A Natural History of the Southern Gastric-brooding Frog

Southern Gastric-brooding Frog (Rheobatrachus silus) -- A Natural History


Order: Anura
Family: Myobatrachidae
Subfamily: Rheobatrachinae


Scientific: Rheobatrachus silus
Common: Southern Gastric-brooding Frog


Description

The Southern Gastric-brooding Frog was a medium sized frog (1.3-2.1 inches) that had large, bulging eyes with a brown iris along with a short and blunt snout.  The tympanum was not visible.  Its skin was a dull grey to brown color that was granular, moist, and covered with mucous.  Its belly was often white, yellow, or cream colored.  It lacked a dorsolateral fold.  It had long, pointy fingers with fully webbed toes.  Each digit had small terminal discs.  Their call sounds a bit like a creeky swing.  Take a listen here.

Distribution and Habitat


The Southern Gastric-brooding frog was officially discovered in Queensland, Australia in 1973.  Based on current evidence, this frog's range was restricted to the Blackall Range and Conondale Ranges in southeast Queensland at elevations between 350-800 meters; making its total (known) range less than 540 square miles.  Predominately an aquatic species, Rheobatrachus silus were typically found in and around permanent streams and waterbodies.  Specifically, it was known to inhabit the streams of the Mary, Stanley, and Mooloolah Rivers in Australia.  To date, this species has never been found more than 4 meters from a watersource.  They were known to reside in cool, clear, and fast-flowing streams; hiding away between and underneath boulders. 

Lifespan/Lifecycle

Not much is known regarding the lifespan of these animals.  In captivity, they have lived up to 3 years.  Reproductive maturity is reached around 2 years of age.  However, the process of egg deposition and amplexus has never been observed in this species.

Behavior

These frogs were not known to be very active.  Although capable of fast swimming, they were more often than not found drifting with currents or sitting in the same position for hours at a time.

Diet

Their diet consisted mostly of small live insects.  However, they have been observed to hunt both on land and in water!  In fact, stronger prey would be taken underwater to be consumed.

Conservation Status

Extinct (2002).  These guys were abundant when they were discovered in 1973, and had seemingly vanished less than a decade later.  To date, no one is quite sure what the exact cause for extinction was, but researchers have proposed it was a combination of habitat destruction, pollution, drought, over-collection, and potentially even the chtryid fungus that ultimately led to their demise.

What Made Them Unique?

Now for the cool part.  Rheobatrachus silus’ claim to fame is how it took parental care to a previously undescribed level.  This species is one of two (the other being the closely related R. vitellinus) described to swallow its own fertilized eggs, not for energy, but to brood young in its stomach.  At least 8 weeks after ingestion, the mother moves to the water where she rests in a vertical position, snout just sticking out of the water.  Eventually, with a forceful abdominal muscle contraction, she gives birth to her frogs (both tadpoles and juveniles) through her mouth in a projectile fashion (Corben et al, 1974).  These frogs have been observed to give birth to over 22 live young over the course of days to several weeks.

As it is easy to imagine, there are a number of problems with raising young in a stomach.  Normally, stomach acid and other digestive secretions would destroy these eggs before they ever had a chance to develop.  Therefore, is it logical to hypothesize that these secretions are reduced or inhibited in some way.  How is this accomplished?

It turns out there are multiple methods of gastric secretion control in these frogs.  It has been hypothesized that stretch receptors in the stomach stop the feeding drive of the mother.  Because the female “feels” full, she doesn’t eat (and doesn’t start the digestive process). 

In addition, newborn tadpoles have been observed to release cords of mucus from their mouths.  This mucus has been shown to significantly decrease the rate of spontaneous acid secretion.  It has been hypothesized that the compound responsible for this inhibition is known as PGE₂, a prostaglandin. PGE₂, synthesized in the mucosa, stimulates the production of bicarbonate and mucus to combat acid.  While it is not known what kicks off the inhibition process (the eggs contained little prostaglandins), it has been proposed that inhibition is maintained by PGE₂ production in the young throughout their development, and that this inhibition stops as soon as birth takes place (Tyler et al, 1983; Leong et al, 1986). 

It is unfortunate for us (and them) that these little guys have gone extinct.  They are among some of the most unique animals to have ever lived.  One can only hope that somewhere in Queensland, a small surviving population waits to be discovered.

Suggested Further Reading and Links

http://www.iucnredlist.org/apps/redlist/details/19475/0
http://animaldiversity.ummz.umich.edu/site/accounts/information/Rheobatrachus_silus.html
http://en.wikipedia.org/wiki/Gastric-brooding_frog#cite_note-Hines1999-5

Corben, C.J., Ingram, G.T., and Tyler, M.J. (1974). Gastric Brooding: Unique Form of Parental Care in an Australian Frog.  Science: 186, 946-7.

Tyler, M.J., Shearman, D.J.C., Franco, R., O’Brien, P, Seamark, R. F., and Kelly, R. (1983). Inhibition of Gastric Acid Secretion in the Gastric Brooding Frog, Rheobatrachus silus.  Science: 220, 609-610.

Leong, A.S.Y., Tyler, M.J., and Shearman, D.J.C. (1986). Gastric Brooding: A New Form in a Recently Discovered Australian Frog of the Genus Rheobatrachus.  Aust. J. Zool.: 34, 205-9.