Telmatobius culeus, commonly known as the Titicaca water frog or Lake Titicaca frog, is a medium-large to very large and endangered species of frog in the family Telmatobiidae. It is entirely aquatic and found only in the Lake Titicaca basin, including rivers that flow into it and smaller connected lakes like Arapa, Lagunillas and Saracocha, in the Andean highlands of Bolivia and Peru. In reference to its loose skin, it is sometimes jokingly referred to as the Titicaca scrotum frog.
It is closely related to the more widespread and semiaquatic marbled water frog (T. marmoratus), which also occurs in shallow, coastal parts of Lake Titicaca, but lacks the excessive skin and it is generally smaller (although overlapping in size with some forms of the Titicaca water frog).
Appearance Size In the late 1960s, an expedition led by Jacques Cousteau reported Titicaca water frogs up to in outstretched length and in weight, making these some of the largest exclusively aquatic frogs in the world (the exclusively aquatic Lake Junin frog can grow larger, as can the helmeted water toad and African goliath frog that sometimes can be seen on land). The snout–vent length of the Titicaca water frog is up to , and the hindlegs about twice as long.
Most individuals do not reach such sizes, but are still big frogs. Titicaca water frogs of the largest and typical form, upon which the species was first described, usually have a snout–to–vent length of and weigh less than . This typical form tends to inhabit relatively deep water in eastern Lake Titicaca, but a minority of the individuals in the Coata River (which flows into far western Lake Titicaca) are similar. Several other forms are found at shallower depths in Lake Titicaca, in smaller lakes that are part of the same basin, and in rivers and streams that flow into Titicaca. These tend to be smaller in size with a snout–to–vent length of and historically they were recognized as separate species (T. albiventris and T. crawfordi), but there are extensive individual variations (sometimes even at a single location), no clear limits between the forms (they intergrade) and taxonomic reviews have found that all are variants of the Titicaca water frog. Females generally reach maturity at a slightly larger size than males, they average larger and they also have a larger maximum size than males.
Morphology and color In addition to total size, the various forms of the Titicaca water frog differ in the relative size of the dorsal shield (a hard structure on the back), relative width of the head and other morphological features, with most bays in Lake Titicaca having their own type.
Compared to similar-sized frogs, the lungs of the Titicaca water frog only are about one-third the size. Instead it has excessive amounts of skin to help the frog respire in the cold water in which it lives. The baggy skin is particularly distinct in large individuals. In living individuals the skin folds are swollen with fluids, but if deflated the frog is actually relatively thin.
The color is highly variable, but generally gray, brown or greenish above, and paler below. There are often some spots, which can form a marbled pattern. Animals in coastal southernmost Lake Titicaca typically have striped thighs and relatively bright orange underparts. If teased, Titicaca water frogs can secrete a sticky whitish fluid from their skin in defense.
Habitat and ecology Titicaca water frogs live exclusively in lakes and rivers in the Lake Titicaca basin. Adults of the typical form generally live deeper than in Lake Titicaca itself, but the maximum limit is unknown. While exploring this lake in a mini submarine, Jacques Cousteau filmed individuals and their prints in the bottom silt at , which is the record depth for any species of frog. The other forms of the Titicaca water frog are found at no more than . A study that surveyed depths from the shore to near Isla del Sol found that adults were most common at . In general, Titicaca water frogs prefer a mixed bottom; either a muddy or sandy bottom with some rocks, or with plenty of aquatic plants and some rocks.
Temperature The Titicaca water frog spends its entire life in water that typically is , with the average annual temperature being near the middle and the minor seasonal variations being matched or even exceeded by the daily variations. The frogs can regulate their own temperature by moving between different microhabitats with slightly different water temperatures and adults will sometimes position themselves on top of underwater rocks to bask in the sun that penetrate the lake's clear water.
Respiration The water in which it lives generally is very rich in oxygen, but limited by the low atmospheric pressure at the high altitude, from about at Lake Titicaca to at least in associated river and smaller lakes. It respires by its skin, which absorbs oxygen, functioning in a manner that is comparable to gills. It sometimes performs "push-ups" or "bobs" up–and–down to allow more water to pass by its large skin folds. The skin is very rich in blood vessels that extend to its outermost layer. Of all frogs, it has the lowest relative red blood cell volume, but the highest count (i.e., many but small red blood cells) and with a high oxygen capacity. The frog mainly stays near the bottom and it has never been observed to surface in the wild, but captive studies indicate that it may surface to breathe using its diminutive lungs if the water is poorly oxygenated.
Behavior Social behavior and breeding Although a good swimmer, several individuals can often be seen laying inactively next to each other on the bottom. Titicaca water frogs tend to be most active during the night.
The Titicaca water frog breeds year-round in shallow coastal water where the female lays about 80 to 500 eggs. Amplexus lasts one to three days. The "nest" site is typically guarded by the male until the eggs hatch into tadpoles, which happens after about one to two weeks. The tadpole stage lasts for a couple of months to a year. The tadpoles and young froglets stay in shallows, only moving to deeper water when reaching adulthood. Maturity is typically reached when about three years old.
Feeding
The Titicaca water frog mostly feeds on amphipods (especially Hyalella) and snails (especially Heleobia and Biomphalaria), but other food items are insects and tadpoles. Adults also regularly eat fish (primarily Orestias, up to at least long) and cannibalism where large frogs eat small individuals has been recorded. It has an extremely low metabolic rate; below that of all other frogs and among amphibians it is only higher than that of a few salamanders.
In captivity, the tadpoles will feed on a range of tiny animals, such as copepods, water fleas, small worms and aquatic insect larvae.
Call and hearing Similar to at least some other Telmatobius species, male Titicaca water frogs will call underwater when near the shore. The simple and repeated call can only be detected with a submerged microphone from a relatively short distance. The function is not clear, but calling primarily occurs during the night and it is likely related to attracting females, courtship or aggression.
The ears are greatly reduced and several of the structures, including the tympanic membrane and the Eustachian tubes, are absent. How the Titicaca water frog hears is unconfirmed, but it probably involves the lungs (as known from some other frogs).
Conservation status and threats The Titicaca water frog has declined drastically, leading the IUCN to rank it as endangered.
It was once common, with a survey in the late 1960s by Jacques Cousteau and colleagues counting 200 individuals in an only plot of the huge lake. Although not directly comparable, a survey in 2017 of three transects at 38 locations only detected a total of 45 Titicaca water frogs at 6 of the locations (none at the remaining). It is estimated that it declined by more than 80% in just 15 years, from 1990 to 2004, equalling three Titicaca water frog generations. Several other species in the genus Telmatobius are facing similar risks.
The causes of the precarious status of the Titicaca water frog are over-collecting for human consumption, pollution and introduced trout, and it may also be threatened by disease.
Capture for food The species is consumed as a traditional food or blended drink known as frog juice, and as traditional medicine that is claimed to be an aphrodisiac, and treat infertility, tuberculosis, anemia, asthma, osteoporosis and fever, but this is entirely unsupported by evidence. Dishes with Titicaca water frogs are also sold by some local restaurants as a novelty to tourists.
On a month-long expedition to Lake Titicaca 100 years ago, this frog was not seen in any of the markets in the region, no locals were seen hunting for it and when asked locals said they considered it inedible. Whether this reporting was incomplete or there has been a significant change is unclear, but by the 2000s tens of thousands were caught for food and traditional medicine each year, and even though now illegal the trade has to some extent continued. Smaller numbers have been exported to other countries as food, for frog leather (skin) and the pet trade.
Pollution and mass deaths Pollution from mining, agriculture and human waste has become a serious problem in the range of the Titicaca water frog. Breathing through their skin, Titicaca water frogs easily absorb chemicals from the water. Additionally, nutrient-rich pollution from agriculture can cause algae blooms where oxygen levels plummet, asphyxiating the fully aquatic frog.
Historically, smaller mass deaths occasionally occurred in this species, but they are now fairly common and since 2015 there have also been large mass deaths. In April 2015, thousands of dead Titicaca water frogs were found in Bolivia on the shore of Lake Titicaca, and in October 2016 an estimated 10,000 were found dead in the Coata River (a Lake Titicaca tributary). At least in the latter case, scientists believe pollution killed the frogs. This is also supported by the timing of the mass deaths, which mostly occur in the rainy season where pollution likely is washed into the lake from the surroundings.
Die-offs possibly are reversible. It has been observed that small Titicaca water frogs may appear in the vicinity of an affected area later, possibly recolonizing it.
Introduced trout The introduced, non-native rainbow trout likely feed on tadpoles of the Titicaca water frog, the frogs are caught as bycatch in fishing nets set for trout, and coastal pens for farming trout overlap with the frog's breeding habitat and may impact it. Lake Saracocha was home to Titicaca water frogs of the "albiventris" form, but they have not been found in later surveys and it is suspected that introduced trout were implicated in their apparent disappearance from this location.
Rainbow trout was introduced on a US initiative from their original North American range to Lake Titicaca in 1941–42 to aid the local fisheries that had relied on the smaller native fish. The fast-growing and relatively large non-native fish (trout and Argentinian silverside) are now the most important species in local fisheries, far exceeding the fisheries for the smaller natives (Orestias and Trichomycterus). Because of the economic importance, these fisheries, along with trout farming, are supported by the local governments, making it unlikely that they would support any initiatives to reduce or even remove the trout from the region.
Disease