Middle American Frogs of the Hyla microcephala Group

Chapter II: Part 2

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+Costa Rica+: Alajuela: 12.4 km. N Florencia, MVZ 76108-10, USC 2628; *Las Playuelas, 11 km. S Los Chiles, USC 7216; Los Chiles, USC 7217, 7219; 3 km. NE Muelle de Arenal, USC 2644 (2); *"San Carlos," USNM 29970. Cartago: Chitaria, KU 103690; *1.6 km. E Rio Reventazon Bridge, east of Turrialba, UMMZ 119978 (2); *Tunnel Camp, near Peralta, KU 32456, 32458-69, 41098 (skeleton); Turrialba, FMNH 101794, 103188-9, KU 25725-9, 32439-48, 41095-7 (skeletons), 64797-827, 68300-2 (skeletons), 68403 (eggs), 68404 (tadpoles), MCZ 29224-5, 29310-2, UMMZ 119979 (6), USC 31, 256 (2), 458 (2), 580, 594, 599 (7), 7074 (2), USNM 29933. Guanacaste: Arenal, USC 6254; *Finca San Bosco, USC 62724, 6276 (3), Guayabo de Bagaces, USC 7022 (3), 7023; *Laguna Arenal, USC 6262 (4); 3 km. NE Tilaran, USC 524; *5 km. NE Tilaran, USC 6269; *6 km. NE Tilaran, UMMZ 122653 (6), S-2680 (skeleton), USC 523 (8). Heredia: Puerto Viejo, KU 64828-63, 68303-7 (skeletons), 68405-6 (tadpoles), 104099-100 (tadpoles); *1.5 km. N Puerto Viejo, KU 64871; *1 km. S Puerto Viejo, KU 86432-40; *4.2 km. W Puerto Viejo, KU 64864-5; *5.9 km. W Puerto Viejo, KU 64866-70; *7.5 km. W Puerto Viejo, KU 86431. Limon: Batan, UMMZ 119980 (2); La Castilla, ANSP 23707; Puerto +Limon+, KU 32449-55.

+Panama+: Bocas del Toro: 3.2 km. NW Almirante, KU 96026; Cayo de Agua, KU 96027-31; Fish Creek, KU 96032-4. Canal Zone: Barro Colorado Island, AMNH 69790, ANSP 23244-50; FMNH 13380, 22972-4; Juan Mina, AMNH 55429, UU 3899; *8.6-13.8 km. N Miraflores Locks, TNHC 23439, 23477, 23484-8, 23491, 23494-9, 23501-2, 23504-8, 23510-17, 23519-30, 23532-8, 23541-54, 23561. *Rio Chagres, AMNH 55431-4; Rio Cocoli, 3.5 km. N Miraflores Locks, TNHC 23461, 23489-90, 23493, 23500, 23503, 23509, 23518, 23531, 23539-40; *Summit, ANSP 23361, KU 97788; *Three Rivers Plantation, SU 2130. Cocle: El Valle de Anton, AMNH 55435, 69786-9, ANSP 23506-9. Colon: Achiote, KU 77215-78; Ciricito, CAS 71499-500, 71505-6. Darien: Rio Canclon at Rio Chucunaque, UMMZ 126733; Rio Chucunaque, near Yavisa, AMNH 51783. Panama: Cero La Campana, FMNH 67847-50.

+Colombia+: Choco: Andagoya, FMNH 81856; Boca de Raspadura, AMNH 13570-8.

+Hyla sartori+ Smith

_Hyla underwoodi_ (in part), Smith and Taylor, Bull. U. S. Natl.
Mus., 194:85, June 17, 1948.

_Hyla microcephala sartori_ Smith, Herpetologica, 7:186,
December 31, 1951 [Holotype.--UIMNH 20934 from 1 mile north of
Organos, south of El Treinte, Guerrero, Mexico; H. M. Smith and
E. H. Taylor collectors]. Duellman, Univ. Kansas Publ., Mus. Nat.
Hist., 15:124, December 20, 1961. Porter, Herpetologica, 18:168,
October 17, 1962. Davis and Dixon, Herpetologica, 20:230,
January 25, 1965. Duellman, Univ. Kansas Publ. Mus. Nat. Hist.,
15:652, December 30, 1965.

_Diagnosis._--Dorsum tan with broad dark brown chevrons or transverse bars; shanks marked with two or three broad transverse bars; dorsolateral stripes absent.

_Description and variation._--No noticeable geographic variation is apparent in either structural features or coloration in this species. All specimens lack a dorsolateral dark stripe and white line, although a dark line is present on the canthus and dissipates in the loreal region. A broad interorbital brown bar is present in all specimens. The color pattern on the dorsum invariably consists of a broad, dark, chevron-shaped mark in the scapular region and a broad dark chevron or transverse bar in the sacral region. The shanks invariably have two or three dark brown transverse bars.

When active at night individuals are yellowish tan above with chocolate brown markings (Pl. 14). The belly is white, and the thighs are pale yellowish tan. The iris is dark bronze-color. In breeding males the vocal sac is yellow. By day some individuals were observed to change to creamy gray with distinct darker markings.

_Remarks._--Although tadpoles of this species have not been found, observations on the breeding sites indicate that the tadpoles probably develop in ponds. Except for calling males observed around a pool in a stream-bed 11.8 kilometers west-northwest of Tierra Colorada, Guerrero, all breeding congregations have been found at temporary ponds.

Smith (1951:186) named _Hyla sartori_ as a subspecies of _Hyla microcephala_. This subspecific relationship seemed reasonable until analysis of the mating calls showed that the call of _H. sartori_ is more nearly like that of _H. phlebodes_ than that of _H. microcephala_. The broad hiatus separating the ranges of _H. microcephala_ and _H. sartori_ is additional evidence for considering _H. sartori_ as a distinct species.

_Distribution._--_Hyla sartori_ occurs in mesophytic forests to elevations of about 300 meters on the Pacific slopes of southern Mexico from southwestern Jalisco to south-central Oaxaca (Fig. 4). The lack of specimens from Colima and Michoacan probably reflects inadequate collecting instead of the absence of the species there. On the basis of available habitat the species would be expected to occur in Nayarit, but extensive collecting there has failed to reveal its presence. The semi-arid Plains of Tehuantepec apparently limit the distribution to the east.

_Specimens examined._--190, as follows: +Mexico+: Guerrero: 5 km. E Acapulco, AMNH 54611-2; 23.2 km. N Acapulco, UIMNH 26404-7; Colonia Buenas Aires, 23 km. E Tecpan de Galeana, UMMZ 119223 (7); *El Limoncito, FMNH 75785, 100390-402, 104631, 104633, UMMZ 117250, USNM 134266; El Treinte, FMNH 100403, UIMNH 20935-7; Laguna Coyuca, AMNH 59686; La Venta, MCZ 29635; *Morjonares, UIMNH 26392-402; 1.6 km. N Organos, FMNH 100404-5, UIMNH 20933-4; 19.2 km. S Petaquillas, UIMNH 26408; 6.1 km. E. Tecpan de Galeana, TNHC 23396-408; *11.2 km. N Tierra Colorada, UIMNH 26403; 11.8 km. WNW Tierra Colorada, UMMZ 119225 (51), S-2677-9 (skeletons); Zacualpan, UMMZ 119224 (6). Jalisco: 6.4 km. NE La Resolana, KU 67853-69; 24 km NE La Resolana, KU 67870-3. Oaxaca: 3 km. N Pochutla, KU 57539; 13.4 km. N Pochutla, UMMZ 123495 (40).

CRANIAL OSTEOLOGY

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

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.

_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 Canal 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 grasses and sedges; _phlebodes_ usually calls from taller and more dense grasses than does _microcephala_. Except for some minor differences in calling location observed by the junior author (Fouquette, 1960b) in the Canal Zone, the differences in density and height of grasses 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 grasses, 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 grass 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 masses 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.

PHYLOGENETIC RELATIONSHIPS

The evidence already presented on osteology, external structure, coloration, mating call, and life history emphatically show that the four species under consideration are a closely related assemblage. Now the question arises: To what other groups in the genus is the _Hyla microcephala_ group related? Furthermore, it is pertinent to this discussion to attempt a reconstruction of the phylogeny of the group as a whole and of the individual species in the _Hyla microcephala_ group. With regard to the relationships of the group we must take into account certain species in South America. Our endeavors there are hampered by the absence of data on the mating calls and life histories of most of the relevant species.

As mentioned in the account of _Hyla m. microcephala_, the species _microcephala_ possibly is subspecifically related to _Hyla misera_, a frog widespread in the Amazon Basin. _Hyla misera_ resembles _microcephala_ in coloration, external structure, and cranial characters. The frontoparietals are equally poorly ossified, and the frontoparietal fontanelle is extensive. Our principal reason for not considering the two taxa conspecific at this time is our lack of knowledge concerning the color of living _H. misera_, the structure of the tadpoles, and the characteristics of the mating call. Even with the absence of such data that we think essential to establish the nomenclature status of the taxa, we are confident that the two are sufficiently closely related that any discussion of the phylogenetic relationships of one species certainly must involve consideration of the other.

_Hyla misera_ possibly is allied to other small yellowish tan South American _Hyla_ that lack dark pigmentation on the thighs. Probable relatives are _Hyla elongata_, _minuta_ (with _goughi_, _pallens_, _suturata_, _velata_, and possibly others as synonyms), _nana_, and _werneri_. The consideration of the interspecific relationships of these taxa is beyond the scope of this paper, but we can say that each of these species has a pale yellowish tan dorsum, relatively broad dorsolateral brown stripe, and narrow longitudinal brown lines or irregular marks on the dorsum. Furthermore, examination of the skulls of _elongata_, _nana_, and _werneri_ reveals that they are like _misera_ and _microcephala_ in the nature of the frontoparietal fontanelle and in having a greatly reduced quadratojugal. Thus, on the basis of cranial and external characters the _Hyla microcephala_ group can be associated with _Hyla misera_ and its apparent allies in South America. This association can be only tentative until the mating calls, tadpoles, and chromosome numbers of the South American species are known.

Among the Middle American hylids, only the _Hyla microcephala_ group and _H. ebraccata_ have a haploid number of 15 chromosomes (Duellman and Cole, 1965). All other New World _Hyla_, for which the number is known, have a haploid number of 12; the only other _Hyla_ having 15 is a Papuan _Hyla angiana_ (Duellman, 1967).

_Hyla ebraccata_ occurs in the humid tropical lowlands of Middle America and the Pacific lowlands of northwestern South America. It is the northernmost, and only Central American, representative of the _Hyla leucophyllata_ group, which is diverse (about 10 species currently recognized) and widespread in tropical South America east of the Andes. This group is characterized by having broad, flat skulls with larger nasals and more ossification of the frontoparietals than in the _Hyla microcephala_ group. The quadratojugal is present as a small anteriorly projecting spur that does not connect with the maxillary. Externally, the _Hyla leucophyllata_ group is characterized by having a well-developed axillary membrane, uniformly yellow thighs, and a dorsal color pattern in many species consisting of a dark lateral band, a pale dorsolateral band or dorsal ground color, and a large middorsal dark mark. In some species, the dorsal pattern consists of small dark markings or is nearly uniformly pale. At least in the Central American _Hyla ebraccata_, the mating call consists of a single primary note followed by a series of shorter secondary notes, the tadpoles have xiphicercal tails and lack teeth, and the haploid number of chromosomes is 15. On the strength of these observations it seems imperative to consider the _Hyla leucophyllata_ group as a close ally to the _Hyla microcephala_ group. Successful artificial hybridization supports the close relationship of _H. m. microcephala_ and _phlebodes_; partial success of artificial hybridization of these two with _ebraccata_ (Fouquette, 1960b) provides further evidence for close relationship between the _Hyla leucophyllata_ and _Hyla microcephala_ groups.

In Mexico and northern Central America two small species, _Hyla picta_ and _Hyla smithi_, comprise the _Hyla picta_ group. These frogs resemble members of the _Hyla microcephala_ group by having a yellowish tan dorsum with a dorsolateral white stripe and uniformly yellow thighs. Furthermore the mating call is not unlike those of the species in the _Hyla microcephala_ group. Despite these similarities, the _Hyla picta_ group differs from the _Hyla microcephala_ group by having a well-developed quadratojugal that connects to the maxillary, tadpoles with teeth present and caudal fins completely enclosing the caudal musculature, and a haploid number of 12 chromosomes. In all of these characteristics the frogs of the

_Hyla picta_ group more closely resemble other Middle American _Hyla_ than they do the _Hyla microcephala_ group. Therefore, it can best be presumed that the superficial resemblances of coloration and the mating call are the result of convergence.

Since the _Hyla microcephala_ and _leucophyllata_ groups apparently are related and since the greatest diversity of these frogs is in South America (if _Hyla misera_ and its relatives are placed with the _Hyla microcephala_ group), it seems appropriate to place the centers of origins of these groups in South America. Therefore, the _Hyla microcephala_ group and _Hyla ebraccata_ of the _Hyla leucophyllata_ group either have immigrated into Central America, or they are representatives of those groups that were isolated in Central America during most of the Cenozoic when South America was separated from Central America.

The interspecific relationships of the species in the _Hyla microcephala_ group are not clear. On the basis of coloration, _H. m. microcephala_ and _H. robertmertensi_ are close, and _H. m. underwoodi_ and _H. phlebodes_ are nearly identical. The mating calls of _H. phlebodes_ and _sartori_ closely resemble one another, whereas the call of _robertmertensi_ is intermediate between these and _microcephala_.

In most respects _Hyla microcephala_ is distinct from the other species, and with the exception of the amount of ossification of the frontoparietals, the other species can be easily derived from a _microcephala_-like ancestor. Possibly the slightly increased ossification of the frontoparietals in _robertmertensi_, _phlebodes_, and _sartori_ is secondary, or possibly after differentiation of the species the amount of ossification was further reduced in _microcephala_. If so, the species fall into a reasonable phylogenetic scheme that has _microcephala_ as the extant species most like the ancestral stock.

We visualize the evolutionary history of the group to have followed a course that began with the invasion of Central America by a _microcephala_ ancestral stock that differentiated into two populations in lower Central America--a _microcephala_-like frog on the Pacific lowlands and a _phlebodes_-like frog on the Caribbean lowlands. Differentiation could have been brought about by isolation by montaine or marine barriers. The population on the Pacific lowlands either was preadapted for subhumid conditions or became so adapted and dispersed northward onto the Pacific lowlands of northern Central America. Simultaneously the frogs on the Caribbean lowlands, which were adapted to humid environments, dispersed northward in the humid forested regions to southern Mexico and crossed the Isthmus of Tehuantepec onto the Pacific slopes of Oaxaca and Guerrero northward to Jalisco. Subsequent development of arid conditions, possibly in the Pliocene, Pleistocene, or even as late as the Thermal Maximum in post-Wisconsin time, resulted in a restriction of the ranges in northern Central America, thereby isolating part of the _phlebodes_-stock on the Pacific slopes of Mexico, where it adapted to drier conditions and evolved into _sartori_. The rest of the _phlebodes_-stock was restricted to the humid forests on the Caribbean lowlands of lower Central America. The increased aridity on the Pacific lowlands eliminated the _microcephala_-stock from southern Honduras and northwestern Nicaragua and in so doing left an isolated population on the lowlands of Chiapasand Guatemala, which differentiated into _robertmertensi_. The original stock on the Pacific lowlands of Panama and southeastern Costa Rica became _microcephala_.

If the _microcephala_-stock was, as we believe, better adapted for existence under subhumid conditions than was the _phlebodes_-stock, the development of subhumid conditions in much of the lowland region of northern Central America and southern Mexico would have permitted the expansion of the range of _microcephala_ into the area now inhabited by _H. m. underwoodi_, while _phlebodes_ was being eliminated from this area by climatic conditions that were unsuited to its survival there. Perhaps the similarity in coloration of _H. m. underwoodi_ and _phlebodes_ is the result of convergence or possibly hybridization occurred at the time the former was expanding its range and the latter's range was being restricted. If hybridization did occur, the differences in mating call subsequently were enhanced, thereby providing a valid isolating mechanism in sympatric populations.

_Hyla microcephala_ and _phlebodes_ range into northern South America. Probably both species entered South America in relatively recent times after they had differentiated from one another in Central America.

LITERATURE CITED

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_Transmitted July 11, 1967._

Transcriber's Notes

This file was derived from scanned images. With the exception of the list of typographical errors that were corrected below, the original text is presented.

In the copy of the original, the Plate text contains the notation "X 2" after the caption to let the reader know that the image was enlarged by a factor of two.

Emphasis Notation

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Typographical Errors Corrected:

Several minor typographical corrections were made (missing periods, commas, incomplete italicization, etc.); but are not indicated here. More substantial changes are listed below:

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Middle American Frogs of the Hyla microcephala GroupChapter II: Part 2

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