Interesting things are afoot at the BBFC it would seem.
Ruggero Deodato is currently working on a director's cut of Cannibal Holocaust that will feature less animal cruelty. It seems a copy of the film was submitted to the BBFC and the only one of animal cruelty scenes that was going to require cuts was the killing of the small rodent type animal. A reason this was unacceptable was that whereas the other animals killed on camera were consumed for food, this was not.
There's something of an irony here in that within the diegesis of the film this animal is one which is eaten (the guide who kills it actually remarks 'tonight we eat meat' or something like that) whereas a pig that gets shot later is presented as not being eaten ("that pig would have been food for those people').
Context based guidelines just get harder to understand the longer you think about them contextually...
[There's loads more on the topic at the Cult Labs forums]