The geographical isolation and recent glacial history of Shetland have resulted in a depleted mammalian fauna. The field mouse (Apodemus sylvaticus L.), along with the brown rat (Rattus norvegicus Berkenhout) and the house mouse (Mus musculus domesticus Schwartz & Schwartz), are one of only three recorded types of rodent present on the island. Based largely on moprphological studies of epigenetic variations, the source of the original founding population has been attributed to Norway with the most obvious date of introduction being presumed to be around the 9th century AD with the arrival of the Vikings. However, archaeological evidence now suggests that this species was present during the Middle Iron Age (around 200 BC - AD 400), and one theory proposes that Apodemus was in fact introduced from Orkney where a population had existed since at the least the Bronze Age.[3]