The secret to why grey squirrels outcompete their red counterparts may lie in their gut bacteria, scientists believe.

Grey squirrels are an invasive species from North America that was introduced to the UK and Ireland in the late 19th century.

They out-compete the native red squirrels for food and space and carry a disease, called the squirrel pox virus, which kills the reds but has no known lasting effect on the greys.

Not only are grey squirrels larger and more robust than red squirrels, we have now identified a significant difference in their gut bacterial microbiota, potentially giving them another advantage over reds

Grey squirrels also pose a threat to the sustainable management of woodlands because of the damage they cause to trees by bark stripping, a behaviour that was not fully understood by experts.

Now an analysis of the gut microbiome – the ecosystem of microbes that live in intestines – of both red and grey squirrels has revealed the latter to have a more diverse gut bacteria.

The researchers used DNA sequencing methods to identify the different types of bacteria in the guts of both red and grey squirrels.

The team found a particular type of microbe in the digestive systems of grey squirrels that helps break down a plant compound known as oxalate.

According to the researchers, the presence of this bacteria suggests grey squirrels can digest calcium from tree bark, which could also explain their destructive bark stripping behaviour.

The team said its findings, published in the Journal of Medical Microbiology, show grey squirrels have not only better general health and immunity compared to red squirrels but they also have a broader diet and are able to access a wider range of resources.

The more we know about grey squirrels, the more equipped we're going to be in the future to tackle the threats they pose to red squirrels and our native trees, which is one of the biggest problems for forest conservation in the UK

Lead author Roberto La Ragione, professor of veterinary microbiology and pathology at the University of Surrey, said: “Red squirrels are now an endangered species in the UK.

“Not only are grey squirrels larger and more robust than red squirrels, we have now identified a significant difference in their gut bacterial microbiota, potentially giving them another advantage over reds.”

Study co-author Chris Nichols, conservation evidence manager at the Woodland Trust, said: “The more we know about grey squirrels, the more equipped we’re going to be in the future to tackle the threats they pose to red squirrels and our native trees, which is one of the biggest problems for forest conservation in the UK.”

He added: “More research needs to be done to further understand the relationship between red and grey squirrels and their gut microbiota, but perhaps in the future we could develop this research into methods that could promote healthy gut bacteria in red squirrels or new ways of preventing squirrels from damaging trees.”