Jan Hamrsky / Swan mussel (Anodonta cygnea) burrowing into the sand, Europe, August. Controlled conditions.

Jan Hamrsky / Swan mussel (Anodonta cygnea) burrowing into the sand, Europe, August. Controlled conditions.

Jan Hamrsky / Solid orb mussel (Sphaerium solidum), Europe, July. Controlled conditions.

Jan Hamrsky / Solid orb mussel (Sphaerium solidum) burrowing into the sand, Europe, July. Controlled conditions.

Jan Hamrsky / Solid orb mussel (Sphaerium solidum) burrowing into the sand, Europe, July. Controlled conditions.

Markus Varesvuo / Willow grouse / Ptarmigan (Lagopus lagopus) making a half snow cave, Inari Kiilopaa, Finland, April.

Charlie Summers / Western white-tailed prairie dog (Cynomys leucurus) excavating a new burrow in the side of an old burrow. Arapahoe National Wildlife Refuge of North Park, Colorado, USA, June.

David Kjaer / Sand wasp (Ammophila sabulosa) at burrow entrance, Arne, Dorset, UK, July.

David Kjaer / Sand wasp (Ammophila sabulosa) at burrow entrance, Arne, Dorset, UK, July.

David Kjaer / Sand wasp (Ammophila sabulosa) at burrow entrance, Arne, Dorset, UK, July.

Markus Varesvuo / Grey partridge (Perdix perdix) buried in deep snow, Liminka, Finland, February.

Markus Varesvuo / Black grouse (Lyrurus tetrix) making a snow cave, Kiilopaa, Inari, Finland, January.