The half-life of any chemical, either in the Creatures series or our own universe, is the time it takes for a given amount of any chemical to decay to half the original value. In Creatures, altering this can have profound effects on biochemistry.
In creatures, the half-life of any chemical is determined by a big gene called, oddly enough, the half-life gene. The half-life gene is the longest gene in a creature, as it contains the decay rates of all the chemicals in a creature's biochemistry. A common mutation of the half-life gene produces a longer half-life for ageing or life, leading to longer-lived creatures.
As of Creatures 2, if you hex edit a gene file, and change the decay rate to values that are not in the included table, you will just get the same behaviour as the nearest lower value in the table. For example, any value from 0 to 7 will behave as 0 does, and any value from 64-71 will behave the same as 64. source
See biological half-life on Wikipedia for more on the real-life topic.
- Genetics Lesson: Examining a Creatures Half-Life at Discover Albia - contains a table of half-life numerical values compared to real-world time for C1, C2 and C3/DS.
- Half-Lives and Other Genetic Mysteries at Naturing::Nurturing - critiques the official Genetics Kit's information about halflife times.