Prepared by Harry Hines
Queensland Parks and Wildlife Service
and the South-east Queensland Threatened Frogs Recovery Team, 2002
Appendix 1. Species Profiles
Southern Gastric-brooding Frog Rheobatrachus silus Liem 1973
A moderately large, aquatic frog, males 30-44mm, females 41-54mm (Ingram 1983, Tyler and Davies 1983a). The dorsal surface is brown, or olive brown to almost black, usually with obscure darker blotches on the back. A dark streak runs from the eye to base of the forelimb. There are darker cross-bars on the limbs, and pale and dark blotches and variegations on the digits and webbing. The ventral surface is white or cream with yellow markings on limbs. Skin is shagreened or finely granular above, and smooth below. Snout is blunt and rounded, with the eyes and nostrils directed upwards. Eyes are large and prominent, located close together and close to the front of head. The tongue is largely adherent to the floor of mouth and the tympanum is hidden. Fingers lack webbing, while toes are fully webbed. Digits have small discs. (Cogger 1996, Liem 1973, Tyler and Davies 1983a)
Call is a loud staccato, consisting of 30-34 pulses repeated in a long series, lasting 260-290 ms. Dominant frequency is 1000Hz, with less emphasised frequency bands at 500,700,1200 and 1400Hz (Tyler 1983b).
Breeding activity occurs between October and December and appears dependent upon rains (Ingram 1983). This species has a unique reproductive mode in which eggs or early tadpoles are swallowed by the female and complete their development in the stomach (Tyler and Carter 1982). Hormones produced by the young inhibit the digestive secretions of the stomach and inactivate the upper intestine, a process of special interest to the medical community (Tyler 1985). Tadpoles rely on yolk reserves throughout development (Tyler and Davies 1983b). Up to 25 young are brooded in this fashion, emerging from the mother's mouth as fully formed metamorphs after about six or seven weeks (Tyler and Davies 1983b). The digestive tract returns to its normal state and the female recommences feeding within four days (Tyler 1983a). Maximum longevity is at least three years (Ingram 1983).
The tadpoles are reared in the stomach of the female frog and are therefore quite unusual. They are bulbous, pale and low finned without keratinised mouthparts (Tyler and Davies 1983b).
Rheobatrachus silus is an aquatic species and has never been located more than four metres from water. This species is restricted to rocky perennial streams, soaks and pools in rainforest and tall open forest with a closed understorey. It prefers rock pools and backwaters with leaf litter and rocks in which to shelter (Ingram 1983)
Rheobatrachus silus is most active during the warmer months, between September and April, with abundance decreasing as conditions become drier in winter (Ingram 1983). It is not known where these individuals go during winter, but it is believed they hibernate in deep crevices in the rocks (Ingram 1983, Liem 1973). Individuals may be active night or day, particularly after rain. They establish home ranges in and around suitable pools, spending extended periods partly submerged and immobile. When heavy rain falls the males move away from the water, sometimes up to four metres, and call from sheltered hollows or crevices above the pools (Ingram 1983).
Restricted to elevations of 400 800m in the Blackall and Conondale Ranges, south-east Queensland, between Coonoon Gibber Creek (26° 33'S, 152° 42'E) and Kilcoy Creek (26° 47'S, 152° 38'E) (Hines et al. 1999) (Figure 3).
|Figure 3. Distribution of southern gastric-brooding frog Rheobatrachus silus.
Shaded areas are: light grey - State forest or timber reserve, dark grey - national parks and conservation parks. Some towns and larger streams are shown.
Not sighted in the wild since 1981 despite continued efforts to relocate the species. Since Ingram and McDonald's (1993) review, the following surveys and monitoring for this species have been undertaken (summarised by Hines et al. 1999):
- Regular monitoring at Ingram's (1983) study site - Beauty Spot 100 on Booloumba Creek, Bundaroo, Peters and East Kilcoy Creeks in the Conondale Range and at Picnic Creek (the type locality near Kondalilla) on the Blackall Range.
- 1995 intensive "frog search" of Conondale Range.
- 1997 "frog search" of the headwaters of Kilcoy, North Booloumba and Bundoomba Creeks, Conondale Range.
- Since 1996, systematic surveys of many streams in the Conondale and Blackall Ranges. Some sections of streams were visited on many occasions over a range of weather conditions. Poorly surveyed streams in the Upper Stanley River were targeted.
- Opportunistic surveys by various frog biologists.
The species declined rapidly in late 1979, with only a single specimen located after that, in 1981 (Richards et al. 1993).
The reason(s) for the disappearance of this species remains unknown (Tyler and Davies 1985). Populations of R. silus were present in logged catchments between 1972 and 1979. Although R. silus persisted in the streams during these activities, the effects of timber harvesting on this aquatic species were never investigated. Its habitat is currently threatened by feral pigs, invasion of weeds (especially mistflower Ageratina riparia), and altered flow and water quality due to upstream disturbances.
It is listed as Presumed Extinct nationally, and as Endangered in Queensland and in the Action Plan. Rheobatrachus silus meets IUCN (2001) criteria for Presumed Extinct.