The book of numbers reading chapter 0 2/7/11

Quote: " The trick they used was very simple: as warriors left to go to battle, they put a rock in a pile. When they got home again, they each removed one. The number of rocks left equaled the number of men lost. The chief would then remove each rock in turn and pick up a stick for each rock her removed (sticks are easier to carry). He then would walk to the other tribe with his sticks and demand a buffalo in exchange for each one. So without actually counting or even understanding the notion of numbers, it was possible for very precise trades and transactions to take place."

Question: Why would the tribe leaders agree to giving away their buffalos to the tribe that they just fought?

Comment: This quote was interesting to me because I was wondering why the tribe chiefs would agree to give away their buffalos to their rival tribes. And I also found it interesting how their putting a rock, taking a rock, and getting buffalos method worked out. Because they might have lost men, but they also gained more buffalos so more food. But what about when all of their warriors die? How would they fight and would they still keep the trading buffalos treaty?