Since the grey squirrel, Sciurus carolinensis, was introduced from north America in 1876 it has driven the native red squirrel, Sciurus vulgaris, from most of England and Wales.
The number of greys has risen to more than 2.5 million while reds have dwindled to 160,000, three-quarters of which are in Scotland.
Greys rob the reds of hazelnuts, which they can eat earlier in the season, consume a wider range of seeds, and are more adaptable.
But the pair seem to have resolved their differences in Craigvinean Forest, near Dunkeld, Perthshire. Here the two species are sharing the same habitat, nesting in each others' territory and using the same feeding grounds.
The discovery was made by researchers from the Forestry Commission and Scottish Natural Heritage using radio collars to track the animals' movements, New Scientist magazine reported. Scottish National Heritage spokesman George Anderson said: "It seems that reds and greys have been living together in this area for around 40 years."Reuse content