T. macrostomus is completely aquatic, and restricted to lakes and waters that are directly connected to them like rivers, streams and canals. It has also been introduced to the upper part of the Mantaro River, but it is unclear if this population persists. It is found both in large marshy lakes and smaller deep lakes. This smooth-skinned, dark brownish frog generally ranges from shallow water to a depth of, but prefers places with extensive aquatic vegetation and deep. In its native habitat the water temperature typically is between. Captives have been kept in aquariums for more than 5 years at around. It feeds on aquatic snails, amphipods, aquatic insects and small fish. T. macrostomus is among the largest frogs in the world and the largest exclusively aquatic frog, a title sometimes incorrectly awarded to its somewhat smaller relative, the Titicaca water frog. T. macrostomus measures up to in snout–to–vent length, and in outstretched length. It can weigh up to about, and the hindlegs alone can surpass in length. Very large individuals are rare; most adults have a snout–to–vent length of and weigh less than. Females grow larger than males. The tadpoles also grow very large, up to about in length.
Conservation and threats
Once common, this frog is now rare and considered endangered by the IUCN. During a survey in 2012 it was only found at about 10% of the visited sites and in low densities, leading to the suggestion of recognizing it as critically endangered. The primary threats to the species are capture for human consumption, predation by introduced trout, pollution from mining, agriculture and human waste, and water fluctuations caused by the Upamayo Dam. These factors have also contributed to the decline of the native Orestias pupfish, historically an important part of the diet of the frog, but not recorded in a recent study of its diet. Although not yet detected in T. macrostomus, some of its relatives have strongly declined due to diseases like chytridiomycosis and Ranavirus. Mass deaths have occurred in T. macrostomus, with a particular large one in 1996, but the reason for this is unknown. Much of its range is within reserves, but protection is insufficient. Local and national groups, partially funded internationally, have started monitoring and conservation projects for this species, and education for the people living in its range. In 2008, an experimental captive breeding program was initiated at two centers at Lake Junin and it resulted in a few thousand tadpoles, but this project ceased in 2012. Following progress and success with the captive breeding program for the related Titicaca water frog, it is planned to expand it to the Junin species at Huachipa Zoo in Lima.
