Social animals use a suite of preemptive behaviors in anticipation of conflict, including staying quiet, monitoring their surroundings, conducting raids, and bonding through play. In a review publishing in the Cell Press journal Trends in Ecology & Evolution on July 16, researchers describe how environmental cues and memories of past events can trigger these behaviors. Over generations, these pre-war preparations could impact socio-cognitive evolution, population dynamics, and community structures.
"Intergroup conflict is rife throughout the natural world, being found in social species from ants to primates," says corresponding author Andrew Radford of the University of Bristol in the UK.
Conflict over resources such as territory space, food, or mating exerts a powerful evolutionary force on social species, potentially impacting fitness and survival, say the researchers. Traditionally, research has focused on actions between rival groups during contests and the behavioral consequences afterward. But evolution can also select for preemptive behaviors that maximize the chances of winning in a conflict.
"What is becoming very clear is that preemptive behavior is widespread whenever intergroup conflict is found," says first author Josh Arbon of the University of Bristol. "There is growing evidence that the amount of anticipatory behavior displayed is dependent on the current threat level. More is seen when rivals are more likely to be encountered, larger in size, less familiar, or more likely to attack."
Humans have long been known to prepare for warfare by increasing surveillance, using elevated areas to gather information, conducting ambushes and raids, and moving quietly through enemy territory to avoid detection. Recent studies of wild animals provide similar examples of preparation for encounters with rival groups.
"Studying other species experimentally and in natural conditions can not only expand our understanding of a widespread aspect of sociality but also help to provide insights into our own conflict ancestry," Radford says.
Observations of chimpanzees have revealed that groups tend to rest on hilltops in areas where intergroup contests occur rather than engage in noisier activities such as feeding or traveling. In addition, experiments have shown that dwarf mongooses respond to olfactory or vocal cues of rivals by moving more slowly and engaging in sentinel behaviors, which allow them to monitor their surroundings more easily.
The threat of intergroup conflict can also influence the space-use patterns of animals. To signal territorial ownership, dwarf mongooses deposit more scent marks in response to simulated rival intrusions, and meerkats tend to scent mark near burrows examined by intruders. Similarly, black howler monkeys return to locations of past contests, potentially to advertise their presence to neighbors. By contrast, Japanese macaques, chacma baboons, and long-tailed tits avoid areas inhabited by rivals.
Beyond space-use patterns within a commonly used area, a more extreme preemptive behavior is raiding—actively seeking out rivals on their home turf. For example, male chimpanzees silently invade neighboring territories in single file and move toward other groups' vocalizations, apparently preparing to attack rivals. Banded mongooses also engage in lethal gang attacks, conducting raids to kill the offspring of rival groups.
When the threat from outsiders is greater, various mammal species stay closer to one another. For instance, chimpanzees groom and play with one another more in advance of collective territory defense. Such behaviors likely facilitate communication, reduce anxiety, enhance bonding, and promote a stronger fighting force.
"There is increasing evidence that non-human animals adjust various behaviors to enhance information gathering, incentivize contest participation, reduce anxiety, and minimize collective and individual risk in anticipation of encounters with rival groups," Arbon says. "What is notable is that these behaviors occur across a diverse range of social species."
According to the authors, questions for future research include examining how animals assess the intergroup threat level and adjust their preemptive behavior accordingly. The cognitive demands associated with preemptive actions also remain unclear. These responses are shaped by multiple interacting factors, including sensed information and memories of past encounters.
"Intergroup conflict could be an important social driver of cognitive evolution," Radford says. "But this remains an idea that is difficult to test, and teasing apart the relative importance of signals and cues from memories is a challenge."
"To understand fully the influence and importance of intergroup conflict, including for our own evolution, we must study the complete timeline of behaviors, not just those during and after contests but also those that occur in anticipation," Arbon says.
