For the last few decades, amphibian populations around the world have been decimated by a deadly disease. Caused by a fungus, it has been responsible for catastrophic declines in frogs, driving many species into extinction, and threatening around a third of all the remaining species still clinging on. But infection does not always lead to extinction, as some species are found to carry the fungus but still survive – for a period of time at least.
Scientists looking at one species that carries the disease yet manage to survive found something they didn’t expect: infection actually increases the male’s attractiveness to females. Those male frogs that tested positive for the fungus, known officially as Batrachochytrium dendrobatidis (Bd), put more effort into their calls to attract mates. This could have profound implications on how the disease spreads within amphibian populations, and how it has managed to disperse so far and wide.
The disease has been implicated in mass amphibian die-offs. Forrest Brem/Wikimedia Commons
Originally identified in 1993, Bd was found to be the cause of a mass die-off in frog populations in Australia. Bd was identified as a chytrid fungus, leading to the highly infectious disease known as chytridiomycosis, which kills amphibians by stopping them from absorbing water through their skin. This prevents them from breathing or regulating their water levels. Since then, the disease has been found on every continent except Antarctica, heavily impacting hundreds of species of frogs and possibly driving at least 200 of those species to extinction.
But the mass dies-offs that were initially associated with the disease are not the whole story. Many populations of amphibians have been infected, but have then declined slowly over time, meaning that the spread and infection of Bd is seemingly more complex than first thought. It was this that led the researchers to look into how the fungus was impacting the Japanese tree frog (Hyla japonica), which appear to be tolerant to the pathogen.
Their results, published in Biology Letters, found that those male frogs infected with Bd were marginally larger than their uninfected counterparts, but also that they put more effort into their breeding calls. Female Japanese tree frogs prefer males that call for longer and faster, and it turns out that this is exactly what the frogs with the fungus are doing.
Now whether or not the increase in the males’ attractiveness is an intended outcome of the fungus in order to spread itself further and increase its transmission is not known. The researchers suggest that this could be the case, or perhaps the increase in calling has been selected for in infected males that need to reproduce earlier before they die. Either way, it adds to a growing body of evidence that infections of chytridiomycosis are far more complicated than thought.
While there is currently no way to prevent the disease, last year it was reported that scientists had managed to cure a population of Majorcan midwife toads, a species that had previously been decimated by the fungus. This week also saw the news that researchers had managed to find a short-term treatment for the disease, which could extend the time needed to save the amphibians, but not cure them completely.
1
