Cash back?

I have two chocolate labs and a lab mix.

The two purebred labs have a strong retrieve drive, and Penny expecially will play fetch for hours, dropping a slobber-soaked tennis ball into my hand for me to throw, repeating this until her legs are weak and shaking.

This morning, I was putting some cash into my wallet. I set the opened wallet down in the purse and put the purse on the table for a second. Of course, Murphy's Law being what it is, the purse spilled, and coins rolled all over the kitchen.

Penny pounced and gave chase to the money. I started telling her, "Leave it!" and "Mine!", thinking she would swallow the coins IF she could get them up. As I watched her with one eye while scrambling to scrape up the money, she delicately picked up dimes, brought them to me, dropped them into my purse, and raced off for more.

Last night the kids and I watched a PBS special about dogs; the premise of the special was that humans had selected traits that they wanted in dogs and bred selectively, creating all the types, sizes, and breeds of dogs we have today. I wonder which trait was the one which caused Penny to retrieve cash.