Compatible rhesus monkeys provide long-term stimulation for each other (1989)

Ranheim, S., Reinhardt, V.

Abstract

The [12 pair-housed] rhesus monkeys in the present study distracted each other in species-typical ways on average 35% of the time, even though the companions had lived with each other continually for 2.5 years. Apparently, a compatible companion is a very valuable, ever-changing stimulus that does not easily lose its effect over time.