Aubria subsigillata (Duméril, 1856)
There is still some dispute about whether Aubria subsigillata and Aubria occidentalis comprise one or multiple species (Perret 1994, Ohler 1996, Amiet et al. 2004).
Aubria subsigillata is a large, stocky frog with a dark brown back and a relatively small but visible tympanum. The underside is speckled white over a brown background, although in older individuals the throat may be almost entirely white while only the posterior portion of the underside retains the speckled pattern. Distinct yellow-brown glands are present on the underside of the thigh at the midpoint of the femur in all individuals, even juveniles and sub-adults. Snout-vent length is 65-95 mm and females are typically significantly larger than males. It is generally difficult to tell the sexes apart, though, as male secondary sex characteristics such as vocal sacs and nuptial pads are absent. However, compared with males, females may have relatively shorter eyes, and femoral glands closer to the base of the thigh.
The IUCN Red List (2004) categorizes this species as Least Concern in view of its relatively wide distribution, tolerance of a degree of habitat modification, presumed large population, and because it is unlikely to be declining fast enough to qualify for listing in a more threatened category (Amiet et al. 2004).
It presumably occurs in several protected areas (Amiet et al. 2004).
It is an adaptable species that is not likely to be facing any significant threats (Amiet et al. 2004).
Populations of this species are believed to be stable (Amiet et al. 2004).
This is a large species (adult males: SVL 65.1-87.6 mm, n = 13; adult females: SVL 76.0-95.0 mm, n = 9). The distance from the nostrils to the end of the snout is short (64-88‰ of the SVL), and the distance between the nostrils is important (61-74‰ of the SVL). The diameter of the tympanum is relatively small (62-84‰ of the SVL). Round glands are present under the thighs in males, females, and juveniles. The back is uniformly brown without a medio-dorsal line. The throat, chest, and belly have round whitish spots surrounded by a dark brown network in specimens of varying sizes (Ohler and Kazadi 1990, translated/adapted by Dietterich (2010)), although this pattern may disappear from the throat area in adult specimens (Ohler 1996).
This species is nocturnal, becoming active from just after dark until approximately 1:00 a.m. During the day, it burrows down in the mud to depths of 50 cm or more, leaving inconspicuous holes (Knoepffler 1976).
Tadpoles are dark in color and frequently aggregate into ball-shaped schools (Knoepffler 1976). The tail (23 mm) is slightly longer than the body (16 mm). The keterodont rows formula is 2:4+4/3 (Ohler 1996). Schiøtz (1963) gave a more thorough description in Danish.
Aubria subsigillata can be differentiated from its close relative A. masako by the location of femoral glands, foot webbing, and skin coloration. In particular, the femoral glands of A. subsigillata are located midway between the knee and the cloaca on the ventral side of the thigh, whereas in A. masako they are closer to the knee and located more posteriorly. The feet of A. subsigillata are more webbed (I 1 – 2 II 1 – 2 III 1 – 2.5 IV 2.5 – 1 V) than those of A. masako (I 1½ - 2 II 1 – 2½ III 2 – 3 IV 2½ - 1 V). As for skin coloration, the dorsum of A. subsigillata is more uniformly colored than that of A. masako and lacks a mid-dorsal line; additionally, A. masako may have warts but the skin of A. subsigillata is smooth. Ventral coloration develops differently in the two species as well: the mottled pattern of A. subsigillata disappears first on the throat whereas in A. masako it disappears first on the vent (Ohler 1996).
The difference between Aubria subsigillata and A. occidentalis is harder to determine because there is still some disagreement about these species’ identities and the boundaries between them. Perret (1994) argues that they can be distinguished by the position of the femoral gland (in the middle of the femur in A. occidentalis, closer to the knee in A. subsigillata) and possibly by the sound of their calls, but Ohler (1996) claims they are the same species. Amiet (2004) agrees with Perret (1994) that A. occidentalis and A. subsigillata are different species, but does not specify how to differentiate them.
The original description by Duméril (1856) is incomplete, so the following section is adapted from a thorough redescription by Ohler and Kazadi (1990).
The holotype (specimen 1566 at the Muséum national d’Histoire naturelle, Paris) is an adult male with snout-vent length (SVL) 77.8 mm, collected in Gabon by Charles-Eugène Aubry-Lecomte.
Description of the holotype. – The head is longer (32.0 mm) than it is wide (28.4 mm). The snout is rounded and longer (13.7 mm) than the eye (9.5 mm), projecting in front of the buccal opening. The canthus rostralis is rounded and the loreal region is concave. The distance between the nostrils (5.9 mm) is greater than the interorbital distance (3.5 mm), which is less than half the width of the upper eyelid (7.3 mm). The nostrils are closer to the eye (7.4 mm) than the end of the snout (7.7 mm), with their openings somewhat rounded with two small dermal outgrowths on the external border. The tympanum is rounded, with diameter (4.9 mm) about half of the diameter of the eye (9.5 mm) and less than its distance to the eye (6.4 mm). The pineal ocellus is absent. Vomerine teeth are present at the level of the internal side of the choanae in two rows each containing four teeth. The angle between these rows is approximately 100° opening anteriorly, with the distance between their posterior ends equal to their length. Maxillary teeth are well developed. Three points at the end of the lower jaw correspond to cavities in the upper jaw. The tongue is oval and elongated, with a deep indentation in its free posterior part. A weak supratympanic fold runs from the eye to the shoulder. The forearm is slightly longer and wider than the rest of the arm. The fingers are robust, with digit II the shortest, digit I slightly longer than IV, and digit III the longest. The tips of the digits are rounded and not enlarged. There is a dermal crest on the internal edge of digits II and III. There is a terminal bead at the level of the distal joint in dorsal position on each digit. Sub-articular tubercles are rounded and moderately developed. Metacarpal tubercles are elongated. Just one elongated palmar tubercle is present but it is little developed.
The hind legs are short and robust. The heels do not touch when the thighs are placed at a right angle relative to the axis of the body. The leg (30.0 mm) is slightly shorter than the thigh (34.9 mm) or the foot, measured from the proximal edge of the internal metatarsal tubercle to the tip of toe IV (38.2 mm). Toe IV is the longest, with toe III longer than toe V. Toe tips are rounded but not enlarged. There is a terminal bead at the level of the distal joint in dorsal position on each toe. Webbing is moderately developed, up to the proximal edge of the distal sub-articular tubercle of toe IV, and curving in between toes III and IV and between toes IV and V midway between the proximal and intermediate sub-articular tubercles. There is no dermal fringe anywhere on the toes. Sub-articular tubercles are well developed and elongated. Internal metatarsal tubercles are short (2.4 mm) but very prominent, less than half the length of toe I (8.4 mm). There is no external metatarsal tubercle. There is a light tarsal fold from the metatarsal tubercle to the tibio-tarsal joint. The skin on the dorsal surface has miniscule roughnesses, but maintains a smooth aspect. The skin on the sides of the stomach is folded. There is no latero-dorsal fold. A round gland is present on each thigh at two-fifths of the distance between the anus and the knee, of diameter (5.2 mm) larger than that of the tympanum (4.9 mm). There is a heap of glandular cells under the base of each arm. No male secondary sex characteristics are visible.
The color of the dorsal surface is blackish brown, with the ventral surface dirty beige and grey-brown, with a regular design of light round spots on a dark network on the chest and belly. Under the thighs, the tibias, and the feet, there are lighter spots placed in a more irregular manner on a dark background, and there is no marked pattern on the throat.
Specimens determined to be Aubria subsigillata vary in the possession of sub-brachial glands that are not always prominent. Femoral glands are always distinct under the thighs, even in juveniles and sub-adults. A medio-dorsal line has never been observed in this species. The reticulated ventral design is usually very clear on adult specimens. Certain ventral regions can lack this design, but no marked relation between this phenomenon and the age or sex of specimens could be determined. The pineal ocellus is present in almost all specimens but it is absent in the holotype. The medial palmar tubercle, which is also absent in the holotype, is present in most specimens but it is often short and poorly developed.
This species is generally large. Adult males range in snout-vent length from 65.1-87.6 mm, and adult females are larger with snout-vent lengths of 76.0-95.0 mm (Ohler and Kazadi 1990). Two specimens at Harvard University's Museum of Comparative Zoology have snout-vent lengths of 82 mm and 66 mm (Dietterich, unpublished results).
According to Ohler’s review of the genus Aubria (1996), Aubria subsigillata is sister to A. masako. The genus Aubria, in turn, is sister to Pyxicephalus in the subfamily Pyxicephalinae, which is sister to the subfamily Dicroglossinae. Ohler and Kazadi (1990) hypothesized that, since Pyxicephalus appears more derived than Aubria, it either evolved from Aubria, or otherwise Aubria evolved from it in via paedomorphosis. Ecological evidence suggests that Pyxicephalus was the more derived clade (Ohler and Kazadi 1990), but Ohler (1996) suggests that in fact Aubria is more derived than Pyxicephalus, which would imply some paedomorphic events in Aubria’s evolutionary history.
Aubria subsigillata is unusual among frogs in that its adult diet consists primarily of small fish, in particular Epiplatys that leap out of the water to escape predators or to look for different surroundings. They may also eat smaller amphibians and arthropods (Knoepffler 1976, Channing 2001, Rödel et al. 2005).
Aubria subsigillata ranges from southwestern Cameroon through mainland Equatorial Guinea to southwestern Gabon. It occurs on the island of Bioko (Equatorial Guinea) (Amiet et al. 2004). Jackson et al. (2007) have collected A. subsigillata as far east as the northeastern part of the Republic of the Congo. The contact zone between this species and the more recently described Aubria masako and A. occidentalis is poorly understood and requires further investigation. It is possible that this species ranges as far west as Nigeria. An old record from Angola is shown by Perret (1996) to belong to a very different, undescribed species (Amiet et al. 2004).
It lives in swamps or along small streams in lowland rainforest, gallery forest and degraded secondary habitats (farm bush) in the forest zone. It breeds in still-water pools and marshes (Amiet et al. 2004).
Metamorphosis has not been thoroughly studied in this species, but it usually occurs when the tadpole is about 24 days old (Knoepffler 1976).
Little information is available on its abundance (Amiet et al. 2004).
Since these frogs are nocturnal and burrow during the day, they are difficult to observe, but the eggs are numerous, laid in strings, and of the Bufo type (Knoepffler 1976). There is evidence that males might guard their tadpoles in the water but this has yet to be properly documented (Ohler and Kazadi 1990).