Buttermilk and yogurt work. Kefir apparently works too, but not as well. Buttermilk and whey increase tenderness and reduce chewiness in marinated pheasant. Straight up lactic acid also tenderizes, and I'm betting that does much of the work in buttermilk and yogurt marinades.
For fun, you can skip to 13 minutes into the following clip to watch Heston Blumenthal convince someone to give yogurt marinated chicken a MRI. Spoiler: Yogurt appears to dramatically enhance marinade penetration. Why? Who knows!