David Attenborough explores whether there is counting in the natural world.
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The natural world is full of extraordinary animals
with amazing life histories.
Yet, certain stories are more intriguing than others'.
The mysteries of a butterfly's life cycle,
or the strange biology of the emperor penguin.
Some of these creatures were surrounded by fantastic
myths and misunderstandings.
Others have only recently revealed their secrets.
These are the creatures that stand out from the crowd.
The curiosities that I find particularly fascinating.
At the turn of the 19th century,
a German horse called Hans hit the headlines.
It was claimed that he could perform
complicated mathematical calculations.
And Chinese records tell of a species of bamboo
that seems able to count the years,
for all individual plants growing in different parts of the world
flower at exactly the same time.
Can a horse and a plant truly count?
In 1891, a retired German mathematics teacher
called Wilhelm von Osten decided to do a very unusual thing.
He began to teach maths to his horse, Hans.
After four years, Hans was presented to the public
to demonstrate his remarkable abilities.
To everyone's amazement,
he was able to count the number of people in the audience,
perform complex arithmetic, read a clock,
recognise and identify playing cards,
and he knew the calendar of the whole year.
To signal the correct answer, Hans tapped his hoof.
He was accurate and consistent,
and was declared worldwide as the first horse genius.
It all seemed incredible.
Could Hans the horse really count?
And why might animals need to do so anyway?
Well, it could help them keep track of the number within a group.
They might need to know how many offspring they have.
And, for many animals, it's an advantage to know
if one quantity is smaller or larger than another.
Studies of lions in the Serengeti
suggest that they're able to count roars
in order to assess the size of a competing pride.
They estimate the number of challengers from the sound
of the incoming roars, and compare this to the size of their own pride.
They will only decide to defend themselves if their pride is larger.
But it seems their counting is limited
to no more than five or six roars.
Could Hans the horse be drawing on his wild instincts
to use numbers in this way?
Wild horses live in small bands
that can join up to form big herds of more than 100 animals.
They have a dominant stallion, and a firm pecking order.
Horses are not territorial,
and have no great need to know the exact numbers in their own herd.
But they do sometimes make judgments about quantity
when choosing food.
This basic ability to judge differences in amounts
is the first step in the skill of counting.
But what methods might animals use to assess numbers?
There are several ways to keep count.
We can count precisely to very high numbers,
and have created number symbols that indicate exact amounts.
So, we know that the number five, for example, indicates five objects.
Even if we can't see them.
Perhaps animals judge quantity in a similar way,
and have their own pictorial shapes perhaps, in mind,
that match an amount.
Another idea is that animals judge differences in amount
as an accumulation,
just like filling a measuring tube with a liquid.
Alternatively, the mind may have memory slots
that store a limited number of objects
in the same way as a filing cabinet stores cards.
So, there could be several different ways in which animals
judge quantity or amounts.
Hans the horse appeared to count very precisely to high numbers.
This seems an improbable feat for any animal.
But, recently, an extraordinary discovery showed that an insect
with a brain a fraction of the size of a horse's,
can count with great accuracy.
Ants are social animals that use scent trails
and visual reference points to find their way around.
But, in the desert, where scent evaporates,
and the landscape is featureless,
they nonetheless seem able to navigate successfully.
In 2007, researchers investigated how Tunisian desert ants
find their way home from foraging trips across barren plains.
It's known that ants can measure and integrate two parameters -
direction, and distance of travel.
The desert sun helps an ant orientate its direction.
But how do they measure distance?
Experiments were performed to manipulate the ant's stride length
to see if they were counting the number of footsteps that they took.
Based on a featureless environment,
ants learned to travel home to a set point.
Then, stilt-like extensions were glued to their legs
to lengthen their stride.
These ants took the right number of steps,
but, because of the increased leg length,
they marched right past their goal.
So, ants are able to log the number of steps that they take,
perhaps not counting them in the way that we do,
but they do have an internal counter.
Unlike ants, horses have no need to count their steps,
so it's unlikely that they have an internal pedometer.
But they can, nonetheless, assess quantity.
Wild horses are highly social.
And, if one is harassed by flies,
it will seek to join the largest available group for protection.
So, horses can estimate numbers and recognise differences in size.
But this is a much simpler ability
than the counting that Hans the horse was doing.
There seem to be no limit to the complexity of the calculations
that Hans could compute, and his answers were precise.
For many, this seemed too extraordinary to be true.
In 1904, the German Board of Education was so intrigued
that they formed a 13-strong commission
to look closely at how Hans could perform such amazing feats.
They wondered if Hans' owner was using trickery
to feed him the answers.
To test this,
other members of the board were given questions and answers
to pose to Hans.
Incredibly, Hans still answered correctly.
Eminent psychologist Oskar Pfungst
then came to investigate Hans' skills further.
And confirmed that he appeared to understand numerology,
and the abstract idea of counting words associated with numbers.
For Hans to perform complex mathematics,
he would need to understand
much more than just differences in quantities.
He would need to recognise the fact that two
is smaller than six.
And that specific number symbols
always go with the corresponding amounts.
To count, he would also need to realise that numbers occur
in a set, repeatable sequence.
And that the last number counted
represents the total of the whole set.
In effect, that he'd answered the question, "How many do you have?"
It seems that many animals have a sense of number,
but few are conscious of an exact total.
For most animals, the ability to recognise an amount,
and to compare it to others, is all they need.
Usually, a crude estimate, between more or less,
or larger and smaller, is enough.
But could horses be an exception?
Could they have advanced counting skills?
In 2009, experiments showed
that horses could count to a certain level.
I'm going to take false apples,
ones that don't have a smell which might confuse the issue,
and show the horse one...
..four in here.
And one, two...
Now, then, which do you want?
Repeated tests of 14 horses
found that they consistently selected buckets
that contained a higher number of apples.
But that sense of number was limited -
they could only keep track of numbers up to about six,
and no higher.
So, it seemed incredible
that Hans the horse had such advanced counting skills.
Perhaps he was being helped or trained in some way by his owner?
There is little doubt that most horses are very intelligent animals.
And, if they're given clear signals,
they can indeed learn to perform complicated routines.
Hans the horse was schooled for many years,
so, perhaps, he had developed an advanced understanding of numbers
when given clear instructions by his owner.
In the wild,
horses communicate with each other by using quite a rich body language.
Wild Mustang use a complex silent one that scientists call Equus.
It consists of a series of gestures,
that are much like signing for the deaf.
Every part of the horse conveys meaning.
Especially the ears, tongues, lips, shoulders, and necks.
They have the sensitivity and intelligence
to interpret the tiniest of gestures, even breathing patterns.
And, from this, they can judge each other's intentions.
And it's this ability to sense subtle changes
in physical and emotional states of those around them,
that has made horses so responsive to training.
So, did Hans the horse really understand numbers,
or was something fishy going on?
In 1907, after further research,
Professor Oskar Pfungst discovered
that Hans could only get the correct answer
if the questioner knew the answer,
and then, only if he could see his face.
That was a significant discovery.
I have my own clever horse, her name is Millie.
Millie, what is two plus two?
HORSE SCRAPES HOOF FOUR TIMES
There you are.
All right, let me ask you something more difficult, Millie.
Millie, what is eight plus two minus seven?
HORSE SCRAPES HOOF THREE TIMES
Yes! Well done, Millie!
There you are.
Millie is, indeed, a clever horse,
and appears to be able to do arithmetic.
That's because, in fact, she can react to very subtle signals.
If I take my hand off her and step forward...
And, if I step back, she stops.
So, eventually, it was shown that Hans was not a mathematician genius,
he was just extremely skilled at following body language.
Particularly those facial signals that questioners might give
when they reach the right answer to the question.
Some animals can, of course, count.
But, for most animals, knowing the difference between more or less
is all they need for survival.
Hans the horse baffled all the experts for many years.
But true counting is, in fact, a complex concept
that few animals grasp.
There is a plant, however,
that may be able to do so in a surprising way.
It's the fastest-growing plant on earth.
A type of grass we know as bamboo.
Remarkably, some species only flower every 30 or 60 years.
Others do so in cycles of over 100 years.
But how do these plants measure time?
Can they count down the years?
Flowers that bloom in the spring, like these,
are triggered to do so by a rise in temperature
and an increase in the length of the days.
But bamboos flower on an entirely different system.
They don't do so annually,
but at intervals which far exceed the length of a human life.
The function of flowers is to reproduce.
Bright, sweet-smelling blooms attract insects,
which carry pollen from one plant to another, and so fertilise them.
The flowers of bamboos are unassuming and drab.
Because they're pollinated not by insects, but by the wind.
Most grasses, indeed, have flowers that are so small
that they tend to go unnoticed.
Since their pollen is carried by the wind,
they have no need for spectacular blooms.
Bamboos are the largest members of the grass family,
and they were introduced to Britain from Asia during the 1800s.
Many were planted here in Kew.
But, for over 100 years, nobody ever saw them flower.
Bamboos grow in tropical or subtropical climates.
They are, in fact, one of the most widespread plants.
But, despite this, few people ever see them flower.
The reason is, they only do so very rarely.
We know this because early collectors and scholars
have kept careful records that, in some cases, extend over centuries.
Some are still preserved at Kew Gardens in London,
which houses one of the largest historical collections of plants
in the world.
This is a specimen of the giant timber bamboo.
It was collected, as this label shows, in China in 1855.
At the time, bamboo was clearly in flower.
There they are.
Quite small and obscure.
Much like those of other grasses.
And then, in the 1960s,
Phyllostachys bambusoides bloomed again.
And here is the evidence.
This specimen is from 1961.
These are just the records from European collectors.
Chinese and Japanese accounts go back much further, over 1,000 years.
Together, these records show that Phyllostachys bambusoides
flowers in cycles of around 110 years.
And there was another surprise.
Phyllostachys is native to China and Japan.
But, in the 19th century,
it was introduced to other countries as an ornamental garden plant.
And when it flowered, most recently in the 1960s,
it came into bloom not just in its native Asia,
but all around the world at the same time.
It's a most bizarre life cycle.
How do bamboos flower at the same time
when separated by thousands of miles?
The unusually long flowering cycle of bamboo was well-known in China.
But there were other stories about its flowering
that were picked up by European visitors trading in the Orient.
Bamboo was valued by local people for its sturdiness and durability.
The bamboo was held in such respect
that it featured prominently in the paintings and calligraphy
of ancient Chinese and Japanese artists.
Although the bamboo is deeply rooted in local cultures,
one part of the plant has instilled fear since the earliest times.
An old Chinese proverb says when the bamboo flowers,
it means either pestilence or famine.
In 1898, a medical officer called John Mitford Atkinson,
based at a government hospital in Hong Kong,
sent some bamboo seeds to the keeper of the horarium here at Kew.
And, with it, this letter.
In it, he writes that,
"Oddly enough, in the years that the bamboo flowered,
"plague epidemics seemed always to ravage the colony."
So, could there perhaps be some truth in these old sayings?
So, here was another mystery.
Not only does the bamboo flower very rarely, but when it does,
it seemingly causes death and famine.
The bamboo's life cycle is truly puzzling.
How can a plant survive by only flowering every 100 years?
The answer, it seems, can be found underground.
This is the rhizome of a bamboo.
It extends in all directions from the plant.
Putting down roots, and sending up shoots.
It's a very efficient way of spreading.
As gardeners know to their cost,
because you plant one patch of bamboo,
and before you know where you are, it's taken over the entire garden.
It's a way of spreading that has its advantages.
Bamboos don't have to flower and seed every year.
Instead, they grow a whole network of underground rhizomes,
and put their energy straight into producing fast-growing
and strong shoots.
The stems emerge from the ground at their full width,
and shoot to the sky like a periscope.
In just a few weeks, they reach their full height.
After this, they don't get any taller or thicker.
They simply expand outwards, like a family or colony.
This is a giant bamboo.
Some species can reach the extraordinary height of 30 metres,
and, to achieve that in a single season,
they have to grow at the phenomenal rate of a metre a day.
You can literally see them grow.
But, when it comes to flowering,
bamboos are one of the slowest.
What could be the reason for this long interval?
In Southeast Asia, there are trees that may give us a clue.
They're called dipterocarps and, like bamboos,
they also flower and seed synchronously,
but on a shorter timescale.
They produce seeds en masse every two to seven years.
And when they do, they swamp the forest floor
with an abundant supply of food.
This attracts small mammals from all around.
By fruiting at the same time, the trees ensure that,
despite the many predators, some of the seeds will survive,
and grow into new seedlings.
Bamboo seeds are also highly nutritious,
and lots of animals like to feed on them.
Rats, mice, birds, monkeys, even elephants.
They all devour huge numbers of the seeds, given the chance.
So, bamboos may fare better
if they synchronise their reproduction to flower and seed
at the same time.
By overwhelming their enemies with food,
they can ensure that at least some of their seeds will survive.
Once bamboos fell into this flowering cycle,
any that flowered too early would lose all their seeds to predators.
In years when bamboos do flower,
there is often a boom in rodent populations.
But once the small mammals have stripped the forest of seeds,
they swarm into fields and villages to devour people's crops and grain.
The rats carry dangerous diseases,
and the result is often death and starvation among people.
So, paradoxically, the bamboo,
which provides an essential livelihood for so many people,
at times causes death and devastation.
When Atkinson made a connection between bamboo flowering
and plague epidemics in Hong Kong during the 19th century,
he had little idea of the true reasons behind this.
But, as it turns out, he was right.
And the old Chinese proverb contained a deadly prophecy.
While the reason behind the synchronous flowering
may have been explained,
it's still a mystery as to how bamboos actually do it.
Could the plants be counting down the years
in order to all flower at the same time?
It seems the answer may, once again, lie within their roots.
The bamboo's unusual way of reproducing
via a network of underground rhizomes
means that most plants are, ultimately, from the same mother plant.
These clumps have been shared across the world,
and although they're now in different locations,
they still carry the same genetic make-up.
They are effectively clones of the parent plant.
And it may be that they have some kind of internal memory
that is also passed on.
Scientists believe that the bamboo's roots contain some kind of clock
that enables them to count the passing of the years.
How they do that is still a mystery.
But, nonetheless, there is an animal that might give us a clue.
Periodical cicadas in North America spend 17 years underground,
feeding on the sap from tree roots.
Within the space of a few days,
the whole population emerges in their millions.
Their mission is to breed.
But what triggers the cicadas to all emerge at exactly the same time
every 17 years?
We know that, when feeding underground,
they can detect changes in the tree sap each spring,
and so tell the passing of a year.
Could it be that bamboos also count the years in this way?
It's possible that bamboos register the passing of the seasons
in a similar way by changes in their sap.
We just don't know.
But, while the exact mechanism remains a mystery,
it may well prove that these time-measuring plants, bamboos,
are the master mathematicians of the non-human world.
True counting is very rare in nature.
But some animals and plants achieve numerical feats
that are astonishing in their own right.
Can animals count? This is a question that has intrigued and fooled investigators for a long time. Just over 100 years ago, a German horse called Hans was declared a mathematical genius but all was not as it seemed. And strangely, some bamboos around the world flower exactly at the same no matter where they are. Are they counting down the years?