Platymantis desticans, Brown, Rafe M. & Richards, Stephen J., 2008

Brown, Rafe M. & Richards, Stephen J., 2008, Two new frogs of the genus Platymantis (Anura: Ceratobatrachidae) from the Isabel Island group, Solomon Islands, Zootaxa 1888, pp. 47-68 : 49-56

Platymantis desticans sp. nov.

Figs. 2 , 3 A, 4A

Holotype. SAMA R56849, adult male, collected by S. Richards at Barora Faa Island, (off the western tip of Isabel Isl.), 0–10 m above sea level (07° 32.065'S, 158° 21.005'E), Isabel Province, Solomon Islands, 10 May 2000.

Paratypes. SAMA R56850–52, three adult males, same data as holotype except SAMA R56852 collected on 11 May 2000.

Referred Specimens. Solomon Island, Choiseul Province, Choiseul Island, Pavora River: two specimens deposited in the Australian Museum corresponding to ABTC 50323 and 50385 tissue vouchers.

Etymology. The specific epithet is an adjective derived from the Latin verb destico meaning to squeak, and is used in reference to this species’ distinctive advertisement call.

Diagnosis. Platymantis desticans is distinguished from congeners by a combination of (1) body size (36.9–38.0 mm for four adult males), (2) narrowly expanded terminal disks of the fingers and toes ( Fig. 2 ), (3) skin of dorsum shagreened with fine dermal asperities and scattered irregular tubercular ridges, (4) subarticular tubercles of fingers, toes, hands and feet very prominent and raised, (5) plantar surface of pes smooth between subarticular tubercles, (6) digits lacking all vestiges of lateral flange and interdigital webbing, and (7) unique advertisement call.

The new species is morphologically most similar to P. w e b e r i but differs from this species by differences in body size and proportions (most notably hind limb length, reflected in TBL/SVL ratio, Table 1 ), the presence (vs. absence) of dermal asperities on the dorsum, and the presence of short, irregular tubercular ridges (vs. highly prominent, elongate, longitudinally oriented and medially bowed dermal ridges and folds), by a smooth (vs granular) plantar surface of the pes between subarticular tubercles ( Fig. 2 ), by dark brown (vs. light cream) ventral coloration, and by its highly distinctive advertisement call ( Fig. 5 ).

The new species is also phenotypically similar to P. myersi (from Bougainville Island) but differs from that species by the presence of dermal asperities on the dorsum (vs asperities absent, dorsum smooth or weakly tuberculate posteriorly), a short, rounded (vs long, pointed) snout, the complete absence of interdigital webbing (vs minute web present between toes in P. myersi ), thighs smooth (vs strongly granular), plantar surface of pes smooth between subarticular tubercles (vs strongly granular) and an apparently uniform color pattern (vs color pattern variable; Brown, 1949, 1952).

Description of holotype. A mature male, in excellent condition; habitus slender; head slightly distinct, only slightly wider in dorsal aspect than body, length 45% of SVL; head length 114% of head width; snout short, its tip bluntly rounded, protruding slightly beyond lower jaw, rounded in lateral aspect; eyes protrude laterally beyond silhouette of head in dorsal aspect, and moderately beyond dorsal surface of head in lateral aspect; labial region rounded and smooth, only slightly flared, lips not swollen, not extending past eyes when viewed in dorsal aspect; interorbital region flat; eye diameter 162% of interorbital distance; pupil horizontally ovoid; canthus rostralis barely medially bowed; loreal region slightly concave; eye diameter 62.9% of snout length; nostrils not laterally protuberant; eye-narial distance 6 times the distance from nostril to tip of snout; internarial region flat; tympanum distinct, its diameter 72% of eye diameter; dorsal edge of tympanic annulus partially concealed, overlapped by supratympanic fold; the latter extending from dorsoposterior corner of eye, along dorsal edge of tympanum, and terminating nearly at supraaxillary region; cluster of five strongly conical post-rictal tubercles present between posteroventral edge of tympanum and forearm insertion; tongue ovoid, with deep posterior notch and narrow anterior attachment; choanae round, minute, at anterolateral edge of palate, separated by a distance five to eight times greater than their diameter, nearly obscured by palatal shelf; dentigerous process of vomer elongate; vomerine teeth tiny, translucent, numbering six to eight; dentigerous process very slightly angled anterolaterally, with closest (posterior) points separated by a distance two times the diameter of one choana, their most distant (anterior) ends separated by a distance equal to five or six times diameter of choanae; openings to vocal sac moderate slits, at the level of the angle of the jaw.

Skin of dorsal surfaces of body, head, and limbs generally roughened by dense congregation of dermal asperities ( Figs. 3–4 ; giving the skin the texture of fine sandpaper), and with short irregular tubercular ridges scattered at various angles across dorsum; suprascapular dermal folds absent; dorsal surfaces of limbs textured as body but tubercular ridges absent; ventral surfaces of trunk, head, throat, and limbs smooth; manus length 53.9% of foot length; digits of manus narrow ( Fig 2 A) and round in cross-section; lateral edges of digits lack fleshy dermal flanges; terminal disks slightly expanded and protuberant, with short circum-marginal folds on distal tips of digits; minute supra-articular flaps present above penultimate-ultimate phalangeal articulation; decreasing digital length III, I, IV, II; subarticular tubercles extremely prominent and protuberant, rounded on ventral surfaces; one subarticular tubercle on digits I–II, two tubercles under digits III–IV; supernumerary tubercles very prominent and rounded, present at the base of all digits; surface of palmar surfaces basal to supernumary tubercles entirely smooth; thenar (inner metacarpal), medial palmar and outer metacarpal tubercles prominent, highly convex; thenal tubercle greatly enlarged and protuberant, subtriangular and nearly twice the size of (and separated completely from) medial and outer metacarpals; medial palmar ovoid and larger than elongate, outer metacarpal slender; medial and outer metacarpal tubercles not separated; nuptial pads absent, forearm musculature not hypertrophied.

Hindlimbs short; tibia length 48.9% of snout-vent length, foot length 96.6% of tibia length; tarsus smooth, lacking folds, flaps, or tubercles; terminal disks of toes moderately expanded, protuberant, and ovoid; lateral dermal flanges absent along distal phalanges; circum-marginal grooves limited to distal ends of digits; supraarticular cutaneous folds slight; plantar surfaces of pes smooth ( Fig. 2 B), with prominently protuberant subarticular tubercles but lacking supernumerary tubercles or texture on plantar surfaces; subarticular tubercles numbering three under Toe IV, two under Toes III and V, and one each under Toes I and II; decreasing digit length IV, III, V, II, I; outer metatarsal tubercle prominent and circular, pointed; inner metatarsal tubercle moderate, low, elongate, with a slightly sharp venterolateral edge; digits of pes completely free of web; supracloacal tubercles and dermal flaps absent.

Species P. parilis P. guppyi P. neckeri

(4 m) (18 m) (12 m)

SVL (mm) 39.5–41.4 44.5–63.4 38.6–47.4 Range Barora Faa, Isabel, Choiseul Throughout the Solomons Bougainville Measurements of holotype. SVL 38.0; ED 5.0; TD 3.6; HL 17.1; SNL 7.9; IOD 3.1; HW 15.0; FL 18.3; TBL 18.6; TSL 9.9; PL 18.0; ML 9.7; FA 8.2; Toe4L 13.8; Fin1L 5.7; Fin3L 7.2; Fin3DW 0.9; Fin3PPW 0.5; Toe4DW 0.9; Toe4PPW 0.6.

Color of holotype in preservative. In preservative, the holotype exhibits a nearly homogeneous dark brown dorsal coloration, with fainter medium brown blotches irregularly scattered across the trunk and a prominent light parietal spot on the back of the head posterior to the orbits; dark dorsal coloration blends laterally into light and dark brown marbled flanks with irregular and indistinct tan blotches; dorsal and lateral surfaces of snout colored as body, tympanum slightly lighter, and distinct cream labial bars present; post-rictal region heterogeneous, with distinct cream-tipped post-rictal tubercles; dorsal surfaces of limbs lighter brown than body, with darker (similar to trunk coloration) transverse bars (three on forearm, three on humerus, three on tibia, two on tarsus); dorsal surfaces of hands, feet, and digits similarly colored brown with dark spots and white bars on phalangeal articulations; venter pale medium brown with distinct dark brown mottling; underside of throat heavily suffused with brown with dark brown labial bars wrapping on to ventral surfaces of lower lips; ventral surfaces of hands dark brown with prominent white subarticular tubercles; ventral surfaces of limbs very dark brown with only faint vestiges of lighter coloration; ventral surfaces of hands and feet solid dark brown with light gray, velvety digital disks.

Va r i a t i o n. There is very little color variation evident in the type series. One specimen (SAMA R56851) is noticeably lighter in dorsal coloration, has a lighter venter, and pale brown spots on the throat. The remaining paratypes agree almost perfectly with the holotype in every respect.

Color in life. Based on color images of holotype in life, before preservation ( Fig. 3 ). Dorsal surfaces homogeneous dark chocolate brown with distinct white parietal spot. Labial bars alternated black and white, in contrast to the brown coloration on lateral surfaces of head and snout. The tympanum coloration was bluegray and black and lower portions of the flanks were blue-gray, wrapping on to the venter. The iris was gold above and blue below the pupil and post-rictal tubercles were distinctly white-tipped. The dorsal surfaces of the digits were brown with black transverse bars and white on spots on digital articulations. The throat and venter were dark chocolate brown, fading to tan by the groin. Ventral surfaces of limbs were dark reddish brown.

Advertisement call. The advertisement call of P. desticans is markedly distinct from that of any known species of Platymantis in both temporal and spectral characteristics ( Fig. 6 ). The following description is based on two vouchered recordings from SAMA R56849 (holotype) and 56850 (paratype).

The call (defined throughout as vocalizations produced during a single expiration) is a high-frequency tonal note (sounding to the human ear like a shrill “squeak”), and is repeated in a slow train, characterized with variable intercall interval at the beginning (or “warm-up” period) of a calling bout. As the animal begins to call steadily, calling repetition rate structure becomes steadily discernable. During peak calling activity, the call is delivered in groups of paired calls, separated from one another by a nearly invariant intercall interval and with each pair of calls separated by a lengthy period of silence.

Individual call group repetition rate (i.e., rate of delivery of paired call groups) was 0.07 calls/s for the holotype (SAMA R56849) and 0.08 calls/s for the paratype (SAMA R56850). Call repetition rate (not taking into account paired call group structure; = [total number of calls – 1]/time from beginning of first call to beginning of last) was 0.13 SAMA R56849 and 0.16 for SAMA R56850.

For paired calls, mean intercall interval was 585.1–1242 ms (= 975.8 ± 27 SD; n=4) for SAMA R56849 and 627.3–1411 ms (= 945.3 ± 41 SD; n=6) for SAMA R56850. Duration of silence between call groups (i.e., long interval between paired calls) was 9.8– 18.6 s (= 13.0 ± 3.1 SD; n=4) for SAMA R56849 and 21–32 s (= 24.7 ± 6.5 SD; n=6) for SAMA R56850.

Call structure of P. d e s t i c a n s is complex and invariant. The shrill, high frequency single-note squeak call of P. desticans has an elaborate internal temporal and spectral structure ( Fig 6 B, C). Each note contains two distinct components: an amplitude modulated and frequency modulated first component and then a second, frequency modulated, constant amplitude component. There are pronounced differences in call energy contained in the different components although peak amplitude of both components is nearly equivalent ( Fig. 5 B). Although the audiospectrogram reveals cryptic stucture within the call of P. desticans , the impression on the human ear is a single, slightly wavering, shrill squeak.

The first intracall component of P. desticans is a brief frequency arc. The dominant frequency for the arc component began at 2.1–2.3 kHz, climbed to peak at 3.2–3.4 kHz, and then declined to 2.1–2.3 kHz in SAMA R56849. In SAMA R56850, the frequency arc peaked slightly higher at 3.4–3.5 kHz. The shape of this arc was nearly symmetrical in SAMA R56849 ( Fig. 6 B) but contained a slight ascending prefix tail (not shown) in SAMA R56850. The arc component varied in duration by 26.0–29.1 ms (= 27.6 ± 2.1 SD; n=5) in SAMA R56849 and 29.6–32.8 ms (= 30.9 ± 2.7 SD; n=8) in SAMA R56850. In the arc component of all calls, the vast majority of calling energy is packaged into the dominant frequency, described above. However in some calls, a lower frequency (fundamental) frequency component is discernable in addition to one or two higher frequency harmonics (multiples of the fundamental).

In our recording of SAMA R56849, the fundamental frequency begins at 1.08 kHz, climbs to 1.8 kHz, then declines again to 1.1 kHz. Above the dominant frequency are two harmonics that peak at 4.9 and 6.7 kHz, respectively. In the recording of SAMA R56850, the fundamental frequency arc climbs from 1.0 to 1.9, before declining again to 1.1 kHz.

In all available recording segments, the arc prefix component of the call of new species exhibits an inverse relationship between call frequency and call amplitude ( Fig. 6 B). Call energy is most intense when frequency of the arc component is at its lowest, this amplitude falls as frequency increases, the n climbs again as frequency declines

The second component of the P. desticans call is a gradual frequency sweep. In SAMA R56849, frequency of the second component declines from 2.9 to 2.7 kHz and in SAMA R56850 the frequency sweep component declined from 2.9 to 2.7 kHz. The second component varied in length from 66.7 to 80.3 ms (= 75.4 ± 9.3 SD; n=15) in calls of SAMA R56849 and from 71.2 to 84.3 ms (= 77.1 ± 11.1 SD; n=10) in SAMA R56850.

Ecology and Natural History. Platymantis desticans occurs either at extremely low densities or is naturally very inconspicuous, as no additional specimens were seen or heard despite intensive searches during the survey. Males were detected predominantly by their soft squeaky calls, uttered from exposed positions on the forest floor. Two males were calling from muddy banks near streams and one from the base of a fallen log. The call of this species was extremely difficult to hear at distances greater than about 10 m. In contrast Platymantis weberi occurred in the same habitats at extremely high densities and their calls dominated the acoustic environment at this site. Other species occurring in sympatry with the new species were: Ceratobatrachus guentheri , Discodeles guppyi , Palmatorappia solomonis , Platymantis aculeodactylus , P. guppyi , P. neckeri , P. solomonis and P. w e b e r i.

Distribution. Confirmed to date at two localities: Barora Faa Island, Isabel Province and Pavora River, Choiseul Island, Choiseul Province.

Figures

FIGURE 2. Ventral views of left hand (A) and right foot (B) of P. desticans (SAMA R 61908).

FIGURE 3. Photographs of the holotype of P. d e s t i c a n s (A: SAMA R 56849) and a paratype of P. parilis (B: SAMA R 56908) in life.

FIGURE 5. Ventral views of right hands (A) and left foot (B) of P. parilis (SAMA R 56911).

FIGURE 6. The advertisement call of P. desticans (holotype SAMA R 56849) recorded at ambient temperature of 24.7 ° C Barora Faa Island, by SJR. A power spectrum (A: Fast Fourier Transformation; relative amplitude vs. frequency in kHz), a 200 ms audiospectrogram (B: frequency in kHz vs. time in ms), and an expanded waveform (C: relative amplitude vs. time in ms) of a single call. A 2.0 s audiospectrogram (D: frequency in kHz vs. time in ms) and oscillogram (E: relative amplitude vs. time in ms) of a typical pair of calls from the 35 s recording segment waveform depicted in F (showing paired call groups during peak calling activity).

Tables

TABLE 1. Summary of the distribution of selected qualitative (+ present, – absent) and quantitative diagnostic characters and ranges of morphometric ratios in male specimens of Platymantis parilis, P. desticans, and the phenotypically most similar species from the Solomon Islands (number of specimens measured included in parentheses).

Range	Isabel	Throughout the Solomons and Bougainville	Throughout the Solomons and Bougainville
Fine dermal asperities on dorsum	–	–	–
Dorsolateral folds/ridges Terminal digital disks	– wide	– wide	+, – wide
Plantar surface of foot	smooth	smooth	smooth
Advertisement call	rapidly pulsed, constant note structure	slow, repetitive knocking	rapidly pulsed, alternating between tonal and non-tonal syllables
HL/SVL TBL/SVL	0.38–0.42 0.48–0.58	0.38–0.47 0.47–0.54	0.37–0.41 0.47–0.56
HL/HW	1.0–1.1	0.91–1.10	0.92–1.15
PpFin3/Fin3DW	0.43–0.58	0.38–0.48	0.31–0.50
TABLE 1 (continued).
Species	P. desticans (4 m)	P. weberi (16 m)	P. myersi (3 m)
SVL (mm)	35.8–38.0	36.7–44.6	47.2–56.1