How Animals Prepare for War Long Before the Fighting Starts

From ants to apes, social species scout, bond, raid and retreat before conflict.

Long before a major fight between groups of animals begins, a chimpanzee patrol may climb a hill and fall quiet. Dwarf mongooses slow their pace and post guards. Green woodhoopoes preen one another. Army ants assemble into a living wall.

These animals have not yet confronted an enemy. But they are preparing for one.

A new review in Trends in Ecology & Evolution argues that such behavior represents a neglected phase of conflict in the animal kingdom: everything groups do before rivals appear. By synthesizing research on insects, birds and mammals, Josh Arbon and Andrew Radford, behavioral ecologists at the University of Bristol, describe how animals gather intelligence, defend strategic locations, reinforce social bonds and sometimes launch raids in anticipation of a major confrontation.

“Intergroup conflict is rife throughout the natural world, being found in social species from ants to primates,” Dr. Radford said.

The researchers’ definition of intergroup conflict includes any competition between members of the same species over territory, food, shelter, mates or breeding positions. Some encounters end with little more than calls or scent marking. Others cause severe injury, infanticide or death.

Gathering Intelligence Before Contact

Chimpanzees provide one of the clearest examples of advanced reconnaissance.

In Taï National Park in Ivory Coast, researchers followed two neighboring chimpanzee communities for three years. The apes were more likely to stop on high ground when moving toward a contested border than when heading back toward the center of their territory. On those border hills, they rested rather than pursuing noisier activities, apparently creating better conditions for listening.

Afterward, the chimpanzees advanced or retreated in ways that reflected the location and strength of their rivals. The findings, published in a 2023 PLOS Biology study, suggested that the apes used topography as a tactical source of information.

×

Get smarter every day...

Stay ahead with ZME Science and subscribe.

Daily Newsletter
The science you need to know, every weekday.

Weekly Newsletter
A week in science, all in one place. Sends every Sunday.
No spam, ever. Unsubscribe anytime. Review our Privacy Policy.

Thank you! One more thing...

Please check your inbox and confirm your subscription.

Other animals gather intelligence closer to the ground. When dwarf mongooses encounter the scent or calls of another group, they move more slowly, inspect latrines and increase sentinel behavior. An elevated mongoose can watch the surrounding area while also issuing calls that keep its group informed.

A recent study of wild dwarf mongooses showed that these preparations changed according to the opponent. Sentinels became more likely to call when neighboring groups were relatively large, while the mongooses adjusted their movements, signaling and resource defense to the danger posed by particular rivals.

“What is becoming very clear is that preemptive behavior is widespread whenever intergroup conflict is found,” Dr. Arbon said. “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.”

Some animals even gather information without becoming involved. Acorn woodpeckers may travel considerable distances to observe battles between other groups, retaining information they use days or weeks later.

A Mental Map Made of Rivals

The researchers call these collective behaviors the “landscape of intergroup conflict.”

The idea builds on the ecological concept of a “landscape of fear,” in which prey perceive some locations or times as more dangerous because predators are likely to appear there. Animals then balance safety against the need to eat, reproduce and travel.

Rival groups create a similarly uneven terrain and dilemmas. A border may be dangerous, but so can a valuable feeding site, a den inspected by intruders or an unexpected location deep inside a territory.

Animals respond by either avoiding contested space or asserting ownership over it. Japanese macaques and long-tailed tits use shared areas less than expected, while chacma baboons take detours after detecting another group. Dwarf mongooses deposit more scent at communal latrines after simulated intrusions. Meerkats concentrate scent marks around burrows, which outsiders are likely to investigate. Black howler monkeys deliberately return to sites of previous contests, perhaps to advertise that the area remains occupied.

Animals must also decide how much to trust what they know. A call provides fresh evidence that a rival is nearby, but perhaps little information about the group’s size. A scent mark may reveal sex, body size or reproductive condition, but it grows less reliable as it ages.

The review proposes that animals integrate present evidence with previous experience in a process resembling Bayesian updating: an old expectation changes as new information arrives.

Grooming, Recruitment and Raids

Preparation does not stop with surveillance. A threatened group may need to turn scattered individuals into a coordinated fighting force.

Chimpanzees groom and play more before collective territorial defense. Green woodhoopoes increase allopreening. Dwarf mongooses exposed to signs of rivals groom one another immediately and continue doing so in the following days.

Such contact may reduce anxiety, improve communication and strengthen social bonds. It may also solve a practical problem: not every group member has the same incentive to fight. Grooming, play and other social rewards could encourage reluctant participants to join a dangerous contest.

Vervet monkeys use both rewards and coercion. Females may groom males or direct aggression toward them, and males that receive more of either become more likely to participate in the next bout of fighting. In other circumstances, males use aggression to stop females from starting costly encounters.

Groups also close ranks physically. Chimpanzees and several other mammals stay nearer to one another when outsiders threaten them. Some chimps have even been seen drumming on tree roots to attract groupmates. Banded mongooses and meerkats use calls to recruit companions after finding fresh rival scent. Some army ants stand side by side facing the threat, forming a barrier of living bodies.

Then comes the raid: entering another group’s territory specifically to find them and incur a cost.

RelatedPosts

No brain, no problem. Brittle stars can still learn through experience
Agent Orange And Napalm Ravaged Vietnam’s Forests And Mangroves And The Damage Lingers Decades After The War
Zombie Ants, Spider Bait, and a Sneaky Burrowing Worm: The 6 Spookiest Science Discoveries of the Past Year
Flamingos Create Underwater Tornadoes to Suck Up Their Prey

Male chimpanzees may travel silently in single file and move toward rival calls. Banded mongooses attack the offspring of neighboring groups. Greater anis eject eggs from rivals’ nests, often causing the targeted birds to abandon the site. Young honey-ant and fire-ant colonies steal the broods from nearby nests, expanding their future workforce.

These attacks can weaken an opponent before the next direct competition over food, mates or territory.

“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,” Dr. Arbon said. “What is notable is that these behaviors occur across a diverse range of social species.”

The Evolutionary Stakes

Researchers have traditionally examined what animals do during a fight and how they behave afterward. The new review places those studies inside a longer sequence that begins well before contact. It also reveals another dimension of animal cognition that has largely been ignored. Preparing for “war” involves rather complex cognitive tasks, including remembering where encounters occurred, distinguishing neighbors from strangers, estimating numbers and deciding whether to approach, hide or recruit help.

A 2020 Nature Communications paper argued that evolutionary theories of social intelligence had focused too heavily on relationships within groups. Outsiders, the authors proposed, create their own cognitive demands — a capacity they interestingly called “Napoleonic intelligence.”

“Intergroup conflict could be an important social driver of cognitive evolution,” Dr. Radford said. “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,” Dr. Arbon added.