The goliath frog (Conraua goliath), otherwise known commonly as the giant slippery frog and the goliath bullfrog, is a species of frog in the family Conrauidae. The goliath frog is the largest living frog. Specimens can reach up to about in snout–vent length and in weight. This species has a relatively small habitat range in Cameroon and Equatorial Guinea. In a sample of 15 individuals, weights ranged between , and snout–vent lengths were between . In total length, including outstretched legs, the largest specimens can slightly exceed .

Due to its large size, the goliath frog has an extremely selective distribution. This species is primarily located in a dense equatorial forest fringe which is somewhat parallel to the coast and surrounded by rivers. The goliath frog has been located in Sanaga Basin (mainly appearing near the Nachtigal cascades and in the Sakbayeme rapids), Kienke Basin, Ntem Basin (mainly being located near the rapids of the Mensolo and Nsana), and Mbía Basin (where it was found to be very abundant in the rapids and cascades). These distribution patterns emphasize its limited environment which tends to have a clear preference for water territories.

Conservation

thumb|Preserved specimen at the [[Harvard Museum of Natural History]]

The primary threat to the goliath frog is hunting, as it is considered a food source in its native range. Although captives may live longer than their wild counterparts, the species has not been bred in captivity. Due to their classification as an endangered species, the Equatorial Guinean government has declared that no more than 300 goliath frogs may be exported per year for the pet trade, but few now seem to be exported from this country. Fully developed frogs are also believed to prey on fish, small mammals, newts, other salamanders, and smaller frogs.

Reproduction

Like most amphibians, water is vital for their reproduction. Because the goliath frog lacks a vocal sac, it does not produce mating calls, a behavior generally present in frogs and toads. Larval development takes between 85 and 95 days.

Life cycle

Longevity

The goliath frog can live up to 15 years in the wild and up to 21 years in captivity. Due to its large size, Goliath frogs are only known to be preyed on by humans although other predators are possible.

Developmental Stages

thumb|alt=Conraua goliath|Specimen at the [[Cambridge University Museum of Zoology]]

While the reproductive behavior for this species is mostly not well-known, studies by Lamotte, Perret, and Zahl have allowed an overall chronological table of larval development to be created. The construction of these nests can also explain how the goliath frog became the largest frog. Digging out these nests which exceed 1 m in diameter is an extremely arduous task. Other species which perform this task are also quite large in size. This includes: male African Bullfrogs, Gladiator Frogs, and the Bornean Giant River Frog.

Helminth parasites

There are also parasites that mainly target the gastro-intestinal tract of this frog. These parasites are called helminth parasites which are worm-like parasites divided into three main groups: flukes which are leaf-shaped flatworms, tapeworms which are elongated flatworms that inhabit extraintestinal tissues, and roundworms which inhabit intestinal and extraintestinal sites. The Goliath frog, however, was mainly infected by Nematodes (90.5%) which is a specific type of roundworm. The helminth species discovered within the Goliath frog was extremely similar to those discovered in amphibian hosts in other African countries emphasizing that its location/habitat is the main cause of the prevalence of this predator. However, the Goliath frog was also infected by the nymph of Sebekia sp which could be primarily due to these frogs sharing the same habitat of crocodiles (definitive host) and fishes (intermediate host). Frogs that originate in Loum and Yabassi which are places within Cameroon had the largest variety of helminth species whereas frogs from Nkondjock had the smallest variety. This could be explained by the difference in agricultural activities, deforestation, and poaching. Liver weight was examined to reveal that there is a higher accumulation of toxic products at Loum due to the significant increase in agriculture practice. Consequently, land use effects and their impact on water habitats plays a significant role in the pattern of parasitism and land use by goliath frogs. Additionally, the direct life cycles of the helminths may play an important role in species diversity. Parasites which have direct life cycles spend a majority of their adult life within one host which allows their offspring to be spread from one host to another. These types of parasites also often lack an intermediate stage which means that they must be able to survive in the outside environment as well and be able to establish themselves within a new host. This ability to adapt to new environments contributes greatly to the complexity of helminth communities of goliath frogs.

Chytridiomycosis

Chytridiomycosis is an infectious disease affecting amphibians, caused by the chytrid fungi Batrachochytrium dendrobatidis and Batrachochytrium salamandrivorans. It has the capability to cause random deaths due to its high mortality rate. These fungi invade the surface layers of the skin, causing damage to the outer keratin layer. As the tadpole continues to grow, more keratin becomes present on the skin thereby allowing the fungus to spread to many parts of the body resulting in the death of the tadpole. Amphibian skin is vital because it is physiologically active meaning it plays important roles in regulating respiration, water, and electrolytes. While the method of how this fungus kills frogs is not known, it can be hypothesized that its invasion through the skin is related since it can cause an electrolyte depletion, osmotic imbalance, and make it more difficult for the frog to breathe. When infected with this fungus, a frog may have discolored skin, peeling on the outside layers of its skin, be sluggish, and have its legs spread slightly away from itself. This fungus is also transmittable since it can be directly transferred through contact between frogs and tadpoles or through exposure to infected water. Thus, it is highly spreadable and with its high mortality rate among frogs, it is extremely deadly.

Physiology

thumb|alt=Goliath frog skeleton|right|200px|A skeleton of a Goliath frog

The goliath frog contains extensive skin folds to promote respiratory gas exchange at high altitudes. Additionally, the lungs within these frogs are reduced to about one-third of the volume of other frogs and they also contain a smaller heart. This is primarily due to their difference in predation methods. Goliath frogs are typically sit-and-wait predators. This means that they tend to capture/trap their prey either by luring them or using elements of surprise by acting extremely stealthy. As a result, they have a reduced metabolic rate and a unique method of breathing. When attempting to breathe, each buccal movement (a method in which the mouth expands and contracts in order to promote the movement of air into the lungs) pumps air at a rapid rate and the process of getting the oxygen removed from the air is slightly more efficient in this species of frog. These adaptations are very useful for the production of territorial and reproductive calls created by these frogs. Typically, the call of the goliath frog which is produced by the frog opening its mouth is admitted at a high frequency of 4.4 kHz. Furthermore, the goliath frog does not contain a vocal sac which causes the process for making noises/calls for reproduction to be different than most frogs.

Interactions with humans

thumb|In Cameroon

In addition to the impacts of climate change, agriculture, and deforestation, goliath frogs are also threatened by the local practice of hunting them for food. While hunting the frogs, locals will use lanterns to get their attention, then immobilize them using mesh nets. In Nkombia, they may also be captured with nets during the day while they are resting on rocks. However, this method of capture is not very effective, as they are able to jump high, thus easily escaping. Humans are the main predators of this species and the main cause of their endangerment. In order to save the species, hunting and environmental destruction should be limited.

See also

  • Helmeted water toad (Calyptocephalella gayi) – A South American species that can reach a similar length, but with a lower maximum weight
  • Lake Junin giant frog (Telmatobius macrostomus) – A South American species that is the world's largest exclusively aquatic frog

References

Further reading

  • BBC News story about Goliath frogs

Media

  • Photos of goliath frogs with people at Queensland Frog Society