Bioacoustics of Rhombophryne testudo

Definition. Type species of the genus Rhombophryne . Within the R. testudo group, this species is sister to R. matavy (Fig. 2 ; Belluardo et al. 2022 ). It is characterised by having the following unique suite of characters: (1) medium to large body size, adult SVL 33.0– 43.1 mm ; (2) head much wider than long ( HW / HL 2.00–2.60); (3) small eyes ( ED / HL 0.23–0.39); (4) short hindlimbs ( HIL / SVL 1.18–1.41); (5) large inner metatarsal tubercle ( IMTL / FOL 0.15–0.24); (6) generally pronounced outer metatarsal tubercle; (7) second finger slightly shorter than fourth; (8) third toe distinctly longer than fifth; (9) dorsal skin rough with converging paravertebral ridges; (10) presence 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) highly robust skull with very broad squamosal, maxilla, and posterior process of angulosplenial; (15) clavicles absent; and (16) advertisement call (based on 4 analysed calls by one individual) highly pulsed (162–177 pulses per call), with (17) call duration 823–894 ms, (18) inter-call interval 6071–10186 ms, and (19) dominant frequency 508–524 Hz. Rhombophryne testudo 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 ’, ‘ T’ at site 456, ‘ T’ at site 651 and ‘ C’ at site 734; in 16 S 3 ’, ‘ C’ at site 1099, ‘ C’ at site 1179, and ‘ G’ at site 1216.

Natural history. Rhombophryne testudo is a fossorial species found in primary and secondary low-elevation humid forest, where it inhabits burrows 5–10 cm deep beneath the leaf litter ( Glaw and Vences 1994 ). Calls are emitted from January to April, at highest intensity before and during heavy rain, irrespective of the time of day ( Glaw and Vences 1994 ). Calling can occur without rain, but a series of two or three calls is then followed by a long pause of several minutes ( Glaw and Vences 1994 ). Köhler et al. (1997 ) reported finding a 10 cm wide, 15 cm deep nest beneath a rock beside a small creek, containing two adults and 18 juveniles measuring 9–11 mm in SVL , in December 1995 . This suggests development occurs in subterranean burrows with parental care, perhaps by both parents ( Köhler et al. 1997 ). No tadpoles or eggs of the species have yet been observed. A defensive behaviour is known wherein specimens stretch their hindlimbs to prevent them being turned onto their backs ( Glaw and Vences 1994 ). We have also observed individuals puff themselves up significantly when caught, presumably to make them more difficult to swallow — common behaviours in burrowing microhylid frogs. Keefe and Blackburn (2020 ) compared the humerus of Rhombophryne testudo to other burrowing frogs. They noted its similarity to forward-burrowers, suggesting that it may also be able to burrow forwards, but highlighting the need for behavioural observations of this species. The gut of the holotype contains a great deal of soil, as evident from our micro-CT scans (Fig. 11 ). We examined the contents of the digestive system of an adult female specimen (with well-developed eggs), ZMB 51041 . The stomach contained mostly detritus, especially tiny fragments of wood and leaves. Fragments of two insects were present in this detritus: the 4.7 mm abdomen of a small rove beetle ( Staphylinidae ), and the thorax and partial abdomen of an ant. The intestines contained mostly a fine mud with tiny fragments of wood and leaf, the head of an ant, and the thorax and head of the same rove beetle. By volume, the vast majority of gut contents was non-animal material. This suggests that most of the ingested matter of these frogs might be substrate, not insects. Although unlikely, more ecological research is needed to understand if these animals may be partly detritivorous or geophagous. However, it is more likely that insect larvae, annelid worms, or other soft-bodied organisms are the primary prey of these frogs, and are digested quickly and thus absent from the digestive system of preserved frogs.

Call. The advertisement call recorded in January or February 1992 at Nosy Be (air temperature unknown) consists of a long, pulsed note emitted at irregular intervals (Fig. 12 ); calls are not arranged in call series. Pulses within calls are generally narrowly spaced and partly fused, but pulse repetition is distinctly faster in the second half of each call and results in respective parallel frequency bands reflecting pulse rate. Amplitude within calls is constantly increasing from the beginning towards the end where it drops to zero quickly. Numerical parameters of 4 analysed calls are as follows: call duration (= note duration) 823–894 ms (854 ± 30 ms); inter-call intervals 6071–10,186 ms (8370 ± 2100 ms); pulses per call 162–177 (169 ± 7); pulse repetition rate within first half of call approximately 135 pulses / second, within second half approximately 280 pulses / second; dominant frequency 508–524 Hz (519 ± 7 Hz); prevalent bandwidth 500–3200 Hz. Calls from the same recording ( Vences et al. 2006 , CD 3, track 33) have been described by Glaw and Vences (1992 ), D’Cruze et al. (2010 ), and Lambert et al. (2017 ) using slightly different terminological schemes.