Southern torrent frog

The southern torrent frog (Arthroleptides yakusini) is a species of frog in the family Petropedetidae endemic to Tanzania, where it is found in the Uluguru, Udzungwa, and Mahenge Mountains. It is one of many, often taxonomically unrelated, frogs referred to as torrent frogs. It occurs mainly around rocky montane streams surrounded by mature forest at elevations of 300–2,800 m (984.3–9,186.4 ft) above sea level, but also can be found on the forest floor away from water, and may be tolerant of open woodland environments. While it is sometimes claimed to be found in the Nguru Mountains as well, molecular evidence shows that this population may instead represent an unnamed species.

This frog is a nocturnal species, spending the daytime hidden away under rocks and emerging to feed after nightfall. It can grow over 70 mm (2.8 in) in snout-vent length, with males commonly growing larger than females. With its wide jaws, it is believed to prey on smaller frogs. The upper side of its body is mainly gray-brown with mottled patterns and an orange-reddish stripe stretching across each eye, camouflaging it against the leaf litter, while the underside is pale. During the breeding season, mature males produce advertisement calls to attract mates and develop a number of secondary sexual characteristics, including swollen forearms, spines on the throat and chin, knobs on the metacarpal area and a soft protrusion called a tympanic papilla on each tympanum. Eggs are laid on rocks over which a film of water is present, either in small clusters of four to five eggs or large groupings of about 200. The tadpoles remain attached to the rocks, developing out of water for 8 to 10 weeks until their metamorphosis is complete.

This species was named and described in 2002, with an adult male specimen collected in 1995 being designated as the holotype. Prior to this, it was thought to represent a population of the Usambara torrent frog, now known to be a closely related yet separate species. The specific name originates from the Swahili words "ya kusini" which mean "of the south", and the common name "southern torrent frog" has been given to this species, though this common name is also used for the unrelated Asian species Amolops australis. This African species has been listed as Endangered by the IUCN, as it has a severely fragmented distribution and its forest habitat is under threat from encroaching agriculture. Various diseases such as strongyloidiasis and chytridiomycosis are known to occur in this frog, the latter of which is known to cause drastic amphibian population declines, though these illnesses are not always fatal to the infected frog.