Since this question has not been answered.
Hersenen/Hersens (both are valid) is both plural and singular. Similar to pants/trousers in English. Hence:
- Ik heb hersens - I have a brain/I have brains (both are possible)
- I have pants/trousers - Ik heb een broek/Ik heb broeken (both are possible)
Hmm, I do see "breinen" used here and there though. For example in (scientific) articles where the writers choose to use the word "brein" throughout, they also often use the corresponding plural. Or for the fixed phrase "het brein achter" ("the brains behind") I sometimes also see the plural used ("Wie zijn de breinen achter de aanslagen?" = "Who are the brains behind the attacks?").
Otherwise, as it often goes with synonyms (apart from fixed phrases), in some sentences one of the options may simply "sound better" to whoever is saying them, so there aren't really any hard and fast rules I'm afraid xD.
The website does give the plural breinen: http://www.vandale.nl/opzoeken?pattern=Brein&lang=nn#.VnWdUejTVSA
However, in every day use breinen is hardly used in my personal experience.
This confuses me, also.
The English translations where "brain" should be used are ridiculous. In English, the only way an individual person or animal has more than one brain would be if the brains came from other biological entities. The only humans who would have "brains" would be those who work in medical or veterinary science and serial killers.
Actually, I don't believe it is that hard to grasp the use of brains in English.
For once, as a more archaic sense of the term, it can mean intelligence/smarts, which would be a completely legitimate meaning in the context of this sentence. The animal did not fall into the trap, but still got the bait. It has brains.
Also, there are some popular idioms which use the word: beat/rack one's brains out, blow one's brains (i.e. to shoot in the head) and pick someone's brains.
A little farther is the informal meaning of leader: He was the brains of the operation, with a clear metaphorical reference to the cerebral organ.