Female capuchins have linear dominance hierarchies. In contrast to many Old World monkeys such as macaques, in which females socially inherit the rank just below their mothers and just above their next oldest sisters, capuchins do not have a highly predictable ranking within their matrilines. Males are typically dominant to females. The alpha male is always easy to discern, but there are sometimes ambiguous rankings among subordinate males. Male-male relationships are tense, and affiliation between males is typically expressed by resting in contact, playing, or non-conceptive sex rather than by grooming. Males cooperate in coalitions against potential predators, and also in defense of the group against other males. Occasionally male coalitionary aggression becomes so violent that males are killed, particularly if they are encountered roaming the forest unaccompanied by allies. Because aggression from other male capuchins is the leading cause of death (aside from poaching by humans, where there is contact between humans and capuchins), male allies are critical for self-defense during migration, and to assist in taking over other groups. Male emigration to a new troop typically occurs about every 4 years, so most males are in constant danger of having to defend themselves against other groups of males.
Immigrating males often kill young infants when they take over a group. Females band together to defend their infants from infanticidal males, but they rarely succeed in saving their infants. Because infants inhibit their mothers from ovulating by nursing frequently, males are able to bring females into estrus earlier by killing the infants and thereby terminating nursing; this has the effect of increasing their breeding opportunities. Females do often mate with the killers of their infants, and with time, they typically become as supportive of the new alpha male as they had been of the previous one. The alpha male helps defend females from subordinate males within the group as well as from infanticidal males from other groups.
White-faced capuchin troops occupy home ranges of between . They travel between daily, averaging per day. Although they engage in activity that has been described as "territorial", more recent research indicates that White-faced capuchin troops tend to behave aggressively to other White-faced capuchin troops regardless of where they meet, and the aggression is not necessarily intended to exclude the other troops from a specific home range.
Home ranges overlap extensively, so groups are not territorial in the strictest sense of the word. Perhaps because of the intensity of male-male competition and the threat of infanticide, interactions between groups are typically hostile: the males display aggressively toward one another and sometimes engage in physical aggression (even killing an opponent), while females grab their infants and run. Typically, males are the primary participants in aggressive intergroup encounters, and it seems likely that males are defending access to the females in their groups. Alpha males, who have the largest reproductive stake in the group, participate at a higher rate than subordinate males. Groups with more males have an advantage over groups with fewer males, but the location of the encounter within the home range matters as well; smaller groups defeat larger groups when the contest occurs in the core or center area of the smaller group's home range.
The white-headed capuchin sometimes interacts with other sympatric monkey species. White-headed capuchins sometimes travel with and even groom Geoffroy's Spider Monkeys. However, aggressive interactions between the capuchins and spider monkeys also occur. Interactions between the white-headed capuchin and mantled howler are infrequent, and sometimes result in the capuchins threatening the larger howlers. However, affiliative associations between the capuchins and howlers do sometimes occur, mostly involving juveniles playing together.
Although South American capuchin species often travel with and feed together with squirrel monkeys, the white-headed capuchin only rarely associates with the Central American squirrel monkey. This appears to be related to the patchier, more dispersed distribution of food resources in Central America and the fact that there is less dietary overlap between the Central American squirrel monkey and the white-headed capuchin than between their South American counterparts. Therefore, there is less benefit to the Central American squirrel monkey in associating with the white-headed capuchin in order to exploit the capuchin's knowledge of food resource distribution. In addition, compared to their South American counterparts, male white-headed capuchins are relatively more alert to rival males than to predators, reducing the predator detection benefits that the Central American squirrel monkey receives from associating with the white-headed capuchin compared to its South American counterparts. Since the squirrel monkeys generally initiate interactions with the capuchins in South America, the fact that similar associations would impose higher foraging costs and impart fewer predator detection benefits to the Central American squirrel monkey leads to fewer associations with the white-headed capuchin.
Several non-primate animal species tend to follow troops of White-faced Monkeys or are otherwise attracted by their presence. white-lipped peccaries and common agoutis are attracted by feeding white-headed capuchins, looking for fruit that the capuchins drop. Several species of bird are also known to follow white-headed capuchins looking for food. These include the double-toothed kite, the white hawk and the sharp-shinned hawk.
The white-headed capuchin is an omnivore. Its primary foods are fruit and insects. It forages at all levels of the forest, and also forages on the ground. Methods for finding food include stripping bark off of trees, searching through leaf litter, breaking dead tree branches, rolling over rocks, and using stones as anvils to crack hard fruits. Its prehensile tail assists with feeding, helping support the monkey when foraging for food below the branches.
Fruit can make up between 50% and 67% or more of the capuchin's diet. In one study in Panama, white-headed capuchins ate 95 different fruit species. Among its favorite fruits are figs from the family Moraceae, mangos and related fruits from the family Anacardiaceae, the bean-like fruits from the family Leguminosae and fruits from the family Rubiaceae. It generally only eats ripe fruit, testing for ripeness by smelling, tasting and prodding the fruit. It typically eats only the pulp and juice, spitting out the seeds and fibers. Other plant matter eaten includes flowers, young leaves, seeds of certain plants, and bromeliads. It also uses the bromelids as a water source, drinking the water that gets trapped inside. In Carara National Park the capuchins have a varied diet in addition to the above of banana fruits and flowers, heliconia seeds, huevos de caballo fruits and anacardiaceae stems.
Insect prey eaten includes beetle larvae, butterfly and moth caterpillars, ants, wasps, and ant and wasp larvae. It also eats larger prey, such as birds, bird eggs, frogs, lizards, crabs, mollusks and small mammals. The population in Guanacaste, Costa Rica in particular is noted for hunting squirrels, magpies, White-crowned Parrots and baby coatis. The amount of vertebrate prey eaten varies by troop. Even neighboring troops can show significant differences in their diets.