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Hedging their bets


 
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Dr Arthur White delves into a major difference in Australian frogs, a hatching process known as developmental plasticity.    
Unusually, when Australian tadpoles hatch it is a staged process. Most hatch within the first few days but the remaining quarter would gradually hatch over the following weeks or even months. In fact, there are still unhatched eggs when the first tadpoles are near metamorphosis and it can take months for the final hatchings to occur.

 
The hatching process is not haphazard, as scientists have demonstrated. There is a genetic variation that controls hatching rates, suggesting a survival mechanism is at play. The Sydney-based Red crowned toadlet (see left), normally lays its eggs on land and in relatively small clusters numbering from 15 to 50. Because the eggs are laid on land, they can be maintained in a dish with a wet cloth on the bottom, allowing the developmental rates of the eggs to be followed.
A staggered hatching process was observed, one which remained independent of any variables such as temperature or amount of water present. The genetic determination was to spread the hatching process out over a period as long as possible and investigations proved that there were genetic differences between the hatching tadpoles.

 
The tadpoles that hatched first were smaller and developed faster, eventually becoming frogs which were smaller than those that developed form the delayed hatchings. The smaller frogs had a lower survival rate than the larger frogs, so the delayed tadpoles had a better survival chance.

How does all this benefit the frogs? Many of our frogs lay their eggs in non-permanent water sources and have no idea how long that water will remain so, although the first eggs become smaller frogs, there is still a good survival rate. If more water arrives, such as rain, then the slower developing frogs begin to hatch. In this way, the frogs are maximising the survival rate for any circumstances through this staggered hatching cycle.

Interestingly, frogs from northern hemisphere places like the USA and Europe develop in more reliable weather conditions. The climatic conditions are much more predictable, as is the availability of water like rainfall. The duration of their spawning ponds is hence longer and hatching is synchronous, with all tadpoles developing at the same rate.

Australian frogs have adapted to our own particular conditions in a unique way.

Text: V.B.  
Image from Arthur White

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