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