Description
Assa wollumbin are small frogs with male snout-vent lengths ranging from 13.1 - 19.3 mm and female snout-vent lengths ranging from 14.7 - 20.5 mm. The head is moderately wide, ranging from 1.2 - 1.7 times the head length. The snout is slightly pointed when viewed from above, and appears as a downward wedge when viewed laterally. The nostrils are closer to the tip of the snout than to the eyes, and the internarial distance is much greater than the eye to naris distance, ranging from 1.6 to 2.5 times as long. The canthus rostralis is slightly concave. Eye size is moderately long and variable, with eye diameter ranging from approximately less than a third to half of the head length. The eyes do not protrude in dorsal and ventral views. The pupils are horizontal and elliptical. The tympanum is obscured by granular skin, and much smaller than the eye. The body is pear-shaped and flattened dorso-ventrally. The skin along the dorsal and lateral surfaces including the limbs is finely granular. The skin along the ventral surfaces including the limbs is coarsely granular, with small tubercles, lateral edges, and more prominent tubercles on the margin of the jaw. The body has a prominent dorso-lateral margin marked by a strong contrast in coloration and a line of rounded tubercles that extends from behind the eye to around the posterior third of the dorsum. These tubercles vary in size between individuals, ranging from small to prominent. Males have pouch openings on both upper lateral posterior flanks on the dorso-lateral line in the anterior groin; these are usually hidden by the thigh when at rest. There is a glandular ridge of enlarged tubercles from the rear of the jaw to the axil. Some individuals have one or two larger rounded tubercles coincident with v-shaped markings on the dorsum. The urostyle is prominent, rounded, and upturned above the cloaca. The forelimbs are moderately long, thin, and have a row of tubercles on the posterior dorso-ventral margin of the forearm. The palmar surfaces have a round, flattened tubercle at the “outer heel (sic)”. The fingers are short and lack webbing, with subarticular tubercles on each phalanx of the fingers. The 3rd finger is the longest, followed by the 2nd, 4th, and 1st. The 1st finger is greatly reduced with only one phalanx. All fingertips are slightly expanded laterally, smooth, and rounded below. The hind limbs are moderately long, approximately slightly shorter than or equal to half of the snout-vent length, with robust and muscular thighs. The posterior, hidden surface of the thighs has many rounded tubercles. There is a row of prominent tubercles on the posterior dorso-lateral margin of the tarsus and heel. The foot is relatively long with unwebbed toes. The 4th toe is the longest, followed by the 3rd, 5th, 2nd, and 1st. The tips of all the toes are slightly expanded laterally and smooth and rounded below. There are rounded subarticular tubercles at the base of each phalanx of the toes, and a small, rounded tubercle on the outer metatarsal (Mahony et al. 2021).

DIAGNOSIS:

With the exception that Assa wollumbin females are smaller than their sister taxon, A. darlingtoni, the two species are morphologically indistinguishable. However, they can be distinguished based on their range with the former only being found on Wollumbin (Mount Warning), New South Wales, Australia and the latter found on the surrounding Tweed caldera region and in four other localities ranging from the Conondale range in Queensland to the Dorrigo Plateau in New South Wales. It is possible their ranges overlap in the Tweed Volcanic region, however, if they do, those populations have not been identified as of 2023. Unambiguous differentiation requires molecular analysis (Mahony et al. 2021).

COLORATION:

In life, Assa wollumbin individuals have highly variable dorsal and ventral coloration and patterning. Variation in the base color of the dorsal surface of the head, body, and limbs include light cream with an orange tinge, reddish brown, darker brown, and brown-black. Many individuals with lighter coloration also feature distinct darker brown to black markings, which often occur as two anterior direct v-shaped darker chevrons on the upper and lower dorsum. Other individuals have a broad, darker, mid-dorsal stripe with distinct broad lighter cream and black margins. Most individuals also have a faint interorbital bar that does not extend up onto the orbit. The face may be the same color as the flanks, or a darker hue, and may be immaculate or have ill-defined black mottling. Some individuals have a darker marking on the canthus rostralis between the anterior margins of the eye to the nare. Most individuals have a dark vertical bar beneath the eye to the maxilla, occasionally with a cream lined edge; this may be obscured in darker individuals. Most individuals have a distinct delineation across the dorsal-lateral margin with a ridge featuring highly contrasting coloration between the dorsum and flanks. There is often a thin, lighter colored line on the ventral side of the ridge. The upper lateral surface is dark brown to black, diffusing into a lighter brown towards the ventral surface. The ventral surface including the groin, limbs, hands and feet, posterior surface of the thigh, and chin are all the same color within an individual, but the color varies significantly across individuals. Some examples are darker brown-black with small lighter colored granules that are prominent on the edges of the mandibles, and light burnt-yellow with darker flecks (Mahony et al. 2021).

VARIATION:

Females are slightly larger than males, however, the most distinctive form of sexual dimorphism is the occurrence of hip-pockets in the male, one on each side. The lateral pouch opening is up to 3 mm long and inclined posteriorly at a slight angle on the upper dorso-lateral margin. It is continuous with the dermis and extends forward into the lymph sac beneath the lateral and ventral surfaces. The pouches on each side are independent and do not join (Mahony et al. 2021).

Distribution and Habitat

Country distribution from AmphibiaWeb's database: Australia

View distribution map in BerkeleyMapper.

Assa wollumbin is restricted to Wollumbin, also known as Mount Warning, in northeastern New South Wales, Australia. Wollumbin is the eroded central volcanic cone in the Tweed Volcano region. Assa are commonly found in the cooler temperate forests at higher altitudes in the region, above 800 m elevation. However, they also occur at lower altitudes in subtropical rainforest on Wollumbin, around 420 m above sea level (Mahony et al. 2021).

Life History, Abundance, Activity, and Special Behaviors
Assa wollumbin inhabit terrestrial leaf litter. There is no quantitative information available on the numbers of adult A. wollumbin (Mahony et al. 2021).

Males call by expanding the skin below their chin, but their vocal sacs are not loose. Their advertisement calls have a duration of about 1.2 s, and is comprised of a series of fully modulated repeated notes of the same duration separated by slightly longer inter-note intervals. The number of notes range from 5 to 14. The first few notes are very slightly lower in amplitude than the subsequent notes. The call is broadband with a relatively high dominant frequency ranging from 3593 to 3992 Hz; there is no frequency modulation across the call or within notes. Call duration, inter-call duration, and note repetition rate are significantly negatively correlated with temperature; the number of notes in the call is positively correlated with temperature. Synchronous calling among males has been observed (Mahony et al. 2021).

The courtship call is made by the male when a female approaches closely, during amplexus, and sometimes when males are in very close proximity. It is often produced independently, but it may also precede the advertisement call by a short time interval less than 0.2 s. The courtship call has a short duration and is produced at the same dominant frequency as the advertisement call. The courtship call’s individual pulses cannot be distinguished. There is amplitude modulation with a rapid rise to a plateau prior to a rapid decay, and no frequency modulation (Mahony et al. 2021).

Males establish a calling position, often in secluded areas beneath dense leaf litter, and females are attracted to the male who does not move away from the calling position. After a female has been attracted, the male quickly moves to an inguinal amplexus embrace. The female then moves, with the male attached, to the oviposition site which is up to 20 cm from the male’s calling position — this represents the preferred oviposition site and indicates that the male was not calling from a microhabitat that the female preferred. Oviposition often occurs beneath leaf litter, in a location that is moist but not in standing water. A small clutch of large eggs is placed closely together; there is adherence between the eggs and usually at least two layers of eggs. Eggs have a thick and transparent jelly coat, and embryonic development proceeds for at least seven days (Mahony et al. 2021).

The male displays interesting parental care behavior. The male stays close to the egg clutch during embryonic development, about two or three body lengths away, under the leaf litter in a position where the clutch is in view. Sometimes, the male moves to the clutch and over it. At hatching, the male moves over the clutch, and the eggs’ jelly coat liquifies. The embryos wriggle vigorously during hatching with their head arched downward. The male slowly rotates within the clutch, opening a crevice between his flanks and legs by holding his legs slightly away from the body. The newly hatched tadpoles wriggle up the crevice into the male’s hip pouches. The male does not assist tadpoles into the pouch other than displaying this rotating behavior. There are usually an equal number of tadpoles that end up in the left and right pouch. While carrying tadpoles, males have a rounded and robust body habitus, but their pouch openings are not more distinct than in non-carrying males. Metamorphosis occurs in the pouches, and the young emerge without any prompting or assistance by the male. Males can be found carrying two groups of tadpoles at different developmental stages, showing that males can incorporate separate clutches that were fertilized at different times. During mating, females do not inspect males to see if he is already holding tadpoles in his pouches, and males carrying young can mate a second time during one breeding season (Mahony et al. 2021).

Larva
Tadpole development in A. wollumbin has been observed to be very similar to tadpole development in A. darlingtoni (Mahony et al. 2021).

Trends and Threats
As of 2023, population trends are unknown because no quantitative information on the numbers of adult A. wollumbin is available (Mahony et al. 2021).

Laboratory observations have shown that Assa are highly susceptible to chytridiomycosis, but there are no reports of declines in Assa populations consistent with this threat. One possible explanation is that since Assa inhabit terrestrial leaf litter, they do not use shared bodies of water with other frogs and the likelihood of chytrid spore transmission is low (Mahony et al. 2021).

Land clearing is not a likely threat to A. wollumbin; there is good evidence that they are dependent on native forests, but all of their habitat (Wollumbin) is located within Wollumbin National Park, which is a protected area (Mahony et al. 2021).

Climate change is the most significant threat to Assa, since it is restricted to cool, moist montane forest areas that are predicted to warm in the coming decades. Model investigations with projected climate change predict that the species will remain stable until around 2040 - 2050, after which declines are expected at different rates based on different model scenarios. The most severe projection predicts Assa will become extinct in the wild by 2095 (Mahony et al. 2021).

The recommended conservation status for A. wollumbinby the authors of its description is “Critically Endangered” following IUCN Red List criteria (Mahony et al. 2021).

Possible reasons for amphibian decline

Climate change, increased UVB or increased sensitivity to it, etc.

Comments

PHYLOGENETIC RELATIONSHIPS:

ETYMOLOGY:

The species epithet, “wollumbin” refers to the distribution of the frog. Given by the First Nation Traditional Custodians for the Tweed, Wollumbin (also known as Mount Warning) is the name of the mountain that is the central volcanic cone of the Tweed volcano region, and the only place where A. wollumbin is known to be found (Mahony et al. 2021).

OTHER INTERESTING INFORMATION:

Molecular data indicates that there has likely been long-term reproductive isolation between A. wollumbin and A. darlingtoni, with the former found in the central Wollumbin volcanic cone and the latter found on the surrounding caldera and slopes. The caldera is about 32 km in diameter with Wollumbin in the center. The natural barrier to gene flow between the two species is unknown and it is possible they may come in contact in areas that have yet to be sampled (Mahony et al. 2021).

References
Mahony, M. J., Hines, H. B., Mahony, S. V., Moses, B., Catalano, S. R., Myers. S., Donnellan, S. C. (2021). “A New Hip-Pocket Frog from Mid-Eastern Australia (Anura: Myobatrachidae: Assa).” Zootaxa, 5057, 4, 451–86. [link]

Species Account Citation: AmphibiaWeb 2023 Assa wollumbin: Mount Wollumbin Hip-pocket Frog <https://amphibiaweb.org/species/9452> University of California, Berkeley, CA, USA. Accessed Jan 14, 2025.

Citation: AmphibiaWeb. 2025. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 14 Jan 2025.

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