Description
Allobates caldwellae is a medium-sized, brown nurse frog with a robust body where the head is wider than it is long. It was described from 22 males with a snout-vent length range of 14.4 - 16.9 mm and three females with a snout-vent length range of 16.6 - 16.9 mm. In both the dorsal and lateral view, the snout is rounded. The nostrils are positioned behind the tip of the snout and face out to the side. Additionally, the nostrils are visible from both the lateral and ventral views. The canthus rostralis is slightly straight in the dorsal view but is rounded in the cross-section. The loreal region is slightly flat. The interorbital region is also flat, and slightly more than twice the eye length. The eye length is about the same as the internarial distance and more than twice the diameter of the tympanum. The tympanum is distinct, small, and rounded with a poorly developed membrane. Males have single, round-when-expanded, subgular vocal sacs that cover two-thirds of the subgular area (Lima et al. 2020).

The forearms of A. caldwellae are almost as long as the upper arm length. There is no metacarpal fold. The palmar tubercle is slightly rounded. The relative length of the fingers are III > IV > I > II. There are no lateral fringes or basal membranes between the fingers. On fingers I, II and IV, there is one round subarticular tubercle, and there are two subarticular tubercles on finger III. Allobates caldwellae has robust and long hind limbs with the thigh being shorter than the tibia. A tarsal keel is present with a short, curved nature, and is separated from the inner metatarsal tubercle. The inner metatarsal tubercle is raised, with a longer length than width, and the outer metatarsal is rounded and raised. The relative lengths of the toes are IV > III > V > II > I. There is one subarticular tubercle on toes I and II, two subarticular tubercles on toes III and V, and three subarticular tubercles on toe IV. However, the proximal subarticular tubercle on toe IV is poorly developed. Supernumerary tubercles are absent. There are basal membranes present between toes III and IV, but there are none present between any other toes (Lima et al. 2020).

The skin of this toadlet is granular on the head, upper eyelid, and dorsum. The dorsal surfaces of the forelimbs are smooth. The dorsal surfaces of the thigh and tibia are generally smooth with a few scattered granules. The skin of the ventral surfaces of the groin, throat, chest, belly, and forelimbs is smooth. The ventral surfaces of the thigh are areolate (Lima et al. 2020).

DIAGNOSIS:

Allobates caldwellae can be differentiated from other species within its genus by body size, coloration, bioacoustics, habitat, and the amount of egg pigmentation. Specifically, male A. caldwellae have a maximum snout-vent length of 16.9 mm, which is larger than A. bacurau and A. grillisimilis but smaller than A. fuscellus, A. masniger, A. nidicola, A. nunciatus, A. paleovarzensis, A. trilineatus, and A. vanzolinius. The snout-vent length range of male A. caldwellae is similar to the snout-vent lengths of A. caeruleodactylus, A. conspicuus, A. marchesianus and A. subfolionidificans. However, A. caldwellae lacks blue fingers and toe discs, which are present in A. caeruleodactylus; A. caldwellae has a wider than long head that differentiates it from A. marchesianus; and A. caldwellae lacks a dark hind limb stripe that is found in A. conspicuus and A. subfolionidificans. The lack of a hind limb stripe also differentiates A. caldwellae from A. tapajos (Lima et al. 2020).

Coloration allows A. caldwellae to be differentiated from A. carajas, A. crombei and A. flaviventris, with A. caldwellae having a light brown dorsum featuring small dark spots. Additionally, male A. caldwellae have a golden yellow throat and vocal sac, which separates this species from the pink to translucent throat of A. carajas, grey or purplish throat of A. crombiei, and the gray throat of A. flaviventris and A. gasoconi and the grey vocal sacs of A. caeruleodactylus, A. conspicuus, A. fuscellus, A. grillisimilis and others (Lima et al. 2020).

Allobates caldwellae is most similar to A. tinae from which it was split, but differs from A. tinae by the yellowish belly in female A. caldwellae, and the lack of melanophores on the throats and vocal sacs in male A. caldwellae or the restriction of a few scattered melanophores to the lower jaw. Additionally, finger III of male A. caldwellae has a uniform width across all phalanges, and it has a longer upper arm length relative to the forelimb length than in A. tinae (Lima et al. 2020).

Bioacoustically, A. caldwellae has a trilling call composed of 3 - 7 single notes that differentiate it from A. bacurau, A. caeruleodactylus, A. crombiei, A. flaviventris, A. grillisimilis, A. magnussoni, A. marchesianus, A. masniger, A. nidicola, A. nunciatus, A. paleovarzensis, A. subfolionidificans, A. tapajos, and A. carajas. From the populations that are confirmed to be A. tinae, A. caldwellae’s call is shorter with a shorter note and call duration, fewer notes, and shorter internote intervals. Additionally, an undescribed species from the upper Madeira River that has a similar appearance as A. tinae can also be distinguished by having pulsed notes, whereas A. caldwellae has unpulsed notes (Lima et al. 2020).

Finally, A. caldwellae can be differentiated from confirmed populations of A. tinae based on habitat differences, with the former being found in dense ombrophilous forests with solid ground in the northern Purus–Madeira Interfluve, whereas A. tinae is found in open ombrophilous forests with palm trees in the southern Purus–Madeira Interfluve. These different environments are suspected to be the cause of the differences in egg pigmentation of the two species, with A. caldwellae having less pigmented eggs, presumably in response to less solar radiation exposure (Lima et al. 2020).

COLORATION:

In life, the dorsum is light brown with small dark brown granule-shaped pigments on the back and delimited by a conspicuous, lighter brown dorsolateral stripe that begins at the eyelids and extends to the urostyle area. Immediately below the dorsolateral stripe is a well-defined, dark brown lateral stripe that spans the same distance. The lateral stripe is narrow at the nostrils, but widens and has a relatively uniform width with well-defined edges starting at the posterior region of the eye sockets. An interrupted, irregular, iridescent white ventrolateral stripe is apparent between the tympanum and mid-body or inguinal region; this stripe is absent in preservation. The dorsal surface of the arms is brownish orange and the background color of the dorsal surface of the hind limbs is brown with darker brown granules. The anterior and posterior areas of the thighs are darker brown. There are no transverse bars on the hind limbs. The ventral surface of the hind limbs is translucent in males but yellowish in females. There is a pale-colored moon-shaped paraclocal mark. Males have gold yellow chests, throats, and vocal sacs with a few scattered melanophores along the jaw and rarely on the vocal sac. Male abdomens are also yellow, but without melanophores. Female throats, chests, and bellies are yellow without melanophores. The irises are golden with black reticulations (Lima et al. 2020).

Newly metamorphosed individuals are light brown with pale dorsolateral and brown lateral stripes in life (Lima et al. 2020).

In preservation, the dorsal background color ranges from light to dark brown. From the snout to the urostyle area, the dorsum has many dark brown, flat granules. A light dorsolateral stripe extends from the snout to the inguinal region. Beneath the dorsolateral stripe is a dark brown lateral stripe that extends from the snout, over the upper portion of the tympanum, to the inguinal region. The lateral stripe varies in width and intensity, being thinner in the snout area and thicker from the posterior corner of the eyes to the inguinal region, and darker along the anterior ⅔ of its length, but becoming lighter and more patchy closer to the groin. The lower part of the tympanum is cream colored. The dorsal surface of the forelimbs is an immaculate brownish-cream. The dorsal surface of the hind limbs are brownish cream colored with both the anterior and posterior thighs being dark brown. The hind limbs have dark brown spots but no transverse stripes. The paracloacal region is dark brown. The ventral surface of the throat, chest, abdomen, legs, and arms are cream colored. The belly is a translucent cream. The ventral surface of the feet and hands are cream colored with several tiny dark spots. (Lima et al. 2020).

VARIATION:

Males and females A. caldwellae differ slightly in coloration and finger structure, but appear to have overlapping body sizes. The 22 male specimens have a snout-vent length range of 14.4 - 16.9 mm, whereas the three female specimens have a snout-vent length range of 16.6 - 16.9 mm. In life, females have a yellowish chest, throat, and abdomen without melanophores whereas these components are gold yellow in males and have few melanophores. Males have a wider finger III, alongside a uniform width for all of its phalanges. Females have an irregular finger III width with the basal phalanx being wider than the medial and distal phalanges (Lima et al. 2020).

Distribution and Habitat

Country distribution from AmphibiaWeb's database: Brazil

View distribution map in BerkeleyMapper.

Allobates caldwellae is distributed throughout the northern Purus–Madeira Interfluve (IPM) in Brazil where the frog inhabits solid ground in dense ombrophilous forests. The northern IPM is characterized by high silt, shallow groundwater soils and compact lowland forests primarily consisting of trees and shrubs with a small basal area. The frog's overall habitat is considered a tropical lowland forest as in most of the Amazon in Brazil. The A. caldwellae holotype was collected at an elevation of 50 m, however, no further information was given about the specific elevation in the species description (Lima et al. 2020).

Life History, Abundance, Activity, and Special Behaviors
The reproductive season begins at the start of the rainy season, which occurs in December up until May. The males vocalize between 5:30 - 9:30 A.M. and 4:00 - 6:30 P.M. from leaves in the litter or up to 40 cm from the ground on small branches or nearby vegetation. However, males can vocalize all day under light rain (Lima et al. 2020).

Males use their calls to attract females to nests within small rolled, folded, or overlapped leaves on the forest floor where cephalic amplexus occurs. After approximately 5 minutes, the males temporarily leave the nests and the females will remain in the leaf until oviposition occurs; the duration of oviposition for this species remains unknown. Only one clutch is laid per nest (Lima et al. 2020).

Females lay the eggs in transparent jelly. The freshly laid eggs have a dark gray animal pole, which covers about ½ of the animal hemisphere. The eggs also have a conspicuous edge with a whitish equatorial portion (Lima et al. 2020).

The males take care of spawning and the transfer of tadpoles into pools of water by carrying the tadpoles on their backs (Lima et al. 2020).

Larva
The description of A. caldwellae tadpoles are based upon 12 tadpoles in Gosner developmental stage 36, however five individuals in earlier stages were also found (See Lima et al. 2020 for more details). At Gosner stage 36, the body length range is 6.5 - 7.0 mm and the total length range is 21.5 - 22.5 mm. The body is slightly wider than it is tall. From the dorsal view, the body is ovoid shaped, truncated at the end, but appears triangular in the lateral view. The width of the body is about 5 mm. The total width of the head is around 4.5 - 4.7 mm. The snout is rounded in both the lateral and dorsal views. The nostrils are visible, widely spaced, located on the dorsal side, and are directed anteriorly. The distance from the eyes to the nostrils is less than the internarial distance and interorbital distance. The eye diameter is slightly smaller than the interorbital distance and directed dorsolaterally. The oral disc is emarginated and located anteroventrally. Its average width is greater than the interorbital distance. There are 3 - 4 short, triangular papillae on each side of the anterior lip that are separated by a gap that is about half the width of the disc. The posterior lip has a single row of 12 - 14 short, triangular papillae. There are no submarginal papillae. The labial tooth row formula is 2(2)/3(1), with the first and second anterior row being about the same length, almost as wide as the oral disc.The first and second posterior row are also about the same length, slightly shorter than the anterior rows, while the third posterior row is shorter than both. The upper jaw sheath is arched and the lower jaw is V-shaped. Both jaw sheaths are narrow and knurled. The spiracle, positioned centrally on the left side of the body, measures around 0.6 mm in length with the last third being free of the body and the opening directed posterodorsally. Allobates caldwellae has a long vent tube with a length of about 1.4 mm that is attached to the right side of the ventral fin. The length of the tail is around 14.8 - 15.5 mm. The caudal muscle is robust and tapers from its highest at the body insertion with a height of 2.0 mm to the tail tip with a height of 0.2 mm; the midpoint is about 1.0 mm high. The dorsal fin begins about 4 mm after the insertion with the body. Although the upper fin arches slightly, it has a uniform width for the majority of the tail. The ventral fin is narrower than the upper. The tail has an acuminated tip and no flagellum. Tadpoles do not appear to have a lateral line (Lima et al. 2020).

Allobates caldwellae larva can be differentiated from other species based on their oral disc characters, coloration and patterning. Specifically, the presence of small, triangular labial papillae on the lips differentiate A. caldwellae larva from A. caeruleodactylus, A. grillisimilis, A. marchesianus, and A. tapajos, which have very long papillae on both lips (A. caeruleodactylus, A. grillisimilis, and A. marchesianus,) or rounded papillae on the anterior and long papillae on the posterior lip (A. tapajos). The number of papillae on the lips is also diagnostic with A. caldwellae having four on each side of the anterior lip and a maximum of 14 on the posterior lip while A. subfolionidificans has six papillae on each side of the anterior lip and 40 on the posterior lip; A. grillisimilis has five on each side of the anterior lip and 29 on the posterior lip; A. paleovarzensis has 16 – 19 posterior lip papillae; A. magnussoni has 12 – 13 papillae on each side of the anterior lip and 32 – 35 posterior papillae; and A. nidicola and A. masniger have no papillae. In preservative, the anteroventral region of the body in A. caldwellae is cream colored, which differentiates it from the translucent anteroventral regions in A. carajas, A. magnussoni, A. palevarzensis and A. tapajos. The absence of transverse tail bars in A. caldwellae differentiate them from A. caeruleodactylus, and A. marchesianus. And lastly, A. nidicola and A. masniger are fully terrestrial, endotrophic larva, completing development in the nest while A. caldwellae is exotrophic and aquatic, being transported to water by the male parent (Lima et al. 2020).

In life, the dorsal and lateral background color is brown. Otherwise the coloration is similar to that in preservative. In preservative, A. caldwellae larva have a cream background on the dorsum, lateral region, and tail musculature that is covered by dark brown, irregular shaped and sized melanophores. The anteroventral region is a lighter cream color, but also covered with irregular, dark spots. The posteroventral region is translucent, showing the intestines inside. The fins are also translucent but have whitish reticulations and clusters of irregular melanophores (Lima et al. 2020).

Males carry larva on their back into water (Lima et al. 2020).

Trends and Threats
At the time of the species description, no trends or threats were evaluated for this species. However, the authors note that there is ongoing, intense deforestation within the Purus–Madeira Interfluve (Lima et al. 2020).

Possible reasons for amphibian decline

Habitat modification from deforestation, or logging related activities

Comments
Bayesian inference analysis of 12S, 16S, COI, and CytB mtDNA and six nuclear genes (28S, HH3, RAG1, RHO, SIA, TYR) found that A. caldwellae was a unique species and split from A. tinae. The analysis also found that A. caldwellae is sister to a clade composed of confirmed A. tinae specimens and specimens from the municipality of Tefé that have a similar appearance to A. tinae but likely represents an undescribed species. The next most closely related clade includes specimens from the east bank of the upper Madeira River that also have a similar appearance to A. tinae but likely represent an undescribed species (Lima et al. 2020).

References
Lima, A. P., Ferrão, M., and Lacerda Da Silva, D. (2020). Not as widespread as thought: integrative taxonomy reveals cryptic diversity in the Amazonian nurse frog Allobates tinae Melo‐Sampaio, Oliveira and Prates, 2018 and description of a new species. Journal of Zoological Systematics and Evolutionary Research, 58(4), 1173–1194. [link]

Species Account Citation: AmphibiaWeb 2025 Allobates caldwellae <https://amphibiaweb.org/species/9743> University of California, Berkeley, CA, USA. Accessed May 10, 2025.

Citation: AmphibiaWeb. 2025. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 10 May 2025.

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