Part 6

The frogs of the Hyla microcephala group have a minimal amount of cranial ossification as compared to more generalized hylid skulls, such as Smilisca (Duellman and Trueb, 1966). In the Hyla microcephala group the sphenethmoid is small and short, and a large frontoparietal fontanelle is present. The quadratojugal exists only as a small spur and is not in contact with the maxillary. The prootics are poorly developed. The anterior and posterior arms of the squamosal are short; the anterior arm extends no more than one-fourth of the distance to the maxillary, and the posterior arm does not have a bony connection with the prootic. The nasal lacks a maxillary process, and the medial ramus of the pterygoid lacks a bony connection to the prootic.

Teeth are absent on the parasphenoid and palatines, but present on the maxillaries, premaxillaries, and prevomers. The teeth are simple, pointed, and slightly curved. Although the number of teeth varies (Table 3), no consistent differences between the species are apparent.

Table 3.--Variation in the Number of Teeth in the Species of the Hyla Microcephala Group. (N=Number of Jaws, or Twice the Number of Individuals; Means are Given in Parentheses After the Observed Ranges).

========================+====+=============+==============+========== Species | N | Maxillary | Premaxillary | Prevomer ------------------------+----+-------------+--------------+---------- H. microcephala | 32 | 31-47(37.8) | 4-13(8.9) | 2-4(3.2) | | | | H. phlebodes | 10 | 38-45(40.1) | 8-13(10.3) | 2-5(3.9) | | | | H. robertmertensi | 6 | 23-43(32.8) | 7-12(10.5) | 2-3(2.7) | | | | H. sartori | 6 | 27-43(38.2) | 9-10(9.3) | 3-4(3.7) ------------------------+----+-------------+--------------+----------

Table 4.--Comparative Cranial Osteology of Hyla microcephala Group

===============+=======================+========================+ Character | H. microcephala | H. robertmertensi | ---------------+-----------------------+------------------------+ Frontoparietal | Minimally ossified | Ossification extensive | | with large fontanelle | anteriorly with narrow | | extending from | medial separation; | | sphenethmoid to | fontanelle largest in | | occipital ridge. | parietal region. | | | | | | | Nasals | Moderately long and | Moderate in size; | | slender; arcuate in | slightly wider | | dorsal view. | anteriorly than | | | posteriorly in dorsal | | | view. | | | | Sphenethmoid | Extremely short in | Moderately short in | | dorsal view. | dorsal view. | | | | | | | | | | Columella | Distal and greatly | Distal and slightly | | expanded. | expanded or not. | ---------------+-----------------------+------------------------+ Table 4. (Continued) ===============+========================+======================== Character | H. phlebodes | H. sartori ---------------+------------------------+------------------------ Frontoparietal | Ossification extensive | Ossification moderately | anteriorly with narrow | extensive anteriorly; | medial separation; | medial separation of | fontanelle largest in | about uniform width | parietal region. | throughout length of | | fontanelle. | | Nasals | Moderate in size; | Long and broad; | slightly wider | arcuate in dorsal | anteriorly than | view. | posteriorly in dorsal | | view. | | | Sphenethmoid | Moderately short in | Moderately short in | dorsal view. | dorsal view; ossified | | anteriorly between | | nasals. | | Columella | Distal and not | Distal and not | expanded. | expanded. ---------------+------------------------+------------------------

Despite the great reduction in the ossification of the cranial elements, certain apparently consistent differences exist between the species seem to be consistent. The most notable differences are: 1) amount of ossification of the frontoparietals and consequent shape and size of the frontoparietal fontanelle, 2) shape of the nasals, 3) shape and extent of the sphenethmoid, and 4) shape of the columella (Table 4, Figs. 5-6). On the basis of these characters, Hyla microcephala can be set apart from the other species and characterized as having a poorly ossified frontoparietal and correspondingly large frontoparietal fontanelle; long, slender, arcuate nasals; extremely short sphenethmoid; and expanded distal end of the columella. The other species in the group (phlebodes, robertmertensi, and sartori) have more ossification of the frontoparietals, broader nasals, only a moderately short sphenethmoid, and an unexpanded distal end of the columella. Among these three species, the skulls of phlebodes and robertmertensi are most nearly alike, whereas the skull of sartori differs by having a differently shaped frontoparietal fontanelle, broader nasals, and an ossified anterior extension of the sphenethmoid between the nasals (compare Fig. 5b with Fig. 6 a-b).

Although all skulls examined belong to breeding adults, the extent of the ossification of the frontoparietals and the resulting shape of the frontoparietal fontanelle might be correlated with the age of the frog. Nevertheless, in the 24 skulls of Hyla microcephala examined, the frontoparietals are less extensively ossified than in the skulls of the other species. The trivial differences among the other three species certainly are suggestive of close relationship, but on the basis of present knowledge of the evolutionary trends in hylid cranial osteology, the differences offer little evidence for determining phylogenetic lineage.

ANALYSIS OF MATING CALLS

Calls of all five taxa were compared in several characteristics, of which three are deemed most significant systematically. These are 1) the pattern and duration of the notes of a call-group, 2) the fundamental frequency, and 3) the dominant frequency. Air temperatures were noted at the time the calls were recorded, but no valid correlation could be determined between this factor and any of the parameters of the calls; consequently recordings made at all temperatures (21-29 deg. C.) were grouped together.

Pattern and duration of notes.--In all five taxa the basic pattern consists of a call-group made up of one primary note followed by a series of shorter secondary notes. In some species the secondary notes differ from the primary in other characteristics. Both subspecies of Hyla microcephala have a long, unpaired primary note followed by 0 to 18 (usually about 4) somewhat shorter paired secondary notes. In calls of Hyla m. microcephala the mean duration of the primary is 0.131 (0.10-0.16) second and that of the secondaries is 0.101 (0.05-0.14) second, whereas in H. m. underwoodi the mean duration of the primary is 0.018 (0.05-0.15) second and that of the secondaries is 0.086 (0.06-0.11) second.

Hyla robertmertensi has a reverse of this pattern in that the primary note is paired and the secondaries are unpaired. In the sample studied a call-group contains 0-28 secondary notes (generally about 3). The mean duration of the primary is 0.091 (0.07-0.11) second and that of the secondaries is 0.040 (0.025-0.06) second.

Hyla phlebodes and sartori have call-groups composed of a rather short, unpaired primary and several short, unpaired secondaries (0-28 in phlebodes, 0-23 in sartori). The mean duration of the primary of phlebodes is 0.105 (0.07-0.16) second and that of the secondaries is 0.067 (0.035-0.12) second. The mean duration of the primary of sartori is 0.080 (0.07-0.09) second and that of the secondaries is 0.053 (0.035-0.07) second.

The two subspecies of H. microcephala are identical in call pattern and agree closely in duration of notes, although those of the nominate subspecies tend to be slightly longer. Hyla robertmertensi is distinctive in call pattern in that it is the only species having a paired primary; the duration of the primary is completely overlapped by that in the other species, but the secondaries tend to be the shortest in the group. The call patterns of H. phlebodes and H. sartori are identical and the range of duration of notes of phlebodes completely overlaps that of sartori, although both the primary and secondary notes of the latter tend to be somewhat shorter (Table 5, Pl. 16).

Fundamental frequency.--This parameter was analyzed for the primary notes. It was measured for the secondaries as well and was found to differ in magnitude in the same way as the primary note. In a few examples of both subspecies of H. microcephala a high primary note, in which the fundamental frequency is exceptionally high, is sometimes emitted (Fouquette, 1960b). None of these notes was used in this analysis; only the fundamental frequencies of normal primary notes are compared (Table 5, Fig. 7).

Table 5.--Comparison of Normal Mating Calls in the Hyla microcephala Group. (Observed Range Given in Parentheses Below Mean; Unless Otherwise Noted Data Are for Primary Notes.).

----------------+--+---------+---------+-------------------+-------------- | |Dominant | Funda- |Duration of notes | Repetition | | | mental| (seconds) | rate of Species |N |frequency|frequency+---------+---------+ secondaries | | (cps) | (cps) | Primary |Secondary|(notes/minute) ----------------+--+---------+---------+---------+---------+-------------- H. m. |44| 5637 | 205 | 0.13 | 0.10 | 268 microcephala | |(5150 |(184-244)|(0.11 |(0.05 | (192-353) | | -5962)| | -0.16)| -0.14)| | | | | | | H. m. |47| 5772 | 220 | 0.11 | 0.09 | 283 underwoodi | |(5177 |(192-275)|(0.05 |(0.06 | (197-384) | | -6200)| | -0.15)| -0.11)| | | | | | | H. |25| 5388 | 162 | 0.09 | 0.04 | 418 robertmertensi| |(5150 |(140-178)|(0.07 |(0.03 | (368-570) | | -5785)| | -0.11)| -0.06)| | | | | | | H. phlebodes |34| 3578 | 148 | 0.11 | 0.07 | 284 | |(3220 |(125-158)|(0.07 |(0.04 | (210-350) | | -4067)| | -0.16)| -0.12)| | | | | | | H. sartori |10| 3217 | 126 | 0.08 | 0.05 | 434 | |(2950 |(116-135)|(0.07 |(0.04 | (396-477) | | -3600)| | -0.09)| -0.07)| ----------------+--+---------+---------+---------+---------+--------------

The two subspecies of H. microcephala agree closely in fundamental frequency. There is considerable overlap, but the difference between the means is significant at the 0.001 level of probability (t = 4.2406). The call of H. robertmertensi does not overlap that of H. sartori or either subspecies of H. microcephala in this parameter; but it does overlap that of H. phlebodes, although again the difference between the means is significant at the 0.001 level (t = 9.360). Hyla phlebodes and sartori have the lowest fundamental frequencies, and there is some overlap, but here too the difference between the means is significant at the 0.001 level (t = 4.923).

Dominant frequency.--A dominant band of frequencies cuts across the harmonics of the fundamental, obscuring the harmonic pattern and generally shifting upward in frequency. The midpoint of this band is measured at the terminal border as the dominant frequency. As with the fundamental frequency, only the normal primary notes were utilized in the comparisons (Table 5, Fig 8).

The two subspecies of H. microcephala agree more closely in this parameter than in fundamental frequency. The overlap is great, but the difference between the means is significant at the 0.001 level (t = 3.658). The calls of both subspecies completely overlap that of robertmertensi in this parameter, but the difference between the means is significant at the 0.001 level. The calls of H. phlebodes and H. sartori overlap considerably in this characteristic, although the difference between the means is significant at the 0.001 level (t = 7.504) (Fig. 9). The call of neither species overlaps those of H. microcephala and robertmertensi.

Repetition rate.--The repetition rate of the secondary notes, in calls consisting of more than one secondary, was measured for each form. A considerable amount of variation in this parameter was found in all of the taxa (Table 5). This variation probably is due in part to the effect of temperature differences. Repetition rate is the only parameter analyzed for which there is a correlation with the air-temperature, but even here the correlation is weak, probably due to the microenvironmental effects of humidity, air-movement, and other factors in addition to the ambient air temperature that influences the body temperature of the frogs. These rates are nearly alike in both subspecies of H. microcephala and in phlebodes. The repetition rates in H. robertmertensi and H. sartori are considerably faster than in the other three taxa. Hyla sartori has the fastest repetition rate of the group.

In all characteristics of the mating calls the two subspecies of H. microcephala agree closely, as might be expected, although the differences are statistically significant. Hyla robertmertensi is distinctive in call pattern and seems to be closer to microcephala in dominant frequency but closer to H. phlebodes in fundamental frequency. Thus, it is somewhat intermediate between microcephala and phlebodes. The identical pattern and similarity in fundamental and dominant frequencies of the calls of H. phlebodes and H. sartori possibly indicate close relationship.

Geographic variation in call.--Hyla m. microcephala has higher fundamental and dominant frequencies in Costa Rica than in Panama. In Costa Rican H. m. underwoodi the fundamental and dominant frequencies are lower than in other parts of the range. Frogs of this subspecies recorded in Nicaragua and Honduras have slightly lower dominant frequencies and higher fundamental frequencies than those recorded in Guatemala or Oaxaca. The duration of both primary and secondary notes decreases to the south; samples from Nicaragua and Costa Rica have the shortest notes. Comparison of duration of notes in the two subspecies shows that the Panamanian H. m. microcephala have slightly longer notes than do any H. m. underwoodi; the more northern populations of H. m. underwoodi from Mexico most closely approach H. m. microcephala in this characteristic.

The calls of H. robertmertensi in Oaxaca have higher dominant and fundamental frequencies and longer secondary notes than do those in Chiapas.

The calls of H. phlebodes recorded at Puerto Viejo, Costa Rica, have slightly lower dominant frequencies than do those recorded at Turrialba, Costa Rica, and in Panama, whereas those recorded at Turrialba have lower fundamental frequencies than in other samples. The duration of notes is slightly shorter in both Costa Rican samples than in those recorded in Panama.

LIFE HISTORY

The frogs of the Hyla microcephala group breed in shallow grassy ponds. In some places they breed in permanent ponds, but usually congregate around temporary pools, such as depressions in forests, flooded fields, and roadside ditches. At the height of their breeding season, usually in the early part of the rainy season, the congregations are made up of large numbers of individuals. In April, 1961, and in June, 1966, the senior author noted nearly continuous choruses of H. m. microcephala in roadside ditches along the 75 kilometers of road between Villa Neily and Palmar Sur, Puntarenas Province, Cost Rica; on June 20, 1966, at Puerto Viejo, Heredia Province, Costa Rica, he estimated approximately 900 Hyla phlebodes in one pond, and two nights later noticed that the number of individuals had increased substantially. Other observations by the first author on size of breeding congregations include nearly continuous choruses of H. m. underwoodi between Villahermosa and Teapa, Tabasco, in July of 1958, an estimated 400 Hyla robertmertensi in a road side ditch 7.2 kilometers west-northwest of Zanatepec, Oaxaca, on July 13, 1956, and approximately 150 Hyla sartori around a rocky pool in a riverbed, 11.8 kilometers west-northwest of Tierra Colorada, Guerrero, on June 28, 1958.

The length of the breeding season seemingly is more dependent on climatic conditions in various parts of Middle America than on behavioral differences in the various species. Thus, Fouquette (1960b) found in the Canal Zone that H. m. microcephala formed breeding choruses from May through January, the entire rainy season in that area. In the wetter coastal region of Puntarenas Province, Costa Rica, the species breeds as early as mid-March, whereas in the drier region encompassing Guanacaste Province, Costa Rica, and southwestern Nicaragua breeding activity is initiated by the first heavy rains of the season, usually in June.