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.

[Ill.u.s.tration: Fig. 5. Dorsal views of the skulls of (a) _Hyla m.

microcephala_ (KU 68293) and (b) _H. sartori_ (UMMZ S-2677).

Both 12.]

[Ill.u.s.tration: Fig. 6. Dorsal views of skulls of (a) _Hyla phlebodes_ (KU 68303) and (b) _H. robertmertensi_ (KU 59917). Both 12.]

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.

a.n.a.lYSIS 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 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 a.n.a.lyzed 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 a.n.a.lysis; 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 | Repet.i.tion | | | 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).

[Ill.u.s.tration: Fig. 7. Variation in the fundamental frequency of the normal primary notes in the _Hyla microcephala_ group. The horizontal lines = range of variation, vertical lines = mean, solid bars = twice the standard error of the mean, and open bars = one standard deviation. The number of specimens in each sample is indicated in parentheses after the name of the taxon.]

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_.

[Ill.u.s.tration: Fig. 8. Variation in the mid-point of the dominant frequency band of the normal primary notes in the _Hyla microcephala_ group. The horizontal lines = range of variation, vertical lines = mean, solid bars = twice the standard error of the mean, and open bars = one standard deviation. The number of specimens in each sample is indicated in parentheses after the name of the taxon.]

[Ill.u.s.tration: Fig. 9. Scatter diagram relating the dominant and fundamental frequencies of the normal primary notes in the _Hyla microcephala_ group. Each symbol represents a different individual.]

_Repet.i.tion rate._--The repet.i.tion 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. Repet.i.tion rate is the only parameter a.n.a.lyzed 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 repet.i.tion rates in _H. robertmertensi_ and _H. sartori_ are considerably faster than in the other three taxa. _Hyla sartori_ has the fastest repet.i.tion 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 gra.s.sy 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 Ca.n.a.l 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 encompa.s.sing Guanacaste Province, Costa Rica, and southwestern Nicaragua breeding activity is initiated by the first heavy rains of the season, usually in June.

_Hyla phlebodes_ inhabits regions having rainfall throughout the year.

Although large breeding congregations are most common in the early parts of the rainy season, males probably call throughout the year. At Puerto Viejo in Costa Rica the senior author has heard _Hyla phlebodes_ in February, April, June, July, and August. Charles W. Myers noted calling males of this species in the area around Almirante, Bocas del Toro Province, Panama, in September, October, and February. An exception to the correlation between rainfall and breeding activity was noted by the junior author in _Hyla phlebodes_ in the Ca.n.a.l Zone, where he noticed a decrease in activity of that species in October and November, when the rains are heaviest and most frequent. Furthermore, independent observations made by both of us indicate that _H.

phlebodes_ does not reach peaks of activity during or immediately after heavy rains, but instead builds up to peaks of activity two or three days after a heavy rain. This is in contrast to the other species, all of which characteristically inhabit drier environments than does _H. phlebodes_. Peaks of breeding activity in the other species occur immediately after, or even during, heavy rains.

The calling location of the males generally is on vegetation above, or at the edge of, the water. _Hyla microcephala_ and _H. phlebodes_ call almost exclusively from gra.s.ses and sedges; _phlebodes_ usually calls from taller and more dense gra.s.ses than does _microcephala_.

Except for some minor differences in calling location observed by the junior author (Fouquette, 1960b) in the Ca.n.a.l Zone, the differences in density and height of gra.s.ses utilized for calling-locations probably is dependent primarily on the nature of the available vegetation. Although bushes and broad-leafed herbs are usually present at the breeding sites, males of these species seldom utilize them for calling locations. Both _H. robertmertensi_ and _H. sartori_ have been observed calling from gra.s.ses, herbs, bushes, and low trees. Calling males of _robertmertensi_ have been found two meters above the ground in small trees.

Daytime retreats in the breeding season sometimes are no more than shaded clumps of vegetation adjacent to a pond or in clumps of gra.s.s in a pond. Individuals of _H. m. underwoodi_ were found by day under the outer sheaths of banana plants next to a water-filled ditch. Dry season refuges are unknown.

Amplexus is axillary in all four species. Egg deposition has been observed in _H. m. microcephala_, _m. underwoodi_, and _phlebodes_.

In all three the eggs are deposited in small ma.s.ses that float near the surface of the water and usually are at least partly attached to emergent vegetation. Each clutch does not represent the entire egg complement of the female.

Tadpoles are definitely known of only _H. m. microcephala_ and _phlebodes_; these have been described in the preceding accounts of the species. The tadpoles of these two species can be distinguished readily (Pl. 15). The tadpole of _H. microcephala_ has a uniformly white venter and nearly transparent tail, whereas in _H. phlebodes_ the venter is flecked anteriorly and the tail is mottled. In life, _H.

microcephala_ is easily recognized by the orange posterior half of the tail, whereas the tail in _H. phlebodes_ is mottled tan and grayish brown.