Ptenopus sceletus, Becker & Alexander & Tolley, 2025

Becker, François S., Alexander, Graham J. & Tolley, Krystal A., 2025, Singing on key: An integrative taxonomic revision of barking geckos (Gekkonidae: Ptenopus) with six additional species and keys for morphology and advertisement calls, Vertebrate Zoology 75, pp. 277-323 : 277-323

Ptenopus sceletus sp. nov.

Figures 18 , 19 , 20 K

Common names.

Skeleton Coast barking gecko

Afrikaans: Skedelkus blafgeitjie

Chresonymy.

Ptenopus carpi – Gramentz (2008)

Ptenopus carpi (in part) – Haacke (1975: 230)

Comment.

Ptenopus sceletus sp. nov. is closely related to P. carpi but occurs predominantly north of the Swakop River, while P. carpi only occurs to the south. A single call from one individual in captivity was previously described by Gramentz (2008) as ‘ P. carpi ’, although the call and its variation is herein re-assessed and fully described in a standardised taxonomic framework.

Holotype.

NMNW R 12100 , adult male, collected from NE of Swakopmund (Skeleton Coast), Erongo Region, Namibia (–22.6259, 14.5457), by Francois S. Becker on 21 April 2023.

Paratypes.

NMNW R 12101 –3, adult males except for NMNW R 12101 (adult female, allotype), same collection details as the holotype .

Additional material examined.

See Table S 1 for vouchered (2) and unvouchered photographed (50) specimens, DNA samples (34 available, 8 sequenced), and call recordings (7) included (total n = 62 excluding types).

Etymology.

This species is named in reference to the Skeleton Coast, which generally refers to the coastal Namib Desert between the Swakop River and the Kunene River, encapsulating the species’ entire distribution and type locality. In addition, the white or grey dorsal surfaces of the head and feet give the gecko a ghostly or skeletal appearance (Fig. 19 B ). We therefore use the specific epithet “ sceletus ”, the Latin noun in apposition meaning “ skeleton ”.

Diagnosis.

A moderately large Ptenopus (SVL max. 57.1 mm, mean 52.7 mm, n = 18) with a moderate tail (TL 70 % [range 62–80] of SVL, n = 12), an overall lean appearance, and comparatively slender limbs. In preserve state it is morphologically indistinguishable from P. carpi except by having a generally broader or shorter head / snout, when viewed from beneath (Fig. 19 E ). Live specimens are more easily distinguished, with only males possessing a bright yellow gular patch, as is typical for the genus (Fig. 19 E , vs. gular patch in both sexes in P. carpi sensu stricto); by silver to brown iris colouration (Fig. 19 F , vs. red or ochre in P. carpi ). It is distinguished from all other congeners except for P. carpi , by a combination of the following characters: Banded dorsal colour pattern on the body and tail (vs. spotted or speckled pattern in P. garrulus and P. kochi , and large, paired, ovoid light markings interspaced by darker mottled patches in other species); weakly fringed toes, with fringe length being generally less than half the breadth of the toe between fringes (vs. more than half for other species); nostrils not covered internally by a projection from the upper nasal scale (vs. is covered partially or completely in other species); being larger than most congeners (except P. kochi ), and having longer, leaner limbs.

Holotype description.

(Fig. 18 ). Adult male, SVL 52.4 mm with original tail 37.2 mm (70.0 % SVL). Body and head covered with minute hexagonal to round scales of a similar size, dorsally and ventrally, with scales on limbs notably larger. Body slender, MBSR 123, IOS 44, HL 16.9 mm, HW 12.3 mm, HH 7.5 mm, EED 6.6 mm. Upper labials 9 enlarged, the last being smaller at the angle of the jaw; lower labials 9 enlarged and one smaller scale posteriorly; four granules bordering the mental. Clear, but not prominent, superciliary ridges tapering from above mid-eye level. Around the eye is a single row of elongated scales around the anterior, dorsal, and posterior margins, but more rounded and smaller along the ventral margin. Nasal scales very slightly swollen, the prenasals barely separated by one tall, thin, triangular internasal scale in narrow contact with the somewhat round, septagonal rostral. Eyes large, EYE 3.9 mm. Ear opening oblique (~ 45 °), short and relatively broad, set on the far posterior corner of the head, with small, rounded, projecting scales at the anterior margin. The neck region behind the cheeks is barely swollen with internal calcium deposits. Arms, legs, and tail lean. Toes elongate, moderately flattened and weakly fringed; small, pointed fringed scales on finger margins; strong nails on fingers and toes, being larger and thicker on the fingers.

Colouration.

In life, the holotype (very similar to paratype NMNW R 12103 ; Fig. 19 A ) has a dorsal background colour of light yellow to cream, with light orange speckles along the spine and dark purple-brown speckles or mottles all over, which coalesce to form a clearly banded dorsal colour pattern, with three dark bands on the body (the posterior being nearly split into two bars), eight bars on the tail, and a faintly defined bar on the head between the eyes. The ventrum is immaculate white with a bright yellow gular patch that is nearly divided in the middle; some of this yellow colouration is also faintly visible on the dorsal surfaces of the limbs and snout. The iris is silver.

In preservative (Fig. 18 ), the lighter colours have faded to cream or off-white and the darker colours faded to various shades of grey-brown. The yellow and orange colours have completely faded. The iris is a milky blue-white.

Variation.

Refer to Table S 1 for range of morphometric characters, including the paratypes. Morphometric characters vary relatively little among specimens, although the colours vary somewhat (Fig. 19 ). The dorsal colour pattern consists of 3–4 dark brown or purplish bands on a beige, yellowish, or light orange background, densely speckled with dark brown, grey, purple and / or sometimes green; sometimes the speckling is very dense, giving the appearance that the gecko is not or barely banded; an additional six to eight dark bands occur on the tail. Banding tends to be clearer and colouration brighter in younger individuals (Fig. 19 D ). Males have bright yellow gular patches, sometimes partially or entirely split in the middle, while females do not, or have very faint yellow on the sides of the throat (Fig. 19 E ). Males also have light yellow infusions on the head, limbs, and body, while females tend to be paler overall (Fig. 19 A – B ); in populations further north, dorsal colouration tends to be duller (Fig. 19 C ). Iris colouration varies from silver to brown (Fig. 19 F ), with individuals in the southern populations tending to have silver eyes.

Advertisement call.

The advertisement call (Figs 3 , 20 J ) consists of 10 (8–13) notes uttered in slow succession, with a note rate of 2.60s - 1 (1.25–4.32). Note duration is short (28 ms [range 21–40]) and regular, note 1 duration deviance 8.7 % (range 0.5–16.2). The inter-note intervals may be very long (387 ms [range 204–661]), but are shorter and more regular at the beginning, then tending to longer intervals as the call progresses: Call starts fast, then slows down. Hence the inter-note interval range is usually high (102 % [range 26–163]). Median call density very low (28.3 [range 20.9–40.3]) and call duration is both long and variable (3.9s [range 2.5–7.2]). The basal frequency is 405 Hz (range 357–516) but very soft and usually inestimable, with harmonic bands louder towards the dominant frequency. The upper dominant frequency is at 4.3 kHz (3.8–4.8), and a clear (sometimes louder) lower dominant frequency is evident at around 2.0 kHz (1.4–1.9), usually less than half the upper dominant frequency. Frequency appears to remain constant throughout the notes and call, as is the case with the (human) perceived pitch, which is low and monotonous compared to most other species. Bandwidth (90 %) is difficult to estimate consistently, but is broad: approximately 0.3–6.9 kHz.

The call appears to vary more than the calls of other species, except for P. carpi , with calls uttered late at night sometimes having only one to three notes. Call period (mean 365 s) varies greatly, but is usually lower than other species, and has not been recorded any shorter than 104 s during peak chorus activity. This species does not chorus as notably as most other species. Calls can be heard from after sunset to late at night. Seasonal phenology of calling activity is not known, but calls have been recorded in May and October. Calling activity is more pronounced on warmer evenings, but may continue despite cold, foggy conditions.

As with other species, P. sceletus sp. nov. calls from the burrow entrance, although it may also call occasionally while roaming. This could not be observed directly, but a wandering gecko was often spotted in the area where a call was just heard – although wandering geckos are easier to detect than geckos in a burrow.

Distribution and habitat.

This species occurs from the southern banks of the Swakop River, northwards in a narrow strip along the Skeleton Coast (northern Namib Desert), to the southern tip of the northern Namib erg, near the Kunene River (Namibia-Angola border; Fig. 5 ). Its habitat falls within the coastal fog zone and it occurs on compact, gravelly substrates.

This species occurs parapatrically with P. carpi across the Swakop River and sympatrically in the Rössmund area, just south of the river. Ptenopus sceletus sp. nov. occurs in sympatry or parapatry with P. maculatus in the eastern portions of its range ( P. maculatus occurs only on silts in the northern limits of its range, while P. sceletus sp. nov. remains on gravels in that area).

Natural history.

The peak calling and breeding season of Ptenopus sceletus sp. nov. is probably between May and October (span of current call recordings), and juveniles have been found in November. This is slightly later than the apparent breeding season for P. carpi , but also coincides with months of somewhat lower fog incidence.

This species does not call nearly as often as most congeners, but appears to spend more time wandering on the surface, even far from the burrows (as in P. carpi and P. adamanteus sp. nov.). It is active much later into the night than most congeners, often only emerging well after dark, and its activity is not dampened by cold or foggy conditions (as in most other species).

The burrow is usually shallower and simpler than those of other species, but may have two to three side-branches and typically has an escape exit (as in other species). As in P. carpi , it utters two different calls: The advertisement call and the single-pulse call, the latter usually uttered late at night after the main calling activity has ceased, and especially during dense fog.

Figures

Figure 3. Box and whisker plots of the bioacoustic characters analysed using ANOVA / Kruskal-Wallis tests, for species of Ptenopus. Numbered characters (numbered as in Methods) and their units are indicated on y-axis labels. Species colours correspond to those on Figure 2 and indicated on the key. Significant differences (according to pairwise post hoc tests) are indicated through homogenous groupings: Species not significantly different in the present character are one group, as indicated by lower-case letters. Species that share the same letters not differing significantly from each other; species with multiple letters are not significantly different from any other species sharing those letters. Approximate divisions between the groups are indicated by dotted lines perpendicular to the y-axis, and the group letters are indicated to the right of each graph.

Figure 5. Revised, interpreted distributions of Ptenopus species in southern Africa (high resolution map, can be zoomed for detail). Some species’ ranges overlap, shown with translucent overlapping shapes on the map. Others likely occur parapatrically along extended contact zones, represented by opaque, touching shapes. For further clarity, co-occurrence patterns for all species pairs are shown on Table 5, and confirmed individual records on Figure 2.

Figure 18. Holotype (NMNW R 12100) of Ptenopus sceletus sp. nov. from NE of Swakopmund, Skeleton Coast, Erongo Region, Namibia (– 22.6259, 14.5457). Scale bar = 1 cm relative to full ventral and dorsal views. Photos by Francois S. Becker.

Figure 19. Life colour variation in Ptenopus sceletus sp. nov.: A NMNW R 12103 (paratype) from N of Swakopmund, Erongo Region, Namibia (– 22.6259, 14.5457); B NMNW R 12101 (allotype) from locality A; C NMNW R 11763 from Hoanib River, Kunene Region, Nambia (– 19.3539, 13.1453); D NMNW R 11754 from Henties Bay, Erongo Region, Namibia (– 22.1584, 14.3086); E NMNW R 12100 (holotype) and NMNW R 011771 from the same locality as A, showing sexual dichromatism in gular patches of this species (male gular patch may be divided or undivided, while female only has slight shades of yellow on lateral edges); F NMNW R 11771 from N of Swakop River near Swakopmund (– 22.6652, 14.57619) and NMNW R 11755 from N of the Omaruru River near Henties Bay (– 21.7703, 14.5520), showing iris colour variation from silver (most common in the sourthern extreme of the range) to brown (more common further north). Photos by Francois S. Becker.

Figure 20. Sonograms (top graphs) and oscillograms (bottom graphs) of representative advertisement calls of Ptenopus species (A – J, species names indicated on graphs). Both x and y axes are directly comparable / equally scaled across species. Specimen numbers and localities: A NMNW R 11585, NE of Koës, Hardap Region, Namibia; B iNaturalist 140169978 (Jacobus Retief), Merweville, Western Cape Province, South Africa; C NMNW R 11622, E of Lüderitz, || Karas Region, Namibia; D NMNW R 11643, SW of Gobabeb, Erongo Region, Namibia; E NMNW R 11657, NE of Keetmanshoop, || Karas Region, Namibia; F NMNW R 11704, Gobabeb; G NMNW R 11716, Hoanib River, Kunene Region, Namibia; H NMNW R 12114, Gobabeb; I NMNW R 11843, NW of Gobabeb; J NMNW R 11790, N of Swakopmund, Erongo Region, Namibia. The audio files on which this figure is based, are made available (File S 2).