A Scottish keep that appeared in the 1975 film Monty Python and the Holy Grail has become caught up in a mystery about where bats hibernate.

Experts have been intrigued about where pipistrelle, which roost in large numbers in Doune, Central Scotland, in summer, spend the winter.

Just over 30 have been counted in Doune Castle, which was used in the film.

Meanwhile, bat watchers in the far north are monitoring for a species which may spread with global warming.

Pipistrelle come in two varieties - soprano and common

Anne Youngman, the Bat Conservation Trust's Scottish officer, said as many as 1,600 pipistrelle have been counted in Doune during summer months.

They use a telephone exchange as a maternity roost and the area's sizeable population makes it a prime place for the trust to train volunteers.

Mrs Youngman said: "The telephone exchange is cosy and safe, while Doune Castle is a cold, humid place for them to hibernate.

"But we are lucky to find 30 there in winter, so where do the rest go."

Pipistrelle in Scotland are not known for travelling far unlike bats in other countries, she said.

Mrs Youngman added: "In Poland there are World War II bunkers no longer used by the military but contain thousands of bats.

"Workers found one species that appeared in one spot year after year.

"They ringed one and found that it came from Germany to hibernate in Poland."

Computer analysis

Meanwhile, North Highland Bat Network is trying to monitor for Natterer's bat which has not been seen further north than the south side of the Dornoch Firth.

David Patterson, who surveys bats with wildlife artist Lyn Wells, said climate change could see the mammals finally make the flit north.

Natterer's are difficult to distinguish from Daubenton's bat and often computer analysis of their call is needed to identify them.

Mr Patterson said there was suitable habitat, including oak woodland where Natterer's could forage on the wood floor for spiders.