Bank vole  (Clethrionomys glareolus)

Hibernation: a winter survival strategy characteristic of some mammals in which an animal's metabolic rate slows down and a state of deep sleep is attained. Whilst hibernating, animals survive on stored reserves of fat that they have accumulated in summer. In insects, the correct term for hibernation is 'diapause', a temporary pause in development and growth. Any stage of the lifecycle (eggs, larvae, pupae or adults) may enter diapause, which is typically associated with winter.
Ovulation: in female mammals, the release of a ripe egg from an ovary (one of the paired reproductive organs).
Subspecies: a different race of a species, which is geographically separated from other populations of that species.

1. National Biodiversity Network Species Dictionary (May 2002)
   http://www.nhm.ac.uk/nbn
   
2. Burnie, D. (2001) Animal. Dorling Kindersley, London.
   
3. The Environment Agency (1998) Species and Habitats Handbook. The Environment Agency, Bristol.
   
4. Macdonnald, D. W. & Tattersall, F. T. (2001) Britain's mammals- the challenge for conservation. The Wildlife Conservation research Unit, Oxford University.
   www.wildcru.org
   
5. Macdonnald, D. W. (2001) The New Encyclopedia of Mammals. Oxford University Press, Oxford.