Du Toit's torrent frog

Du Toit's torrent frog
Conservation status
Critically Endangered  (IUCN 3.1)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Amphibia
Order:	Anura
Family:	Petropedetidae
Genus:	Arthroleptides
Species:	A. dutoiti
Binomial name
Arthroleptides dutoiti Loveridge, 1935
Synonyms
Petropedetes dutoiti (Loveridge, 1935)

Du Toit's torrent frog, the Mt. Elgon torrent frog, or the Kenya rocky river frog (Arthroleptides dutoiti) is a species of frog in the family Petropedetidae endemic to Mount Elgon in Kenya; it has not been found on the Ugandan part of the mountain. It is one of many, often taxonomically unrelated, frogs referred to as torrent frogs.

The species was found by Cornelius Albertus du Toit, who collected the frog holotype in 1934. Dr. du Toit was a member of the Cape Town University, a founder member of the Zoological Society of South Africa, and Professor of Zoology at Stellenbosch University in South Africa. He wrote Zoological research in South Africa in 1961.

Discovery and naming

From 1933 to 1934, the Museum of Comparative Zoology conducted surveys of the reptiles and amphibians in the rainforests of Kenya and Uganda. During a survey on January 8, 1934, Cornelius Albertus du Toit discovered a species of frog in the Koitobos River on the eastern slopes of Mount Elgon in Kenya, about 7,200 ft (2,194.6 m) above sea level. He collected three individuals, which would later be studied and recognized as a new species by British biologist Arthur Loveridge in 1935. Loveridge named this species Arthroleptides dutoiti after its discoverer, designating a gravid female specimen as the holotype of the species, while the other two specimens (an adult male and a young individual) were designated as paratypes. The species was next recorded in April 1962, when US herpetologist Ronalda Keith collected an additional eight individuals about 7,000 ft (2,133.6 m) above sea level at the Suam River, around 10 km (6.2 miles) from the type locality, and recorded her observations on the frogs in her field book. The specimens collected by Keith have been placed in the collection of the American Museum of Natural History.

Distribution and habitat

View of a montane stream on Mount Elgon

The Du Toit's torrent frog is endemic to Mount Elgon, with the only known sightings recorded from the Koitobos and Suam rivers on the Kenyan side of the mountain, between 2,100–2,200 m (6,889.8–7,217.8 ft) above sea level. As such, the species has a small extent of occurrence of only 973 km² (375.7 sq mi). The frog's natural habitat consists of rocky streams with cold, fast-flowing water surrounded by dense montane forest. However, surveys from 2001 to 2014 in both the wet and dry seasons have failed to record the species on Mount Elgon, and it has likely been extirpated from the surveyed locations (including the locations it had formerly been observed in), or possibly become extinct entirely.

Description

This species is a small frog, with the adult female type specimen collected in 1934 measuring 31 mm (1.2 in) in snout-vent length, and the adult male paratype specimen being slightly smaller, with a snout-vent length of only 25 mm (0.98 in). The head is slightly broader than it is long, with distinctly visible tympana, each with a diameter a little less than two thirds that of the eyes. No vomerine teeth are present in this species. The snout tapers slightly, with nostrils located midway between the eyes and the snout tip, and a very prominent canthal ridge. The width of the upper eyelids is approximately equal to the distance between the eyes. The digits widen at the tips to form disks, which each possess a groove down the middle. The toes but not the fingers are half-webbed. The hind legs are long, exceeding the snout-vent length, and in the male paratype specimen are twice the snout-vent length. The skin of the back is distinctly warty and pitted, but smooth elsewhere.

The coloration of live individuals was recorded by Ronalda Keith in her field book when seeing this species in 1962, writing that they are mostly dark grey or black with speckled patterns on their upper surface. These speckles are usually khaki but can vary in color, sometimes being more greenish grey in juveniles, dark grayish black in females or tanish grey in males. The iris is horizontal, with a golden line above it. The underside is lighter, being grey with blue spots in females, and a lighter shade of grey with brown spots in males. A line of pale dots is present over the anus, as are triangular patterns on the snout between the eyes and nostrils. The legs possess vertical bands of color, which are more prominent in juveniles, and the digit tips are slightly white-edged.

Biology

Like other members of the genus Arthroleptides, the Du Toit's torrent frog occurs in and around rocky montane streams, with the dark, mottled color of this species providing camouflage against the volcanic rocks in its habitat. Upon finding live individuals in 1962, Ronalda Keith recorded that the frogs would sit on rocks or in the water beneath overhanging rocks, particularly favoring long wet moss growing on rocks and logs as sitting locations, and would even sometimes hang from rocks and roots like in insect. The amphibians are powerful swimmers, capable of emerging onto rocks several feet away after diving into rapids and whirlpools. Keith observed the frogs for five days in the rainy season, during which she documented the male frogs calling from shelters in the banks and logs for the entire duration of her observation, each occupying a territory of at least 5 sq in (32.3 cm²), while the female and juveniles frogs remained in the water. The species is oviparous, with an adult female kept in captivity by Keith recorded to have laid around 12 eggs, each encased in a strong jelly-like substance and adhered to each other to form a blob. The eggs are rather large, around 3 mm (0.1 in), and are pigmented. Although eggs and tadpoles have not been observed in the wild, it is presumed that since the streams this frog inhabits lack shallow, quiet backwaters, the eggs are laid on wet rocks close to torrential streams and waterfalls, and the tadpoles develop out of the water on the rocks, as this is the case for closely related species such as the southern torrent frog.

Conservation

A. dutoiti is an EDGE species, reflecting its evolutionary distinctness and endangered status. It was last seen in 1962, despite later surveys. Because the habitat of this species appears to be generally in good condition, its disappearance might have been caused by disease such as chytridiomycosis.