Duration.—The average duration of call-groups consisting of two or more notes is 1.18 seconds in S. baudini; 1.02 in cyanosticta, 0.91 in phaeota, 1.32 in puma, 1.48 in sila, and 1.29 in sordida. Although there is considerable variation in the lengths of the notes (only primary notes in S. puma and sila are considered here), S. cyanosticta, phaeota, and sordida have noticeably longer notes than do the other species (Table 8). The secondary notes are longer than the primary notes in S. puma (average 0.27 secs. as compared with 0.13 secs.) and in S. sila (average 0.25 secs., as compared with 0.16 secs.).
Note repetition rate.—The rate at which notes in call-groups containing two or more notes are produced varies in S. baudini from 2.5 to 7.1 (average, 3.7) calls per second; cyanosticta, 1.8-2.1 (1.9); phaeota, 2.0-2.4 (2.2); puma, 1.9-2.9 (2.2); sila, 1.3-2.4 (1.8); and sordida, 1.5-2.6 (2.1). Smilisca baudini, which has notes of short duration (0.09 to 0.13 seconds), has the fastest note-repetition rate. Although the individual notes of S. cyanosticta and S. phaeota are relatively long (average, 0.38 and 0.31 seconds, respectively), the intervals between the notes is short; consequently, their note-repetition rates do not differ greatly from those of S. puma and S. sila, which have shorter notes (average, 0.13 and 0.16 seconds, respectively) but longer intervals between notes.
Pulse rate.—Pulses vary in frequency from 78 to 240 per second in the calls analyzed (only primary notes in S. puma and S. sila), but the variation in any given species is much less than that in the entire genus (Table 8). Smilisca puma is outstanding in having a high pulse rate, which is approached only by that of S. baudini. Even in the species having the lowest pulse rates, the pulsations are not audible. The secondary notes produced by S. puma and S. sila have a slower pulse rate than the primary notes; often the pulses are audible. In S. puma the pulse rate of secondary notes is sometimes as low as 48 pulses per second, and in S. sila still lower (as low as 40 pulses per second). The upper limits of pulse rate in the secondary notes in these species merge imperceptibly with the rates of the primary note; consequently, on the basis of pulse rate alone it is not always possible to distinguish primary from secondary notes.
Frequency.—Smilisca produces noisy (as opposed to more musical) calls, and the energy is distributed throughout the frequency spectrum; the calls are poorly modulated, except in S. sordida, in which two usually discrete bands of frequency are present (Pl. 11C). For the most part the calls of Smilisca consist of little modified energy of the fundamental frequency and of its harmonics, some of which are emphasized.
The upper frequency range varies within each species and even within the calls of one individual. Smilisca phaeota has the lowest upper frequencies; no calls ranged above 4400 cycles per second (cps.), and half of the calls never exceeded 3000 cps. Smilisca cyanosticta produces calls in which the upper frequency is below 7000 cps. and usually below 6000 cps. Likewise, S. puma produces calls that are below 7000 cps., whereas S. sila has frequencies of up to 8400 cps. In both S. baudini and S. sordida, the highest frequencies attained are about 9100 cps. Variation in the highest frequencies in a series of consecutive calls by one individual frog was noted in all species. Such variation is especially prevalent in S. puma; for example one individual (KU 87771; Tape No. 376) recorded at a temperature of 24° C. at 7.5 kilometers west of Puerto Viejo, Heredia Province, Costa Rica, on July 31, 1964, produced three consecutive primary notes having upper frequencies of about 6000, 4000, and 4000 cps., respectively. Apparently in a given species the production of the higher frequencies in some notes and not in others is correlated with the amount of distention of the vocal sac and is not dependent upon the structure or tension of the vocal cords.
Although the dominant frequency in S. sordida is lower than that in S. baudini and S. cyanosticta, the call of the former is audibly higher-pitched. This is due primarily to the emphasis on certain harmonics at a high frequency (sometimes as high as 9000 cps.) in S. sordida, whereas in S. baudini and other species, if harmonics are present at those frequencies, they are not emphasized.
The fundamental frequencies are as low as 90 cps. in S. sila and S. sordida and as high as 200 cps. in S. puma (Table 8). The fundamental frequency seemingly is relatively unimportant in determining the general pitch of the call, a characteristic most dependent on the dominant frequency and emphasized harmonics in the higher-frequency spectrum. In none of the species is the fundamental the dominant frequency. In the low-pitched call of S. phaeota the dominant frequency is the third harmonic (the second harmonic above the fundamental frequency, which is the first harmonic). In all other species a much higher harmonic is dominant; for examples, in S. cyanosticta harmonics from 10 to 15 are dominant; in S. baudini, 15-19; and S. sila, 20-30.
A glance at the audiospectrographs and their accompanying sections (Pls. 10 and 11) reveals the presence of two emphasized bands of frequency in all species except S. phaeota, in which only the lower band is present. These two bands of emphasized harmonics are part of a continuous, or nearly continuous, spread of energy throughout the frequency spectrum, except in S. sordida in which the bands are usually distinct. As shown in the sections, certain harmonics in each of the bands are emphasized with nearly equal intensity. Therefore, with the exception of S. phaeota, the calls of Smilisca are characterized by two major frequencies, one of which is the dominant frequency and the other is a subdominant frequency (Table 8). The upper major frequency is dominant in all calls in S. baudini and S. cyanosticta, but either major frequency may be dominant in other species. The upper major frequency is dominant in 65 per cent of calls by S. puma, 87 per cent in S. sila, and 68 per cent in S. sordida. Individuals of these three species sometimes produce a series of calls in which the dominant frequency changes from one of the major frequencies to the other. Four consecutive notes emitted by an individual of S. sordida recorded 13 kilometers east-northeast of Golfito, Puntarenas Province, Costa Rica, had dominant frequencies of 910, 1950, and 750 cps., respectively. In each case, an alternation of major frequencies took place in respect to dominance. An individual of S. puma from 7.5 kilometers west of Puerto Viejo, Costa Rica, produced a primary note followed by one secondary note; each note had major frequencies at 600 and 1800 cps.; the dominant frequency of the primary note was at 1800 cps., whereas in the secondary note the dominant frequency was at 600 cps. The difference in emphasis on the major frequencies is so slight that shift in dominance is not audible.
Effect of temperature on calls.—The present data are insufficient to test statistically the correlation between temperature and variation within certain components of the calls in Smilisca, but even a crude graph shows some general correlations. The widest range of temperatures is associated with the recordings of S. baudini. Three individuals recorded at a temperature of 30° C. at Tehuantepec, Oaxaca, had pulse rates of 180 pulses per second and fundamental frequencies of 160-180 cps., as compared with an individual recorded at a temperature of 17° C., which had a pulse rate of 140 and a fundamental frequency of 135 cps. All individuals of S. baudini recorded at higher temperatures had faster pulse rates and higher fundamental frequencies. Pulse rates differ in the other species in the genus but less strikingly (probably owing to narrower ranges of temperatures at which recordings were made). In five recordings of S. sordida made at 20° C. the pulse rate is 80-90, as compared with four recordings made at 25° C. having pulse rates of 120-135. Thirteen recordings of S. sila made at 17° C. have pulse rates of 97-112 (average 105); one individual recorded at 26° C. has 120 pulses per second. Seemingly no correlation exists between temperature and other characteristics of the calls, such as duration and rate of note-repetition.
The breeding call as an isolating mechanism.—Blair (1958), Bogert (1960), Duellman (1963a), Fouquette (1960), Johnson (1959), and others have provided evidence that the breeding calls of male hylids (and other anurans) serve as isolating mechanisms in sympatric species. In summarizing this discussion of the breeding calls of Smilisca we want to point out what seem to be important differences in the calls that may prevent interspecific hybridization in sympatric species of Smilisca.