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Translator: Joseph Geni Reviewer: Morton Bast
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.
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)


【TED】マーカス・バーン: フンコロガシの踊り (Marcus Byrne: The dance of the dung beetle)

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Zenn 2017 年 11 月 5 日 に公開
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