Frogs at risk of deadly virus ‘breed younger’
FROGS from groups exposed to a deadly virus are breeding at younger ages than those free from the disease, researchers in the West Country have found.
Scientists found the youngest breeding frogs in populations affected by ranavirus were aged two, while those in disease-free areas bred from the age of four.
The team, led by researchers at the University of Exeter and ZSL’s Institute of Zoology, warn this decrease in breeding age means disease-exposed populations are at greater risk of local extinction from environmental changes.
Frogs gather at breeding spots such as ponds and then disperse, with most returning to the same ponds year after year.
Dr Lewis Campbell, who conducted the research during his PhD at the University of Exeter, said: “We found significantly fewer old frogs and significantly more young frogs at populations which have ranavirus.
“It’s possible that the more times an older frog returns to the same infected breeding pond, the more likely they are to become diseased and die.
“The absence of older frogs may then create an opportunity for younger – and therefore smaller and less competitive – frogs to successfully breed.
“With high mortality among older frogs, it’s also possible that natural selection pressure has favoured those that are genetically disposed to breed younger.”
Ranavirus, which was first recorded in the UK in the 1980s and is usually fatal, can cause severe skin sores and internal bleeding.
In the study, which incorporated data collected by citizen scientists, breeding frogs in disease-free populations were usually aged between six to eight years old.
But those in populations where ranavirus was present were mostly aged three to six years old.
Dr Campbell, now of the University of Wisconsin-Madison, said: “Our models suggest both infected and uninfected populations can continue to thrive in normal conditions.
“But disease-exposed populations appear to depend heavily on younger breeders that don’t produce as many offspring as older, larger frogs.
“For these populations, an environmental change such as a late frost – that would kill frogspawn and further lower the number of offspring produced – could cause that population to collapse.”
Ranavirus is believed to be spreading in the UK, with human activity such as movement of animals and soils believed to be the major cause, researchers said.
Amphibians are the most threatened group of vertebrates globally, they added, with emerging diseases playing a large role in their population declines.
Dr Xavier Harrison, from ZSL, said: “We often think of the negative consequences of wildlife disease as being the death of infected individuals in the short term.
“But this study shows that even when a population seems to have survived a disease outbreak and appears otherwise healthy, there are still lingering consequences of that disease months or years afterwards.
“If we really want to understand the full impact of wildlife diseases in nature we need to monitor diseasechallenged populations over much longer timescales.”
The paper was published in the journal PeerJ.