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Winter breeding frogs

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Not all Australian frogs breed during the warmer months, and Arthur White looks at the the advantages for two Sydney species that will only breed in winter.    
It's hard to imagine that there could be any advantages for a cold-blooded animal like a frog to lay its eggs only in winter when food is less plentiful and the cold slows the development of tadpoles. However two species of frogs local to the Sydney region behave in this way and their populations are doing well. These are the Jervis Bay Tree Frog and Hazwell’s Froglet which is a ground dweller.

Indeed, given that there are so few frogs that breed in winter, their calls (a reedy high-pitched call of three or four notes for the Jervis Bay Tree Frog and a dull, duck-like call for Hazwell’s Froglet) are easily heard on winter nights.

Litoria jervisiensis (shown left) and Paracrinia haswelli (tadpole shown below) are the two winter-breeding frogs Arthur discusses.
all photos from Arthur White.

Arthur investigated possible reasons for this unusual and extreme choice (the winter frog breeding research being much more uncomfortable than the usual springtime investigations) given that there were just as many predators at that time of the year. He supposed that their eggs might not be so affected by the cold and that somehow the tadpoles might develop relatively quickly in the colder water. He reared Jervis bay Tree Frogs in a laboratory under different temperature conditions to test those suppositions. However neither proved to be the case, the eggs and tadpoles being just as affected by the colder temperatures as other frog species.

The breeding advantages lay elsewhere and illustrate the adaptation that evolution brings to animal species. The first advantage is that both species lay their eggs in permanent water bodies that are quite deep which, since the eggs are laid in winter, have little chance of drying out from evaporation. Even if they did dry out the eggs would survive in the mud left on the bottom of the water source.

The second advantage is that by laying the eggs in winter, the tadpoles that hatch at the onset of springtime will be the first to develop and metamorphose into baby frogs. In turn they will be at the head of the queue to take advantage of the growing food supply that spring brings and not have to compete with the spring breeding frog species, yet another advantage.
These two species highlight that wherever there is an environmental niche that could be turned to a survival advantage some species will evolve to do just that - take advantage.

Dr Arthur White was interviewed for A Question of Balance by Ruby Vincent. Photos from Arthur White. Summary text by Victor Barry, re edited July 2016.

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