Seychelles frogs

(Sooglossidae)

Class Amphibia

Order Anura

Family Sooglossidae

Thumbnail description
Small, secretive frogs with varying, generally subdued coloration and generalized body form

Size
Snout-vent length of adults ranging from 0.41 to 2.2 in (10.5–55 mm)

Number of genera, species
2 genera; 3 species

Habitat
Tropical rainforest, including both undisturbed and disturbed forest

Conservation status
Endangered: 1 species; Vulnerable: 2 species

Distribution
Granitic islands of the Seychelles, western Indian Ocean

Evolution and systematics

The three species of sooglossid frogs were discovered at the turn of the nineteenth century and were described from specimens sent to European museums by field biologists. These frogs mistakenly were thought to be bufonids or ranids until 1931, when Noble placed them in a separate subfamily (Sooglossinae), which he thought was a subgroup of the Pelobatidae. The taxonomic history of sooglossids has been tortuous, and even today the phylogenetic history and classification are uncertain. However, all frog systematists today rank them as a full family. Sooglossids have no fossil record, but it is believed that they originated many millions of years ago and may be transitional between the more primitive arciferal frogs (with separate shoulder girdles) and the more advanced firmisternal groups (with fused shoulder girdles). No subfamilies are recognized.

Sooglossids are confined to the high granitic islands of the Seychelles archipelago, which are isolated from major landmasses in the western Indian Ocean. The islands are 1,000 mi (1,600 km) distant from Africa (Mombasa), 580 mi (930 km) northeast of Madagascar, and 1,800 mi (2,900 km) southwest of India (Bombay). Because amphibians are intolerant of saltwater and have no obvious means of transoceanic dispersal, the presence of endemic frogs in the Seychelles was somewhat of a mystery until the history of these islands was elucidated. The main islands of the Seychelles are composed of granite rocks, which are of a continental nature. The islands are the mountaintop remnants of a partially submerged microcontinent that was left behind as India drifted northward toward Asia during the Cenozoic. The exact date that the Seychelles microcontinent separated from India is unknown, but it probably occurred sometime between 55 and 65 million years ago. The geological history of the Seychelles suggests that the ancestors of the modern sooglossids drifted to their present position and have been isolated for many millions of years. The observation that these frogs have no obvious sister group also suggests they have been isolated for a very long time. These facts indicate that the Seychelles microcontinent has never been submerged fully since it detached from India; otherwise there would be no surviving endemic frogs and other ancient endemic groups, such as the Seychellean caecilians (Amphibia, Gymnophiona).

Physical characteristics

Sooglossid frogs are small to medium-small, ordinary frogs. They have subdued colors that generally make them difficult to see where they live on the forest floor among litter and rocks and on low vegetation. The smallest species, Sooglossus gardineri, or Gardiner's frog, is among the smallest frogs in the world, with adults growing to only about 0.39–0.47 in (10–12 mm) in snout-vent length. Nesomantis thomasseti, or Thomasset's frog, is much larger at about 1.8 in (45 mm) in snout-vent length. Females are slightly larger than males in all three species. There are no obvious differences in coloration between the sexes, and young are colored nearly the same as adults.

Distribution

Sooglossids are restricted to two granitic islands, Mahé and Silhouette, of the Seychelles in the western Indian Ocean. The islands lie just south of the equator between 4° and 5° south latitude and 55° and 56° east longitude.

Habitat

Sooglossids occur in the rainforests above the 656 ft (200m) contour line. They have not been observed on coastal plains. Presumably, their ancestral habitat was undisturbed forest, but they obviously survive in disturbed and even highly disturbed forests. The Seychelles frog, Sooglossus sechellensis, and Gardiner's frog are not associated with streams, whereas Thomasset's frog usually is found near streams.

Behavior

Sooglossids are secretive frogs, generally hiding in leaf litter, hollow stems, rock crevices, and leaf axils of low vegetation. Generally, they are not active on the surface except during rainy weather.

Feeding ecology and diet

These frogs eat a wide variety of small invertebrates, including mites, fruit flies, moths, mosquitoes, and other forest floor insects. Thomasset's frog often perches on rocks near streams at night and feeds on flying insects.

Reproductive biology

Sooglossids call day or night from hiding places; each species has a distinctive call. Gardiner's frog has a high-pitched "peep" and the Seychelles frog a "wrracck toc toc toc toc"; Thomasett's frog produces a call similar to that of the Seychelles frog, which sounds like "wrracck wrracck wrracck toc toc toc."

These frogs have the primitive form of the mating embrace (inguinal amplexus), in which the male clasps the female just in front of her hind limbs with his forelimbs. The Seychelles frog and Gardiner's frog deposit their eggs in hidden nests on the forest floor. Both species engage in parental care, in which the female remains with the eggs until they hatch. This finding is contrary to statements in the early literature, which claimed that the male Seychelles frog guards the young. In the latter species, the eggs hatch into tadpoles, which climb onto their mother's backs and are carried around until they metamorphose into froglets. The froglets remain on their mother's backs a short time but soon jump off to live an independent life.

The eggs of Gardiner's frog hatch directly into tiny froglets about the size of a grain of rice, which soon leave the nest. There is no post-hatching tadpole stage in this species, and the mothers do not transport the young on their backs. Nothing is known about the reproduction of Thomasset's frog; presumably, they deposit their eggs in hidden nest sites on land, and the females guard the eggs until they hatch directly into small froglets. This is a suggestion based on the reproductive biology of the two other species and the fact that no unidentified tadpoles have been found in aquatic habitats in the Seychelles.

Conservation status

Gardiner's frog and the Seychelles frog are listed as Vulnerable by the IUCN; both occur in dense populations and are distributed widely at higher elevations. Thomasset's frog, however, which is listed as Endangered by the IUCN, is less common and has a more restricted range. All three species occur in a national park on one of the islands. Although there appears to be no immediate threat to their survival, the fact that they are restricted to two tiny islands with expanding human populations is reason for concern.

Significance to humans

None known.

Species accounts

List of Species

Gardiner's frog

Sooglossus gardineri

taxonomy

Nectophryne gardineri Boulenger, 1911, Mahé, Morne Pilot, 2,700 ft (823 m), and Silhouette, highest jungle.

other common names

None known.

physical characteristics

These are among the world's smallest frogs. The average snout-vent length of adult males is 0.4 in (10.2 mm), with a maximum of 0.43 in (11 mm); the average for females is 0.47 in (11.9 mm), with a maximum of 0.5 in (13 mm). The coloration varies widely. Some frogs are uniformly reddish brown on the dorsum, and others are tan; some have scattered spots, and others have stripes on the dorsum. The sides of the head and body are usually darker than the dorsal and ventral surfaces.

distribution

The species occurs at elevations above 660 ft (200 m) on Mahé and Silhouette, Seychelles Archipelago, in the western Indian Ocean.

habitat

They inhabit the forest floor and low vegetation.

behavior

The frogs are active night and day during the rainy season.

feeding ecology and diet

This species feeds on tiny ground and litter-layer invertebrates.

reproductive biology

The call is a high-pitched "peep." The female deposits eight to 15 eggs in hidden nests on the forest floor. The female guards the eggs until they hatch into tiny froglets about 0.12 in (3 mm) long. There is no larval stage.

conservation status

The IUCN lists the Gardiner's frog as Vulnerable.

significance to humans

None known.

Seychelles frog

Sooglossus sechellensis

taxonomy

Arthroleptis sechellensis Boettger, 1896, Auf den Seychellen.

other common names

None known.

physical characteristics

This is a medium-sized sooglossid. The average snoutvent length is about 0.59 in (15 mm) in males and about 0.79 in (20 mm) in females. The dorsum is golden brown, and the sides and upper surfaces of the legs have scattered black spots. There is a large, often triangular black spot on top of the head between the eyes.

distribution

This species occurs at elevations above 660 ft (200 m) on Mahé and Silhouette, Seychelles archipelago, in the western Indian Ocean.

habitat

The Seychelles frog inhabits leaf litter on the forest floor and at the edges of rainforest.

behavior

These secretive frogs are seldom seen at the water surface.

feeding ecology and diet

The Seychelles frog feeds on small insects, mites, and other invertebrates that live in forest litter and rotten logs.

reproductive biology

Males call day or night from hidden sites on the forest floor during the rainy season: "wrracck toc toc toc toc." Females deposit six to 15 small white eggs in hidden nests. They remain with the eggs until they hatch into tadpoles. Tadpoles are transported on the mother's back until they metamorphose into tiny froglets. There is no aquatic tadpole stage.

conservation status

The IUCN classifies this species as Vulnerable.

significance to humans

None known.

Resources

Books

Noble, Gladwyn K. The Biology of the Amphibia. New York: McGraw-Hill, 1931.

Nussbaum, R. A. "Amphibians of the Seychelles." In Biogeography and Ecology of the Seychelles Islands, edited by D.R. Stoddart. Hague: Dr. W. Junk, 1984.

Periodicals

Green, D. M., R. A. Nussbaum, and Y. Datong. "Genetic Divergence and Heterozygosity Among Frogs of the Family Sooglossidae." Herpetologica 44, no. 1 (1988): 113–119.

Griffiths, I. "The Phylogenetic Status of the Sooglossine." Annals and Magazine of Natural History 2, no. 22 (1959): 626–640.

——. "The Phylogeny of the Salientia." Biological Review no. 38 (1963): 241–292.

Noble, Gladwyn K. "An Analysis of the Remarkable Cases of Distribution Among the Amphibia, with Descriptions of New Genera." American Museum Novitates no. 212 (1926): 1–24.

Nussbaum, R. A. "Mitotic Chromosomes of Sooglossidae (Amphibia: Anura)." Caryologia 32, no. 3 (1979): 279–298.

——. "Phylogenetic Implications of Amplectic Behavior in Sooglossid Frogs." Herpetologica 36, no. 1 (1980): 1–5.

——. "Amphibian Fauna of the Seychelles Archipelago." National Geographic Society Research Reports no. 18 (1985): 53–62.

Nussbaum, R. A., A. Jaslow, and J. Watson. "Vocalization in Frogs of the Family Sooglossidae." Journal of Herpetology 16, no. 3 (1982): 198–203.

Ronald A. Nussbaum, PhD