Two troops of baboons have been filmed going to war, with hundreds of monkeys entering into a pitched battle.
The fight, filmed by the BBC Natural History Unit, appears to be triggered by male baboons attempting to steal females from the harems of rivals.
Usually, the two troops live relatively peacefully alongside one another on a 1km-long cliff in the Awash National Park in Ethiopia.
But they violently clash in a sequence broadcast as part of the series Life.
"The scale of the fight and the way the males are so dominant is just unparalleled in primate society," says Miss Rosie Thomas, a member of the Life production team who filmed the sequence.
Baboons are one of the most aggressive primates out there
Miss Rosie Thomas BBC Life production team member
Baboons live in complex male-led societies.
Scientists have identified four levels at which baboons organise themselves.
At the smallest level, a dominant male baboon will control a harem of females.
A number of these one-male units, as scientists call them, may organise into clans of monkeys.
Units and clans can gather into much larger social groupings, which are called bands.
The monkeys within each band coordinate their activities, acting as a cohesive social unit.
Many bands also hang out as part of a huge troop.
A single troop of Hamadryas baboons (Papio hamadryas) can contain several hundred individuals.
At a site called Filoha in the Awash National Park of lowland Ethiopia, scientists including Dr Mathew Pines have been studying how the interactions between these different groups of baboons play out.
A film crew from the BBC Natural History Unit spent five weeks at the site recording the action, alongside Dr Pines and other researchers working for the Filoha Hamadryas Baboon Project.
Four baboon troops live at the cliff in Filoha.
Two are relatively small and are difficult to follow and study as they are not habituated to the presence of researchers.
Ready for battle
However, two troops that live alongside one another are huge, with over 200 monkeys in each.
These troops occasionally clash as they move down from the cliff upon which they sleep to find water, researchers have discovered.
It is the demand for females that usually triggers inter-troop warfare.
Male baboons are either dominant, controlling a harem, or they are 'followers' - helping to protect the harem in exchange for occasional access to the females and mating rights.
However, a number of young males are solitary, having no access to females.
They cannot steal them from the dominant males in their troop.
"If they are trying to steal them from within their group, they have to overthrow the dominant male to keep them, or the dominant male will just steal them back," says Miss Thomas.
"But if they can see an opportunity to steal them from another group, it is much more difficult for the male to steal the female back."
And when these males raid another troop, it sparks a pitch battle.
Once the fight is over, the females often suffer further.
Males will often attack females in their harem for having considered the attentions of interloping solitary males.
"Baboons are one of the most aggressive primates out there," says Miss Thomas.
"What's interesting about hamadryas baboons is the way they control their social structure through aggression. Just seeing some of the males disciplining their females - it really is quite nasty," she says.
Such huge troops form for a number of reasons.
They offer the baboons protection against predators such as lion, leopard and hyena, researchers believe.
Scarred by battle
Large amounts of doum palms in the region also allow such large groups of baboon to live together.
When food becomes scarce, the monkeys then split into smaller bands and units.
The complexity of baboon society is also reinforced by a study published last month in the American Journal of Primatology.
Dr Amy Schreier of Duke University in Durham, North Carolina and Dr Larissa Swedell of the City University of New York in Flushing, New York have discovered more about a fourth level of social organisation among the baboons.
As well as troops, bands and single-male units, Drs Schreier and Swedell have confirmed that baboons organise themselves into clans, a discovery first made in the 1970s by researcher Jean-Jacques Abegglen at another site, Erer Gota.
The researchers cannot yet be sure, but they suspect that clans are collections of related males.
When a band splits up, usually because of scare food, males tend to break away along clan lines, forming inter-related groups, say the researchers.
Clan members are also more likely to secure access to females.
The hamadryas baboons at Filoha are only the second population of hamadryas known to organise into clans, though Dr Swedell says it is likely that all hamadryas baboons form clans.
"Baboon wars" is broadcast within the Primates episode of the BBC series Lifeat 2100GMT on BBC One on Monday 14 December.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.