[Pg 517]

March 20, 1968

University of Kansas Publications, Museum of Natural History

Editors: E. Raymond Hall, Chairman, Henry S. Fitch,
Frank B. Cross

Volume 17, No. 12, pp. 517-557, 4 pls. 9 figs.

Published March 20, 1968

University of Kansas
Lawrence, Kansas

PRINTED BY
ROBERT R. (BOB) SANDERS, STATE PRINTER
TOPEKA, KANSAS
1968

31-9419

The small yellow tree frogs, Hyla microcephala and its relatives, are among the most frequently heard and commonly collected frogs in the lowlands of southern México and Central America. The similarities in size, proportions, and coloration of the different species have resulted not so much in a multiplicity of specific names, but in differences of opinion on the application of existing names to the various taxa. For example, the populations on the Atlantic lowlands have been known by three names, two of which have been applied to other taxa. Much of the confusion has been the result of previous workers' unfamiliarity with the animals in life and unawareness of the intraspecific geographic variation in the most widespread species.

Independently we undertook studies of these frogs in the field. The second author worked on the interspecific relationships and isolating mechanisms in Panamá (Fouquette, 1960b) and later studied the species in southern México. As part of his survey of the hylids of Middle America, the first author accumulated field and laboratory data on the frogs throughout their ranges in México and Central America. The purpose of this report is to present our findings on the four species of Middle American frogs that we place in the Hyla microcephala group. In addition to conventional taxonomic characters, we have utilized the features of the cranial osteology and have relied heavily on the data obtained from an analysis of the mating calls. Furthermore, we have included ecological and distributional data in our synthesis of interspecific relationships.

Examination of specimens was made possible by the provision of working space at various institutions or through the loan of specimens. For their generosity in this manner we are grateful to Richard J. Baldauf, Charles M. Bogert, James E. Böhlke, Doris M. Cochran, Robert F. Inger, John M. Legler, Alan E. Leviton, Gerald Raun, Jay M. Savage, Hobart M. Smith, Robert C. Stebbins, Wilmer W. Tanner, Charles F. Walker, Ernest E. Williams, and Richard G. Zweifel.

Duellman is especially grateful to Charles W. Myers, Linda Trueb, Jerome B. Tulecke, and John Wellman for their assistance in the field and to Linda Trueb for her work on the cranial osteology that is incorporated in this report. Fouquette is indebted to H. Morgan Smith and A. C. Collins for assistance in the field, to A. J. Delahoussaye for assistance in the laboratory, and to W. Frank Blair for use of the facilities of the sound laboratory at the University of Texas and for much help in the early stages of this study.

The research reported herein was accomplished mainly through support by the National Science Foundation (grants NSF G-9827 and GB-1441 to Duellman and GB-599 to Fouquette). The latter's field work in México was assisted in part by NSF Grant G-4956 to W. Frank Blair. Some of the field studies carried out in Panamá by Duellman were supported by a grant from the National Institutes of Health (NIH GM-12020).

We are grateful to many persons, too numerous to mention, who in various ways aided our field work in Middle America. We are especially indebted to Dr. Rodolfo Hernandez Corzo and the late Ing. Luis Macías Arellano of the Dirección General de la Fauna Silvestre of the Mexican government for providing permits to collect in México.

For this report, data has been obtained from 2829 preserved frogs, 42 skeletal preparations, 8 lots of tadpoles and young, and 4 lots of eggs. Much of the material was collected in our independent field work, which has extended over a period of 11 years.

Measurements were taken in the manner described by Duellman (1956). Osteological data were obtained from specimens that were cleared in potassium hydroxide, stained with alizarin red, and stored in glycerine. Recordings were made by means of Magnemite portable tape recorders (Amplifier Corp. America). The calls recorded by Fouquette were analyzed on a Sonagraph (Kay Electric Co.) at the University of Texas; those recorded by Duellman were analyzed mainly on a Vibralyzer (Kay Electric Co.) at the University of Kansas and in part on a Sonagraph at the University of Southwestern Louisiana. Sample calls were analyzed on all three instruments; the slight differences in results were found to be less than the error in measurement, so the data from all sources were combined without correction. The techniques and terminology of the calls are those defined by Fouquette (1960a, 1960b).

In the accounts of the species we have attempted to give a complete synonymy. At the end of each species account the localities from which specimens were examined are listed alphabetically within each state, province, or department, which in turn are listed alphabetically within each country. The countries are arranged from north to south. Localities preceded by an [Pg 521] asterisk (*) are not plotted on the accompanying maps due to the crowding of symbols that would have resulted. Abbreviations for museum specimens are listed below:

[Pg 522] Discussion. —The color pattern is the most useful character in distinguishing the species of the Hyla microcephala group from one another. Except in Hyla microcephala , little geographic variation in color pattern is noticeable. The features of color pattern that are helpful in identifying the species are: 1) presence or absence of lateral dark brown stripe; 2) longitudinal extent and width of lateral stripe, if present; 3) presence or absence of a narrow white line just dorsal to the lateral dark stripe; 4) presence or absence of an interorbital dark mark; 5) the arrangement of dark markings on the back, either as longitudinal lines or series of dashes, or in the form of various kinds of transverse markings; 6) presence of dark flecks, longitudinal line, or transverse marks on shanks.

Few consistent differences in measurements and proportions exist among the species ( Table 1 ). The most obvious morphological difference is that the head is noticeably narrower in H. robertmertensi than in the other species. Hyla phlebodes is the smallest species; adult males attain snout-vent lengths of only 23.6 mm. The body is slender in H. microcephala and robertmertensi , slightly wider in phlebodes , and noticeably broader in sartori .

Distribution. —The composite range of the Middle American frogs of the Hyla microcephala group includes the lowlands of southern México and Central America, in some places to elevations of 1200 meters, southeastward from southern Jalisco and southern Veracruz, excluding arid regions (northern Yucatán Peninsula, Balsas-Tepalcatepec Basin, Plains of Tehuantepec, Grijalva Valley, Salamá Basin, and upper Motagua Valley) to the Pacific lowlands and the Cauca and Magdalena valleys in Colombia.

Fig. 1. Map showing locality records for Hyla microcephala .

Locality	N	Snout-vent length (S-V L)	Tibia length S-V L	Foot length S-V L	Head length S-V L	Head width S-V L	Tympanum Eye
	H. m. microcephala
Panamá: Canal Zone	25	21.5-24.1	50.2-56.0	40.9-46.6	28.5-32.8	28.1-30.9	44.0-54.1
		22.8±0.20	52.9±0.37	43.5±0.28	31.0±0.22	29.4±0.11	49.0±0.55
Costa Rica: Golfito	25	18.5-24.5	49.1-54.4	41.8-48.0	30.2-35.5	29.0-32.7	40.0-57.8
		22.4±0.27	51.6±0.26	45.1±0.32	33.1±0.25	30.8±0.16	48.4±1.10
	H. m. underwoodi
Nicaragua: La Cumplida	25	23.0-25.6	51.0-55.7	41.3-46.5	29.7-33.5	28.9-31.8	42.3-60.0
		24.1±0.19	52.9±0.25	43.7±0.25	31.6±0.19	30.4±0.17	49.3±0.97
Guatemala: Finca Chamá	25	21.8-25.0	51.0-57.2	41.2-47.8	30.8-35.3	29.6-33.6	37.5-56.4
		23.5±0.16	54.3±0.39	44.4±0.30	33.0±0.16	31.3±0.36	45.2±0.89
Tabasco: Teapa	25	22.7-25.8	48.0-54.5	40.7-46.8	29.5-33.0	28.7-31.8	40.7-53.8
		24.3±0.14	51.5±0.29	43.3±0.25	31.7±0.17	30.3±0.16	45.5±0.38
Oaxaca: Donají-Sarabia	25	22.1-25.9	49.8-55.6	40.5-46.6	30.4-34.8	28.9-32.6	37.0-54.1
		23.8±0.19	52.8±0.33	43.4±0.27	32.8±0.19	30.8±0.17	45.1±0.76
Veracruz: Alvarado	25	21.9-25.4	49.6-54.4	40.7-47.5	29.9-33.8	29.1-32.9	40.7-53.8
		24.1±0.17	51.1±0.28	42.6±0.34	31.4±0.18	30.5±0.17	46.6±0.65
	[Pg 525] H. robertmertensi
Guatemala: La Trinidad	21	21.8-24.6	47.1-52.8	40.9-51.3	30.0-33.3	27.3-29.8	44.4-50.0
		23.4±0.15	49.9±0.34	43.5±0.17	31.3±0.20	28.5±0.23	47.4±0.46
Chiapas: Acacoyagua	25	21.4-25.7	47.8-52.4	41.7-46.3	29.1-32.7	26.0-30.3	42.8-53.8
		24.1±0.20	50.4±0.45	43.9±0.23	31.2±0.29	28.1±0.20	46.5±0.50
Oaxaca: Tapanatepec	25	22.4-26.4	44.1-48.3	39.1-44.5	26.1-30.4	25.4-28.1	45.8-58.3
		24.7±0.18	46.4±0.23	41.7±0.23	28.4±0.16	26.8±0.14	52.9±0.77
	H. phlebodes
Panamá: Canal Zone	25	19.6-23.2	49.1-56.9	41.9-47.1	33.6-37.4	32.3-36.0	37.9-46.4
		22.2±0.16	52.8±0.35	45.4±0.26	34.8±0.18	33.8±0.18	41.6±0.49
Costa Rica: Turrialba	25	19.7-23.6	47.4-55.7	38.1-46.4	32.6-35.9	30.5-35.0	35.7-48.2
		22.0±0.18	51.1±0.35	42.8±0.38	34.1±0.16	32.9±0.17	40.1±0.53
	H. sartori
Guerrero: Tierra Colorada	25	23.7-26.0	47.2-51.4	42.4-47.8	29.4-31.8	28.9-31.0	42.3-52.0
		24.8±0.13	49.6±0.23	45.2±0.27	30.6±0.13	30.0±0.12	47.4±0.59

[Pg 526]

Diagnosis. —Brown lateral stripe narrow, extending from nostril along canthus, along upper edge of tympanum to groin, bordered above by narrow white line; pair of dark brown longitudinal lines on dorsum extending to vent; shanks having dark longitudinal line or flecks, no transverse bars; interorbital dark mark lacking.

Description and Variation. —The color pattern is nearly constant. Of 103 males from the Canal Zone, all lack an interorbital dark bar, and all have a dark longitudinal line on the dorsal surface of the shank and a narrow lateral dark stripe, bordered above by a narrow white line, extending to the groin. The longitudinal dark lines on the dorsum are continuous to the groin in 95 specimens and fragmented in two specimens. In two others the lines converge and fuse in the scapular region, and in four specimens auxiliary, fragmented lines are present dorsolaterally.

In all specimens from southeastern Costa Rica (Golfito, Palmar Sur, and Villa Neilly) the pattern is constant, except that in about 10 per cent of the specimens the longitudinal line on the dorsal surface of the shank is replaced by a row of brown flecks.

Of the limited number of Colombian specimens examined, all are patterned normally, except three from Sautata, Chocó, three from Curumani, and three from Arcataca, Magdalena, which have flecks on the dorsal surfaces of the shanks, and one from Espinal, Tolima, which has no markings on the shanks.

When active at night most individuals are pale yellowish tan dorsally; the white dorsolateral line is noticeable, but the brown lateral stripe, dorsal brown lines, and lines on shanks are so pale that often they are barely discernible. By day the dorsum changes to tan or pale reddish brown; the stripes are dark brown, and the dorsolateral stripe that is white at night becomes creamy yellow ( Pl. 13 ). Small brown flecks are present on the dorsum of most individuals. The venter always is white, and the iris is pale bronze with a brown tint immediately anterior and posterior to the pupil. In breeding males the vocal sac is pale yellow.

Tadpoles. —Tadpoles of this species have been found in weed-choked ponds in eastern Panamá Province. The following description is based on KU 104097, a specimen in developmental stage 34 (Gosner, 1960).

[Pg 527] Total length, 20.5 mm.; body length, 8.2 mm.; body slightly wider than deep; snout pointed; nostrils large, situated dorsally, much closer to snout than eyes, directed anteriorly; eyes moderately small, situated dorsolaterally and directed laterally; spiracle sinistral, located just posteroventral to eye; anal tube dextral. Tail xiphicercal; caudal musculature moderately deep, becoming slender posteriorly, extending beyond caudal fin; fins deepest at about one-third distance from body to tip of tail; dorsal fin extending onto body, deeper than deepest part of caudal musculature; ventral fin slightly shallower than musculature. Mouth small, terminal, lacking teeth and fringing papillae, but having finely serrate beaks. In preservative, top of head pale brown; dark stripe from tip of snout through eye to posterior edge of body, narrowing to thin line on proximal one-fourth of tail; venter white; tail creamy tan with fine black flecks most numerous posteriorly; posterior two-thirds of fins edged with black. In life, top of head yellowish tan; lateral stripe brown; belly white; anterior half of tail lacking pigment; posterior half deep orange; iris pale bronze ( Pl. 15 ).

Distribution. — Hyla microcephala microcephala inhabits coastal lowlands from the area of Golfo Dulce (apparently absent from the Osa Peninsula) in southeastern Costa Rica eastward in Panamá, including the Azuero Peninsula to northern Colombia and thence southward in the valleys of the Río Cauca and Río Magdalena in Colombia ( Fig. 1 ). Except for the central area of the Canal Zone the subspecies is unknown from the Caribbean drainage in Central America, but in Colombia the subspecies occurs only in the Caribbean drainage. In Central America this frog occurs mostly on the coastal lowlands; the highest recorded elevation is 560 meters at El Valle, Coclé, Panamá. Throughout most of its range Hyla microcephala microcephala occurs in disturbed habitats—cut-over forests, secondary growth, and pastureland. It does not seem to be an inhabitant of either primary forest or of Curatella -savanna.

Specimens examined. —522, as follows: Costa Rica : Puntarenas : Golfito, KU 32172-207; 3 km. E Golfito, KU 86399, USC 2757-8; Palmar Sur, KU 64591-608, USC 2650 (14), UU 3907-32; *1.5-2.5 km. ESE Palmar Sur, KU 68293-7 (skeletons), 93957-62; Parrita, USC 8254 (2), 8255, 8256 (4), 8258 (2) [intergrades with H. m. underwoodi ]; 3 km. NW Piedras Blancas, KU 103689; 6.1 km. NE mouth of Río Tárcoles, USC 818 (2), 6081-2 [intergrades with H. m. underwoodi ]; Villa Neilly, USC 2651; *1-5 km. WNW Villa Neilly, USC 6182-4, 8003 (4), 8031 (3), 8032; *10.5 km. WNW Villa Neilly, KU 64609-27, 68398 (eggs).

Panama : Canal Zone : Albrook Air Base, TNHC 23389, 23497; Balboa, ANSP 19555-6; *Fort Clayton, UIMNH 42008-12; *2.8 km. SW Fort Kobbe, KU 96015-25; *Frijoles, MCZ 19208; *Bamboa, MCZ 21507; *8.3 km. N Gatún Locks, TNHC 23441; *Juan Diaz, MCZ 13747; *Juan Mina, AMNH 55436-7, ANSP 21811-2, UMMZ 126734, 126735 (6), UU 3900-6; *8-14 km. N Miraflores Locks, TNHC 23374-88, 23390-409, 23411-38, 23440, 23442-60, 23462-76; 23478-83, 23492, 23555-60, 23562-76; *Río Chagres, AMNH 55430, 55439; *Río Cocolí, 3.5 km. N Miraflores Locks, TNHC 23410; *Summit, ANSP 23365-71, FMNH 22966-9, KU 97783-87. Chiriqui : 5.5 km. E Concepción, AMNH 69772; *14.4 km. E Concepción, AMNH 69773-8; 2 km. S David, AMNH 69779; *Progreso, UMMZ 58252, 58253 (2), 58254, 58436; Río Gariché, 8.3 km. ESE Paso Canoas, KU 103065-8. Coclé : 1 km. SE El Caño, KU 103042-51; El Valle de Antón, AMNH 59614-18 (10), 69785, ANSP 23502-5, KU 77201-14, MVZ 66578-83, UIMNH 46532. Colón : Cement Plant, Transisthmian Highway, FMNH 60394-5. Darién : El Real, KU 80454-5, 103052-64, UMMZ 125036 (10), USNM 140567-8; Río Canclon at Río Chucunaque, UMMZ 125035; *Río Chucunaque, near Yavisa, AMNH 59523. Los Santos : Tonosí, KU 101606-9. Panamá : 5 km. S Bejuco, AMNH 69782; 3 km. W Chepo, KU 77172-4, 104097-8 (tadpoles); *6 km. WSW Chepo, KU 77175; *Chico, Río La Jagua, USNM 129070; *La Joya, Cacora, ANSP 25129-33; Madden Dam, FMNH 67819; Nueva Gorgona, AMNH 69780-1; *1.6 km. W Nueva Gorgona, AMNH 69783-4; 1.5 km. W Pacora, 77176-200; *Río La Laja, near Chamé, ANSP 21845; *Río Tapia, MCZ 10048; *Tapia, AMNH 18930, 18950, 18952-3; *18 km. E Tocumen, MVZ 78662.

[Pg 529] Colombia : Chocó : Sautatá, Atrato, FMNH 74918 (2), 74919. Magdalena : Aracataca, ANSP 19755-7; Curumani, MCZ 21465-74, UIMNH 28855; UMMZ 90168, USNM 118247; El Banco, Río Magdalena, ANSP 25061; Fundación, UMMZ 48281-2. Tolima : Espinal, MCZ 15068; Mariquita, FMNH 81822-3. Valle : Sevilla, MCZ 13751-3.

Diagnosis. —Brown lateral stripe narrow, extending to groin or only to sacral region, bordered above by narrow white line; dorsal pattern bold, consisting of X- or )(-shaped mark in scapular region or pair of interconnected dark lines on back; interorbital dark mark usually present; shanks usually having dark transverse bars.

Description and Variation. —The dorsal color pattern is highly variable. The various permutations of the X-shaped scapular mark and dark sacral marks differ proportionately in different samples. The variation in color pattern in 12 samples is summarized in Table 2 . In samples from the southern part of the range (southern Nicaragua and Guanacaste Province, Costa Rica) more (40-93%) individuals have the lateral stripes extending to the groin than in northern samples (0-42%) from southern México and Guatemala. Likewise, the percentage of specimens lacking bars on the shanks and a dark interorbital bar is higher in the Costa Rican samples than elsewhere in the range. The X- or )(-shaped scapular markings and /\- or / \-shaped sacral markings are most prevalent in northern samples, whereas to the south the dorsal markings are more commonly arranged in a pattern of paired lines, which usually are discontinuous and usually extend posteriorly only to the sacral region. Thus, the color pattern in H. m. underwoodi in the southern part of its range shows trends towards the pattern characteristic of H. m. microcephala . Intergrades between these two subspecies have been discussed in the account of the nominate subspecies.

[Pg 530]

Fig. 2. Map showing locality records for Hyla robertmertensi .

Specimens examined. —490, as follows: Mexico : Chiapas : Acacoyagua, USNM 114754-61; *2 km. W Acacoyagua, UMMZ 87843 (28), 87844 (50), 87845 (50), 87846 (45), 87847 (27), 87848 (3); 32 km. N Arriaga, KU 57619-24, 59917-8 (skeletons); Asunción, FMNH 100413, 100501-4, UIMNH 26989-90, USNM 134267; *La Esperanza, USNM 114737-48, 114750-3, 17 km. S Las Cruces, KU 57625-49, 59997 (eggs); 8.5 km. N Puerto Madero, UMMZ 119981 (2); *11.7 km. N Puerto Madero, UMMZ 119982; Tapachula, FMNH 100096, UIMNH 26987; *11 km. S Tapachula, KU 57605-18, 59916 (skeleton); Tonolá, FMNH 27073, 100505-10, UIMNH 26988. Oaxaca : Tapanatepec, UMMZ 115245 (2), *1.6 km. E Tapanatepec, UMMZ 115244 (14); *4.3 km. E Tapanatepec, UIMNH 38368-9; *7.5 km. W Tapanatepec, UMMZ 115246 (39); 12.8 km. W Tapanatepec, KU 65007-14; 7.2 km. WNW Zanatepec, UMMZ 115243 (77); *13.6 km. WNW Zanatepec, TNHC 25213-22; 22.7 km. WNW Zanatepec, TNHC 25203-9.

Guatemala : Jutiapa : Jutiapa, UMMZ 106848; La Trinidad, UMMZ 107733 (23). Retalhueleu : Casa Blanca, UMMZ 107732.

El Salvador : La Libertad : 16 km. NW Santa Tecla, KU 44112. San Salvador : 21.9 km. N San Salvador, UMMZ 119983 (6).

[Pg 536]

Diagnosis. —Dark brown lateral stripe, if present, usually extending only to insertion of forearm, never posteriorly to sacral region; white line above brown stripe absent or faint; dorsal pattern weak, usually consisting of irregular dashes or interconnected lines; interorbital dark mark present; shanks having weakly defined transverse bars.

Description and variation. —In the majority of specimens (70%) the lateral dark stripe extends from the nostril to the eye and thence above the tympanum to a point above the insertion of the arm; in 17 per cent the stripe extends to the mid-flank, whereas in 13 per cent the stripe is absent. A narrow and faint white line is present on the canthus in some specimens, but no distinct white stripe is present above the lateral dark line posterior to the eye. An interorbital bar and transverse marks on the shanks are invariably present. The dorsal markings are variable, but in most specimens (92%) consist of either an X- or )(-shaped mark in the scapular region; in the other specimens the markings are irregular short lines or absent. Approximately equal numbers of specimens have a transverse bar, chevron, or broken lines in the sacral region, whereas about eight per cent of the specimens lack markings in the sacral region.

When active at night, individuals are pale yellowish tan with faint brown dorsal markings. By day they are tan with more distinct brown markings ( Pl. 14 ). The thighs are pale yellow; the belly is white. The iris is pale creamy tan with brown flecks. In breeding males the vocal sac is yellow.

Tadpoles. —Tadpoles of this species have been found in an extensive grassy pond at Puerto Viejo, Costa Rica. The following description is based on KU 104099, a specimen in development stage 36 (Gosner, 1960).

Total length, 21.0 mm.; body length, 6.7 mm.; body slightly wider than deep, snout pointed; nostrils large, directed anteriorly, situated near end of snout; eyes small, situated dorsolaterally, directed laterally; spiracle sinistral, located just posteroventral to eye; anal tube dextral. Tail xiphicercal; caudal musculature moderately deep, extending far beyond posterior edge of fins; fins deepest at about midlength; dorsal fin extending onto body, slightly deeper than caudal musculature; ventral fin slightly shallower than musculature. Mouth small, terminal, lacking teeth and fringing papillae, but having finely serrate beaks. In preservative top of head olive-tan with brown flecks; dark stripe from snout through eye to posterior edge of body; belly white, flecked with brown anteriorly; tail creamy tan with grayish brown blotches. In life, dorsum of body reddish tan mottled with darker brown; lateral stripe dark brown; belly white, mottled with brown and black; caudal musculature heavily pigmented [Pg 537] with grayish tan; posterior tip of tail marked with dark gray; caudal fins heavily blotched with grayish tan; iris orange-tan peripherally, red centrally ( Pl. 15 ).

Fig. 3. Map showing locality records for Hyla phlebodes .

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 [Pg 539] 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.

Fig. 4. Map showing locality records for Hyla sartori .

Distribution. — Hyla sartori occurs in mesophytic forests to elevations of about 300 meters on the Pacific slopes of southern México from southwestern Jalisco to south-central Oaxaca ( Fig. 4 ). The lack of specimens from Colima and Michoacán 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.

[Pg 540] Specimens examined. —190, as follows: México : Guerrero : 5 km. E Acapulco, AMNH 54611-2; 23.2 km. N Acapulco, UIMNH 26404-7; Colonia Buenas Aires, 23 km. E Tecpán 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. Tecpán 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); Zacualpán, 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).

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 proötics 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 proötic. The nasal lacks a maxillary process, and the medial ramus of the pterygoid lacks a bony connection to the proötic.

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.

PLATE 13



Upper figure, Hyla microcephala microcephala (KU 64593);
middle figure, H. microcephala underwoodi (KU 64565);
lower figure, H. microcephala underwoodi (UMMZ 115247).
All approximately ×3.

PLATE 14



Upper figure, Hyla robertmertensi (UMMZ 115243);
middle figure, H. phlebodes (KU 64798);
lower figure, H. sartori (UMMZ 119225).
All approximately ×3.

PLATE 15




Tadpoles of Hyla microcephala group:
upper figure, H. m. microcephala (KU 104097);
lower figure, H. phlebodes (KU 104099).
Both ×4.

PLATE 16



Audiospectrograms and sections of mating calls of Hyla microcephala group:
(a) H. m. microcephala (KU Tape No. 19);
(b) H. robertmertensi (KU Tape No. 41);
(c) H. phlebodes (KU Tape No. 6);
(d) H. sartori (KU Tape No. 190).

[Pg 542]

Fig. 5. Dorsal views of the skulls of (a) Hyla m. microcephala (KU 68293) and (b) H. sartori (UMMZ S-2677). Both × 12.

[Pg 543]

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 [Pg 544] 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.

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 [Pg 545] 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 ).

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 [Pg 547] 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 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 ).

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 .

[Pg 548]

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.

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.

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 [Pg 549] 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 Panamá. 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 México 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 Panamá, 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 Panamá.

[Pg 550]

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, Panamá, 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 [Pg 551] 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.

[Pg 552]

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.

[Pg 553] 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 México 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 [Pg 554] 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 México and crossed the Isthmus of Tehuantepec onto the Pacific slopes of [Pg 555] 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 México, 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 Chiapas and Guatemala, which differentiated into robertmertensi . The original stock on the Pacific lowlands of Panamá 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 México 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.

[Pg 556]

[Pg 557]

Transmitted July 11, 1967.