Bioacoustics of Platypelis tsaratananaensis

Diagnosis and comparisons. Assigned to the genus Platypelis in the Cophylinae based on enlarged terminal discs on fingers and toes, absence of finger-like prepollex, absence of nuptial pads, occurrence in Madagascar , and molecular phylogenetic relationships. Distinguished from all other cophyline species with enlarged discs on fingers and toes by combination of the following character states: medium body size (adult SVL 22–33 mm ), most specimens with colored patches above the eyes, absence of reddish or yellowish ventral color, no sharp border between dorsal and lateral color, toe 3 distinctly shorter than toe 5, vomerine teeth present, males with prepollical tubercle but lacking a finger-like prepollex as typical for Anodonthyla . Distinguished from all other species of Platypelis and Cophyla by relative toe length (third toe distinctly shorter than fifth toe, vs. only slightly shorter, of equal length, or longer), and by advertisement call (consisting of double-note calls vs. single-note calls). Further distinguished from Platypelis alticola , P. grandis and P. tuberifera by usually smaller body size (adult SVL 22–33 mm but usually below 30 mm , vs. 30–105 mm ); from C. occultans , P. ravus , and P. tetra by larger size (SVL 22–33 mm vs. 16–21 mm ; from P. barbouri , P. grandis , P. mavomavo , and P. tetra by a completely smooth dorsum (vs. tuberculated), from P. barbouri , P. milloti , P. mavomavo , and P. ravus by always uniform greyish ventral side (vs. reddish or yellow color on venter and/or ventral side of hindlimbs). Furthermore distinguished from all other arboreal microhylids included in our analysis by a substantial genetic differentiation as indicated by mitochondrial DNA (>10% uncorrected pairwise divergence in the 16S fragment studied herein).

Redescription. Because of the poor state of preservation of the holotype , we here provide a redescription based on a newly collected adult male, ZSM 1649/2010 (ZCMV 12391). Specimen in good state of preservation, some muscle tissue removed from right thigh, snout–vent length 26.4 mm . Body very slender; head slightly longer than wide, very slightly wider than body; snout rounded in dorsal and lateral views; nostrils directed dorsolaterally, not protuberant, slightly nearer to tip of snout than to eye; canthus rostralis indistinct; loreal region plain; tympanum distinct, 54% of eye diameter; supratympanic fold very indistinct, virtually absent; tongue ovoid, not bifid or notched; maxillary teeth distinct; vomerine teeth distinctly recognizable as two round groups on a bony ark; choanae rounded. Forelimbs very slender; subarticular tubercles moderately distinct; outer metacarpal tubercle moderately distinct, relatively small and flat; inner metacarpal tubercle large, forming distinct protuberance at base of first finger; hand with only traces of webbing between fingers; fingers distinctly flattened and broad along entire length; relative length of fingers 1<2<4<3, fourth finger distinctly longer than second; finger discs distinctly enlarged, slightly triangular; nuptial pads absent. Hindlimbs slender; tibiotarsal articulation reaching between forelimb and tympanum when hindlimb adpressed along body; tibia length, 40% of SVL; lateral metatarsalia strongly connected; inner and outer metatarsal tubercles small, difficult to recognize; distinct webbing between toe 3 and 4 and less developed between toe 4 and 5; subarticular tubercles on toes moderately well recognizable; toes moderately broad along their entire length; relative length of toes 1<2<3<5<4, toe 5 distinctly longer than toe 3. Dorsal skin smooth, without dorsolateral folds. Ventral skin smooth on throat and chest and moderately granular on belly. After two years in 70% ethanol, dorsum, thighs and shanks grey with numerous small and well delimited blackish spots; head grey with much less dark spotting; blackish skin above eyes. Forelimbs and hindlimbs without crossbands. Ventrally, throat and limbs uniformly yellowish, belly uniformly greyish. Variation and distribution. Specimens of P. tsaratananaensis were characterized by a remarkable variability in color and pattern in life ( Fig. 4 ). Of 156 individuals, nine (5.6%) had a yellow to orange vertebral stripe. Most individuals had yellow to beige patches above the eyes which sometimes were large, continuous and contrasting, and sometimes were rather formed as an irregular network of small colored markings. Often a dark line was present between the eyes, and sometimes this black line formed the border of a large light patch on the anterior head. The flanks sometimes had distinct light spotting. The ventral side was always uniform greyish and somewhat translucent. Based on 156 specimens all measured at Matsabory Maiky in the field, males were slightly smaller than females: male SVL was 25.2 ± 1.8 mm ( 22.1–29.8 mm , N= 58) and female SVL was 25.9 ± 2.4 mm (21.0– 32.5 mm , N= 98) (Mann-Whitney U-test, Z=2.281, P = 0.023). All specimens had a smooth dorsal skin without tubercles. Habitat and natural history. Platypelis tsaratananaensis was almost exclusively found on bamboo, and occurred in high densities in bamboo forest with relatively large plants that we provisionally identified as belonging to the genera Arundinaria and Nastus . Bamboo shrubs were found commonly on the Tsaratanana but at high altitudes the plants were small and the segments probably unsuitable for Platypelis reproduction because of the small diameter of internodes which furthermore did not appear to contain water; P. tsaratananaensis was consequently very rare in these areas. Typically, the species was found in bamboo segments with a small hole, probably made by insects emerging from the bamboo stem after completing their larval development therein, and in some cases opened by the nocturnal lemur Daubentonia madagascariensis ( Rakotoarison et al. 2010 ) ; especially in living bamboo, these holes were water-filled ( Fig. 9 ). In various cases we observed embryos probably belonging to P. tsaratananaensis ( Fig. 8 ). Sometimes the embryos contained in the same bamboo segment were in distinctly different stages, suggesting that they belonged to separate clutches. The number of embryos found in one segment varied from 1–27 (mean 13 ± 8.1). Adult specimens were observed in bamboo segments of 30.2 ± 7.7 mm ( 30.2–48.9 mm ) diameter, located at 151 ± 80 cm ( 30–400 cm ) height above the ground. The average number of adult individuals found within one occupied bamboo segment was 2 ± 1 (1–6, N= 87) individuals. Gravid females were larger than non-gravid females (Mann-Whitney test, p = 0.001). We counted 16 ± 5 (1–24, N= 46) oocytes per female. The number of oocytes was correlated with female size (correlation coefficient, r = 0.625, p = 0.001). In those 13 holes where embryos or larvae were found, these were always accompanied with metamorphosed individuals: in the respective segments we found either two males (4 cases), one female and one male (2 cases), one female and 2 males (2 cases), three females and one male (2 cases), one female, one male and one juvenile (1 case), one female (1 case), or three females (1 case). Hence, in most cases the embryos were accompanied by at least one male, and only rarely by only one or several females. The characteristics of the bamboo segment occupied were related to the size of the frogs found therein ( Fig. 11 ). For this comparison, also some possibly subadult specimens ( 15–21 mm SVL) were included that morphologically could be identified as probably being of male or female sex (based on prepollex size). We found a positive linear correlation between height above the ground and SVL (correlation coefficient, r = 0.295, p = 0.026 for males and r = 0.248, p = 0.012 for females), i.e., larger specimens were found in higher bamboo segments. No correlation was observed between the diameter of the occupied bamboo segment and SVL (correlation coefficient, r = -0.247 with p = 0.64 for males and r = -0.122, p = 0.224 for females). Equally we found no significant correlation between the vertical diameter of the entrance hole to the bamboo segment and SVL of males (correlation coefficient, r = -0.136, p = 0.313), whereas this correlation was highly significant and negative in females (correlation coefficient, r = -0.311, p = 0.002), i.e., paradoxically small females were mostly found in segments with large entry holes. No correlation was observed between the horizontal diameter of the hole and SVL of males and females (correlation coefficient, r = -0.18, p = 0.179 for males and r = - 0.003, p = 0.979 for females). Many of the encountered specimens of P. tsaratananaensis were infected by mites probably of the genus Endotrombicula (e.g., similar to those visible in Fig. 3 a and 6b for P. alticola ). We recorded 0–54 parasites per individual in males (average 5.3) and 0–14 mites per individual in females (average 3.1). These differences were statistically mildly significant (P = 0.04) in a t-test but not in a non-parametric U-test. The number of mites was not correlated with the size of the specimens (P>0.4). Advertisement call. Calls of Platypelis tsaratananaensis were heard at Matsabory Maiky and Andrevorevo. They are unusual for cophylines in general because each call consists of two distinct notes which however are emitted without a clear silent interval between them ( Fig. 10 ). Calls were recorded from an adult male, ZSM 1649/ 2010 (ZCMV 12391) on 13 June 2010 in the early evening (precise time not recorded, but between 19:00 and 22:00 h) at Matsabory Maiky, at an air temperature of about 13°C. The specimen was not seen calling but clearly located on top of a small palm within a forest with numerous bamboo stands, about 3.5 m above the ground; upon collection it still emitted some faint vocalizations. Spectral call frequency ranged from about 2320–3240 Hz, with a dominant frequency between 3057–3186 Hz (3116±51; N= 8). A weak frequency band could be recognized at 1500 Hz, but it is uncertain whether this represents the fundamental frequency or rather an artefact of the recording. Temporal call parameters are as follows: note length of first note in note pairs 79–100 ms (92±9 ms; N= 4); note length of second note in note pairs 116–145 ms (128±13 ms; N= 4); interval between notes within a note pair 93–110 ms (100±8 ms, N= 4); interval between note pairs 4854–5218 ms (4983±203 ms; N= 3).