For a small frog, the alpine tree frog (Litoria verreauxii alpina) packs a lot of surprises.

For one, this tree frog lives in snowy gullies and high mountain crags across the Australian Alps, far from the tropical areas we normally associate with tree frogs.

But these frogs have another surprise. Their numbers have been decimated by a deadly fungal disease, chytridiomycosis, which spreads in water, enters the frog’s skin, and kills by causing cardiac arrest. The chytrid fungus has wiped out almost all alpine tree frogs, whose numbers have fallen more than 80% since the 1980s. The species now occurs in only a few fragmented and highly isolated sites. Even here, the fungus kills almost all alpine tree frogs in their first breeding season.

Given these odds, it begs the question – how is the species not extinct? To find out, we used lab and field studies to investigate whether the threat of chytrid infection was forcing these frogs to change.

To our surprise, we found clear signs of change. When infected with the fungus, male frogs set about fathering more offspring.

Before the emergence of the fungus, brisk spring nights across the Australian Alps would have been filled with the songs of male alpine tree frogs.

These choruses are long gone across most of the species’ range. The alpine tree frog is now critically endangered.

The call of the alpine tree frog. Laura Brannelly,

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