Litoria insularis, Richards & Oliver, 2022

Richards, Stephen J. & Oliver, Paul M., 2022, A new species of insular treefrog in the Litoria thesaurensis species group from the Nakanai Mountains, New Britain, Papua New Guinea, Vertebrate Zoology 72, pp. 1067-1076 : 1067

Litoria insularis sp. nov.

Holotype.

SAMA R64781 (FN SJR10776), adult male with nuptial pads, Vouvou Camp, Nakanai Mountains, East New Britain Province, Papua New Guinea (5.4456°S, 151.4640°E, 850 m a.s.l.), collected by S. Richards on 16th April 2009.

Paratype.

SAMA R66896 (FN SJR10738), adult male with nuptial pads, same collector and locality details as holotype, collected on 12th April 2009.

Diagnosis.

A species of Litoria that can be distinguished from all other taxa by the following unique combination of characters: moderately small size (male SVL 38.7-41.7 mm); finger webbing moderate, not extending beyond base of second phalanx on any fingers; toe webbing moderate, extending to base of second phalanx on both sides of toe 4; finger discs moderately expanded (3FD/SVL 0.059-0.062); toe discs moderately expanded (4TD/SVL 0.052-0.058); bones of limbs green in life; lower forelimbs and hindlimbs with low crenulated dermal ridges along lateral edges; male throat and neck smooth or ridged, but lacking numerous obvious round bumps in life; venter plain off-white in life; dorsal colouration mottled pale grey or brown with scattered very small dark-brown flecks; head lacking dark lateral band or mask; iris reddish-brown with prominent sky blue outer rim; and call very long (> 13 s), consisting of more than 90 notes repeated at a rate of 6.7 notes/s.

Description of holotype.

Habitus moderately robust (Fig. 1A-B ), limbs moderately long (TL/SVL 0.53); snout narrowly rounded in dorsal aspect, distinctly sloping in lateral aspect. Canthus poorly defined, broadly rounded, slightly curved in dorsal view; loreal region sloping, slightly concave; nostrils near tip of snout, oriented laterally. Eyes large (EYE/SVL 0.127). Tongue broadly oval with distinct posterior notch; vocal slits short, extending anteriorly from level of angle of jaw. Vomero-palatine ridges located midway between internal nares, prominently raised, about 0.6 mm apart, each with 3-4 distinct teeth. Tympanum moderately large (TYM/SVL 0.070), annulus distinct except for postero-dorsal edge obscured by thick, curved, postocular fold extending from posterior edge of eye to above axillary junction. Skin finely but distinctly granular dorsally; abdomen coarsely granular; throat and chest coarsely ridged.

Fingers moderately long, relative lengths 3>4>2>1, terminal discs moderately wide (3FD/SVL 0.062), with circum-marginal grooves (Fig. 2A ); webbing reaches to ultimate subarticular tubercle on inner side of Finger 4 and outer side of Finger 2, to base of third phalanx on inner side of Finger 3, to halfway along third phalanx on outer side of Finger 3, restricted to thin basal strip between Fingers 1 and 2. Subarticular tubercles prominent, circular, undivided. Inner plantar tubercle prominent, ovoid. Nuptial pads low, brown, granular, extending 3.5 mm along base of Finger 1. Prominent tubercles along posterior-ventral edge of forelimbs form crenulated ridge extending from base of Finger 4 to elbow.

Toes moderately short, relative lengths 4>5>3>2>1, terminal discs moderately wide (4TD/SVL 0.058) with circum-marginal grooves (Fig. 2B ); webbing reaches penultimate phalanx on Toes 1-3 and 5, and to base of third phalanx on both sides of Toe 4. Inner metatarsal tubercle and subarticular tubercles prominent, not divided (Fig. 2B ). Tubercles along outer edge of lower limbs form weak crenulations. Skin on postero-ventral surface of thighs coarsely granular.

In life at night dorsal and upper-lateral surfaces of torso varying shades of mottled light to medium brown (Fig. 1B ), but during photography colour quickly changed to varying shades of mottled light grey overlain with extensive fine brown stippling (Fig. 1A ); scattered small dark-brown flecks present across torso. Head with same variable base colouration as torso but lacking dark-brown flecks and having denser dark-brown stippling on snout, around canthus rostralis and on anterior surface of orbitals. Exposed surfaces of limbs and digits same colouration as torso but with more obvious and intricate mottling, denser brown stippling and lacking dark-brown flecks. Lateral and ventral surfaces of torso, throat and limbs unpatterned, largely off-white with small areas of near translucent skin, especially on thighs and upper arms (Fig. 2C-D ). Iris patterned with dense light-brown stippling and a prominent light-blue outer ring.

In preservative dorsal and upper lateral surfaces of head and torso mottled with varying shades of light brown and light grey, anterior portion of head distinctly greyer than remainder of dorsal surfaces, scattered small dark-brown flecks on dorsal surfaces of torso clearly apparent, with smaller number of small white flecks. Exposed surfaces of limbs show similar base colouration to dorsum, but also have areas of off-white mottling and lack dark-brown flecks. Vent bordered dorsally by dark brown blotch and then irregular white band. Dorsal surfaces of digits buff with varying amounts of fine brown stippling, stippling especially dense on outer digits, but tending to very sparse on inner digits. Ventral surfaces buff, largely unpatterned, except for small areas of dense stippling close to lower jaw, and along extremities of forelimbs and hindlimbs.

Summary meristic data for holotype (all measurement in mm).

SVL 41.7; TL 22.1; HW 15.3; HL 14.8; EYE 5.3; TYM 2.9; IN 3.3; EN 4.2; 3FD 2.6; 3FP 1.4; 4TD 2.4; 4TP 1.8.

Variation.

The sole paratype (SAMA R66896) is an adult male (Fig. 3A ) with the following measurements: SVL 38.8; TL 21.9; HW 13.4; HL 12.4; EYE 5.2; TYM 2.5; IN 2.9; EN 3.8; 3FD 2.3; 3FP 1.2; 4TD 2.0; 4TP 1.3. It is similar in colour and pattern to the holotype, with the following differences, base colouration in both life and preservative lighter grey with less obvious mottling, dark-brown flecks much more numerous and extending onto limbs, and dorsum with two small off-white blotches.

Advertisement call.

We recorded a single complete call produced by the holotype at an air temperature of 22.0°C. It was recorded from approximately 2 m distance and the call of this species is very quiet so detailed analysis of some features was not possible due to reduced resolution and high background noise. The call is a long (13.9 s) series of 94 short, distinctly pulsed notes repeated rapidly (6.7 notes/s) at relatively uniform inter-note intervals of 0.106-0.144 s (mean = 0.120, SD = 0.012, n = 25). Twenty-five notes measured in detail are 0.027-0.050 s long (mean = 0.034, SD = 0.005) and contain 3-5 pulses (3 pulses = 4 notes, 4 pulses = 17 notes, 5 pulses = 4 notes). Energy in the call is distributed broadly between ~0.5 and 3 kHz and dominant frequency is between 1260 and 2156 Hz in five randomly selected notes. The call produced by the holotype is illustrated and compared with two calls of a sympatric L. thesaurensis in Fig. 4 .

Comparisons with other species.

Litoria insularis sp. nov. was found in sympatry with L. thesaurensis and is most likely to be confused with that species, but differs in having wider finger discs (3FD/SVL 0.059-0.062 versus 0.037-0.052) and toe discs (4TD/SVL 0.052-0.058 versus 0.038-0.047); dorsum with scattered small dark-brown flecks (versus base colouration and pattern highly variable, but usually not with small scattered dark-brown flecks) (Fig. 3B-D ); skin on male throat smooth (Fig. 2C ) (versus with prominent unpigmented bumps (Fig. 3E )); and in having an extremely distinct call comprising a series of 90 rapidly repeated short (~0.03 s), distinctly pulsed notes produced at a rate of 6.7 notes/s (versus a single drawn out note lasting ~0.6 s (Fig. 4 ; Kraus and Allison 2004).

It has been suggested that Litoria thesaurensis as currently construed may be a complex of several species ( Kraus and Allison 2004). The characters listed above serve to distinguish the new species from all populations referred to L. thesaurensis from East Melanesia, including near-topotypic material from Bougainville in the biogeographic (versus political) Solomon Islands. Comparisons of photographs of L. insularis sp. nov. with L. thesaurensis from East Melanesia further suggest that the new species differs in having plain off-white ventral and hidden surfaces of the limbs (versus typically with large areas of yellow) and in having a distinctive rim of sky-blue pigmentation on the outer edge of the iris (versus brown to bright red iris with no obvious pale blue rim). However, given the propensity for frogs in the Litoria thesaurensis species group to undergo rapid shifts in colouration and for iris colouration to be obscured by the degree of pupil dilation, we note these potential differences here, but do not include them as diagnostic features at this stage.

The new species differs from other species in the L. thesaurensis species group as follows; from L. flavescens Kraus & Allison, 2004 by its smaller size (male SVL 38.7-41.7 mm versus 44.5-45.6), by having green limb bones (versus white), and by having a light grey and/or brown dorsum (versus yellow); from L. lutea by its smaller maximum size (adult male maximum 42 mm versus 62 mm), and dorsum with scattered small dark-brown flecks (versus dark brown flecks absent (Fig. 5F )); and from L. impura (Peters & Doria, 1878) by having green limb bones (versus white), in having only moderate webbing on the fingers, by having a chin without dark pigmentation in breeding males (versus chin and throat black), and by lacking a prominent dark-brown canthal stripe (versus present). The call of Litoria insularis is the most divergent among currently recognised members of the group (the call of L. lutea has not been described). In contrast to the multi-note call of the new species, Litoria flavescens , L. impura and L. thesaurensis all produce single-note calls containing> 50 pulses. Vocalising males can therefore be readily distinguished from these taxa.

Only two other species of Litoria occur in East Melanesia. Litoria insularis sp. nov. can be readily distinguished from these as follows: from L. infrafrenata ( Günther, 1867) by its much smaller size (male SVL 41.2 mm versus up to at least 135 mm), mottled brown or grey dorsal colouration (versus typically plain green or brown), and by lacking a prominent white labial strip (versus present); and from L. lododesma Menzies, Richards & Tyler, 2008 by its much larger size (max male SVL 41.2 mm versus 23.8 mm), dorsum brown or grey with scattered brown flecks (versus typically green without pattern), and the absence of white lateral stripe on the head (versus usually present).

Distribution and natural history.

Known only from the type locality in the Nakanai Mountains in the eastern portion of the island of New Britain (Fig. 5 ). The species was not heard or recorded at nearby sites at higher (~1500 m. a.s.l.) or lower elevations (~200 m a.s.l.) during the same expedition ( Richards 2011). As far as we are aware it has also not been recorded on other surveys elsewhere in New Britain, suggesting it may be endemic to moderate elevations in the Nakanai mountains.

The type series was collected in relatively undisturbed Hill Forest (Fig. 6A, B ) at around 890 m a.s.l. in an area of complex and heavily dissected karst. When collected the holotype was calling sporadically from an elevated position on vegetation approximately 1 metre above the ground along a small slowly flowing stream (Fig. 6C ). The paratype was collected from low vegetation along the same stream. Litoria insularis occurs in sympatry with L. thesaurensis and L. lododesma , both of which were much more abundant at the type locality. Other species encountered at the type locality were the microhylid Oreophryne brachypus (Werner, 1898), the ranid Papurana novaebritanniae (Loveridge, 1948) and a diverse community of at least nine species of Ceratobatrachidae ( Richards 2011).

Suggested IUCN Conservation Status.

Litoria insularis is known only from a single locality despite moderately intense survey effort elsewhere in New Britain, suggesting that it may have a localised distribution. The type locality is immediately adjacent to a road that has been constructed into the interior of the island from Pomio on the coast and this will undoubtedly improve access to, and increase pressure on, the surrounding forest. Rainforests across New Britain are also under threat from rapid expansion of Oil Palm plantations so, given the combination of very limited knowledge about the new species’ distribution, ecology and habitat requirements, and potential threats, we recommend that it be listed as Data Deficient under the IUCN Redlist Criteria.

Etymology.

The name Litoria insularis alludes to both the new species’ likely endemic status on the island of New Britain, and the potential that it is further restricted to the Nakanai Mountains, which are themselves a 'sky island’.

Figures

Figure 1. Litoria insularis sp. nov. holotype SAMA R 64781 from the Nakanai Mountains, East New Britain, Papua New Guinea in life. Images A and B show temporal variation in dorsal colouration, but consistent pattern of mottling and dark-brown flecks. Photographs S. J. Richards.

Figure 2. Details of holotype of Litoria insularis sp. nov. SAMA R 64781. A Ventral view of fingers and webbing; B ventral view of toes and webbing (scale bars = 5 mm); C ventral colouration; D hidden colouration of thighs. All photographs by S. J. Richards.

Figure 3. Details of species in the Litoria thesaurensis Group in life. A Litoria insularis sp. nov. paratype SAMA R 66896 in life; B Litoria thesaurensis SAMA R 64668 from New Britain showing yellow lateral pigmentation; C ventral view of L. thesaurensis from New Britain, with round tubercles on the throat faintly visible; D L. thesaurensis from Bougainville showing dark brown weakly striped pattern; E Litoria thesaurensis from Bougainville showing plain green dorsum; F Litoria lutea from Bougainville showing light green dorsum colouration with off-white flecks. All photographs by S. J. Richards.

Figure 4. Waveform (top) and spectrogram (bottom) of (A) an entire call of Litoria insularis sp. nov.; B a three second section from the middle of this call illustrating the distinctly pulsed nature of the notes; C two consecutive calls of L. thesaurensis recorded in sympatry with the new species, showing strikingly different call structure. The strong signal at ~ 2.2 s on A) is a call of the New Britain endemic microhylid frog Oreophryne brachypus.

Figure 5. Map of Papua New Guinea with type locality for Litoria insularis sp. nov. on New Britain indicated by a red star.

Figure 6. Habitat at type locality of Litoria insularis sp. nov. in the Nakanai Mountains, East New Britain Province, Papua New Guinea. A Aerial view of upper hill forest on rugged karst; B forest profile; C slow-flowing stream along which the type series was collected.