Animal reproduction - how the rez box/HUD works

When you use the Rez box/HUD to get a new animal, the selected animal is rezed out of the object and a copy of the script called animal is placed in the new animal. The animals all contain a copy of themselves (without the animal script) so they can reproduce by doing the same as the rez box/HUD i.e. rez out the animal and send it their version of the animal script

This means that if you update the animal script, new rezzed animals will use that new script. Also, the 'update' feature replaces the animal script in existing animals with the new version.
As the animal script is stored as 'not running' in the rez box/HUD, it's important to test any changes to the animal script first before using the update feature, as syntax errors will mean all the updated animals will stop working when the faulty script is sent to it and you will have to either delete them or manually replace the script in every animal!