1) Japanese macaques are known to throw super-intellects. Imo's invention of the sand-free sweet potato and the floating rice trick have been documented. Scientists gave the monkeys pieces of sweet potato to attract them; Imo thought of washing hers. It made the process faster, so the scientists switched to rice. Imo again put hers in water, skimmed off the floating rice, and had a quick snack.
2) They have different traditions in different places. This implies that brilliant monkeys occur repeatedly. This is a big deal. People tend to overlook how big a deal it is, but Homo ergaster or African Homo erectus had one inventor, ever. They kept his or her brilliantly made hand-axe for about a million years ... and basically never changed it or added any other tools based on the same principles.
3) Domestication is extremely rare outside of humans. Social insects do it -- there are ants that farm fungi and others that herd aphids. But what the Japanese macaques are doing is much more sophisticated. They're selecting deer with a tolerance for monkey behavior. Some of those deer are so phlegmatic they put up with being pulled, chewed on, and dry-humped. Seriously, we have horses that won't put up with that much hassle. The deer definitely get something out of the exchange: grooming and food scraps. The monkeys are making very deliberate choices about which deer to approach. Which means if this sticks, those tamer deer are going to get better health and better food, which boosts their reproduction, which will pass on those traits. That means monkey sex is influencing deer sex. You might not see the effects for a while, because this seems to be a new thing, but somewhere out there is a super-intellect who thought a deer would make a dandy fucktoy and paid for it. Yes, the deer are effectively prostitutes. They do make a choice -- some will walk away, others will tolerate the riding and humping for the perks. Judging from the way one monkey was pulling on fur and antlers, it probably won't take long to figure out that you can steer a mount that way.