褒美よりも遊びたい？　ネズミはかくれんぼ好き、独研究

【ワシントンＡＦＰ＝時事】ネズミが隠れ場所を探してこっそりと移動している時は、かくれんぼで遊びたいだけなのかもしれないと推定した研究が12日、米科学誌サイエンスに発表された。（写真はかくれんぼをするネズミ。コンスタンティン・ハルトマン氏ら提供）
　独フンボルト大学の神経科学者チームが箱だらけの小さな部屋の中でネズミたちと数週間過ごした結果、驚くべきことにネズミがかくれんぼの名人であることが分かった。
　褒美として餌を与えなかったにもかかわらず、ネズミは隠れまわる人間を見つけたり、人間に見つけられたりすることを純粋に楽しんでいる様子だったという。このことはネズミがうれしい時に見せる「跳躍」や、過去の研究で喜びを示すものと判明した超音波の笑い声から分かったという。
　この研究はかわいらしさ（見方によっては気味の悪さ）以上に、哺乳類の間に見られる重要な進化的特徴である遊ぶという行為に関する新たな見識を示すものだ。
　共同執筆者の一人で、フンボルト大学のコンスタンティン・ハルトマン氏はＡＦＰの取材に対し、「長年にわたってたくさんの仕事を一緒にしていれば、ネズミがいかに知能が高く社交的な動物か分かる」「それでも、ネズミがあんなにもうまく（かくれんぼを）やれるというのは大変な驚きだった」と語った。
　実験は複数の若い雄ネズミを放した３０平方メートルの部屋で行い、研究者１人が隠れる役で段ボール箱の陰にしゃがんだり、ネズミに先に隠れる場所を見つけさせて鬼役を演じたりしたという。
　ネズミたちは１、２週間のうちに、閉じた箱の中でスタートしてから遠隔操作でふたを開けられる場合が鬼役で、開いた箱の中でスタートする時が隠れる役だと学習した。するとすぐに高度な戦略を編み出し、鬼役の時には人間が以前に隠れていた場所に行ったり、隠れる役の時には透明な箱よりも不透明な箱を選んだりするようになった。
　ハルトマン氏によると、研究チームは訓練を促すため、餌や水ではなく、子猫や子犬と遊ぶ時になでるように、肯定的な社会的交流を褒美とした。
　だが研究チームは、ネズミこうした社交だけを目当てにかくれんぼをしていたわけでなく、かくれんぼで遊ぶこと自体を好んでいたのではないかと推測している。
　喜びの跳躍や笑い声の他にも、見つかってから別の場所に「隠れなおす」ことを時には数回繰り返し、遊ぶ時間を長引かせ褒美の受け取りを遅らせようとする行動もみられたからだ。
　遊ぶという行為は若い哺乳類の認知発達における重要な一部で、進化においてヒトに近いネズミは、ヒトの脳の活動の研究における理想的なモデルとなっている。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2019/09/17-09:28）

The next time you come across a rat darting furtively for cover, consider this: It might just want to have a playful game of hide-and-seek.
A group of neuroscientists in Germany spent several weeks hanging out with rodents in a small room filled with boxes, finding the animals were surprisingly adept at the cross-cultural childhood game -- even though they weren't given food treats as a reward.
Instead, the rats appeared to genuinely enjoy both finding their sneaky human companions and being caught by them, as shown by their joyful leaps (what the Germans called freudensprung) and ultrasonic giggles that previous work has found is a sign of happiness.
The researchers' paper was published in the influential journal Science on Thursday, and beyond the cuteness factor (or creepiness, depending on one's perspective), it offers new insight into play behavior, an important evolutionary trait among mammals.
When you work a lot with rats over the years, you see how intelligent these animals are and how social, co-author Konstantin Hartmann from the Humboldt University of Berlin, where the other members of the team are also based, told AFP.
But it was still very surprising to us to see how well they did, he said.
Working with adolescent male rats in a room of 30 square meters (320 square feet), a scientist would either find a cardboard box to crouch behind in a hiding role, or give the rat a headstart to find cover while the scientist searched.
Over a period of one to two weeks, the rats were taught that starting the game inside a closed box that was opened remotely meant they were seeking, while starting the game with the box open meant they were hiding.
They quickly developed advanced strategies, including re-visiting spots humans had previously hidden when they were seeking, and choosing to take cover in opaque rather than transparent boxes when they were hiding.
To help train them, the authors rewarded the rats not with food or water, which would invalidate the experiment, but with positive social interaction in the form of physical contact, explained Hartmann.
They chase our hand, we tickle them from the side, it's like a back and forth a little bit like how you play with small kittens or puppies, he said.
The scientists suspect though that the rats were motivated not just by this interaction but that they also liked to play for the sake of play itself.
The animals would let out high-pitched giggles three times above the human audible range and would execute so-called joy jumps during the game -- both associated with feelings of happiness.
Once they were discovered, the rats often jumped away and playfully rehid at a new location, sometimes repeating the process several times -- indicating they wanted to prolong the play session and delay the reward.
- Ethics questions -
Play is an important part of cognitive development for adolescent mammals, and rats make for ideal models to study brain activity in humans because of their evolutionary proximity to us, which is also why they are often used in the study of disease.
Scientists are therefore keen to learn what parts of the brain's prefrontal cortex that is linked to social behaviors, are involved -- but because play is a free-flowing activity, it had been difficult to study.
The team therefore attached microwires to the rats' heads that recorded their brain activity, allowing them to identify which individual neurons were linked to specific game events.
This in turn could be used for future study: for example, to look at neural development when play activities are restricted during adolescence.
But the more we learn about rat and mice social behavior, the more human-like they seem, raising difficult ethical questions about their use in medical trials and other experiments.
I think, being aware of the cognitive abilities of an animal is really important, said Hartmann, adding it was always important to judge the value of the expected outcome against the use of animals.
This type of research will also help other scientists to see in rats more than what you usually see when you just get the rat and use it for standard experiments, when you're not aware of what these animals can do.