Doug,
This is so adorable and something to really enjoy. I'm going to show this to my grandchildren who'll be thrilled. It shows family and extended family with security and bonding. I love it.
Is this anywhere near you? If so, the investment in a series would be so rewarding. The great thing about this place is that unlike birds, they're not able to fly away at the first click of the shutter or sight of you!
Asher
P.S. As for cropping, (and Don's approach does work very well indeed ), I'd personally only go for a thin strip from the lower edge. Leave enough grass as a border and then immediately the two of them. That way, all the other bison belong! OTOH, as you have framed it, it works already very well and big animals balance the extra space one might think of removing.