Darwin's frog (Rhinoderma darwinii), also called the Southern Darwin's frog, is a species of frog of the family Rhinodermatidae. It was discovered by Charles Darwin during his voyage on HMS Beagle on a trip to Chile. In 1841, French zoologist André Marie Constant Duméril and his assistant Gabriel Bibron described and named Darwin's frog. The diet of R. darwinii consists mostly of herbivore invertebrates. R. darwinii is currently classified as an endangered species by the International Union for Conservation of Nature.

Darwin's frog has an unusual method of brooding, in which the male will facilitate development of its tadpoles inside its vocal sac. This male brooding may make Darwin's frog unique among extant frog species, as the only other frog that has this behavior is the R. rufum (northern Darwin's frog), which has been presumed extinct since 1981.

Characteristics

left|thumb|Ventral view of trunk

Size

Darwin's frog is a small species with a snout–to–vent length of . The snout is elongated into a fleshy proboscis which gives the head a triangular shape. The limbs are relatively long and slender. The front feet are not webbed, but some of the toes on the back feet usually are.

Coloration

The upperparts of this species are variably colored in brown or green. Some brown individuals may have weakly defined V-shaped markings on the back, have central upperparts that are distinctly lighter brown than the flanks, or have whitish front limbs. Females are typically brown and tend to stay on substrates that match this color. Males are far more variable and occur on a wider range of substrates; in particular, brooding males often are often partially or completely green above. The throat is brownish and the remaining underparts are black with large white blotches in a pattern that is unique to each individual. In captivity, male R. darwinii have been shown to change color. These frogs that were initially brown changed to green over a year. This change in color is believed to be due to the green color of the environment these frogs were kept in.

Habitat and distribution

Darwin's frog is found in Chile and Argentina at elevations as high as 1350 meters above sea level. R. darwinii is mainly found in the Valdivian Temperate Rain Forest which covers parts of both Chile and Argentina. In Chile, its range extends from Concepción Province to Palena Province and in Argentina from Neuquén Province and Río Negro Province. It is found in glades and forested areas at altitudes of up to about above mean sea level, in bogs and near slow-moving streams, and in a variety of vegetation types. It appears that a mixture of grassland, mossy areas, coarse woody debris, and young trees and bushes in a mature native forest provides its optimum habitat requirements. Short vegetation increases the retention of water while decreasing the temperature of the soil and providing concealment from predators. The population is fragmented and this frog has poor dispersal ability.

This frog has been observed in many protected parks: Lanín National Park and Nahuel Huapi National Park in Argentina and many more in Chile.

Conservation efforts

Due to its decline in the wild, captive colonies have been established as a precaution at two zoos in Chile, the National Zoo (working with the US Atlanta Botanical Garden) and Concepción Zoo (working with the University of Concepción and Germany's Leipzig Zoo).

In 2017, the IUCN SSC Amphibian Specialist Group formed a Binational Conservation Strategy that brought together 30 different countries. The goal of this group is to study R. darwinii in order to improve conservation efforts. The group details the unique characteristics (mouth brooding) as one justification for this increased conservation effort. The goal of the group is to understand key aspects of information related to R. darwinii by the year 2028.

Diet

The diet of R. darwinii's consists of detritivore, herbivore, and carnivore invertebrates. It has been observed to consume each type of invertebrate at a percentage consistent with their prevalence in the environment. The percentage at which carnivorous invertebrates are consumed is lower than herbivore or detritivore invertebrates. This difference can be explained because spiders are the predominant type of carnivore invertebrate prey that R. darwinii encounters. These spiders are able to evade the predation of R. darwinii effectively due to their evasion ability.

In the habitats where R. darwinii have been observed, there seem to be relatively high percentages of herbivore invertebrates. This could mean that R. darwinii seek environments with enriched herbivore invertebrates as a food source.

Rhinoderma darwinii style of predation has been characterized as "sit and wait". This method seems to conserve energy and allows R. darwinii to evade predators effectively.

Reproduction

Male R. darwinii will call to attract females in an attempt to mate. It has even been shown that male R. darwinii will call when brooding. R. darwinii use non-linear vocal phenomena (NLP) in order to attract and communicate with mates. Darwin's frog has been shown to have distinct mating patterns based on population and body size. More research needs to be conducted in order to further explore the mating of R. darwinii.[[File:Ranita de Darwin (Rhinoderma darwinii).jpg|thumb|Most green individuals are brooding males

Territoriality

R. darwinii exhibit rare behavior in terms of territoriality and parental care. In anuran species, parental care and territoriality are positively related. In R. darwinii, there is high parental care due to the ingestion of tadpoles by fathers. However, there is low territoriality exhibited by these R. darwinii males. In fact, neither females nor males of R. darwinii have been observed guarding eggs. These observations of R. darwinii were used to further develop the relationship between territoriality and parental care by establishing the role of oviposition defense. These observations suggest that anurans will exhibit territoriality with parental care if they defend oviposition sites.

Threats

Rhinoderma darwinii has been shown to be highly susceptible to Batrachochytrium dendrobatidis infection. The amphibian disease chytridiomycosis, caused by Batrachochytrium dendrobatidis fungal infection, is also a probable factor.

Rhinoderma darwinii seems to be less affected by chytridiomycosis when compared to other amphibians. However, Chytridiomycosis can still infect and kill R. darwinii. Previous studies have supported the idea that chytridiomycosis could be somewhat responsible for the decrease in R. darwinii populations observed in Chile and Argentina.

In recent years, studies have shown that R. darwinii have shown variation in Batrachochytrium dendrobatidis infection across populations. In populations where there were high Bd infection rates, there are observed to be higher population growth rates. Similarly, in populations with low Bd infection rates, there are observed to be lower population growth rates. Populations with higher Bd infections rates seemed to have the highest reproductive rates. This means that even though many individuals are dying of Bd infection, there are more individuals being born. There exists a positive relationship between Bd infection rate and the number of juvenile individuals for R. darwinii.

This odd feature of Bd infection and R. darwinii population growth prompted further investigation. The explanation for this observation is termed the "parasite-induced plasticity" hypothesis. This hypothesis says that individuals will devote more resources towards reproduction as opposed to survival. This increased devotion towards reproduction takes place over a single generation. This plasticity is beneficial to the infected individual because the infection will take over at some point so until that point the individual will try to have as many offspring as possible.

Use in research

Rhinoderma darwinii have been used to study size variation in body size of ectotherms. Previous research supported the hypothesis that larger body sizes were tied to higher seasonality because of an idea termed starvation resistance. Starvation resistance is the idea that the larger the size of an ectotherm, the less likely it is to "starve", as it can use its body's mass for fuel.

However, work done on R. darwinii supports another hypothesis. The name of the hypothesis supported by the experiment that shows greater seasonality leads to longer periods of time in the cold is termed the hibernation hypothesis. These animals in the cold are likely to hibernate and under hibernation will have a lower basal metabolic rate. This will lead to lower amounts of energy expended, and thus less of the ectotherm's body mass will be lost. This explanation offers an alternative to the starvation resistance hypothesis.

See also

  • Gastric-brooding frog
  • Mouthbrooder

References

Sources

  • Crump, M.L. (2003). Grzimek's Animal Life Encyclopedia, 2nd ed., Vol. 6 Amphibians, 175, Gale.
  • Duellman, W.E., ed. (1999). Patterns of Distribution of Amphibians: A Global Perspective, 325, The Johns Hopkins University Press.
  • Frost, D.R., ed. (1985). Amphibian Species of the World: A Taxonomic and Geographical Reference, 551, Allen Press, Inc. and the Association of Systematics Collections, Lawrence, Kansas.