Bioacoustics of Cacosternum cederbergense

Description of the holotype . An adult male in breeding condition, SVL 23.2 mm ( Fig. 6 ). Body slender, urostyle posterior to vent, widest around the angle of the jaw (HW = 9.7 mm ). The head is bluntly rounded from above, truncated in profile. Head length is about one quarter of the body length (HL/SVL 0.26). Canthus rostralis rounded, straight from eye to nostril, loreal region slightly concave; nostrils small, rounded, directed anterolaterally, raised marginally. The nostrils are placed closer to the snout than to the eye (EN/SL 0.66). Internarial distance is slightly less than distance between eye and nostril (NN/EN 0.94). Eyes directed anterolaterally, the eyes protrude, and barely visible from below, relatively large (ED/SL 1.02). Distance between anterior corners of eyes more than double the internarial distance (NN/EE 0.45). The angle of the jaw is posterior to a line drawn vertically from the back of the eye. Tympanum not visible. The dorsal surfaces of the head, trunk and limbs are smooth, with small indistinct protuberances; the rictal gland is unbroken, continuing around the posterior angle of the jaw, expanded posteriorly. The supratympanic fold continues posteriorly, thick and glandular to the arm insertion, forming a thickened saddle. Subcutaneous dorsolateral glands running from urostyle along the flanks, small, do not reach the insertion of the forelimbs. The forelimbs are slender, hands small (HAN/SVL 0.26), fingertips bluntly rounded without discs. Relative finger lengths I<II<IV<III; subarticular tubercles distinct, rounded, with one on fingers I and II, two on fingers III and IV. No webbing between fingers, although each digit tapers laterally to a thin margin. Thenar tubercle moderate size, partially obscured by nuptial pad that reaches the distal phalanx of the first finger; palmar tubercles large, heart shaped. Four supernumerary tubercles on palm. Hind limbs moderately long (TIB/SVL 0.42; FOT/SVL 0.45), foot slightly longer than tibia (TIB/ FOT 0.93); thighs are poorly developed, granular on the posterior faces with a diffusion of small glands; relative toe lengths are I<II<V<III<IV. The toe tips are not expanded; subarticular tubercles: one on toes I and II, two on toes III and V, and IV. No webbing between the toes. The inner metatarsal tubercle is large and conical, protruding, equal in width to the tip of the first toe. Outer metatarsal tubercle present as a small bump. Colour in life ( Fig. 6 ). Dorsum with olive mottling over paler beige patches; small protuberances covered in black blotches; pale beige patch between eyes and in the area between the scapula and sacral regions; limbs with transverse dark and pale bands, the darker bands contain olive-green centres; iris hazel with a pale rim around the pupil. Ventrum white with large black blotches that fuse together and are tinged with orange; throat with smaller black spots anteriorly; diffuse grey beneath lower lip that extends to the insertion of the forelimbs; palms, feet and underside of limbs orange with many dark reticulations. Light intensity has a substantial effect on colour shade, where adults become dark in the absence of light and turn paler when exposed to light. Colour in preservative. Dorsum dark grey with pale grey mottling; pale grey crossbar anterior to the eyes, with anterior pale grey line to snout; supratympanic and subcutaneous dorsolateral glands orange-mustard colouration, with olive darker blotches; pale grey and dark grey transverse bands on limbs. Ventrum white with grey blotches that fuse together; gular grey anterior and white with grey blotches posteriorly; palm of hands dark grey, with fingers and tubercles white; sole of feet uniform dark grey, except subarticular tubercles white. Paratype variation. SVL in females ranges from 23.0– 29.7 mm , and in males 20.7–26.6 mm . The condition of the subcutaneous dorsolateral glands varies in size, correlating with SVL, from prominent in large individuals to indistinct in smaller ones. The density and intensity of dark mottling on the ventrum, the size of individual blotches, and the degree to which they fuse differs from specimen to specimen ( Fig. 7 ). Males have grey throats ( Fig. 7 ab, 8a), and nuptial pads that reach the distal phalanx of the first finger ( Fig. 10a ). Supernumerary tubercles may be prominent to indistinct, and vary in number, from three to five. Those located beneath the basal subarticular tubercle on the first finger are often obscured by the thenar tubercle. Meristic differences between the holotypes and paratypes , and males and females, are negligible. Advertisement call. A total of 20 calls from the holotype specimen ( PEM A15391 ) were recorded at the type locality over a period of 18.5 minutes. Air temperature during this time was 10º C. The call of C. cederbergense sp. nov. can be described as a short, pulsed bray emitted singly at long intervals (58.3 ± 97.23 s , Fig. 9a , Table 3 ). It has a mean dominant frequency of 1.8 ± 0.03 kHz, and a dominant frequency range of 0.92 kHz. Calls have between 18 and 26 pulses at a rate of 90.6 ± 7.35 pulses per second. No territorial call was heard, but no more than a single male was heard calling in a pool. Habits and Habitat ( Fig. 11 ). All records come from Nardouw and Cederberg Sandstone Fynbos sensu Mucina and Rutherford (2006) , who describe the landscape as flat to gently sloping tableland dominated by rocky outcrops and gullies that are surrounded by deep sandy flats. Restios are common in moist areas, and small shrubs in drier sections. Breeding takes place atop large sandstone boulders and rocky outcrops in rock pools that form after the first winter rains. Cacosternum cederbergense sp. nov. appears to favour highly heterogeneous habitat with rocky and vegetated sections, often on scree covered slopes. During the day, adult and juvenile frogs have been found sheltering under rocks, or in small cracks in the pool edge. At night, adults may be observed resting entirely submerged on the pool bottom, or partially submerged clinging to the rock. If disturbed, they will dart downwards and walk along the substrate away from the perceived threat. When picked up they show similarities to the death-feigning that has been observed in C. capense and C. namaquense , whereby they tuck their limbs and head inwards, stiffening up in the process (De Villiers 1929; OA personal observation). On occasion, individuals may secrete a white substance from their large dorsolateral glands. As is the case with C. capense , this secretion is likely toxic, acting as a defensive adaptation to avoid predation ( Rebelo 2021 ). Natural History. Calls have only been heard on two occasions despite multiple night excursions. In June 2023 , on the same day as a significant rainfall event ( 54 mm ), only three males were heard calling at the type locality. Males began calling one hour after darkness, uttering soft creaks at long intervals. Call sites were limited to the pool edge, whereby males call half submerged holding onto the pool edge with their forelimbs. No territorial call has been heard, although two or more males have never been observed in the same pool. Pairs in amplexus have not been observed. A single egg clump ( 30 eggs ) was found resting on the bottom of a pool away from the water’s edge. Individual eggs have been found scattered across the rock pool, presumably as a result of wave action caused by wind that dissembles the egg clump.

Diagnosis. Referable to the genus Cacosternum based on the absence of webbing between fingers and toes, concealed tympanum and smooth ventrum with brown mottling on white background, and molecular monophyly of the 16S mitochondrial marker within Cacosternum . This new species differs from other Cacosternum species with regard to the 16S fragment, by a net uncorrected p-distance value of 1.9–7.1% (Table 1). This new species forms part of the morphologically and phylogenetically distinct C. capense group, which can be separated from other species of Cacosternum on the basis of their large size, pulse count of their ‘creak’ call, and possession of dorsolateral glands that produce defensive secretions. Morphologically, C. cederbergense sp. nov. is intermediate between C. capense and C. namaquense but can be diagnosed based on the following character states: (1) rictal glands: enlarged, forming an uninterrupted arc with the angle of the jaw (interrupted in C. capense ; continuous with the upper lip and reaching the base of the arm in C. namaquense ), (2) supratympanic fold: continues posteriorly as a thickened fold (similar to C. namaquense ; indistinct in C. capense ), (3) nostrils: when viewed from above they are within one nostril diameter of the anterior snout edge (similar in C. capense , but located on the anterior margin of the snout in C. namaquense ), (4) pupil of the eye: absence of a dark streak of variable length running downward, from the pupil into the iris (present in C. capense and C. namaquense ), (5) dorsum: smooth, with a diffusion of small dark protuberances concentrated between the scapular and sacral regions (smooth to granular in C. namaquense , usually free of warts; covered with large rounded warts in C. capense , each surrounded by dark flecks), (6) dorsal glands: a pair of dorsoventral subcutaneous glands run longitudinally from the vicinity of the urostyle to the scapula region (in C. capense glands are bean-shaped and two pairs are present, one above the vent in a semi-fused condition, another anterior to the thighs; condition similar in C. namaquense ), (7) dorsal colouration: orange and olive-green tinges common (also present in C. capense but rare in C. namaquense ), (8) a pale interorbital patch present but diffuse (in C. namaquense the patch is free of mottling and forms an uninterrupted interorbital bar; absent in C. capense , replaced by a dark triangle with a posterior vertex), (9) pale patch present between the scapular and sacral regions, diffuse (in C. namaquense the patch on the back often extends to the flanks and is usually free of mottling; absent in C. capense ), (10) ventrum: breeding males with a rim of diffuse grey colouration around the lower jaw, brown spots underneath, extending to the base of the forearms (blotches rapidly coalesce towards the throat in C. namaquense ( Fig. 8c ), becoming paler and smaller; throat black in C. capense ( Fig. 8b )), (11) belly white with dark blotches often fusing together but never forming stripes (broad stripes often present in C. namaquense and sometimes in C. capense ), (12) supernumerary tubercles: count of three to five, rounded, outer row closer to palmar tubercle than to proximal subarticular tubercles (same in C. capense , in C. namaquense conical, count of four to five, outer row closer to proximal subarticular tubercles than to palmar tubercles), and (13) palmar tubercle: heart-shaped (bifid in C. namaquense ; depressed in C. capense ). Meristic differences indicate the head of C. cederbergense sp. nov. is proportionately wider than C. namaquense , but roughly equal to C. capense ( Fig. 3 ). Cacosternum cederbergense sp. nov. also has proportionately larger eyes than C. capense and C. namaquense . Snout length is longer in C. capense than in C. namaquense and C. cederbergense sp. nov. The advertisement call consists of a brief pulsed creak emitted at long intervals ( Fig. 9a , Table 3 ). It differs from the harsh creaks produced by C. capense and C. namaquense which always have a mean dominant frequency greater than 2 kHz. The call of C. namaquense can have more than 26 pulses and often a duration greater than 0.30 s , whereas that of C. cederbergense sp. nov. ranges from 18 to 26 pulses and lasts less than 0.30 s .