico and northern Central America and not in any of the other approximately 500 species of hylids in the world is untenable. It is more logical to assume that the development of the glands took place only once in a stock of hylids that gave rise to the five species herein recognized as members of the genus Ptychohyla.

Generic Relationships

The affinities of Ptychohyla apparently are not with any of the other groups that have been generically separated from Hyla. Of the daughter genera in Middle America only Plectrohyla has stream-adapted tadpoles, but these large frogs are not closely related to Ptychohyla. Stuart (1954:169) suggested that certain montane species of Hyla in lower Central America and Hyla salvadorensis in El Salvador may be related to Ptychohyla or even congeneric. I have had experience with most of these species in the field and believe that Stuart was correct in his suggestion of relationships. The species concerned are four red-eyed stream-breeding Hyla in Costa Rica--H. alleei, H. legleri, H. rufioculis, and H. uranochroa, plus Hyla salvadorensis in the mountains of El Salvador. Morphologically all of the species are similar; Hyla uranochroa, H. legleri, and H. rufioculis have a lateral white stripe that is expanded to form a spot beneath the eye, as in Ptychohyla schmidtorum. The tadpoles of Hyla rufioculis and H. uranochroa have large funnel-shaped mouths and long slender tails like those of Ptychohyla schmidtorum. Lips of the tadpoles of H. legleri and H. salvadorensis are folded laterally, in this respect resembling those of the Ptychohyla euthysanota group. I do not know the tadpoles and the breeding call of Hyla alleei. The breeding calls of Hyla rufioculis and H. uranochroa consist of high melodious notes; the calls of H. legleri and H. salvadorensis consist of series of short notes that have the general characteristics of the call of Ptychohyla schmidtorum. Affinities of the genus Ptychohyla seem to me to be with the red-eyed species forming the Hyla uranochroa group in Costa Rica. All of the species in the Hyla uranochroa group have large frontoparietal fontanelles, rather small ethmoids, and small nasals that are not in contact with one another or with the ethmoid. Some species have a complete quadratojugal-maxillary arch; others do not. Assuming that the parental stock that gave rise both to the Hyla uranochroa group and to Ptychohyla was widespread in Central America at a time of cooler, more humid conditions, it is possible that with subsequent warming temperatures and seasonal rainfall in the lowlands the parental stock was restricted to the Costa Rican highlands, where the Hyla uranochroa group developed, and to the Chiapas-Guatemala highlands, where Ptychohyla evolved.

Interspecific Relationships

Ptychohyla schmidtorum is thought to resemble more closely the parental stock of the genus than does any other species of Ptychohyla now extant. This parental stock is discussed above in the account of the generic relationships. Ptychohyla schmidtorum has a red eye, white lateral stripe, frontoparietal fontanelle, funnel-shaped mouth in tadpoles, and lacks nuptial spines; in all of these characters it resembles members of the Hyla uranochroa group. Probably during times of glaciation during the Pleistocene, when climates in Mexico and Central America were depressed, the Ptychohyla stock was more widespread than it is now. Subsequent elevation of climatic zones during interglacial periods would have isolated populations as they are today in regions of cloud forests. Thus, through geographic isolation populations could have differentiated and evolved into the present species. Climatic fluctuation in the Pleistocene must have been of sufficient magnitude to permit the spread of cool, moist forests containing Ptychohyla across the Isthmus of Tehuantepec into the mountains of Oaxaca.

Because of its small nuptial spines, small triangular vomers, coloration, and absence of a rostral keel, Ptychohyla euthysanota, more than any of the other species in the P. euthysanota group, resembles P. schmidtorum. At the present time P. euthysanota and P. schmidtorum are sympatric.

As I have mentioned previously, ecological segregation and interspecific competition probably is highly developed in the tadpoles of Ptychohyla. If this ecological segregation resulted from intraspecific competition in a stock of Ptychohyla, possibly P. euthysanota and P. schmidtorum differentiated sympatrically in this way. Specific identity is maintained, at least in part, by different breeding calls in males.