Gorilla mobs attacking single individuals suggests new type of behavior for them

November 28, 2016 by Bob Yirka, Phys.org report
Credit: CC0 Public Domain

(Phys.org)—A trio of researchers studying gorillas in Karisoke Research Center in Rwanda has reported on a developing trend observed in mountain gorillas—mobs attacking single individuals for unknown reasons. In their paper published in Scientific Reports, Stacy Rosenbaum, Veronica Vecellio and Tara Stoinski describe three mob attacks that have been observed by several human witnesses over the past decade and offer some possible explanations.

For most of the modern study of gorillas in their native environment, the consensus has been that they are generally docile with one another—there have been observations of males fighting, sometimes to the death, but for the most part, the life of the gorilla was thought to be one of mostly peaceful. But now, it appears that the peace can be disturbed by the occasional mob attack on a single individual or, as the researchers note, two individuals.

In the first witnessed attack, back in 2004, Rosenbaum was actually one of the witnesses. She describes the incident as arising seemingly out of nowhere. A single male the team had named Inshuti approached a group of gorillas the researchers had named the Beetsme. After some initial rebuffs, the lone male continued to seek acceptance. Then one of the gorillas screamed—the witnesses could not say if it was Inshuti or a member of the group. That was followed by three adult males chasing Inshuti until they caught him and pinned him to the ground. Soon thereafter, the rest of the Beetsme group arrived and all of them (including females and youngsters) participated in causing harm to Inshuti—from pulling hair to scratching and kicking. The leader of the Beetsme sunk his teeth into the gorilla's flesh and shook it like a fighting dog. The mob attack continued for just a few minutes, but then stopped just as quickly as it had started. The attackers walked away and Inshuti slunk into the underbrush to attend to his wounds.

The researchers report on two other similar incidents, one of which included an attack on Inshuti and another male. They note that mob attacks by other apes, including chimps, is common, as in humans, but until these recent incidents, it was thought gorillas were gentle giants, unlikely to engage in such violence. The team admits they do not know why the have begun acting like mobs at times but note that it has occurred during a time when the mountain gorilla population has grown due to conservation efforts.

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More information: Stacy Rosenbaum et al. Observations of severe and lethal coalitionary attacks in wild mountain gorillas, Scientific Reports (2016). DOI: 10.1038/srep37018

Abstract
In humans and chimpanzees, most intraspecific killing occurs during coalitionary intergroup conflict. In the closely related genus Gorilla, such behavior has not been described. We report three cases of multi-male, multi-female wild mountain gorilla (G. beringei) groups attacking extra-group males. The behavior was strikingly similar to reports in chimpanzees, but was never observed in gorillas until after a demographic transition left ~25% of the population living in large social groups with multiple (3+) males. Resource competition is generally considered a motivator of great apes' (including humans) violent intergroup conflict, but mountain gorillas are non-territorial herbivores with low feeding competition. While adult male gorillas have a defensible resource (i.e. females) and nursing/pregnant females are likely motivated to drive off potentially infanticidal intruders, the participation of others (e.g. juveniles, sub-adults, cycling females) is harder to explain. We speculate that the potential for severe group disruption when current alpha males are severely injured or killed may provide sufficient motivation when the costs to participants are low. These observations suggest that the gorilla population's recent increase in multi-male groups facilitated the emergence of such behavior, and indicates social structure is a key predictor of coalitionary aggression even in the absence of meaningful resource stress.

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Mark Thomas
1 / 5 (1) Nov 28, 2016
Maybe Beetsme showed Inshuti what happens when you don't pay your debts on time. :-)
HeloMenelo
2.3 / 5 (3) Nov 28, 2016
Maybe monkey goracle just wants more bananas :)
Osiris1
1 / 5 (2) Nov 28, 2016
Sometimes names are one's destiny. The lady named the assaulting group: "Beetsme"..... as in BEAT me; and that is just what they did. They did not want "Inshuty" (make what you want out of that name....a little imagination can get several words that probably would get censored by the speech controller here so this writer need not mention the obvious). When "Inshuty" insisted on acceptance, and continued to nag, the others gave him the answer that he had stubbornly refused to accept. Now forced to accept his rejection after his 'spanking', he slunk off like a miffed Trump, first to tend his wounds; then to plot revenge.
Osiris1
Nov 28, 2016
This comment has been removed by a moderator.
jeffensley
4 / 5 (2) Nov 29, 2016
Not sure why we continue to label something "new" when we observe it for the first time. Just because we haven't witnessed it doesn't mean it hasn't occurred before. Organization and tribal behavior isn't unique to the human species.
Jayarava
not rated yet Dec 05, 2016
What the report crucially leaves out is whether or not Inshuti was known to the Beatsmes or whether he was a stranger, With social primates it makes all the difference! If he was a stranger, then there is no mystery. If he was known to them, then some further explanation is required.
katesisco
3 / 5 (1) Dec 05, 2016
I have noticed that studies tend to see the 'current' as a genetic pattern without any qualifiers. Here you see the qualifiers.
I noticed in the NYT the other day an article where researchers explored the 'lasting' permanence of attained democracy. The article claimed without any basis whatsoever that democracy has long thought to be permanent once attained. I guess their point was to direct support to their claim that militarism can upset democracy. Where in history has democracy which must have been the initial form for any civilization lasted?
gkam
Dec 05, 2016
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