Donkeys are not as able to keep warm as horses in the UK's cold, damp winters, according to a new study.
The study, by scientists at the Universities of Portsmouth and Canterbury Christ Church University, recommends the animals are given extra winter protection.
The study was funded by the Donkey Sanctuary and published in Equine Veterinary Journal.
One of the authors is Dr Leanne Proops, a specialist in animal behaviour and cognition in the University of Portsmouth's Department of Psychology.
She said: "The common perception is donkeys are hardy and capable of enduring challenging environments. While it's true they're highly adapted to the harsh, semi-arid environments, it would be wrong to assume this hardiness allows them to thrive under all conditions.
"Our results showed that unlike horses, donkeys are not able to adjust their hair coat weight, hair length and width in response to colder, winter weather, which suggests donkeys need welfare guidelines separate to those for horses.
"We'd suggest they need manmade protection from wind and rain."
The researchers also found the intermediate nature of mule hair coat properties should also be considered.
The study aims to provide the first scientific assessment of the extent to which donkeys require protection from the elements across the range of environmental conditions typically experienced in the UK.
The research examined the insulation properties of the hair samples (weight, length and thickness) which showed that donkeys' coats do not change significantly across the seasons and that their coats were significantly lighter, shorter and thinner than that of horses and mules in winter. In contrast, the coats of horses and ponies changed significantly between seasons, growing much thicker in winter.
The Donkey Sanctuary has been working with Dr Proops and Dr Britta Osthaus, at Canterbury Christ Church University, both specialists in animal behaviour and cognition.
Dr Faith Burden from the Donkey Sanctuary said: "For many years it has been the 'common sense' advice given by The Donkey Sanctuary to ensure that donkeys and mules are given the right protection from our cold winters. This study now provides us with scientific evidence to show why the welfare needs of donkeys and mules differ slightly to those of horses and ponies, and how we can act to give them better protection from the elements."
Further publications from the project are planned, looking at heat loss and the behavioural responses of donkeys and horses to different weather conditions.
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B. Osthaus et al. Hair coat properties of donkeys, mules and horses in a temperate climate, Equine Veterinary Journal (2017). DOI: 10.1111/evj.12775