Bioacoustics of Rhombophryne mangabensis

Call. The advertisement call recorded on 13 March 1991 at Nosy Mangabe (air temperature unknown) consists of a tonal note of moderate duration emitted in regular call series containing a variable number of calls (2–5) repeated at rather regular succession (Fig. 20 ). Each call exhibits some irregular amplitude modulation, with 2 or 3 peaks recognizable within each call, with the terminal peak having the highest energy. Slight upward frequency modulation is evident in each call until the last third of the call’s duration, followed by a terminal drop in frequency. Numerical parameters of 20 analysed calls are as follows: call duration (= note duration) 180–237 ms (198 ± 15 ms); inter-call intervals within regular call series 419–822 ms (484 ± 113 ms); duration of regular call series 820–2895 ms (1779 ± 925 ms); dominant frequency 3300–3550 Hz (3434 ± 85 Hz); prevalent bandwidth 2800–7500 Hz; weak frequency bands recognizable at approximately 5250 and 6760 Hz. Calls from the same recording ( Vences et al. 2006 , CD 3, track 31) have been described by Glaw et al. (2010 b ) using a slightly different terminological scheme.

Diagnosis. Within the genus Rhombophryne , R. mangabensis can easily be distinguished from all members of the R. serratopalpebrosa species group by the absence of superciliary spines (vs presence); from members of the R. minuta species group by less gracile body and much shorter limbs; from R. proportionalis by its much larger body size ( SVL 19.6–23.2 mm vs 11.0– 12.3 mm ); from members of the R. ellae species group by the absence of inguinal spots and reddish orange colouration on the hindlimbs (vs presence); from members of the R. laevipes species group by the absence of inguinal spots or ocelli (vs presence in most species), and smaller adult body size ( SVL 19.6–23.2 mm vs 24.1–57.2 mm ). Within the R. testudo species group, see accounts of R. testudo , R. coudreaui , and R. matavy above for distinction from those species. From its sister species, R. savaka , it can be distinguished by the less pronounced diastema in vomerine teeth, slightly longer relative tibia length ( TIBL / SVL 0.44–0.46 vs 0.37–0.42), absence of inguinal spots (vs presence), and by advertisement call with slightly longer call duration (180–237 ms vs 121–142 ms), generally longer inter-call interval (419–822 ms vs 302–451 ms), and lower dominant frequency (3300–3550 Hz vs 4144–4210 Hz). For distinction from newly described species, see accounts of those species, below.

Definition. Within the R. testudo group, this species is sister to R. savaka based on molecular phylogenetic data (Fig. 2 ; Belluardo et al. 2022 ). It is characterised by the possession of the following unique suite of characters: (1) small body size, adult SVL 19.6–23.2 mm ; (2) head wider than long ( HW / HL 1.60–1.86); (3) moderately small eyes ( ED / HL 0.34–0.38); (4) short hindlimbs ( HIL / SVL 1.54–1.60); (5) small inner metatarsal tubercle ( IMTL / FOL 0.09–0.12); (6) indistinct outer metatarsal tubercle; (7) second finger roughly equal in length to fourth; (8) third toe distinctly longer than fifth; (9) dorsal skin finely granular with a few scattered warts, especially a suprascapular pair outlined in black; (10) absence of chin barbels; (11) absence of pronounced superciliary spines; (12) absence of inguinal ocelli or spots; (13) absence of distinctive markings along the posterior thigh; (14) unfortified skull without expansion of maxilla or posterior process of angulosplenial; (15) clavicles reduced, thin, curving; and (16) advertisement call (based on 20 analysed calls by one individual) tonal, with (17) call duration 180–237 ms, (18) inter-call interval 419–822 ms, and (19) dominant frequency 3300–3550 Hz. Rhombophryne mangabensis is furthermore distinguished from all other Rhombophryne species analysed by the following robust diagnostic nucleotide combination in the mitochondrial 16 S rRNA gene (positions relative to the full 16 S sequence of Anilany helenae MZ 751042 ): in 16 S 5 ’, ‘ A’ at site 383, ‘ G’ at site 567 and ‘ C’ at site 641; in 16 S 3 ’, A’ at site 917, ‘ A’ at site 1088, ‘ G’ at site 1090, and ‘ A’ at site 1127.

Natural history. Rhombophryne mangabensis is a semi-fossorial or detriticolous species found in low-elevation humid forest. Specimens were collected at night during heavy rain, during which many calls were heard in an intense chorus. Males called from concealed positions in the leaf litter of ‘ apparently primary low-altitude rainforests’ ( Glaw et al. 2010 b : 240). Calling has not been directly observed, but the call described above certainly belongs to this species, as specimens caught while searching for the source of calling were inflated with air, typical for calling Rhombophryne . Active specimens were encountered on the forest floor during the day, in the evening, and at night. One individual was captured by pitfall traps set up in a valley with closed-canopy humid forest associated with thick leaf litter and many rotten logs.