Here is a link to a little video about Original Play by Playing By Heart author, O. Fred Donaldson . He talks about there being a big difference between how a lot of people play with their kids (because we just didn't know any other way), and how animals in the wild play with their own kind. A lot of play with kids, especially boys, is based on rough-housing, wrestling, tickling - with an underlying feeling of it being controlling and agressive. In nature, animals will still wrestle, but in a much more fluid manner - imagine dolphins gliding over and around each other, rather than ramming into each other.
There are three parts to this series, if you have limited time, just watch the second part.
Click on Original Play, then Original Play again and then the part you want to watch.