平和の象徴ボノボ、実は「ワル」がもてる？ チンパンジーと比較研究

【ワシントンＡＦＰ＝時事】霊長類の中でも人類に最も近いボノボとチンパンジーは、よく「平和を愛するボノボ」、「攻撃的なチンパンジー」といった具合に対照的に語られる。（写真は、ベルギーのメッヘレン近郊にあるプランケンデール動物園で撮影されたボノボ）
　だが、実はボノボの雄はチンパンジーの雄よりも１対１のけんかをする頻度が高く、よくけんかをする雄ほど交尾率が高いとする研究結果が１１日、米科学誌カレント・バイオロジーに１１日発表された。
　論文の筆頭著者を務めた米ボストン大学のモード・ムジェノ氏は、先行研究でボノボの雄の間に「繁殖の偏り」、すなわち子どもの数に大きな差があることが判明し、ボノボの攻撃性について調査を行ったと話す。
　研究チームはコンゴ民主共和国のココロポリ・ボノボ保護区の三つのボノボコミュニティーと、タンザニアのゴンベ国立公園の二つのチンパンジーコミュニティーに焦点を当てた。ボノボとチンパンジーはＤＮＡの９９．６％が共通している。
　そしてボノボの雄１２頭、チンパンジーの雄１４頭の行動を２年間にわたって追跡し、それぞれが攻撃的になる頻度、相手、状況などについてデータを収集した。
　その結果、予想に反してボノボの雄の方が、チンパンジーよりも高い攻撃性を示すことが判明。敵対的な交渉は２．８倍、身体的なけんかは３倍の割合で起きていた。
　ムジェノ氏によるともう一つ、ボノボではより攻撃的な雄の方が、『性皮最大腫脹（しゅちょう）』と呼ばれる状態にある排卵期の雌との交尾を勝ち取っていた。
　■けんかをする姿が魅力的？
　チンパンジーの雄は雌に対して攻撃性を示す傾向が強かったのに対し、ボノボの雄はほぼ他の雄にしか攻撃性を示さなかったのは予想通りだった。
　チンパンジーは極めて雄優位社会で、雄が団結して雌に交尾を強要することもある。一方、ボノボはしばしば雌が群れのリーダーで、交尾を強要しようとする雄に対しては、雌同士が連帯して阻止しようとする。
　またチンパンジーの雄のけんかは複数が関わり、群れの中、あるいはライバルの群れとの縄張り争いで、死ぬこともある。けんかのコストが大きいということでもあり、そのため頻度が少ないのかもしれない。
　一方、ボノボの雄のけんかは圧倒的に１対１が多い。またボノボが殺し合いをしたという報告はこれまでにない。
　「素行がいい」雄の方が交尾できないという点については、「攻撃的な雄ほどライバルを負かし、より多くの時間を雌と過ごすことができるということが関係している可能性はある」とムジェノ氏はいう。
　ただし、「ワルっぽい」男性の方が女性を魅了する傾向があるという人間での見方が、そのままボノボに当てはまるかどうかについては、ムジェノ氏は懐疑的だ。
　雌のボノボの力は絶大で、雄の攻撃性が自分たちに向けられたときは容赦なく封じ込めるが、それが他の雄に向けられた場合には魅力的に映るのかもしれないとムジェノ氏は述べた。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2024/04/15-20:57）

Humankind's two closest primate relatives are often said to embody contrasting sides of our nature: peace-loving bonobos versus violence-prone chimpanzees.
But a new study out Friday in Current Biology says it's not that simple. Male bonobos in fact fight each other more often than male chimps do -- and the bonobo bad boys who have more dust-ups also see greater mating success.
Lead author Maud Mouginot of Boston University told AFP she decided to investigate the question of aggression among bonobos after prior research revealed a reproductive skew among males, meaning some fathered far more offspring than others.
So the question was, if bonobos are not that aggressive, how can they have such a high reproductive skew? she said.
Since their recognition as a species distinct from chimpanzees, bonobos have been romanticized for their free-spirited nature.
Part of their reputation as hippies stems from how they use sex as a means of conflict resolution and often have same sex couplings, especially among females. They're also more likely to share food than chimps.
Researchers had previously attempted to compare aggression between the two species, which share 99.6 percent of their DNA with each other, but these studies were limited because they used differing methods in the field.
Mouginot and her colleagues focused on three communities at the Kokolopori Bonobo Reserve in the Democratic Republic of Congo, and two chimpanzee communities at Gombe National Park in Tanzania.
By tracking the individual behavior of 12 male bonobos and 14 male chimpanzees over two years, the team was able to compile data on how often each engaged in aggressive interactions, who these encounters involved, and whether there was physical contact such as biting and pushing or simply charging at a rival.
Surprisingly, the researchers discovered that male bonobos exhibited higher levels of aggression than chimpanzees. Specifically, bonobos engaged in 2.8 times as many aggressive encounters and three times as many physical altercations as did their chimpanzee counterparts.
That's, I think, the big finding of the paper, said Mouginot. And the other thing is, we actually found that more aggressive male bonobos win more copulation with what we call 'maximally tumescent females,' meaning females whose genitals have swollen because they are ovulating.
- More time with females? -
Male bonobos almost exclusively reserved their aggression for other males, while male chimpanzees were more likely to become aggressive with females.
Both these findings aligned with expectations. Bonobo females are often leaders in their groups and form alliances to stop lone males who may attempt to coerce them sexually, so it makes little sense for males to challenge them.
Conversely, chimpanzees are strongly male-dominant societies, and it's the males that band together, coercing females into sex or punishing male adversaries that challenge their authority.
The fact that male bonobo disputes are overwhelmingly one-on-one, rather than one-against-many, might explain why they happen more often, said Mouginot, as the stakes are lower. Bonobos have never been reported to kill each other.
Chimpanzee altercations, on the other hand, involve multiple males and can result in fatalities -- either within their own group, or in territorial battles against rival groups. The greater costs associated with chimp combat might therefore limit how often it occurs.
As for why nicer bonobo males fared worse with females -- it's possible that those aggressive males can also spend more time with females by vanquishing rivals, said Mouginot, but this would require further confirmation.
But Mouginot, who now focuses her anthropological work on humans, is skeptical about whether bad boy tropes in people -- the idea that men who are troublemakers tend to attract more women -- map directly onto bonobos.
Female bonobos, she emphasized, wield significant power and won't hesitate to shut down male aggression when directed at them. But it's possible they might find it attractive when it is directed at others.