Bibron's toadlet

The genus Pseudophryne encapsulates 14 different species of frogs localized to Australia.[4] These frogs are morphologically similar to small bufonid frogs not localized to Australia. The first known nomenclature of the Pseudophryne genus dates back to 1835 with the characterization of the Bombinator australis[4]. The types and localities of different Pseudophryne species has been debated, P. bibronni being localized to South Eastern Australia and other Pseudophryne localized to different regions in Australia.[4]

This species is variable and may represent more than one species. It grows to about 30mm in length and is brown, grey, or black above, often with scattered red spots. It is normally strongly marbled black and white on the ventral surface, however in some areas this marbling can be faint. There is always an orange, red or yellow patch in the armpits. This patch varies from yellow to orange between sites. At the Jervis Bay region this patch is always yellow, this may be a result of hybridization with the Dendy's toadlet (Pseudophryne dendyi) which has a strong yellow patch in the armpits. There is also a yellow/orange patch or raised bump on the back of the thigh. Its fingers and toes do not show any webbing nor do they contain any discs. The species has horizontal pupils and the iris displays a gold coloring.[5]

The species shows high sexual dimorphism in terms of size. Females are 14% longer than male toadlets and 15% heavier than male toadlets.[6] This is due to the hypothesized longer growing period of female members of the species compared to male members of the species. The cause of female longevity is unknown but it is speculated that it is due to decreased predation risk and increased lifetime expectancy.[6]

The diet of this species is very similar to the diet of another Australian toadlet, Pseudophryne coriacea.[6] They primarily feed on small invertebrates present in leaf litter, such as small ants and greenflys. However, there seems to be no selection in the type of prey that this species will consume. They will consume any arthropod that becomes available to them in the leaf litter present in their habitat.[6] Small black ants are the most common source of food for this species. This species also produces pumiliotoxins (PTXs) that deter predators through their diet.[7] While the mechanism of this acquisition is unknown, consumption of certain alkaloids in their diet allows the toadlet to produce PTXs that deter predators like snakes.[6][7] These toxins are similar to the compounds secreted by the Poison Dart frogs from South America.[6][7]

Image of Bibron's toadlet with coloration and markings

Because of the toxic nature of PTXs, snake species avoid feeding on these frogs. The only real known predator of the P. Bibronii is Limnodynastes peronii, also known as the two striped grass frog.[6] Bones of the Bibron's toadlet have been found in the gut content of two striped grass frogs in the Wallingat State Forest in New South Wales, Australia.  

This species inhabits areas that are likely to be flooded after rain. This can be anything from coastal swamps, creeks, temporary ponds/roadside ditches in forest, cleared land, heathland and even sub-alpine areas. When threatened, the Bibron's toadlet will often lie on its back, unresponsive, pretending to be dead. This performance had been reported to last for up to an hour.

Reproduction

The males attract the females by making a grating "cre-eek" noise from a concealed area, in mud, under rocks, or within damp leaf litter.[8] Breeding season begins after the heavy rains during the Australian autumn months of March, April, and May, and proceeds for four to five months after.[9] Most breeding and mating occurs at night and can occur at cold temperatures.

The species is polyandrous, with each female mating with several males. The female visits up to eight males, mates with each, and deposits eggs in the shallow nest he has dug.[10][11][12] As the female releases the eggs in the nest, the male grabs the female by the inguinal amplexus and fertilizes the eggs as they are being released by the female.[9] The female moves on and the male tends to the nest. The nest, which may contain moist leaf litter or sphagnum moss, floods during rain. The eggs hatch and the tadpoles develop in the water. If no sufficient rain happens soon after laying the eggs can remain unhatched for many weeks, with the tadpoles developing inside. The nest must stay wet so the eggs remain moist, but not too wet or they will be washed away. Female frogs that mate with more males, thus depositing eggs in more nest sites, are more likely to have some young survive.[10][11]

Image of the Bibron's Toadlet in leaf litter

The eggs of P. bibronii are heavily regulated by the water content and partial pressure of oxygen in the atmosphere.[14][15] After being fertilized, the eggs respond to changes in the environment to modulate growth. The terrestrial growth period of the eggs lasts about four to five months and after heavy flooding the eggs hatch into tadpoles.[14] If no flooding occurs the eggs can regulate growth for up to three months before they have a lower chance of survival. During this terrestrial period, the frog is subject to many different hydrous and anhydrous conditions that have different effects on growth.[15] P. bibronii develop faster in relatively wet conditions at higher water potentials. P. bibronii eggs have a lower tolerance range compared to other flexible shelled reptiles that are also sensitive to water conditions.[15] Thus, P. bibronii males make nesting sites in areas that are moist and are likely to flood to initiate hatching.

P. bibronii eggs hatch in response to partial pressure of oxygen in the surroundings. Eggs develop in burrows that are buried under vegetation or leaf litters. The atmospheric oxygen reaches the eggs through small tunnels and oxygen pressure remains stable until eggs are mature.[14] Eggs reach maturation after 39 days of age and if they are flooded at this point they will be ready to hatch.[14] Once the nest is flooded with water the partial pressure of oxygen in the nest decreases as the oxygen consumption of the embryos increases. The low partial pressure of oxygen triggers the hatching of the egg and the tadpoles are released.[14] If partial pressure of oxygen never decreases due to delayed flooding, development of the egg stalls.[14]

P. bibronii are widespread throughout Queensland, Australia. The Flora and Fauna Guarantee Act of 1988 designated the species as threatened and the International Union for Conservation of Nature (IUCN) designated the frog as Least concern.[16] Increasing drought in the Australian grasslands has caused a decrease in frog populations overall. The drought has caused once lush paddocks to turn to barren fields, creating fewer places for the P. bibronii to burrow.[8] There has been increasing habitat loss for these frogs causing a significant decline in the population.[16] However, P. bibronii populations are enduring and can still be heard in the woodlands during mating season.[8] Tracking these frogs is also very difficult as they are very secretive and cryptic, thus it is hard to get an accurate population count of the species.

On February 7, 2004, wildfires burned down many natural areas in the Melbourne area.[17] This fire burned down many natural habitats of P. bibronii causing a steep decline in their population levels in disturbed habitats. The impact of these fires is thought to have affected the mortality rates of adult frogs, amount of territory for breeding, and overall habitat loss.[17] Efforts have been made to track changes in P. bibronii population size before and after the fires, but the lack of pre-fire data has made it hard to make any conclusions about the effects of the fire on the population.[17]

• Anstis, M. 2002. Tadpoles of South-eastern Australia. Reed New Holland: Sydney.
• Robinson, M. 2002. A Field Guide to Frogs of Australia. Australian Museum/Reed New Holland: Sydney.
• Frogs of Australia Network — frog call available here.
• SA EPA's Frogs of South Australia website