Written by Roger Downie, Froglife and University of Glasgow
On 3rd February 2025, many media outlets reported news of the ‘birth’ of 33 juvenile Darwin’s frogs at London Zoo. Why was this event so newsworthy? In this article, we look into two unusual modes of parental care in frogs and the plight of the species involved.
Among the more than 70 reproductive modes so far described from amphibians (see Croaking Science December 2021 and April 2019), perhaps the strangest are where the eggs are incubated either in the mouth or the stomach of a parent.
Since mouth-brooding is known from nine families of fish, most commonly in the cichlids, it is perhaps not so surprising that it has also evolved in frogs. However, biologists were astonished in the 1970s to learn of a genus of Australian frogs (only two species) where eggs are incubated in the stomach of the mother.
After fertilisation of the large (about 5mm diameter) eggs, they are swallowed by the mother and pass to her stomach. A prostaglandin E signal in the mucus coating of each egg (and later released from tadpole gills) inhibits secretion of stomach acid and enzymes. Tadpoles develop and metamorphose in the stomach, entirely fuelled by egg yolk, and froglets are born by regurgitation at about six weeks.
The two gastric-brooding frogs Rheobatrachus silus (southern species) and R. vitellinus (northern, known as the Eungella gastric-brooding frog, from the national park where it was found) lived in rainforest creeks in restricted areas of eastern Queensland. Sadly, both species became extinct soon after their discovery. R. silus has not been seen, despite repeated surveys, since the late 1970s, R. vitellinus since the mid-1980s. A well-planned expedition in 2021 checked previous locations in Eungella and found plenty of suitable habitat, and also populations of two endangered endemic frogs, the Eungella tinker frog and the tusked frog, but no R. vitellinus. Habitat destruction and chytrid infection are both thought to have contributed to steep population declines and eventual extinction.
All may not be lost. Australian scientists have access to frozen Rheobatrachus specimens, and have had some limited success so far in attempting to regenerate embryos by cloning methods, where Rheobatrachus DNA is injected into host nuclei. Whether this work will result in viable reproductively-capable adults has yet to be seen.
The conservation position is a little better for the mouth-brooders. Again, only two species are known, Darwin’s frog (Rhinoderma darwinii ) and the closely related R. rufum. These frogs became known to science from specimens collected by Darwin during the voyage of The Beagle in 1834: the first species was described and named by Dumeril in 1841; the second species was described in 1902. Both species are endemic to the temperate rainforests of central and southern Chile, extending a little into neighbouring Argentina, with R. rufum having a more northern distribution than R. darwinii; there is a small area of overlap where both have been found. In Rhinoderma, after fertilisation of eggs, deposited on to land, the male takes them into his vocal sac and incubates them there: in R. darwinii, they develop through metamorphosis and are ‘born’ as froglets; in R. rufum, they emerge as tadpoles and complete development in water.
The original forested areas inhabited by the two Rhinoderma species have been much altered by anthropogenic development: urbanisation, commercial forestry and agriculture. Sadly, R. rufum has not been seen since 1981 despite repeated surveys, and is now classed by IUCN as ‘possibly extinct’. The effort to save this unusual kind of frog now concentrates on R. darwinii.
After several decades where steep declines in R. darwinii populations were recorded, the Chilean section of the IUCN’s Amphibian Specialist Group convened a meeting in 2017 aimed at developing a comprehensive conservation strategy. Thirty experts, including the UK’s Andrew Cunningham, contributed to the resulting plan, launched in 2018 ( Azat et al. 2021). The species was estimated to occupy 65 extant populations in Chile and 10 in Argentina with an overall area of occupancy of only 224 km2. Threats to its continued existence include a) habitat loss: little of its original habitat (coastal deciduous forest ) remains, having been replaced by exotic pine and eucalyptus plantations or agriculture; b) chytrid infection has been in Chile since the 1970s, according to analysis of museum specimens, and surveys have shown it can be lethal for R. darwinii ; c) modelling of climate change suggests that current areas where the species persists will be unsuitable in the future; while new areas may be suitable, the frog’s low dispersal ability will limit any benefit from that. Chile and Argentina combined have 30 protected areas where R. darwinii occurs; there are also three private parks where in situ conservation projects are in progress, and also two separate ex situ projects in Chile: Bourke (2010) reported on the establishment of both indoor and outdoor terraria at Concepcion University, with successful breeding occurring; Fenolio (2012) described a separate facility at the National Zoo of Chile, also with breeding success. The agreed conservation strategy includes 39 prioritised actions and aims to fill information gaps, reduce the main threats and achieve legal and financial support by 2028.
However, a survey in 2024 of one of the largest R. darwinii populations found a 90% decline compared with the previous year. In response, a London Zoo EDGE (Evolutionarily Distinct Globally Endangered Species) team visited the main island of the Chile Archipelago and over five weeks collected 55 frogs from Parque Tantauco, a 1180 km2 private nature reserve. They kept the frogs in quarantine long enough to detect any chytrid infections (52 were chytrid-free), then, using specially-designed climate-controlled boxes, transported the frogs 7000 miles (by boat, road and plane) to London, arriving on 9th December 2024. Eleven of the males were brooding embryos during the trip, and 33 froglets were born soon after arrival in London. The plan is to breed this population in disease-free conditions as a way of rescuing a species whose future in its normal environment looks extremely gloomy. As with all ex situ conservation projects, the question is: what then? Will it be possible to re-introduce specimens to the wild and then regenerate native populations, and if not, what is the long-term fate of the captive population?
References
Azat et al. (2021). A flagship for austral temperate forest conservation: an action plan for Darwin’s fogs brings key stakeholders together. Oryx 55, 356-363.
Bourke (2010). Darwin’s frog captive rearing facility in Chile. Froglog 94, 2-6.
Fenolio (2012). The Darwin’s frog conservation initiative. Amphibian Ark Newsletter 18, 22-23.