As stated above it could either be quadruplets - rare as indicated but possible - or an 'adoption'. I've seen this happen this time of year with our deer herd around my property - 2 does with fawns are paired up and one gets hit by a car - the remaining doe keeps the orphaned fawn with her. I would expect as mentioned by CJB that this would only occur with familiar relations, but hard to prove.

Dale

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"When tempted to fight fire with fire, remember that the Fire Department usually uses water." - anonymous