Litoria rubella (Gray, 1842)

Litoria rubella (Gray, 1842) View in CoL

Figs. 11–12 View FIGURE 11 View FIGURE 12

Suggested common name: Red Tree Frog

Previously applied common names: Red Tree Frog, Little Red Tree Frog, Desert Tree Frog, Naked Tree Frog, Ruddy Treefrog, Seagull Frog, Brown Tree Frog

Hyla rubella . Gray, J. E. 1842. Description of some hitherto unrecorded species of Australian reptiles and batrachians. Zoological Miscellany, Part 2: 51–57.

Syntypes. Not designated, but BMNH 1947.2.24.7–9 (formerly 42.2.24.19–21, according to Condit, 1964, J. Ohio Herpetol. Soc., 4: 95); BMNH 1947.2.24.9 designated lectotype by Ingram & Corben (1990). Type locality: “North coast of Australia; Port Essington ”, Northern Territory, Australia .

Material examined. Details of the 67 Litoria rubella vouchered specimens measured: 29 females and 38 males are presented in Table 1 View TABLE 1 .

Revised diagnosis. This species is assigned to Litoria based on the presence of a horizontally oriented contracted pupil and the absence of a palpebral reticulum ( Tyler & Davies 1978). It exhibits typical characteristics of the Litoria rubella species complex: small head, rudimentarily webbed fingers, partially webbed toes, dark dorsolateral stripes and a high-pitched, multi-pulsed call. It is morphologically indistinguishable from other species in the L. rubella species complex and must be diagnosed using molecular data and/or location. From a genetic perspective, 17, 16 and 20 sites in the mitochondrial ND4 alignment reliably diagnose L. rubella from L. capitula , L. pyrina sp. nov. and L. larisonans sp. nov. respectively ( Table 3 View TABLE 3 ).

Description. No data on type specimens are presented. Examination of images of the syntype ( Fig. 11 View FIGURE 11 ) are consistent with the description presented here. Measurement ranges and means are presented in Table 5 View TABLE 5 . Moderate body size, females generally larger than males; body slightly to moderately flattened and wide; head small (HDL/SUL = 0.23–0.30), slightly flattened (HD/SUL = 0.11–0.21), and narrow (HDW/SUL = 0.24–0.31); head length equal to width (HDL/HDW = 0.84–1.14); snout triangular in dorsal view, tip of snout blunt to moderately pointed (IND/IOD = 0.32–0.71), rounded in profile; canthus rostralis rounded; loreal region steep, flat to concave when considered in conjunction with upper labial margin; crown of head flat; nares ovate, on anterolateral edge of snout, oriented dorsolaterally; eyes small to moderate (EYE/HDL = 0.23–0.39), pupils oriented horizontally; eye to naris distance most often larger than eye diameter (EN/EYE = 0.69–1.65); tympanum circular with flat to marginally raised annulus; supratympanic fold poorly to well developed, angling posteroventrally towards site of arm attachment; tympanum usually smaller than eye (TMP/EYE = 0.54–1.14); arms short to moderate ([UAL+LAL]/SUL = 0.24–0.35) and thin; fingers with basal webbing; relative length of fingers I<IV<II<III; tips of fingers with well-developed discs, distinct circum-marginal grooves and narrow fringing; discs relatively wide compared to width of the penultimate phalanx (3FDW/3FW = 1.33–3.00); subarticular tubercles rounded; metacarpal tubercles ovate; nuptial pads cover entire basal half of finger I on anterior (inner) surface, continuing on dorsal side only on middle of finger and tapering away from anterior (inner) edge of finger I; hindlimbs short (TIB/SUL = 0.28–0.41); relative length of toes I<II<III~V<IV; toes with well-developed terminal discs, distinct circumarginal grooves and narrow fringing; discs slightly smaller than those of fingers (4TDW/4DW = 1.10–2.67); subarticular tubercles distinct, rounded; inner metatarsal tubercle distinct, spatulate; outer metatarsal tubercle absent; toes partially webbed, extending to or beyond distal tubercles of all toes except IV, webbing generally extending between penultimate and distal tubercle and sometimes beyond distal tubercle on toe IV.

Skin of dorsal surface smooth, lacking dorsolateral folds, parotoid glands, and ridges; glandular tissue generally located between mouth and arm; ventral surface of body finely granular, generally smooth on limbs.

Colour. Colour variation is described from six in-life photographs (see Fig. 12 View FIGURE 12 ), and in situ observations. Dorsal colouration broadly uniform across the head, body and outer surface of limbs; across individuals the dorsal colouration can be fawn, cream, brown, rich chocolate-brown, red-brown, brick-red, grey, or a metallic grey or gold, sometimes whitish in the day; extent of dark brown pigmentation on dorsal surface and appendages highly variable, in some individuals forming small specks or marbling, often with two larger dorsolateral streaks in the inguinal region. Ventral surface uniformly whitish to cream, always lighter than dorsal surface; gradual to sharp contrast between dorsal and ventral colouration over dorsolateral region, generally coincides with lateral head stripe when present. Often an indistinct pale patch below eye, variable in lightness and extent among individuals. Distinctly darker lateral stripe originating from a V-shape hiatus on tip of snout, wrapping around nares, passing through eye and tympanum, dissipating variably from above attachment of arm to more posteriorly towards groin, generally more diffuse posteriorly, contrast and intensity varies from well-defined and dark to pale and poorly contrasted; yellow pigment in inguinal region and occasionally on posterior of arm and foot. Posterior of thighs ranging from yellow to dull orange, with dark marbling in some individuals. Iris golden to copper-brown, sometimes with thin line of red surrounding iris. Male gular pigmentation dark brown; nuptial pads light to dark brown. In preservative, reddish hues over body tend to be lost, colour and contrast of dorsolateral stripe usually fades, colour of thighs tends to be lost, colour of iris lost.

Embryos. Embryos were measured live. Mean diameter of 18 ova (stages 8–9) from 85 km SE Darwin was 0.9 mm (0.9–1.0); mean capsule diameter of 15 eggs (stage 8) was 1.7 mm (1.5–2.1).

Tadpole body and tail shape. Tadpoles are described from Kununurra (WA), near Fogg Dam and Marara (NT), Tennant Creek (NT), Ormiston Gorge, Simpson’s Gap (MacDonnell Ranges, NT) and Girilambone and Cobar, western NSW. Description is as for general description in Systematics Section, but tadpoles from central Australia and NSW tended to grow larger than those from northern Australia ( Table 6 View TABLE 6 ). Mid-dorsal profile may be slightly convex in some specimens (e.g., Tennant Creek area) otherwise profile was not convex in shape, rising gradually to tail/body junction ( Fig. 8A View FIGURE 8 ). Origin of dorsal fin is in proximity of the tail/body junction, rising to maximum height over middle of tail. Specimens from central Australia and NSW tended to have a slight dip preceding rise of dorsal fin.

Tadpole colour in life. Description applies to all localities. Dorsal view: Somewhat transparent in earlier stages with darker eyes, brain, abdomen and vertebral region, becoming increasingly opaque or sandy golden to brown, copper-brown, grey-brown or dark brown with or without darker stippling or mottling as tadpole grows (stage 30 onwards; Fig. 8A, B, C, D View FIGURE 8 ). A darker mid-dorsal patch on either side of vertebral region develops from about stage 33 ( Fig. 8 C–E View FIGURE 8 ), and from about stage 40 onwards, a dark longitudinal patch develops on each side of vertebral region towards tail junction. Dorsal surface of tail muscle may have melanophores of varying concentration forming fine stippling anteriorly ( Fig. 8D View FIGURE 8 ) to darker patches or transverse bands along its length (e.g. Fig. 8E View FIGURE 8 ).

Lateral view ( Fig. 8A View FIGURE 8 ): Dorsal pigment extends laterally down over body meeting opaque white with a copper sheen colouring and fine mottling in some individuals on the lower half of abdomen. Sides of tail muscle with fine melanophores or mottling, may have darker line along midline, and dorsal and ventral edges. Ventral view: Abdomen mostly opaque white with copper sheen, anterior half of body usually clearer, often with copper sheen over heart and scattered fine copper clusters.

Metamorphs. A sample of six metamorphs from northern NT ranged from 11.4–13.4 mm SVL (mean 12.7 mm), seven from western NSW 13.2–18.0 mm (mean 15.3 mm) and 13 from central Northern Territory 14.0– 18.4 mm (mean 16.3 mm). Metamorphs from central NT and western NSW are reddish-brown or lighter brown with a dark streak on either side of vertebral region and a dark patch on each side in inguinal region. Dark lateral stripe as for adult but less defined. The iris is coppery red ( Fig. 8J View FIGURE 8 ). Metamorphs from northern NT ( Fig. 8K View FIGURE 8 ) are noticeably different from those of all other regions (e.g., Fig. 8J View FIGURE 8 ), consistently having a prominent rich brown mid-dorsal band dissected by a fine cream vertebral line and contrasting paler cream sides. The iris is reddish-gold.

Advertisement call. The advertisement call of all populations of L. rubella comprises a single, highly-pulsed note. Populations of L. rubella (n=21) from the monsoon tropics differ from inland arid and semi-arid populations (n=26) in their call characteristics ( Table 4 View TABLE 4 , Figs. 5–6 View FIGURE 5 View FIGURE 6 ). Individuals from the monsoonal northern populations had a mean call duration of 0.36– 0.66 s and an individual average of 25–42 uniformly spaced pulses repeated at a rate of 57.0–80.9 pulses/s. The dominant frequency was 1.72–3.00 kHz. Individuals from the inland populations had a mean call duration of 0.48– 0.87 s and an individual average of 31–56 uniformly spaced pulses repeated at a rate of 43.20–78.04 pulses/s. The dominant frequency was 1.29–2.00 kHz. The call of the New Guinea population is not known.

Habitat and ecology. Litoria rubella is distributed across an enormous precipitation gradient. The species is commonly recorded via FrogID (>7,000 records from 10 November 2017 – 30 July 2023), and is frequently heard calling in disturbed areas, with 27% of FrogID records of the species from suburban or urban habitats and 36% of records in rural areas. Litoria rubella has been detected calling in all months via the FrogID project but has a core calling season from October to March, with a peak in November to February. One of the most encountered and abundant frogs in human-populated areas where they are frequently found in sinks, drainpipes, and toilets (RAC, PD, pers. obs.). In hot and dry environments, they are generally found resting in a water-conserving posture with a whitish colouration to reduce cutaneous water loss. Litoria rubella breeds primarily in lentic waterbodies, with the majority of FrogID records of this species documented at small ponds (32%), flooded areas (21%) and large ponds (17%).

Distribution and conservation status. Litoria rubella is widely distributed over the arid biome and Australian Monsoonal Tropics Biome in Australia, and in southern mainland New Guinea ( Figs. 2 View FIGURE 2 & 4 View FIGURE 4 ); in Australia the range boundary forms east of the Great Victorian and Gibson Deserts, north of the Great Sandy Desert, west of the Carpentarian barrier and Great Dividing Range, and extends south as far as the Flinders Ranges. Although its conservation status requires formal reassessment, it meets the criteria for Least Concern ( IUCN SSC Amphibian Specialist Group 2022) due to its very large distribution (Extent of Occurrence>3 million km 2) and absence of obvious threats. Litoria rubella also occurs in the southern part of New Guinea ( Fig. 2 View FIGURE 2 ).

Comparison with other species. Litoria rubella is allopatric with five members of the L. rubella species group ( L. balatus in south-eastern Queensland, L. dentata in north-eastern New South Wales, L. quiritatus in south-eastern New South Wales, L. larisonans sp. nov. in the Pilbara and L. capitula on the Tanimbar Islands, Indonesia). In Australia L. rubella can be distinguished from the sympatric L. electrica in western Queensland ( Anstis 2017) by the absence versus presence of chocolate-coloured bars across the dorsum.

In New Guinea, L. rubella can be distinguished from sympatric L. congenita by shorter legs (TIB/SUL = 0.28– 0.41 versus 0.477 –0.520 [Tyler 1968]) and plain brown dorsum or more commonly with one, two or three yellowish stripes ( Menzies 2006) and from sympatric L. pygmaea by shorter legs (TIB/SUL = 0.28–0.41 versus 0.50–0.56 [ Menzies 2006]) and arms with light to dark colour tones versus white ( Menzies 2006).

No morphological characters are known to reliably diagnose L. rubella from L. pyrina sp. nov. Litoria rubella from inland Queensland and New South Wales can be distinguished from parapatric L. pyrina sp. nov. by call, having a lower dominant frequency (1.29–2.00 kHz versus 1.75–2.97 kHz) resulting in an audibly lower-pitched call.