Why is it that squirrels prefer nuts with the shell on instead of off?
That makes no sense because shelled nuts provide more energy for less work. With the shell, they have to work to get it out and even then, they only get one or two nuts (depending on the type of nut). With shelled nuts (that humans provide), the squirrel doesn’t have to work to get at the nut meat, and they can fit a lot more of them in their mouths/nests/etc. since no space is wasted on the shell.
In a Nature of Things documentary on squirrels, David Suzuki likened in-shell peanuts to $5 bills; that is, an extra special treat.
My best theory as to why they prefer nuts with the shell is that the shell helps to keep their teeth from overgrowing (they have to continuously gnaw on things to file them down). Of course I can’t imagine that a peanut shell provides much sanding value, so I’m not convinced that is the reason.
About a month after this question, I posted a proposed answer.