Chapter 3

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° 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 secondarynotes differ from the primary in other characteristics. Both subspecies ofHyla microcephalahave a long, unpaired primary note followed by 0 to 18 (usually about 4) somewhat shorter paired secondary notes. In calls ofHyla m. microcephalathe 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 inH. m. underwoodithe 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 robertmertensihas 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 phlebodesandsartorihave call-groups composed of a rather short, unpaired primary and several short, unpaired secondaries (0-28 inphlebodes, 0-23 insartori). The mean duration of the primary ofphlebodesis 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 ofsartoriis 0.080 (0.07-0.09) second and that of the secondaries is 0.053 (0.035-0.07) second.

The two subspecies ofH. microcephalaare identical in call pattern and agree closely in duration of notes, although those of the nominate subspecies tend to be slightly longer.Hyla robertmertensiis 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 ofH. phlebodesandH. sartoriare identical and the range of duration of notes ofphlebodescompletely overlaps that ofsartori, 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 ofH. microcephalaa highprimarynote, 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.).

SpeciesNDominant frequency (cps)Fundamental frequency (cps)Duration of notes (seconds)Repetition rate of secondaries (notes/minute)PrimarySecondaryH. m. microcephala4456372050.130.10268(5150-5962)(184-244)(0.11-0.16)(0.05-0.14)(192-353)H. m. underwoodi4757722200.110.09283(5177-6200)(192-275)(0.05-0.15)(0.06-0.11)(197-384)H. robertmertensi2553881620.090.04418(5150-5785)(140-178)(0.07-0.11)(0.03-0.06)(368-570)H. phlebodes3435781480.110.07284(3220-4067)(125-158)(0.07-0.16)(0.04-0.12)(210-350)H. sartori1032171260.080.05434(2950-3600)(116-135)(0.07-0.09)(0.04-0.07)(396-477)

The two subspecies ofH. microcephalaagree 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 ofH. robertmertensidoes not overlap thatofH. sartorior either subspecies ofH. microcephalain this parameter; but it does overlap that ofH. phlebodes, although again the difference between the means is significant at the 0.001 level (t = 9.360).Hyla phlebodesandsartorihave 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 dominantband of of frequenciescuts 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).

Variation in fundamental frequencyFig. 7.Variation in the fundamental frequency of the normal primary notes in theHyla microcephalagroup. 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 ofH. microcephalaagree 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 ofrobertmertensiin this parameter, but the difference between the means is significant at the 0.001 level. The calls ofH. phlebodesandH. sartorioverlap 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 ofH. microcephalaandrobertmertensi.

Variation in the mid-point frequencyFig. 8.Variation in the mid-point of the dominant frequency band of the normal primary notes in theHyla microcephalagroup. 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.ScattergramFig. 9.Scatter diagram relating the dominant and fundamental frequencies of the normal primary notes in theHyla microcephalagroup. 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 isthe 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 ofH. microcephalaand inphlebodes. The repetition rates inH. robertmertensiandH. sartoriare considerably faster than in the other three taxa.Hyla sartorihas the fastest repetition rate of the group.

In all characteristics of the mating calls the two subspecies ofH. microcephalaagree closely, as might be expected, although the differences are statistically significant.Hyla robertmertensiis distinctive in call pattern and seems to be closer tomicrocephalain dominant frequency but closer toH. phlebodesin fundamental frequency. Thus, it is somewhat intermediate betweenmicrocephalaandphlebodes. The identical pattern and similarity in fundamental and dominant frequencies of the calls ofH. phlebodesandH. sartoripossibly indicate close relationship.

Geographic variation in call.—Hyla m. microcephalahas higher fundamental and dominant frequencies in Costa Rica than in Panamá. In Costa RicanH. m. underwoodithe 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 PanamanianH. m. microcephalahave slightly longer notes than do anyH. m. underwoodi; the more northern populations ofH. m. underwoodifrom México most closely approachH. m. microcephalain this characteristic.

The calls ofH. robertmertensiin Oaxaca have higher dominant and fundamental frequencies and longer secondary notes than do those in Chiapas.

The calls ofH. phlebodesrecorded 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.

LIFE HISTORY

The frogs of theHyla microcephalagroup 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 ofH. m. microcephalain 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 900Hyla phlebodesin one pond, and two nights later noticed that the number of individualshadincreased substantially. Other observations by the first author on size of breeding congregations include nearly continuous choruses ofH. m. underwoodibetween Villahermosa and Teapa, Tabasco, in July of 1958, an estimated 400Hyla robertmertensiin a road side ditch 7.2 kilometers west-northwest of Zanatepec, Oaxaca, on July 13, 1956, and approximately 150Hyla sartoriaround 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 thatH. m. microcephalaformed 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 phlebodesinhabits 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 heardHyla phlebodesin 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 inHyla phlebodesin the Canal Zone, where he noticed a decrease in activity of that species in October and November, when the rains are heaviest andmost frequent. Furthermore, independent observations made by both of us indicate thatH. phlebodesdoes 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 doesH. 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 microcephalaandH. phlebodescall almost exclusively from grasses and sedges;phlebodesusually calls from taller and more dense grasses than doesmicrocephala. 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. BothH. robertmertensiandH. sartorihave been observed calling from grasses, herbs, bushes, and low trees. Calling males ofrobertmertensihave been found two meters above the ground in small trees.

Daytime retreats in the breeding season sometimes are no more than shadedclumpsof vegetation adjacent to a pond or in clumps of grass in a pond. Individuals ofH. m. underwoodiwere 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 inH. m. microcephala,m. underwoodi, andphlebodes. 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 onlyH. m. microcephalaandphlebodes; 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 ofH. microcephalahas a uniformly white venter and nearly transparent tail, whereas inH. phlebodesthe venter is flecked anteriorly and the tail is mottled. In life,H. microcephalais easily recognized by the orange posterior half of the tail, whereas the tail inH. phlebodesis 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 theHyla microcephalagroup 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 theHyla microcephalagroup. 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 theaccountofHyla m. microcephala, the speciesmicrocephalapossibly is subspecifically related toHyla misera, a frog widespread in the Amazon Basin.Hyla miseraresemblesmicrocephalain 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 livingH. 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 miserapossibly is allied to other small yellowish tan South AmericanHylathat lack dark pigmentation on the thighs. Probable relatives areHyla elongata,minuta(withgoughi,pallens,suturata,velata, and possibly others as synonyms),nana, andwerneri. 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 ofelongata,nana, andwernerireveals that they are likemiseraandmicrocephalain the nature of the frontoparietal fontanelle and in having a greatly reduced quadratojugal. Thus, on the basis of cranial and external characters theHyla microcephalagroup can be associated withHyla miseraand 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 theHyla microcephalagroup andH. ebraccatahave a haploid number of 15 chromosomes (Duellman and Cole, 1965). All other New WorldHyla, for which the number is known, have a haploid number of 12; the only otherHylahaving 15 is a PapuanHyla angiana(Duellman, 1967).

Hyla ebraccataoccurs 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 theHyla leucophyllatagroup, 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 theHyla microcephalagroup. The quadratojugal is present as a small anteriorly projecting spur that does not connect with the maxillary. Externally, theHyla leucophyllatagroup 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 AmericanHyla 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 theHyla leucophyllatagroup as a close ally to theHyla microcephalagroup. Successful artificial hybridization supports the close relationship ofH. m. microcephalaandphlebodes; partial success of artificial hybridization of these two withebraccata(Fouquette, 1960b) provides further evidence for close relationship between theHyla leucophyllataandHyla microcephalagroups.

In México and northern Central America two small species,Hyla pictaandHyla smithi, comprise theHyla pictagroup. These frogs resemble members of theHyla microcephalagroup 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 theHyla microcephalagroup. Despite these similarities, theHyla pictagroup differs from theHyla microcephalagroup 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 theHyla pictagroup more closely resemble other Middle AmericanHylathan they do theHyla microcephalagroup. Therefore, it can best be presumed that the superficial resemblances of coloration and the mating call are the result of convergence.

Since theHyla microcephalaandleucophyllatagroups apparently are related and since the greatest diversity of these frogs is in South America (ifHyla miseraand its relatives are placed with theHyla microcephalagroup), it seems appropriate to place the centers of origins of these groups in South America. Therefore, theHyla microcephalagroup andHyla ebraccataof theHyla leucophyllatagroup 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 theHyla microcephalagroup are not clear. On the basis of coloration,H. m. microcephalaandH. robertmertensiare close, andH. m. underwoodiandH. phlebodesare nearly identical. The mating calls ofH. phlebodesandsartoriclosely resemble one another, whereas the call ofrobertmertensiis intermediate between these andmicrocephala.

In most respectsHyla microcephalais 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 amicrocephala-like ancestor. Possibly the slightly increased ossification of the frontoparietals inrobertmertensi,phlebodes, andsartoriis secondary, or possibly after differentiation of the species the amount of ossification was further reduced inmicrocephala. If so, the species fall into a reasonable phylogenetic scheme that hasmicrocephalaas 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 amicrocephalaancestral stock that differentiated into two populations in lower Central America—amicrocephala-like frog on the Pacific lowlands and aphlebodes-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 ofOaxaca 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 thephlebodes-stock on the Pacific slopes of México, where it adapted to drier conditions and evolved intosartori. The rest of thephlebodes-stock was restricted to the humid forests on the Caribbean lowlands of lower Central America. The increased aridity on the Pacific lowlands eliminated themicrocephala-stock from southern Honduras and northwestern Nicaragua and in so doing left an isolated population on the lowlands of Chiapas and Guatemala, which differentiated intorobertmertensi. The original stock on the Pacific lowlands of Panamá and southeastern Costa Rica becamemicrocephala.

If themicrocephala-stock was, as we believe, better adapted for existence under subhumid conditions than was thephlebodes-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 ofmicrocephalainto the area now inhabited byH. m. underwoodi, whilephlebodeswas being eliminated from this area by climatic conditions that were unsuited to its survival there. Perhaps the similarity in coloration ofH. m. underwoodiandphlebodesis 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 microcephalaandphlebodesrange 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

Boulenger, G. A.

1898. Fourth report on additions to the batrachian collection in the Natural-History Museum. Proc. Zool. Soc. London, 1898, pp. 373-482, pls. 38-39. October 1.

1899. Descriptions of new batrachians in the collection of the British Museum (Natural History). Ann. Mag. Nat. Hist, ser. 7, 3:273-277, pls. 11-12.

Breder, C. M. Jr.

1946. Amphibians and reptiles of the Rio Chucunaque Drainage, Darien, Panama, with notes on their life histories and habits. Bull. Amer. Mus. Nat. Hist, 86:375-436, pls. 42-60, August 26.

Cole, L. J. and Barbour, T.

1906. Vertebrata from Yucatan: Reptilia; Amphibia; Pisces. Bull. Mus. Comp. Zool., 50:146-159. November.

Cope, E. D.

1886. Thirteenth contribution to the herpetology of tropical America. Proc. Amer. Philos. Soc, 23:271-287. February 11.

1894. Third addition to a knowledge of the Batrachia and Reptilia of Costa Rica. Proc. Acad. Nat. Sci. Philadelphia, 1894, pp. 194-206.

Duellman, W. E.

1956. The frogs of the hylid genusPhrynohyasFitzinger, 1843. Misc. Publ. Mus. Zool., Univ. Michigan, 96:1-47, pls. 1-6. February 21.

1967. Additional studies of chromosomes of anuran amphibians. Syst. Zool., 16:38-43, March 17.

Duellman, W. E. and Cole, C. J.

1965. Studies of chromosomes of some anuran amphibians (Hylidae and Centrolenidae). Syst. Zool., 14:139-143. July 9.

Duellman, W. E. and Trueb, L.

1966. Neotropical hylid frogs, genus Smilisca. Univ. Kansas Publ., Mus. Nat. Hist., 17:281-375, pls. 1-12. July 14.

Dunn, E. R.

1931. The amphibians of Barro Colorado Island. Occas. Papers Boston Soc. Nat. Hist., 5:403-421. October 10.

1933. Amphibians and reptiles from El Valle de Anton, Panamá.Ibid., 8:65-79. June 7.

1934. Two new frogs from Darien. Amer. Mus. Novit., 747:1-2. September 17.

Fouquette, M. J. Jr.

1960a. Call structure in frogs of the family Leptodactylidae. Texas Jour. Sci., 12:201-215. October.

1960b. Isolating mechanisms in three sympatric tree frogs in the Canal Zone. Evolution, 14:484-497. December 16.

Gaige, H. T., Hartweg, N. and Stuart, L. C.

1937. Notes on a collection of amphibians and reptiles from eastern Nicaragua. Occas. Papers Mus. Zool., Univ. Michigan, 357:1-18. October 26.

Gosner, K. L.

1960. A simplified table for staging anuran embryos and larvae with notes on identification. Herpetologica, 16:183-190. September 23.

Kellogg, R.

1932. Mexican tailless amphibians in the United States National Museum. Bull. U.S. Natl. Mus., 160:1-224. March 31.

Rivero, J. A.

1961. Salientia of Venezuela. Bull. Mus. Comp. Zool., 126:1-207. November.

Smith, H. M.

1951. The identity ofHyla underwoodiAuctorum of Mexico. Herpetologica, 7:184-190. December 31.

Stejneger, L.

1906. A new tree toad from Costa Rica. Proc. U. S. Natl. Mus., 30:817-818. June 4.

Stuart, L. C.

1935. A contribution to a knowledge of the herpetology of a portion of the savanna region of central Petén, Guatemala. Misc. Publ. Mus. Zool., Univ.Michigan, 29:1-56, pls. 1-4. October 4.

Taylor, E. H.

1952. The frogs and toads of Costa Rica. Univ. Kansas Sci. Bull., 35-577-942. July 1.

Transmitted July 11, 1967.

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