An experiment was conducted placing together cows who’d known one another—AKA, their best friend—and those who were strangers. When placed with their best friends, the cow’s heart rates were significantly lower, as were cortisol levels (lower cortisol indicates lower stress). Both of these indicators shot up when cow friends were separated.

Sadly this research was conducted with an eye towards improving milk yields on farms. Not measured were stress levels when newborn calves are separated from their mothers.

https://sentientmedia.org/animal-facts/