Encyclopedia of Animal Cognition and Behavior

Living Edition
| Editors: Jennifer Vonk, Todd Shackelford


  • Jennifer ColbourneEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-3-319-47829-6_621-1



Fear and/or intolerance of strangers, often expressed as aggression or aversion.


Dislike for strangers is pervasive throughout the natural world. The introduction of a strange individual often provokes the strongest aggressive responses in animals; a phenomenon termed the “xenophobic principle” by E. O. Wilson in 1975. Strangers can be a threat to one’s territory, resources, status, or mates, and consequently there is selective pressure against tolerating unknown individuals, as the costs vastly outweigh the benefits (Wilson 1975).

The result is that animals may threaten, harass, or even attack strangers. Animals may go great lengths to avoid areas where strangers may encounter each other; at territory borders, “buffer zones” of increased prey/plant density may be created by such avoidance (Mech 1994). Borders may also be patrolled and marked to ward off intruders, or calls used to advertise the occupation of a territory (Hardouin et al. 2006; Mech 1994).

While territorial species are essentially xenophobic by definition, many group-living species are also prone to aggression towards outsiders. This is because a newcomer represents a threat to each individual’s status in the group hierarchy, thereby upsetting the existing social order (Bernstein and Gordon 1974). It is common for group members to ally with each other against intruders, including with individuals that are typically shunned (Wade 1976). Even if a group is not overtly aggressive, members may increase grooming each other in the presence of strangers, either to consolidate bonding or to make the bond more evident, in what has been term a “social shield” (Antonacci et al. 2010). After a new member joins a group, although they may appear to be tolerated, it can take months for social interactions to return to baseline levels (Fragaszy et al. 1994).

Resource Availability

The cost of tolerating strangers varies according to the resources available. Populations of the same species may be more xenophobic in harsher, resource-poor environments, where protection of resources outweighs any potential benefits from interactions with strangers, such as reproductive opportunities (Spinks et al. 1998).

Similarly, the toleration of neighbors appears to vary depending on the level of threat they pose to an individual. In cases where there is intense competition, the result can be “nasty neighbors” (Müller and Manser 2007). However, when a neighbor poses less of a threat than a stranger, a “dear enemy” situation may arise (Fisher 1954). For example, a stranger may threaten a territory owner’s resources and territory, while a neighbor may only threaten its resources, since the neighbor already has its own territory. In this case, the benefits of tolerating the neighbor outweigh the cost of constant vigilance and aggression (Temeles 1994). Having a neighbor that is a “dear enemy” also increases the value of the territory to the territory holder, compared to a new occupant that will have to negotiate with a new neighbor. Indeed, in great tits (Parus major), it has been found that new occupants initially fight more often with neighbors than the previous residents (Krebs 1982).

Lethal Raiding

Extreme xenophobic aggression towards neighbors may take the form of “lethal raiding,” in which another group’s territory is entered and individuals are attacked by a coalitionary group. Goodall (1986) implied that chimpanzees are motivated to engage in lethal raiding due to the low cost to the aggressor, a hypothesis later developed as the “imbalance-of-power hypothesis” (Manson and Wrangham 1991; Wrangham 1999). This hypothesis predicts that attacks occur only if the raiding party has superior numbers or strength, which is borne out by field observations that have found attacks only on solitary animals or pairs by groups of at least three or more males. For safety, chimpanzees should travel in large groups, which they tend to do near borders. However, the ecological pressures of having to share limited resources (scramble competition) means that in times of reduced food availability, chimps may be forced to travel in smaller groups (Manson and Wrangham 1991). Consequently, large social networks with variation in subgroup size (fission-fusion societies) are necessary for lethal raiding, and indeed coalitionary group killing has been seen in other fission-fusion societies, such as wolves (Mason and Wrangham 1991; Mech 1994).

Sex Differences

Sex differences in xenophobic aggression vary by species. For example, while only chimpanzee males engage in lethal raiding and are primarily the victims of such aggression, wolves of both sexes are equally attacked and show similar aggression levels (Wrangham 1999). Whether or not the males or females of a group are aggressive to a newcomer often depends on the newcomer’s sex. Sometimes, members of the opposite sex are better tolerated, or even protected from same-sex attacks (Wade 1976). Often, aggression towards strangers of the opposite sex will decrease during breeding season (Bernstein and Gordon 1974).


The ubiquity of xenophobia appears to stem from the benefits of protecting territories, resources, mates, and group status from outsiders, although exceptions may be made for breeding opportunities, genetic relatives, or neighbors. Xenophobia is a major source of aggression and stress for individuals and groups, and thus an important factor to be accounted for in studies and management of animal behavior.



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Authors and Affiliations

  1. 1.York UniversityTorontoCanada

Section editors and affiliations

  • Suzanne MacDonald
    • 1
  1. 1.York UniversityTorontoCanada