TED日本語 - マーカス・バーン: フンコロガシの踊り

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TED日本語 - マーカス・バーン: フンコロガシの踊り

TED Talks

フンコロガシの踊り
The dance of the dung beetle
マーカス・バーン
Marcus Byrne

内容

フンコロガシの脳は米粒ほどの大きさです。ところが彼らの食糧である動物の排泄物を転がして巣に持ち帰るとなると目を見張る知性を発揮します。どうやって?その秘密は彼らの踊りにあるのです。TEDxWitsUniversity (ウィッツウォータースランド大学) にて撮影

Script

This is poo, and what I want to do today is share my passion for poo with you, which might be quite difficult, but I think what you might find more fascinating is the way these small animals deal with poo.

So this animal here has got a brain about the size of a grain of rice, and yet it can do things that you and I couldn't possibly entertain the idea of doing. And basically it's all evolved to handle its food source, which is dung.

So the question is, where do we start this story? And it seems appropriate to start at the end, because this is a waste product that comes out of other animals, but it still contains nutrients and there are sufficient nutrients in there for dung beetles basically to make a living, and so dung beetles eat dung, and their larvae are also dung-feeders. They are grown completely in a ball of dung. Within South Africa, we've got about 800 species of dung beetles, in Africa we've got 2,000 species of dung beetles, and in the world we have about 6,000 species of dung beetles. So, according to dung beetles, dung is pretty good.

Unless you're prepared to get dung under your fingernails and root through the dung itself, you'll never see 90 percent of the dung beetle species, because they go directly into the dung, straight down below it, and then they shuttle back and forth between the dung at the soil surface and a nest they make underground.

So the question is, how do they deal with this material? And most dung beetles actually wrap it into a package of some sort. Ten percent of the species actually make a ball, and this ball they roll away from the dung source, usually bury it at a remote place away from the dung source, and they have a very particular behavior by which they are able to roll their balls. So this is a very proud owner of a beautiful dung ball. You can see it's a male because he's got a little hair on the back of his legs there, and he's clearly very pleased about what he's sitting on there. And then he's about to become a victim of a vicious smash-and-grab. (Laughter) And this is a clear indication that this is a valuable resource. And so valuable resources have to be looked after and guarded in a particular way, and we think the reason they roll the balls away is because of this, because of the competition that is involved in getting hold of that dung. So this dung pat was actually -- well, it was a dung pat 15 minutes before this photograph was taken, and we think it's the intense competition that makes the beetles so well-adapted to rolling balls of dung.

So what you've got to imagine here is this animal here moving across the African veld. Its head is down. It's walking backwards. It's the most bizarre way to actually transport your food in any particular direction, and at the same time it's got to deal with the heat. This is Africa. It's hot.

So what I want to share with you now are some of the experiments that myself and my colleagues have used to investigate how dung beetles deal with these problems.

So watch this beetle, and there's two things that I would like you to be aware of. The first is how it deals with this obstacle that we've put in its way. See, look, it does a little dance, and then it carries on in exactly the same direction that it took in the first place. A little dance, and then heads off in a particular direction. So clearly this animal knows where it's going and it knows where it wants to go, and that's a very, very important thing, because if you think about it, you're at the dung pile, you've got this great big pie that you want to get away from everybody else, and the quickest way to do it is in a straight line. So we gave them some more tasks to deal with, and what we did here is we turned the world under their feet. And watch its response. So this animal has actually had the whole world turned under its feet. It's turned by 90 degrees. But it doesn't flinch. It knows exactly where it wants to go, and it heads off in that particular direction.

So our next question then was, how are they doing this? What are they doing? And there was a cue that was available to us. It was that every now and then they'd climb on top of the ball and they'd take a look at the world around them. And what do you think they could be looking at as they climb on top of the ball? What are the obvious cues that this animal could use to direct its movement? And the most obvious one is to look at the sky, and so we thought, now what could they be looking at in the sky? And the obvious thing to look at is the sun. So a classic experiment here, in that what we did was we moved the sun. What we're going to do now is shade the sun with a board and then move the sun with a mirror to a completely different position. And look at what the beetle does. It does a little double dance, and then it heads back in exactly the same direction it went in the first place. What happens now? So clearly they're looking at the sun. The sun is a very important cue in the sky for them.

The thing is the sun is not always available to you, because at sunset it disappears below the horizon. What is happening in the sky here is that there's a great big pattern of polarized light in the sky that you and I can't see. It's the way our eyes are built. But the sun is at the horizon over here and we know that when the sun is at the horizon, say it's over on this side, there is a north-south, a huge pathway across the sky of polarized light that we can't see that the beetles can see. So how do we test that? Well, that's easy. What we do is we get a great big polarization filter, pop the beetle underneath it, and the filter is at right angles to the polarization pattern of the sky. The beetle comes out from underneath the filter and it does a right-hand turn, because it comes back under the sky that it was originally orientated to and then reorientates itself back to the direction it was originally going in. So obviously beetles can see polarized light.

Okay, so what we've got so far is, what are beetles doing? They're rolling balls. How are they doing it? Well, they're rolling them in a straight line. How are they maintaining it in a particular straight line? Well, they're looking at celestial cues in the sky, some of which you and I can't see.

But how do they pick up those celestial cues? That was what was of interest to us next. And it was this particular little behavior, the dance, that we thought was important, because look, it takes a pause every now and then, and then heads off in the direction that it wants to go in. So what are they doing when they do this dance? How far can we push them before they will reorientate themselves? And in this experiment here, what we did was we forced them into a channel, and you can see he wasn't particularly forced into this particular channel, and we gradually displaced the beetle by 180 degrees until this individual ends up going in exactly the opposite direction that it wanted to go in, in the first place. And let's see what his reaction is as he's headed through 90 degrees here, and now he's going to -- when he ends up down here, he's going to be 180 degrees in the wrong direction. And see what his response is. He does a little dance, he turns around, and heads back in this. He knows exactly where he's going. He knows exactly what the problem is, and he knows exactly how to deal with it, and the dance is this transition behavior that allows them to reorientate themselves.

So that's the dance, but after spending many years sitting in the African bush watching dung beetles on nice hot days, we noticed that there was another behavior associated with the dance behavior. Every now and then, when they climb on top of the ball, they wipe their face. And you see him do it again. Now we thought, now what could be going on here? Clearly the ground is very hot, and when the ground is hot, they dance more often, and when they do this particular dance, they wipe the bottom of their face. And we thought that it could be a thermoregulatory behavior. We thought that maybe what they're doing is trying to get off the hot soil and also spitting onto their face to cool their head down.

So what we did was design a couple of arenas. one was hot,one was cold. We shaded this one. We left that one hot. And then what we did was we filmed them with a thermal camera. So what you're looking at here is a heat image of the system, and what you can see here emerging from the poo is a cool dung ball. So the truth is, if you look at the temperature over here, dung is cool. (Laughter)

So all we're interested in here is comparing the temperature of the beetle against the background. So the background here is around about 50 degrees centigrade. The beetle itself and the ball are probably around about 30 to 35 degrees centigrade, so this is a great big ball of ice cream that this beetle is now transporting across the hot veld. It isn't climbing. It isn't dancing, because its body temperature is actually relatively low. It's about the same as yours and mine. And what's of interest here is that little brain is quite cool. But if we contrast now what happens in a hot environment, look at the temperature of the soil. It's up around 55 to 60 degrees centigrade. Watch how often the beetle dances. And look at its front legs. They're roaringly hot. So the ball leaves a little thermal shadow, and the beetle climbs on top of the ball and wipes its face, and all the time it's trying to cool itself down, we think, and avoid the hot sand that it's walking across.

And what we did then was put little boots on these legs, because this was a way to test if the legs were involved in sensing the temperature of the soil. And if you look over here, with boots they climb onto the ball far less often when they had no boots on. So we described these as cool boots. It was a dental compound that we used to make these boots. And we also cooled down the dung ball, so we were able to put the ball in the fridge, gave them a nice cool dung ball, and they climbed onto that ball far less often than when they had a hot ball. So this is called stilting. It's a thermal behavior that you and I do if we cross the beach, we jump onto a towel, somebody has this towel -- "Sorry, I've jumped onto your towel." -- and then you scuttle across onto somebody else's towel, and that way you don't burn your feet. And that's exactly what the beetles are doing here.

However, there's one more story I'd like to share with you, and that's this particular species. It's from a genus called Pachysoma. There are 13 species in the genus, and they have a particular behavior that I think you will find interesting. This is a dung beetle. Watch what he's doing. Can you spot the difference? They don't normally go this slowly. It's in slow motion. but it's walking forwards, and it's actually taking a pellet of dry dung with it. This is a different species in the same genus but exactly the same foraging behavior.

There's one more interesting aspect of this dung beetle's behavior that we found quite fascinating, and that's that it forages and provisions a nest. So watch this individual here, and what he's trying to do is set up a nest. And he doesn't like this first position, but he comes up with a second position, and about 50 minutes later, that nest is finished, and he heads off to forage and provision at a pile of dry dung pellets. And what I want you to notice is the outward path compared to the homeward path, and compare the two. And by and large, you'll see that the homeward path is far more direct than the outward path. On the outward path, he's always on the lookout for a new blob of dung. On the way home, he knows where home is, and he wants to go straight to it. The important thing here is that this is not a one-way trip, as in most dung beetles. The trip here is repeated back and forth between a provisioning site and a nest site. And watch, you're going to see another South African crime taking place right now. (Laughter) And his neighbor steals one of his dung pellets.

So what we're looking at here is a behavior called path integration. And what's taking place is that the beetle has got a home spot, it goes out on a convoluted path looking for food, and then when it finds food, it heads straight home. It knows exactly where its home is. Now there's two ways it could be doing that, and we can test that by displacing the beetle to a new position when it's at the foraging site. If it's using landmarks, it will find its home. If it is using something called path integration, it will not find its home. It will arrive at the wrong spot, and what it's doing here if it's using path integration is it's counting its steps or measuring the distance out in this direction. It knows the bearing home, and it knows it should be in that direction. If you displace it, it ends up in the wrong place. So let's see what happens when we put this beetle to the test with a similar experiment.

So here's our cunning experimenter. He displaces the beetle, and now we have to see what is going to take place. What we've got is a burrow. That's where the forage was. The forage has been displaced to a new position. If he's using landmark orientation, he should be able to find the burrow, because he'll be able to recognize the landmarks around it. If he's using path integration, then it should end up in the wrong spot over here.

So let's watch what happens when we put the beetle through the whole test. So there he is there. He's about to head home, and look what happens. Shame. It hasn't a clue. It starts to search for its house in the right distance away from the food, but it is clearly completely lost. So we know now that this animal uses path integration to find its way around, and the callous experimenter leads it top left and leaves it. (Laughter)

So what we're looking at here are a group of animals that use a compass, and they use the sun as a compass to find their way around, and they have some sort of system for measuring that distance, and we know that these species here actually count the steps. That's what they use as an odometer, a step-counting system, to find their way back home. We don't know yet what dung beetles use.

So what have we learned from these animals with a brain that's the size of a grain of rice? Well, we know that they can roll balls in a straight line using celestial cues. We know that the dance behavior is an orientation behavior and it's also a thermoregulation behavior, and we also know that they use a path integration system for finding their way home. So for a small animal dealing with a fairly revolting substance we can actually learn an awful lot from these things doing behaviors that you and I couldn't possibly do. Thank you. (Applause)

これはフンです 今日はフンに懸ける私の思いを 共感して頂きたいと思います できなくても構いません しかしこの小さな生物とフンの関係には 興味を持てるのではないでしょうか

この生物の脳は 米粒ほどの大きさです しかし人間なら 試みることもないことを彼らはするのです つまり彼らの食糧源である フンの扱い方です

さて何から話しましょう? やはりここはお尻からがいいでしょう これは他の動物が出す排泄物ですが まだ栄養が残っています フンコロガシが生きていくのに 十分な量の栄養素です 彼らはフンを餌としその幼虫も またフンを食べます フンだけを餌にして成長するのです 南アフリカには800種のフンコロガシがいます アフリカ全体では2000種 世界では6000種です 彼らにとってフンは相当のごちそうです

指でフンをほじくれなければ フンコロガシ種の9割は お目にかかれません 彼らはフンの奥に 入り込み地上のフンと 地中の巣との間を 何度も往復するからです

では 彼らはフンをどう扱うのでしょう? 大半は何かしらの形にまとめます 1割方は フンをボールのようにして 元のフンから離れた場所へと転がし 地中に埋めます 彼らは独特の行動を取りますが その行動によってフンが転がせるのです ここに自慢げなフンの持ち主がいます オスです 足の後ろに毛が生えていますからね 彼はこの美しいフンの玉にご満悦のようです ところが悲劇です 卑劣な強盗の犠牲者となりました この行為はフンが貴重な 食糧源であることの表れです 貴重な食糧源は特別な方法で 守る必要があります だからフンを遠くに 転がすのではないでしょうか? フン獲得の競争が 激しいからです このフンの塊--もはや過去形ですね この写真を撮影する15分前は塊でした この激しい競争こそフンコロガシが フンを転がすに至った 理由ではないかと思うのです

ではこの生物がアフリカの 草原を横切る姿を想像して下さい 頭を下に 後ろ向きに歩いています 食糧の運び方としてこれ以上ないほど突飛です しかも熱との戦いもあります アフリカは暑いです

ではここで私と同僚が行った 実験についてお話します フンコロガシは運搬と熱に どう対処しているのでしょう

このフンコロガシを見てください 二つの点に注目して下さい一つは 置かれた障害物に対する 対処法ですちょっと踊りましたね そして最初に向かっていた方向と 全く同じ方向に歩いていきます 一瞬踊ってから目的の方向へと進みます 彼らは行き先が分かっているのです 行きたいところがわかっています これは非常に重要なことです 例えば自分が大きなフンの塊の中にいて ご馳走を独り占めしたい遠くに運びたいわけです 直進するのが一番手っ取り早いですよね そこで私達はこの子に更に課題を与えました 彼の足元を回転させたのです 彼の反応に注目して下さい 彼の世界は足元で今回転しました 90度回転したのです しかし全く動じません行きたい方向に 再び向かっていきます

ではどうやって彼らは 行き先を認識しているのか? 何をしているのか? 私達は気がつきました 彼らは時々ボールの上に登るのです そして辺りを見回します ボールの頂上に登って 彼らは一体何を見ているのでしょう? 動物が利用できる手掛かりは 何でしょう?一番分かりやすいのは 空でしょう では空の何を 見ているのでしょう? 空と言えば太陽です そこで典型的な実験をしました 太陽を動かしたのです 板を使って太陽を遮ります そして鏡を使って全く違う 位置に太陽を動かします フンコロガシの反応を見てください 少し踊って 元の方向と全く同じ方向へ 歩いていきます 彼らが太陽を見ていることが明らかになりました 彼らにとって太陽は重要な手掛かりなのです

でもいつも太陽があるとは限りません 日没後は水平線の下に隠れてしまいます 空には私達には見えない 偏光の大きなパターンがあります 人間の目では感知できません しかし太陽は水平線上にあります 太陽が水平線上にある時は-- 例えばこちら側にあるとすると 南北に大きな帯状の 偏光パタ―ンができます 人間には見えません でもフンコロガシには見えるのです ではそれを証明しましょう 大きな偏光フィルターを用意します その下にフンコロガシを入れます フィルターは 空の偏光パターンの向きと直角に置きます フンコロガシはフィルターの下から 出てきて 右に曲がります 元々の向きのパターンの 空の下に戻ったので 元々の方角へと 体の向きを変えたのです 彼らには光の偏光がわかるのです

ここまでのおさらいをすると フンコロガシはボールを転がします しかも直線で転がします 何故曲がらずに移動できるのか? それは私達の目には見えない 天空の手掛かりを使っているからです

ではその手掛かりはどう感知するのか? それがまさに次の疑問でした そしてこの踊りこそが重要なのではないかと 考えたのです 何故なら このように時々休憩しています そして再び行きたい方角へと向かいます この踊りが持つ意味は何なのか? 何が起こると彼らは向きを変えるのでしょう? こちらの実験では まず無理矢理彼を この溝に誘導します 良く見ると特に 無理強いはされていませんね そして徐々に180度ずらしていき 最終的に元の方向とは 逆方向へと向かわせます 彼の反応を見てみましょう 90度の場所を通り過ぎ 歩き続け 最終的には 元の方向とは180度反対方向を向きます 彼の反応を見てみると 少し踊って 回転して 再び溝の中を進みます 彼は何が問題なのか理解し かつ対処法も心得ているのです そしてこの踊りを転機に 彼らは正しい方向に向き直るのです

以上が踊りについてです しかし灼熱のアフリカで 何年もフンコロガシを観察しているうちに 踊りに関連するもう一つの ある行動に気がついたのです ボールに登ると彼らは時たま 顔を拭くのです またやりましたね これは一体どういうことなのでしょう? 明らかに地面は熱いです 地面が熱いと 踊る回数が増えます そして踊ると 顔の下の方を拭くのです これは温度調節をしているのではないか 熱い砂を払い 顔に唾をかけて 頭を冷やしているのではないかと 私達は考えたのです

そこで二つの舞台を作りました 一つは熱く 一つは涼しい状態です こちらは日陰に もう一つは日向に置きました そして熱探知カメラで撮影しました 今映っているのは熱を映像化したものです そしてフンの中から低温の フンコロガシが出てきました フンコロガシは低温 つまりクール かっこいい奴なのです(笑)

私達の関心はフンコロガシと 周囲との温度差です 周囲はおよそ50℃です フンコロガシとボールはおよそ 30℃から35℃くらいでしょう 例えるなら 大きなアイスクリームの塊を 熱い草原の上を転がしていくようなものです まだ登っていません 踊っていません 体温が比較的低いからです 私達の体温とほぼ同じです そして小さな脳みそもかなり低温です ところが周りが熱くなると 地面の温度を見てください 55℃から60℃です 頻繁に踊り始めましたね 前足を見てください相当熱そうです ボールは熱の影を残しています そしてフンコロガシはボールに登ります 顔を拭き 常に体温を下げ熱い砂を 避けようとしていると考えられます

そこで靴を履かせてみました 地面の温度を足で知覚するのか 確認するためです ここを見て下さい 靴を履くとボールに登る 回数が格段に少なくなります クールブーツと命名しました 靴は歯科治療用の材料で作りました またフンのボールも冷やしました 冷蔵庫で冷やしたフンを与えたのです するとボールが熱い時と比べて 登る回数がかなり減りました 「高床歩き」です 人間も熱い砂浜を 歩くときにします 他人のタオルに飛び乗り 「失礼 踏んでしまいました」と言って 別の人のタオルに飛び移る 足を火傷せずに済みます フンコロガシの行為も全く同じことです

もう一つお話をしたいと思います それはこの種についてです パキソーマ亜属と呼ばれ 13種が存在します 彼らには 興味深い習性があります これはフンコロガシです 見てください 違いが分かりますか? この映像は通常の速度ではありませんが ポイントは前向きに歩いている点です しかも乾燥した小さなフンを運んでいます 同じ属の違う種ですが 全く同じ採食行動を取っています

もう一つ興味深いことがあります 何とフンコロガシは餌を探し 巣に持ち帰るのです この子を見てください 巣作りをしています 最初の位置は気に入らなかったのか 二つ目の位置に巣を構えます 50分後 巣が完成し 食糧調達へと出かけます 乾燥したフンの粒達を目がけます 注目してもらいたいのは 往路と復路の違いです 復路のほうが往路より 遥かに真っすぐです 往路では常に新しいフンが 周囲にないか探しています 復路は自分の巣を目指して 真っすぐ帰るのです 重要なのは普通のフンコロガシと違って 餌場と巣の間を何度も 往復していることです そして南アフリカの犯罪が 繰り返されます お隣が彼のフンを盗みました

ここで目にしているのは 経路積分と呼ばれる行為です フンコロガシには本拠地があります 食糧を求めて外に出るときは 複雑な経路をたどり食糧を見つけると 真っすぐ帰宅します場所が分かっているからです 二つの可能性が浮上します それを確かめるために 餌場にいるフンコロガシを 別の場所に移します 地上の何かを目印にしているなら家に帰れます 一方 経路積分を利用しているなら 巣を見つけることはできません 経路積分を利用するということは この方向に向かって歩数 距離を測っているということです 基準となる巣がどの方角にあるべきか把握しているのです 位置をずらすと場所を間違えます ではこのフンコロガシを使って 実験してみましょう

ここに狡猾な実験者がいます フンコロガシを移動します どうなるか見てみましょう ここに巣があります あそこにあった餌場は 新しい場所に移しました 目印を使っているなら 巣を見つけられるはずです 巣の周りの目印が認識できるからです 経路積分を使っているなら こちらの間違った場所に到着します

実験にかけられたフンコロガシが どうなるか見てみましょう ここにいますね 家に帰ろうとしています どうなるか 残念です 全く分かっていません 餌場から正しい距離にあるはずの家を 探していますが完全に迷子になりました つまり彼らは経路積分を使って 移動しているのです 無慈悲な実験者は 左上に誘導してそのまま去ります

ここに描かれているのは太陽を コンパスにして移動する 生物です 彼らは何かしらの仕組みで この距離が測れるのです これらの種は歩数を数えることが分かっています その仕組みを距離計のように利用して 巣に戻るのです しかしフンコロガシが何を 利用しているのかは解明されていません

米粒ほどの大きさの脳を持つ この生物から学んだことは何でしょう? 彼らが天空の手掛かりを使って ボールを真っすぐに転がせることが分かりました 踊るのは方角を決めるのと 温度調節のためだということも学びました そして巣に戻るときは経路積分を 利用することも知りました 不快な物体を扱うこんな小さな生物からでも 人間なら絶対しない彼らの行為を見て 学べることは多いのです ありがとうございます

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