イルカも遊び相手に「友好的な笑顔」 研究

【ワシントンＡＦＰ＝時事】バンドウイルカは遊び相手に敵意がないことを示すために、友好的な表情をつくるとする研究結果が２日、学術誌「アイサイエンス」に発表された。（写真はブラジル・リオデジャネイロ州ニテロイ沖でジャンプするバンドウイルカ〈資料写真〉）
　ＡＦＰの取材に応じた伊ピサ大学の進化生物学者エリザベッタ・パラジ氏のチームは、イルカの遊び行動を観察中にイルカたちが互いに口を開けてから遊び始めることに気づいた。かみつくのとは異なったという。
　遊びたいときに口を開けるジェスチャーは、霊長類やクマなどさまざまな動物で観察されているが、イルカについては不明だった。
　パラジ氏のチームはこの行動について確認するため、数百回の遊び行動を数か月かけて観察した。その結果、イルカは他のイルカと遊ぶとき、頻繁に歯を見せて笑うことが明らかになった。一方、人間との遊びではこうした表情はたまに見せる程度で、一匹でいるときはほぼ見せなかった。
　人間では、誰かがほほ笑んだり笑ったりすると、その相手に対して好意的な感情を持っている人は１秒以内にそのしぐさを返すことが多い。この反応は、意識的にコントロールできる領域の外にあり、真の感情共有を示すサインと理解されている。
　パラジ氏は「同じことが類人猿やサルといった霊長類の他、ハイエナ、ミーアキャット、犬など、社会性のある肉食動物でも見られる」と説明。「イルカも同じことをするのが分かった」と述べた。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2024/10/04-19:19）

Smile and the whole world smiles with you, goes the expression -- and it seems the same is true for bottlenose dolphins.
The brainy marine mammals use friendly facial signals to signal non-hostile intent to playmates, who respond in kind, a study in the journal iScience said Wednesday -- similar to visual cues used by humans.
Senior author and evolutionary biologist Elisabetta Palagi of the University of Pisa told AFP that she and her colleagues stumbled upon this finding by chance.
While observing dolphin play behavior, they noticed the dolphins would start play sessions by opening their mouths in front of each other without any biting.
Intrigued, Palagi and co-author Livio Favaro of the University of Turin decided to investigate further.
They knew that it's important for dolphins to prevent play -- which can include acrobatics, playing with toys, chasing and tail slapping -- from escalating into real fighting.
A variety of animals, including primates and bears, have been observed offering open-mouth gestures as a visual sign of playfulness. However, it was unclear if dolphins did the same.
To find out, Palagi and her team recorded captive bottlenose dolphins while they played in pairs and interacted freely with human trainers at facilities in Italy and France.
Across hundreds of play sessions stretching over months, they found dolphins frequently showed their toothy grins when playing with other dolphins. They did so a little bit with humans and almost never when alone.
Dolphins were also far more likely to assume this expression when in each other's field of view, and aggressive behaviors like jaw slapping and violent head motions were never associated with the friendly open-mouth gesture.
- Hyenas, meerkats, dogs and humans -
In humans, when one person initiates a smile or laughter, another person who feels positively about them reciprocates the gesture within a second -- something understood as outside voluntary control and a sign that the emotions are genuinely shared.
The same has been found in primates -- great apes and monkeys -- but also social carnivores such as hyenas, meerkats, dogs, said Palagi. And the dolphins did the same.
Indeed, the likelihood of a dolphin mimicking another's open-mouth expression within one second was 13 times higher when the receiver observed the initial gesture, ruling out the possibility of this behavior occurring by chance.
Palagi predicted the findings might be controversial within the wider scientific community, as the primary way dolphins communicate is through whistles, trills and other vocalizations.
But co-author Favaro suggested there could be a good reason for dolphins to switch to visual signals in certain situations.
Dolphins have developed one of the most intricate vocal systems in the animal world, but sound can also expose them to predators or eavesdroppers, he said in a statement.
When dolphins play together, a mix of whistling and visual cues helps them cooperate and achieve goals, a strategy particularly useful during social play when they're less on guard for predators.