It is very difficult, for example, to decide whether the bobac marmot (Marmota bobac), which forages on the ground but spends the majority of its life in a burrow, is terrestrial or subterranean.
Using such an approach the bobac marmot may be described as a terrestrial/subterranean species (T/S), and the yellow-necked mouse as a terrestrial-arboreal/subterranean-arboreal species (TAr/SAr).