Bioacoustics of Ptenopus circumsyrticus

Natural history. The peak calling and breeding season is in the austral spring to summer (August to November), although some calling may take place outside this period. Two eggs (dried out) have been recovered from an abandoned burrow, so clutch size can be up to two. The burrows are generally less complex than the parapatric P. kochi and P. maculatus , and appear to be inhabited solitarily. Very small juveniles have been observed in burrows of their own. While multiple natural predators for Ptenopus spp. have been observed and hypothesised ( Haacke 1975 ), one observation of a P. circumsyrticus sp. nov. being carried off by a large solifuge (le Roux pers. com. 2022) certainly constitutes a novel observation.

Comment. Specimens of P. circumsyrticus sp. nov. were not included in the earliest mentions of the name P. g. maculatus (e. g., FitzSimons 1935 ), but were included in several later works, including notably those of Haacke (1964 , 1969 , 1975 ). Haacke (1969) described the calls of both P. maculatus sensu stricto and P. circumsyrticus sp. nov. from the area surrounding Gobabeb, under the name ‘ P. g. maculatus ’, noting the clear differences. However, Haacke did not manage to match the two distinct calls with particular specimens in the area where they are sympatric (on the gravel plains of the central Namib Desert), and hence was uncertain about whether the different calls reflected intraspecific or interspecific variation. It is also worth noting that, in this sympatric zone, the two species can be readily distinguished by the internasal scale in P. circumsyrticus sp. nov. being large and in broad contact with the rostral, separating the nasal scales clearly, whereas P. maculatus has one or more small and rounded granules, barely separating the nasal scales. This morphological distinction is less clear in allopatric populations.

Advertisement call. The advertisement call (Figs 3 , 20 D ) consists of 4.6 notes (range 3–7), uttered slowly at a rate of 2.44s - 1 (range 1.57–3.68). Note duration is long (48 ms [range 31–57]) and regular (note 1 duration deviance 16 % [range 3–28]), with long, highly regular intervals (inter-note interval range 13 % [range 10–20]) inter-note intervals of 385 ms (range 237–587). Median call density is low (0.129 [range 0.09–0.17]) and call duration is 1.49s (range 1.1–2). The basal frequency is ~ 383 Hz (range 308–474), but very soft and may be inestimable, with harmonic bands louder towards the (upper) dominant frequency at 3.8 kHz (range 3.5–4.1); no clear lower peak frequency is evident. Frequency appears to modulate up by ~ 0.5 kHz in the course of the note, but (human) perceived pitch is usually monotonous throughout the notes and call. Bandwidth (90 %) is difficult to estimate consistently: approximately 1–4.7 kHz. The call strongly resembles the sound of bouncing marbles and on a warm evening on the interdunes thousands can be heard calling in chorus. Call period (mean 133 s) varies greatly, but can be as brief as 16 seconds during peak chorus activity. This species tends to occur (and call) at higher densities than the sympatric congeners P. maculatus and P. kochi . It calls from shortly before sunset and calling activity may continue throughout the night as long as the moon is above the horizon, particularly on warm evenings. This species occasionally calls during the day, especially during foggy or cloudy conditions, but generally not during rainy conditions.