In a set of classic studies in the 1950s and 1960s, Harry Harlow and others investigated the nature of attachment in young rhesus monkeys.
In one study (Harlow & Zimmerman, 1959), the researchers exposed young monkeys to two artificial mothers, one wrapped in toweling cloth, and another made of bare wire, but with a feeding bottle attached to it. The experiment showed that the infants clung to the mother with the toweling cloth for most of the time, and would only approach the other mother when in need of food. This showed that close, warm contact is an important part of bonding with a mother.
Harlow, H. F., Zimmermann, R. R. (1959) “Affectional Responses in the Infant Monkey“. Science, Vol 130, Aug, 421-432
Related lecture: Exploring the Neurobiology of Parent-Child Bonding.