Bison Cow with young calf
The adult bison were going through the stages of shedding their long shaggy brown winter coats – for a much shorter and darker summer coat.
In Yellowstone most of the calves (80%) are born between April-25 and May-25 with almost all calves born by June-5th.
This calf looks quite young but was probably at least 10 days old.
Reference: http://www.bioone.org/doi/abs/10.2981/09-082