Social Traditions

Our interest in social traditions began when a group of 10 capuchin researchers collaborated on a study of traditions in white-faced capuchin monkeys. We concentrated our efforts on three behavioral domains: social conventions, foraging techniques, and interactions with members of other species. We have worked at 4 different research sites in Costa Rica, which are ecologically similar and close enough geographically that there is unlikely to be much genetic variation between sites. This map indicates the locations of the research sites within Costa Rica. We have collectively observed these monkeys for nearly 70,000 hours over a 13-year period. Researchers involved in this cross-site collaborative project included Susan Perry, Linda Fedigan, Katharine Jack, Lisa Rose, Katharine Mackinnon, Mary Baker, Melissa Panger, Joe Manson, Julie Gros-Louis, and Kendra Pyle.

Much of our work thus far has focused on social conventions: dyadic social behaviors or communicative behaviors that are unique to particular groups or cliques. We began this phase of the project by nominating various quirky social interactions or communicative signals that we had witnessed, soliciting data on these behaviors from the researchers at all sites, and subjecting them to a set of operational criteria that allowed us to classify a subset of the nominations as traditions.

What is a Tradition?

We defined a tradition as "a behavioral practice that is relatively long-lasting and shared among members of a group, each new practitioner of the behavior relying to some extent upon social influence to learn to perform the behavior." We subjected candidate behaviors to the following criteria to determine whether they qualified as traditions:

  • There must be intergroup variation in the behavior, such that it is common (i.e. seen at a rate of at least once/100 hours) in one or more group and absent in other groups that have been observed for at least 250 hours.
  • The behavior must be observed to spread through a social network.
  • The behavior must endure in the repertoire for at least a 6-month period.

Observed Traditions at Lomas Barbudal

Five behavior patterns were nominated as likely traditions: hand-sniffing, sucking of body parts, finger-in-mouth game, hair-in-mouth game, and toy game. The last three behavior patterns are referred to as games because they involved two roles and occurred in a relaxed social context, often stemming from slow-motion play wrestling, play biting, or grooming of the mouth. Another variant, eyeball poking, comes close to meeting the criteria. This chart shows the distribution of each behavior pattern across the study groups.


Two basic variants of this behavior are observed: one in which the hand was cupped over the nose and mouth, and another in which the fingers were inserted in the nostrils. At Lomas Barbudal, it was quite common for the behavior to be mutual, with the two participants inserting their fingers in one another’s nostrils simultaneously. The hand-sniffers remain in this pose for several minutes at a time, sometimes swaying gently as they sniff. Observers often describe the facial expressions as "trance-like." In two groups (Cuajiniquil and Station Troop), handsniffing was often combined with finger-sucking behavior.


Since the publication of the original set of articles on capuchin social traditions, we have been documenting the spread of a new tradition in Pelon group – eyeball-poking. In this ritual, one participant inserts its finger in the other’s eyeball, slipping the finger deep between the eyelid and the bottom of the eyeball up to the first knuckle. As in handsniffing, the pair remains in this posture for up to several minutes, and often the one being poked in the eye inserts fingers in the partner’s nostrils or mouth during the eyeball-poking. This video shows adult female Rumor inserting her friend Sedonia’s finger into her eye socket.

Sucking of body parts

In this behavior pattern, one individual inserts some body part of the partner (usually an ear, finger, or tail-tip) into its mouth and sucks on it for several minutes. Sometimes the behavior is mutual, especially with tail-sucking. As in hand-sniffing, the participants are relaxed, often grooming before the onset of the behavior, and a bit isolated from the rest of the group.

Finger-in-mouth Game

This variant involves insertion of a finger into the partner’s mouth (either partner can be responsible for insertion of the finger). Sometimes this game starts in the context of grooming of the mouth or a “dental exam.” The biter usually bites down firmly enough that it is quite difficult to remove the finger, but not hard enough to draw blood. Sometimes this game is combined with elements of the finger-sucking behavior described elsewhere. This variant is observed only at Lomas Barbudal. These two photos show Guapo biting the finger of a juvenile companion, who is trying to retrieve his finger.

Hair-in-mouth Game

In this variant, one individual bites a large tuft of hair from the face or shoulders of the partner. The partner may flinch, but does not exhibit much pain. The partner then typically tries to pry open the mouth and retrieve the hair. The hair is passed with gentle force from mouth to mouth until most of it has fallen to the ground. Then one of the monkeys bites another tuft of hair from the partner so they can continue. The hair game is observed only at Lomas Barbudal. This video clip shows two males from Flakes group at Lomas Barbudal playing the hair game. Adult male Napoleon has juvenile male Jade’s hair in his mouth, and Jade is trying to remove the hair from his mouth. When Jade starts to lose interest, Napoleon once again shows him the remaining hair.

Toy Game

In this variant, an inanimate object (a stick, green fruit, leaf, or piece of bark) is passed from mouth to mouth in the same manner in which the hair or finger is transferred in the other games. No one consumes the toy at the end. This is the one game that is seen in multiple sites (Lomas Barbudal and Curú).