飼い主と再会の犬、涙目に　研究チーム

【ワシントンＡＦＰ＝時事】犬の飼い主にとって、長時間の外出から帰宅した時に愛犬がしっぽを振って勢いよく飛びついたり、顔をなめたりして出迎えてくれることは純粋にうれしいものだ。しかし、犬はじゃれついてくるだけでなく、涙まで浮かべる可能性があるとの説を、日本の研究チームが発表した。（資料写真）
　研究チームの論文は２２日発行の米科学誌カレント・バイオロジーに掲載された。執筆者の一人、菊水健史教授は、飼い主との再会といったうれしい場面では動物も涙を流すという説はこれまで聞いたことがなく、「世界初」の発見かもしれないと語った。
　研究チームは犬の目の縁に特殊な装置を付けて涙の量を測定。飼い主と５～７時間離れ離れになった後で再会すると、５分以内に涙の量が「著しく」増加することが分かった。飼い主と再会した時の涙の量は、それ以外の顔なじみの人と会った時よりも多かった。
　研究チームによると、涙の量が増えるのは「愛情ホルモン」とも呼ばれる、絆を強める働きを持つオキシトシンの分泌が関係している可能性がある。
　また研究では、犬の涙が飼い主の感情に及ぼす影響についても調べた。
　人工涙液を点眼した前後の飼い犬の写真を飼い主に見せ、どの写真に最もいとおしさを感じるか尋ねたところ、涙を浮かべている写真の方が、通常の写真よりもはるかにいとおしく感じるとの回答が得られた。
　菊水教授は、犬にしてみれば飼い主に涙目を見せることでより大切にされる可能性が出てくると推論。研究チームは、人間でも幼児は泣くことでネガティブな感情を伝え、親の世話を促していると指摘する。
　他の動物と違って家畜化された犬は、時間をかけて人間との特殊なコミュニケーション能力を発達させてきた。アイコンタクトは、飼い主との絆を深める役割を果たしているようだ。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2022/08/24-13:55）

Dog owners know the pure joy of returning home from a long trip to be greeted by their tail-wagging, uncontrollably jumping, face-licking companion.
But those ecstatic canines might be shedding more than just fur on your clothes -- they might also be tearing-up, according to a new study published Monday in the journal Current Biology.
We had never heard of the discovery that animals shed tears in joyful situations, such as reuniting with their owners, said Takefumi Kikusui, one of authors of the study, which he called a possible world first.
The scientists measured the amount of tears in the dogs' eyes with the widely-used Schirmer test, which consists of placing a specialized strip under the eyelids. For a baseline reading, they performed the test on dogs during a normal interaction with their owner.
When dogs were reunited with their owners after five to seven hours of separation, they significantly increased tear production in the ensuing five minutes, the researchers found.
They also discovered that the dogs' volume of tears were higher when they reunited with their owner versus other people the dog is familiar with.
According to the researchers, this reaction to tear up is likely linked to the release of oxytocin, nicknamed the love hormone, due to its connection with bond-building.
The scientists then sought to test whether the tears might have an emotional impact on the owners. To do so, they asked the owners to rank various photos of their dogs with and without artificial tears by how much they wanted to care for them.
The dog photos with artificial tears were ranked significantly higher than the normal tearless dog photos, the Japanese research team wrote.
It is possible that the dogs that show teary eyes during interaction with the owner would be cared for by the owner more, hypothesized Kikusui.
In humans, the authors note, infants share their negative feelings by crying, which leads to more care-giving by the parents.
Domesticated by humans like no other animal, dogs have developed specific communication skills over time. Eye contact has been shown to play a role in forming the relationship between a dog and its owner.
In future studies, the researchers would like to test whether dogs similarly produce tears when they meet other canine pals.