Juvenile Tarahumara Frog from the Sierra La Madera, Northern Sonora
The Tarahumara frog (Rana tarahumarae), is a medium-sized (adults range from 64 to 114 mm in snout-vent length), drab green-brown frog with small brown to black spots on the body and dark crossbars on the legs. The hind feet are extensively webbed. This species typically lacks a dorsolateral fold, characteristic of related leopard frogs and other ranid species. Larvae are greenish-yellow with small dark spots over the dorsum and larger spots on the tail. Larvae grow as large as 97 mm prior to metamorphosis. Tarahumara frogs of both sexes have a call consisting of a low grunt of about one-half second in duration, uttered once or twice (sometimes more).
Prey items taken by Tarahumara frogs are diverse, and include juvenile mud turtles, fish, snakes, beetles, moths, water bugs, scorpions, centipedes, grasshoppers, mantids, wasps, spiders, crickets, caddisflies, and katydids. Tarahumara frogs are probably preyed upon by ringtail cats, snakes (especially garter snakes), birds, other frogs, rosy salamanders, fish, and invertebrates, particularly species of water bugs.
The Tarahumara frog is known from 63 localities in montane canyons in extreme southern Arizona south to northern Sinaloa and southwestern Chihuahua, Mexico. The range of the species is centered in the northern Sierra Madre Occidental of Mexico; however, the eastern and southern distributional limits are not clear. The species may not occur south of the Sierra Surutato in Sinaloa. Apparently suitable habitat occurs in Durango, well south of the current known range, but other ranid frogs occur there and specimens of Rana tarahumarae have not been collected in Durango . Rangewide, most localities are in the mountains of eastern Sonora. In the United States the species was known from six locales in Arizona near the Mexican border, including three from the Santa Rita Mountains and three from the Atascosa-Pajarito-Tumacacori Mountains. The species is extirpated from all localities in Arizona. The last observation of a Tarahumara frog in Arizona was in May 1983 in Big Casa Blanca Canyon in the Santa Rita Mountains.
Throughout its range the Tarahumara frog is typically associated with canyons and deep "plunge pools" formed amidst boulders or in bedrock. Plunge pools in canyons with low mean flows (<0.2 cubic feet per second) and relatively steep gradients (> 60 m per km of stream) provide the best breeding sites. Permanent water is probably necessary for metamorphosis. Tarahumara frog habitats are located within oak, pine-oak woodland, or the Pacific coast tropical area (Sinaloan thornscrub and tropical deciduous forest).
The reasons why the Tarahumara frog disappeared from Arizona are not clear. However, the following hypotheses have been presented: 1) winter cold; 2) flooding or severe drought; 3) competition; 4) predation; 5) disease; and 6) heavy metal poisoning. Metals occur naturally in streamside deposits and may be mobilized by acid precipitation events. Acidic rainfall in southeastern Arizona and Former habitat of the Tarahumara frog at the "Bathtubs" in Big northern Sonora may have Casa Blanca Canyon, Santa Rita Mountains, Arizona occurred as a result of atmospheric emissions from copper smelters at Cananea and Nacozari, Sonora and at Douglas, Arizona. Cadmium toxicity may have contributed to observed Tarahumara frog die-offs in Arroyo La Carabina, Arroyo Pinos Altos, and Arroyo La Colonia in Sonora, and Big Casa Blanca and Sycamore canyons in Arizona . Recent evidence suggests a fungal disease, chytridiomycosis, may also be a factor in declines and extirpations of the species from Arizona and some localities in northern Sonora . This same disease has been implicated in declines of frog and toad populations in other areas of North America, Central America, Australia, and elsewhere. Predation by nonnative fish and bullfrogs may have contributed to the disappearance of the species from Pena Blanca Spring and portions of Pena Blanca Canyon, Arizona