Near Hebgen Lake, Montana, researchers watched two bison give birth within an hour of one another in April 2007. The mothers then did something unusual: each helped care for both newborn calves. The accompanying image illustrates the subject rather than documenting that observation.
During five and a half hours of observation, the females cleaned the young and allowed them to nurse from both mothers. They also reacted defensively when other bison approached. The researchers described the combination of nursing and extensive cooperative birth care as the first documented case of its kind in wild bison.
The reason remained uncertain. The females might have been related, or sharing responsibilities might have benefited both, but the observers could not establish either explanation. Their report preserved an uncommon moment without turning it into proof that all bison families behave this way.
Sources: Jones and Treanor: Allonursing and Cooperative Birthing Behavior in Yellowstone Bison (2008)