Life force is a crude measure of the amount of Glycogen present in the bloodstream of creatures in Creatures 1, presented in a percentage form between 0 and 100. 0% means they're dead (or if they're still "alive-seeming", they have the Undead mutation), 75-77% means they're healthy, and 97-100% means you have a Highlander or a Norn with the Zeus mutation. Norns are normally born with 46% life force.