I recently got a short tour of the normally-closed nocturnal house. This now houses an elderly mouse lemur and a thriving aye-aye family. Sabrina and Warlock produced a son, Dobby, who looks just as eccentric as his elders. He is only the 2nd second-generation aye-aye born in the US, so the Zoo is extremely proud of him. Asked how they accounted for this success, our guide said that it seems aye-ayes like to mate high in the trees, and the nocturnal house has a high ceiling which can be utilized for this romantic purpose.
Several months ago, to general surprise, Sabrina and Warlock produced another baby. It had been expected that it might take the couple five years to have their next child, not just one year. (Ah, high ceilings...) This daughter is allegedly still unnamed. (I find this unlikely. I suspect she has a name but it is being kept secret for one or more of the usual bizarre zoo rationales for withholding such non-Nature Facts. Or they are just embarrassed that they named her Morticia or Grimalkin or whatever.)
I asked about the wrench. “Oh yes,” said the guide. “We give them wrenches for their teeth.” It seems that aye-ayes have ever-growing teeth, like rodents, and need hard objects to gnaw on so their teeth don't overgrow. For this purpose they are given bamboo and hard wood. And wrenches.
The guide, who does not appear to have been at the Zoo very long, seemed to find this perfectly mundane, and offered no explanation for how wrenches came to be the dental device of choice.
(“Sure, we give the aye-ayes wrenches for teething. Just like we give the otters eggbeaters for grooming, and we give the pelicans assault rifles for nest-building. It's environmental enrichment.”)
Until I get a different suggestion, I am sticking with the hypothesis that this practice originated in an aye-aye act of pilferage.