Alright, I haven't had the chance to test this yet, but it SOUNDS like what people are doing is thus:
Go to: Program Files\Microsoft Games\Zoo Tycoon 2\ and open up "ai.z2f" with winRAR.
Open ai>animals and go to the tsk file you want.
find the line that says "
" (or whatever the number happens to be.
Change that number to what you want it to be. Save the tsk file (it should be in notepad) and then winRAR will ask you if you want to update the z2f file or whatever and you click yes...
Is this correct? Can anybody else verify?
Do you know what it looks to me like they did? They made the numbers roughly match the ACTUAL lifespans of the real animals, so in the game the animals live about the number of months that they would years, IRL. 0_o Weird.
Anyway, if I got that wrong, please feel free to correct me.
EDIT: This thread addresses the subject a LITTLE bit:
http://zoo.ogresnet.com/forums/showthread.php?t=4584