The (English Rio Grande) rules state:
When a player plays a card with special abilities (!), he can use those abilities.
However the abilities of certain special animals are more like attributes and given the logical consistency of the game, the attributes are likely to be persistent.
That is, if a Hedgehog isn't eaten by larger animals when it is played, it won't be eaten by any later larger animals.
Both the Hedgehog and Turtle's attribute is:
Not bothered by larger carnivores or omnivores, and so can share a cabin with them.
However, the Fox ability is specifically limited to when played.
When a player add the fox to the ark, he takes 2 action discs from the supply.
NB http://www.doris-frank.de/archeFAQ.html points out that whilst herbivore Turtles can share with a carnivore/omnivore of equal size, the omnivore Hedgehog can only share with a larger carnivore/omnivore as the Hedgehog would eat equal sized or smaller animals.
Hope this helps!