If you recall, Bob Hebert presented a very amusing paper at the SEA several years ago (and
I think it was subsequently published) asking why dogs would trade one bone for another
anyway.  From a subjective value perspective, each would have to regard the other's bone
as a more desirable one than the one already possessed.  From Smith's point of view, they
would have to trade because of their propensity to truck and barter.  From the perspective
of a parent of siblings, each would want the other's bone because the other one had it.
  
Sam Bostaph