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Schim­pa­sen über­ja­gen Beu­teaf­fen fast bis zum Aus­ster­ben

Die­se Men­schen­af­fen haben so vie­le rote Stum­mel­af­fen getö­tet und ver­speist, dass deren Popu­la­ti­on um 89 % zurück­ge­gan­gen ist und sie jetzt neue Beu­te fin­den müs­sen.

Humans have made a nas­ty habit of kil­ling off ent­i­re spe­ci­es of ani­mal.

Pas­sen­ger pige­ons once for­med huge flocks in North Ame­ri­ca, but humans hun­ted them on an indus­tri­al sca­le. Lar­ge­ly as a result of this, the spe­ci­es was utter­ly wiped out by the ear­ly 20th cen­tu­ry.

But it seems we’re not the only ani­mal that can’t stop our­sel­ves over-hun­ting our prey.
In a sinis­ter echo of the pas­sen­ger pige­on sto­ry, chim­pan­zees in one area of Afri­ca have over-hun­ted the mon­keys they prey on. As a result, the mon­key popu­la­ti­on has been pushed clo­se to local extinc­tion.

David Watts of Yale Uni­ver­si­ty in New Haven, Con­nec­ti­cut and John Mita­ni of the Uni­ver­si­ty of Michi­gan in Ann Arbor have both spent years stu­dy­ing a sin­gle popu­la­ti­on of chim­pan­zees. They live in an area cal­led Ngo­go in Kiba­le Natio­nal Park, Ugan­da.

They have been so effec­tive as pre­da­tors that they real­ly have kno­cked down the local popu­la­ti­on Alt­hough the chim­pan­zees most­ly eat fruit, they also regu­lar­ly hunt for meat. Their main prey are red colo­bus mon­keys. Bet­ween 1995 and 2014, Watts and Mita­ni obser­ved 556 hunts, 356 of which tar­ge­ted red colo­bus. The­se hunts were very suc­cess­ful: 912 colo­bus were kil­led, an average of 3 per hunt.

It has been clear for several years that the red colo­bus popu­la­ti­on has decli­ned as a result. A 2011 stu­dy found that the popu­la­ti­on fell by 89% bet­ween 1975 and 2007. In the late 1990s the chimps were kil­ling up to half the red colo­bus popu­la­ti­on every year.
“They have been so effec­tive as pre­da­tors that they real­ly have kno­cked down the local popu­la­ti­on,” says Watts.

“Red colo­bus are easier to catch than other mon­keys,” says Watts. That might be part­ly becau­se they tend to stand their ground against chimps, rather than run­ning away. “That can work, but not if the­re are 20 chimps try­ing to catch them.”

The chim­pan­zee popu­la­ti­on in Ngo­go is unusual­ly lar­ge, hovering around 190, which means the chimps hunt in lar­ge groups and can over­whelm the colo­bus.

Watts and Mita­ni have now found that the chimps are star­ting to switch to alter­na­ti­ve prey. Red colo­bus hun­ting reached a peak in 2002, and sin­ce then they have been hun­ting red colo­bus less often, and hun­ting other spe­ci­es more.

The new results are published in the Inter­na­tio­nal Jour­nal of Pri­ma­to­lo­gy.
The second-most-hun­ted spe­ci­es are mon­keys cal­led mant­led gue­re­zas. Watts and Mita­ni have found that the­se seem to be beco­m­ing less com­mon, sug­gesting that the chimps are causing their popu­la­ti­on to decli­ne as well.

Black-and-white colo­bus are also in the chimps’ firing line.

If they kill 2 mon­keys out of a group that only has 10 or 12, that’s a big impact

“I’ve been here a litt­le over a month,” says Watts. “The chim­pan­zees have hun­ted qui­te often in that time. They’ve hun­ted black-and-white colo­bus eight times. I’ve never seen any­thing like that befo­re.”

Black-and-white colo­bus are rela­tively uncom­mon com­pa­red to red colo­bus, and live in smal­ler groups. So the chimps’ deci­si­on to hunt them more is bad news for them.
“One group they’ve tar­ge­ted at least twice,” says Watts. “If they kill 2 mon­keys out of a group that only has 10 or 12, that’s a big impact.”

The good news is, the chimps pro­bab­ly won’t wipe out the popu­la­ti­ons ent­i­re­ly. “I doubt that it will hap­pen, becau­se as the red colo­bus beco­mes scar­cer, then it requi­res more ener­gy to find them,” says Watts. It’s not worth the chimps’ effort to seek out the red colo­bus, so the popu­la­ti­on might sta­bi­li­se – albeit at a much lower level.

It does requi­re a lot of skill to catch the mon­keys

A lack of prey might also chan­ge the chim­pan­zees’ beha­viour.

“One pos­si­bi­li­ty is, if the chim­pan­zees hunt less often, then young chim­pan­zees as they grow up get less expe­ri­en­ced try­ing to hunt, and they don’t learn to be as good as their pre­de­ces­sors,” says Watts. “It does requi­re a lot of skill to catch the mon­keys, and it’s inte­res­ting to see that some chim­pan­zees are bet­ter at doing it than others. They need to learn.”

That said, Watts hasn’t seen any evi­dence of this yet. “My sub­jec­tive impres­si­on this sum­mer is they are get­ting bet­ter at hun­ting black-and-white colo­bus.”

In their ten­d­en­cy to blind­ly over-hunt their prey, chim­pan­zees are rather simi­lar to humans. Perhaps that’s not too sur­pri­sing, as they are our clo­sest living rela­ti­ves. I don’t think chim­pan­zees are capa­ble of thin­king about a long-term future “Peop­le are fond of making claims about simi­la­ri­ties bet­ween chim­pan­zees and humans,” says Watts. “It’s pos­si­ble to push that too far. But one cha­rac­te­ris­tic we have in com­mon is we over-har­vest resour­ces. We’re not good con­ser­va­tio­nists.”

That said, Watts says the­re is also a cru­ci­al dif­fe­rence bet­ween chimps and humans: we are sup­po­sed to be smar­ter than they are. “I don’t think chim­pan­zees are capa­ble of thin­king about a long-term future,” says Watts. If they do switch to a dif­fe­rent prey, it’s not becau­se they are try­ing to con­ser­ve the red colo­bus. “They’re just respon­ding to what they encoun­ter and what they see.”

By con­trast, humans under­stand that we can dri­ve a spe­ci­es to extinc­tion. Unli­ke the chimps, we real­ly can think about a long-term future. But as Watts puts it, “we are too pro­ne to deva­lue it and not care about it.”


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