Extreme weather appears to be disrupting the life cycle of Europe's bats.
Scientists were alarmed to find that some bats in Portugal skipped winter hibernation altogether this year while others gave birth early.
The findings add to growing fears that rising temperatures are having unpredictable effects on bats, birds and other wildlife.
Bats born early in the year may suffer due to lack of insect food.
"It's a phenological mismatch," said Dr Hugo Rebelo of the University of Porto, who is studying the impact of climate change on several Mediterranean bat species.
"What this means is that the bat birth is more or less synchronised with the time of emergence of insects so that when bats give birth there are plenty of resources to feed on and then to feed their own pups.
"With these chaotic weather patterns we are having now in winter and spring we don't know if everything is being mixed up. "
Rare bat species have been routinely monitored in Portugal at their underground roosts since the 1980s.
In order to survive the winter months, bats must hibernate as there are not enough insects flying around in the winter to meet their energy demands.
Dr Luísa Rodrigues, a biologist at The Institute of Conservation of Nature and Forests in Lisbon, said that for the first time in Portugal they found bats that had been born very early.
"In January and February I visited 20 caves and mines and this happened only in one of them," she said.
"It was a rare situation and even in the colony where we found this there were 500 bats and we found only two babies."
These are only isolated cases, she said, but a sign that we need to keep monitoring the situation.
"It's not a red alert but it's something that we need to be conscious of," she added.
The researchers are concerned that mild weather in the south of Portugal is interfering with bat hibernation. They do not know if this will have negative impacts on bat populations.
If the bats emerge from hibernation too early, they struggle to find insect food for themselves and their young, particularly if there is a period of spring rain.
This can lead to malnutrition and "huge mortality events".
"We are completely in the dark," said Dr Rebelo. "We don't know if the loss of hibernation will be beneficial and bats will be overweight and more fit to reproduce or on the other hand they are having early births and they are not adapted to the spring rains."
He said bat roosts have increased in temperature by as much as six to eight degrees in past decades.
In the UK, there is some evidence that climate has had an impact on horseshoe bats.
"The increasing variability of weather conditions is potentially catastrophic for bats," said Dr Fiona Mathews of the University of Exeter.
"Although species in tropical regions do not hibernate, without any ill-effects, in temperate areas the situation is rather different as we cannot rely on there being good weather and plenty of food available in spring.
"In the absence of hibernation, bats begin the process of pregnancy sooner, which can mean that babies are born at a time when their mothers cannot feed sufficiently to support them."
The two species that are most common in Portugal's bat roosts are the bent-wing bat and the greater mouse-eared bat. Both species normally hibernate from December to late February.
The bent-wing bat is present in the southern half of Europe, with the biggest populations found in the warmer Mediterranean area. It is classed as Near Threatened on the IUCN Red List.
The greater mouse-eared bat occurs in western, central and southern Europe. It is one of the first bat species to give birth, but this normally happens in April.
Temperature changes may affect the hibernation of bats in several ways, including the length and timing of hibernation, breeding success of female bats, and the types of insects available for bats to feed on.
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