1
00:00:41,326 --> 00:00:44,523
The biggest predator to walk the Earth today

2
00:00:44,686 --> 00:00:47,154
faces a continuous struggle.

3
00:00:47,326 --> 00:00:50,363
Its prey is heavily armoured, often indigestible,

4
00:00:50,526 --> 00:00:52,915
sometimes even poisonous.

5
00:00:53,086 --> 00:00:57,477
What makes this struggle
between predator and prey more surprising

6
00:00:57,646 --> 00:01:02,322
is that the predators are elephants
and the prey are plants.

7
00:01:03,126 --> 00:01:05,686
These herds are the task force

8
00:01:05,846 --> 00:01:10,397
in a war that has been fought
for millions of years and has produced

9
00:01:10,566 --> 00:01:14,320
some of the most complex
and highly evolved relationships

10
00:01:14,486 --> 00:01:17,637
in the whole of the natural world.

11
00:01:34,726 --> 00:01:39,880
In this tree, there is one of the most
extraordinary plant predators.

12
00:01:42,246 --> 00:01:45,841
It's one animal I don't need to sneak up on.

13
00:01:48,486 --> 00:01:50,078
Boo!

14
00:01:51,086 --> 00:01:56,285
This extraordinary creature is half blind, half deaf,

15
00:01:56,446 --> 00:01:59,882
and this is just about as fast as it can move.

16
00:02:00,886 --> 00:02:05,721
That's what can happen to you
if you live on nothing but leaves.

17
00:02:06,566 --> 00:02:11,959
It's a sloth. It's not exactly
an enthusiastic leaf-eater.

18
00:02:12,126 --> 00:02:14,515
A couple of half-hearted chews

19
00:02:14,686 --> 00:02:18,201
and the leaves go straight down to its stomach.

20
00:02:18,366 --> 00:02:21,915
Leaves, however, are not easily digested.

21
00:02:22,566 --> 00:02:25,603
The sloth's technique is to give them time.

22
00:02:29,366 --> 00:02:33,325
Then, eventually, this mobile compost heap

23
00:02:33,486 --> 00:02:37,638
pulls itself together and starts
on a long, dangerous journey.

24
00:02:39,166 --> 00:02:42,317
This is a very unusual sight.

25
00:02:42,486 --> 00:02:46,638
A sloth in a hurry. It wants to defecate.

26
00:02:46,806 --> 00:02:52,199
The only place it's happy doing that
is down on the ground.

27
00:02:53,086 --> 00:02:58,365
It only does it about once a week,
but why does it come down to do it?

28
00:02:59,366 --> 00:03:04,520
And why does it nearly always do so
in exactly the same place?

29
00:03:04,686 --> 00:03:07,678
Whatever the reason, it must be important,

30
00:03:07,846 --> 00:03:11,759
for a sloth on the ground is almost helpless.

31
00:03:11,926 --> 00:03:16,636
Any predator could attack it,
and it doesn't have the speed to escape.

32
00:03:16,806 --> 00:03:22,403
Why it comes down in this way
is a mystery. Nobody knows.

33
00:03:22,566 --> 00:03:28,243
Now it's finished and back it goes
up to the safety of the canopy.

34
00:03:29,246 --> 00:03:31,635
Leaves are not very nutritious.

35
00:03:31,806 --> 00:03:36,675
The sloth's way of compensating
is not to eat more, but to do less.

36
00:03:36,846 --> 00:03:39,997
Its claws hook over the branches

37
00:03:40,166 --> 00:03:44,125
so the sloth can hang
without any effort of its thin muscles.

38
00:03:44,286 --> 00:03:51,556
To save energy, it spends most of its time
hanging around half asleep in the tree tops.

39
00:03:53,686 --> 00:03:58,965
So, with little muscle and a reaction time
only a quarter as fast as ours,

40
00:03:59,126 --> 00:04:02,914
how does a sloth's day compare with our day?

41
00:04:04,126 --> 00:04:06,879
While I write a few letters,

42
00:04:07,046 --> 00:04:09,879
the sloth manages to groom itself.

43
00:04:11,166 --> 00:04:15,444
While we have our lunch,
the sloth nibbles a few leaves.

44
00:04:15,606 --> 00:04:18,757
Then, as we film a sequence for the series,

45
00:04:18,926 --> 00:04:21,724
it's time for another nap.

46
00:04:24,046 --> 00:04:28,995
Not surprisingly, many mammals
are dependent upon plants.

47
00:04:29,166 --> 00:04:32,158
We live, after all, on a green planet.

48
00:04:33,206 --> 00:04:37,279
Plants capture the energy they need from the sun,

49
00:04:37,446 --> 00:04:42,042
turning much of the Earth's surface
into a vast and varied salad bowl.

50
00:04:47,006 --> 00:04:52,922
The leaf's nutriment is locked away
within a mesh of cellulose walls.

51
00:04:53,086 --> 00:04:55,884
No mammal can digest cellulose.

52
00:04:56,046 --> 00:04:59,641
Leaf-eaters rely on bacteria in their stomachs

53
00:04:59,806 --> 00:05:02,798
to break through this dense lattice.

54
00:05:05,166 --> 00:05:10,160
The first broad-leaved trees
appeared 100 million years ago.

55
00:05:10,326 --> 00:05:14,717
They spread and, eventually,
they formed lush rainforests

56
00:05:14,886 --> 00:05:17,684
like this one in South America.

57
00:05:19,286 --> 00:05:26,362
And it was in places like this that early mammals
first started to eat leaves in a wholesale way.

58
00:05:26,526 --> 00:05:30,314
One of those plant predators, very little changed,

59
00:05:30,486 --> 00:05:32,954
still survives here today.

60
00:05:33,126 --> 00:05:35,117
There's its track.

61
00:05:36,286 --> 00:05:41,121
The prints are very fresh,
so it could be quite close.

62
00:05:42,846 --> 00:05:47,317
The animal I'm following is said to be
as difficult to see as a jaguar.

63
00:05:49,606 --> 00:05:54,157
I must be careful,
because it's also said to be quite dangerous.

64
00:05:57,406 --> 00:05:59,874
There it is.

65
00:06:11,206 --> 00:06:18,078
This is the largest animal in the whole
of the South American rainforests.

66
00:06:18,246 --> 00:06:20,441
It's a tapir.

67
00:06:20,606 --> 00:06:25,805
There's a female on the left
and a half-grown calf on the right.

68
00:06:27,486 --> 00:06:31,081
With a calf there, she could be a bit aggressive.

69
00:06:31,246 --> 00:06:33,237
Better not get too close.

70
00:06:36,286 --> 00:06:38,675
They're feeding on leaves.

71
00:06:38,846 --> 00:06:43,044
In fact, most of their meals
are made on leaves.

72
00:06:43,206 --> 00:06:47,802
You'd think here, they've got
more than enough to choose from,

73
00:06:47,966 --> 00:06:53,120
but they're extremely selective
about which leaves they choose.

74
00:06:59,326 --> 00:07:02,682
And you can see why.

75
00:07:02,846 --> 00:07:06,441
Many of the leaves are protected by spines.

76
00:07:06,606 --> 00:07:11,475
Branches and trunks are armoured,
and spikes can inflict real damage.

77
00:07:17,526 --> 00:07:21,519
Even plants which appear harmless
may have defences

78
00:07:21,686 --> 00:07:23,677
if you look close enough.

79
00:07:26,446 --> 00:07:29,244
Their tissues are loaded with poison,

80
00:07:29,406 --> 00:07:34,196
some of which are really powerful,
such as strychnine.

81
00:07:34,366 --> 00:07:37,563
Tapirs have ways of dealing with that problem.

82
00:07:37,726 --> 00:07:41,116
They eat only a little of any one kind of leaf,

83
00:07:41,286 --> 00:07:46,883
then move on to another so they
don't get a lethal dose of any one.

84
00:07:47,966 --> 00:07:50,526
And they have another defence.

85
00:07:50,686 --> 00:07:54,884
This riverbank is a special place
that has been visited by tapirs

86
00:07:55,046 --> 00:07:57,640
over many generations.

87
00:08:03,406 --> 00:08:08,764
It's eating earth, kaolin,
a special kind of clay that binds to poisons,

88
00:08:08,926 --> 00:08:12,919
neutralising them
before they cause any harm.

89
00:08:13,086 --> 00:08:18,080
The kaolin is a medicine, one that
we ourselves use for the same purpose

90
00:08:18,246 --> 00:08:20,441
when we have stomach ache.

91
00:08:20,606 --> 00:08:24,485
So, in spite of all the defences
plants have evolved,

92
00:08:24,646 --> 00:08:29,003
tapirs manage to find all the food
they need in these forests.

93
00:08:33,686 --> 00:08:39,682
This struggle between mammals and the plants
they feed on is waged all over the world.

94
00:08:43,406 --> 00:08:48,116
The Canadian Rocky Mountains.
The beginning of an autumn day.

95
00:08:57,246 --> 00:08:59,237
(SQUEAKS)

96
00:09:00,726 --> 00:09:05,356
A pika, a member of a small
community that lives among

97
00:09:05,526 --> 00:09:10,725
the boulders bordering a mountain
meadow where they all feed.

98
00:09:12,326 --> 00:09:15,318
(SQUEAKS REPEATEDLY)

99
00:09:19,086 --> 00:09:25,036
That's a warning call, telling other
pikas that this patch is now taken.

100
00:09:26,566 --> 00:09:29,444
Pikas start their foraging early.

101
00:09:29,606 --> 00:09:34,885
They eat all of a plant -
not just leaves, but the flowers as well.

102
00:09:35,046 --> 00:09:37,480
Grazing in the open is dangerous.

103
00:09:37,646 --> 00:09:39,716
There are eagles around,

104
00:09:39,886 --> 00:09:44,676
so pikas never stray very far
from the safety of the rocks.

105
00:09:44,846 --> 00:09:47,838
There seems to be plenty of food now,

106
00:09:48,006 --> 00:09:54,002
but soon snow will come, flowers will
die back, and winter will be upon us.

107
00:09:54,166 --> 00:09:57,556
What happens then when little is growing?

108
00:09:57,726 --> 00:10:02,436
Watch what happens to these
if I leave them just there.

109
00:10:15,526 --> 00:10:18,996
It's not eating my flowers, at least not yet.

110
00:10:19,166 --> 00:10:24,001
It's stacking them in a special larder,
creating a store that will last it

111
00:10:24,166 --> 00:10:28,603
through the hard days
when this valley's covered in snow.

112
00:10:32,886 --> 00:10:37,721
It'll need a stack several feet thick
if it's to survive the winter.

113
00:10:42,566 --> 00:10:47,401
Strangely, many of these leaves
are extremely poisonous.

114
00:10:47,566 --> 00:10:49,682
Why does it collect them?

115
00:10:49,846 --> 00:10:53,555
Well, the poison acts as a natural preservative

116
00:10:53,726 --> 00:10:57,435
and the leaves remain fresh until mid-winter.

117
00:10:57,606 --> 00:11:00,962
The poison works to the pika's advantage.

118
00:11:01,126 --> 00:11:04,596
But the pika's preparations are very subtle.

119
00:11:04,766 --> 00:11:08,805
It takes care to collect a variety of plants.

120
00:11:08,966 --> 00:11:12,038
Those with a little poison become edible quickly,

121
00:11:12,206 --> 00:11:18,281
whereas those with a lot will remain fresh
until almost the end of winter.

122
00:11:18,446 --> 00:11:21,438
Each pika makes several hundred trips a day,

123
00:11:21,606 --> 00:11:24,678
literally making hay while the sun shines.

124
00:12:10,406 --> 00:12:14,797
Sometimes the problem is not
what's in your food, but what is not.

125
00:12:16,246 --> 00:12:20,444
Dealing with dietary deficiencies
has had a dramatic outcome

126
00:12:20,606 --> 00:12:25,202
here on the flanks of Mount Elgon in East Africa.

127
00:12:35,526 --> 00:12:38,324
The first Europeans to visit these caves

128
00:12:38,486 --> 00:12:41,956
noticed marks like these in the walls.

129
00:12:42,126 --> 00:12:46,119
They imagined they'd been made
by ancient Egyptians

130
00:12:46,286 --> 00:12:50,325
who came here to mine
for gold and precious stones.

131
00:12:50,486 --> 00:12:54,399
These grooves do look like
the marks made by a pickaxe.

132
00:12:54,566 --> 00:13:00,163
But to discover what actually made them,
you have to wait till nightfall.

133
00:13:01,486 --> 00:13:06,480
We've set up infrared lights
that the animals can't see, but cameras can.

134
00:13:06,646 --> 00:13:11,436
I'll be able to keep watch
from the safety of a side chamber.

135
00:13:13,646 --> 00:13:15,637
The bats are stirring,

136
00:13:15,806 --> 00:13:21,199
preparing to leave to search for their food
in the night skies outside.

137
00:13:22,406 --> 00:13:27,605
In a few minutes' time,
it'll be as dark outside as it is in here.

138
00:13:30,126 --> 00:13:32,515
Something is moving.

139
00:13:33,526 --> 00:13:38,646
Bushbuck. They're looking extremely nervous.

140
00:13:39,926 --> 00:13:42,918
That's why. There's a buffalo close by.

141
00:13:43,086 --> 00:13:44,804
(CHITTERS)

142
00:13:44,966 --> 00:13:49,562
They're only a few feet apart,
but they can't see one another.

143
00:13:58,086 --> 00:14:01,795
Remember, as far as it is concerned,

144
00:14:01,966 --> 00:14:04,560
it's in pitch blackness.

145
00:14:04,726 --> 00:14:08,799
It seems to be searching for something.

146
00:14:12,606 --> 00:14:17,202
It's eating. I can see its throat as it swallows.

147
00:14:17,366 --> 00:14:22,565
It's understandably
very nervous and apprehensive.

148
00:14:22,726 --> 00:14:24,921
It's licking salt.

149
00:14:29,806 --> 00:14:33,196
The bushbuck has heard something.

150
00:14:33,366 --> 00:14:35,960
It sounds like distant thunder.

151
00:14:41,446 --> 00:14:43,641
It's an elephant.

152
00:14:45,886 --> 00:14:48,446
(ELEPHANT RUMBLES)

153
00:14:54,446 --> 00:14:58,724
Every foot's being placed very carefully.

154
00:15:06,406 --> 00:15:10,194
He bumped his head. Well, no one's perfect.

155
00:15:19,486 --> 00:15:24,879
This deep rumble,
this resonating noise that's coming,

156
00:15:25,046 --> 00:15:29,039
is probably a signal to others
waiting outside the cave.

157
00:15:29,206 --> 00:15:31,322
He's by himself at the moment.

158
00:15:31,486 --> 00:15:34,683
(LOW-PITCHED RUMBLING)

159
00:15:37,526 --> 00:15:40,359
That's the picture at the cave mouth.

160
00:15:40,526 --> 00:15:43,677
The rest are climbing up to the entrance.

161
00:15:43,846 --> 00:15:47,805
How they're managing
this steep slope, I don't know.

162
00:15:49,446 --> 00:15:52,677
There's even a young calf among them.

163
00:16:02,686 --> 00:16:07,680
Maybe the male's rumbles
were messages to say that all is safe.

164
00:16:12,766 --> 00:16:17,556
They're following exactly
the same path that the male took.

165
00:16:17,726 --> 00:16:23,323
The female's using her trunk
to guide her calf over the cave floor.

166
00:16:31,006 --> 00:16:34,601
Has she detected one of our cameras?

167
00:16:36,966 --> 00:16:41,756
Maybe not, but they clearly know
where they're going.

168
00:16:47,606 --> 00:16:53,761
This passage is so narrow, the big male
can only just squeeze through.

169
00:17:02,726 --> 00:17:06,639
And now I can hear that noise.

170
00:17:06,806 --> 00:17:10,196
He's using his tusks to gouge out the salt.

171
00:17:10,366 --> 00:17:13,358
Of course, it's falling to the ground.

172
00:17:13,526 --> 00:17:18,122
So what he does now
is use his trunk to sniff it up.

173
00:17:18,286 --> 00:17:21,995
And then blow it into his mouth.
You can hear that.

174
00:17:22,166 --> 00:17:25,158
(SNIFFING)

175
00:17:26,406 --> 00:17:28,124
(BLOWING)

176
00:17:34,246 --> 00:17:37,795
Elephants must have come here for centuries,

177
00:17:37,966 --> 00:17:41,675
each generation deepening the cave
and passing to the next

178
00:17:41,846 --> 00:17:47,603
its knowledge of the route
through the darkness to the precious salt.

179
00:17:51,046 --> 00:17:55,881
So the marks near the entrance
were not made by ancient Egyptians,

180
00:17:56,046 --> 00:17:58,241
but by elephants.

181
00:18:12,326 --> 00:18:15,921
Could this cavern have been created by them?

182
00:18:18,486 --> 00:18:20,875
It's surely an extraordinary thing

183
00:18:21,046 --> 00:18:25,642
that elephants choose to come
to a cave, go into its depths,

184
00:18:25,806 --> 00:18:29,845
then travel for hundreds of yards
through total blackness.

185
00:18:30,006 --> 00:18:33,396
You couldn't have
a more dramatic demonstration

186
00:18:33,566 --> 00:18:36,558
of how important a mineral can be to an animal.

187
00:18:38,726 --> 00:18:42,719
So the demands of diet
have had the extraordinary effect

188
00:18:42,886 --> 00:18:46,401
of turning elephants into salt miners.

189
00:18:48,286 --> 00:18:51,084
Plants make themselves indigestible,

190
00:18:51,246 --> 00:18:54,238
defend themselves with spines and poisons

191
00:18:54,406 --> 00:19:00,959
and are so poor in nutriment that predators go
to great lengths to get dietary supplements.

192
00:19:01,846 --> 00:19:07,204
Yet, despite all this, plant-eating
mammals are a success story,

193
00:19:07,366 --> 00:19:12,963
nowhere more spectacularly so
than here on the open plains of Africa.

194
00:19:17,406 --> 00:19:21,399
Here plant predators gather
in unparalleled numbers,

195
00:19:21,566 --> 00:19:25,559
the greatest concentration
of mammals found on Earth.

196
00:19:25,726 --> 00:19:31,039
The leaves they seek are those
of one particular kind of plant, grass.

197
00:19:33,206 --> 00:19:38,405
The relationship between them
and their prey is very complex.

198
00:19:38,566 --> 00:19:42,559
Grass is not as passive as it might appear.

199
00:19:48,446 --> 00:19:53,042
The edges of its leaves
are armoured with tiny spines.

200
00:19:56,406 --> 00:20:00,194
Inside its tissues, there are needles of silica.

201
00:20:05,606 --> 00:20:09,997
The grazers, in response,
have developed countermeasures.

202
00:20:11,366 --> 00:20:14,836
They have teeth that grow continuously up

203
00:20:15,006 --> 00:20:17,566
just as fast as they're worn down...

204
00:20:18,406 --> 00:20:20,966
and they digest everything twice.

205
00:20:21,766 --> 00:20:27,682
Each mouthful goes down
into a multi-chambered stomach for processing,

206
00:20:27,846 --> 00:20:31,725
and is then brought up again
for further mastication.

207
00:20:31,886 --> 00:20:36,164
This second chewing can be done
at leisure and in relative safety.

208
00:20:36,326 --> 00:20:40,717
Instead of having your head down
to graze, you can now keep it up,

209
00:20:40,886 --> 00:20:43,275
watching out for danger.

210
00:20:45,086 --> 00:20:50,683
The leaves then go back for a final
treatment in a different chamber.

211
00:20:50,846 --> 00:20:56,557
What nutriment is left refertilises
the plants from which it came.

212
00:20:58,686 --> 00:21:03,760
But there is a season when the rains
stop and the grass shrivels.

213
00:21:04,486 --> 00:21:07,478
The grazers have to find food elsewhere.

214
00:21:15,326 --> 00:21:18,045
The annual migration has started.

215
00:21:31,566 --> 00:21:35,639
Such yearly compulsions
grip grazers all over the world.

216
00:21:35,806 --> 00:21:38,764
In Alaska, caribou also have to move

217
00:21:38,926 --> 00:21:42,521
to escape the privations of the Arctic winter.

218
00:21:50,646 --> 00:21:54,036
But wherever the plant predators travel,

219
00:21:54,206 --> 00:21:57,118
they are beset by animal predators.

220
00:22:14,926 --> 00:22:17,918
Only from the air can you get a real impression

221
00:22:18,086 --> 00:22:21,556
of the vast scale of these annual upheavals.

222
00:22:23,926 --> 00:22:27,919
Every year, millions of animals
travel hundreds of miles

223
00:22:28,086 --> 00:22:31,874
across burning hot plains and freezing cold tundra,

224
00:22:32,046 --> 00:22:36,164
but what is the real reason
for these risky journeys?

225
00:22:37,166 --> 00:22:41,159
Speed up the herds' movements
and a pattern appears.

226
00:22:43,726 --> 00:22:45,921
The wildebeest, for example,

227
00:22:46,086 --> 00:22:49,965
are following special trails in the grass.

228
00:22:57,126 --> 00:22:59,117
Grass may look the same,

229
00:22:59,286 --> 00:23:02,483
but it varies in one particular component

230
00:23:02,646 --> 00:23:06,685
that we now know is essential
for the survival of the wildebeest.

231
00:23:06,846 --> 00:23:08,837
Phosphorus.

232
00:23:12,086 --> 00:23:17,114
Wildebeest can detect which patch
of grass is rich in phosphorus,

233
00:23:17,286 --> 00:23:20,642
so they graze some parts and ignore others.

234
00:23:20,806 --> 00:23:23,798
They time their migration to arrive

235
00:23:23,966 --> 00:23:27,322
on the short grass plains of the Serengeti

236
00:23:27,486 --> 00:23:31,764
just as phosphorus-rich grasses
are beginning to sprout.

237
00:23:33,446 --> 00:23:39,203
Before long, this grass will also dry out,
and the herds will be forced to move again.

238
00:23:44,766 --> 00:23:47,564
Although wildebeest eat the leaves,

239
00:23:47,726 --> 00:23:52,117
they don't damage the stems,
so grass continues to sprout.

240
00:23:52,286 --> 00:23:57,076
A greater threat to its survival
comes from another plant -

241
00:23:57,246 --> 00:23:59,601
small acacia bushes.

242
00:23:59,766 --> 00:24:03,156
In due course, it may grow into a big tree.

243
00:24:03,326 --> 00:24:08,639
If it does, it will compete so effectively
with grass for natural resources

244
00:24:08,806 --> 00:24:12,242
that grass - and therefore grazers -
are driven away

245
00:24:12,446 --> 00:24:15,643
and the trees will extend their territory.

246
00:24:18,846 --> 00:24:21,804
But every plant has its predator.

247
00:24:21,966 --> 00:24:25,754
The dik-dik is the smallest antelope
on the plains,

248
00:24:25,926 --> 00:24:29,714
and it browses on the acacia's lowest leaves.

249
00:24:31,926 --> 00:24:35,839
Its pointed muzzle enables it to avoid the spines

250
00:24:36,006 --> 00:24:41,399
that protect the acacia's branches
from more wholesale browsers.

251
00:24:43,406 --> 00:24:46,443
The dik-dik is so small, it can't reach leaves

252
00:24:46,606 --> 00:24:49,723
more than a couple of feet above ground.

253
00:25:01,366 --> 00:25:04,244
Others attack the higher branches.

254
00:25:06,606 --> 00:25:09,803
The impala, with its larger muzzle
and longer neck,

255
00:25:09,966 --> 00:25:14,323
can reach three times higher
than the dik-dik.

256
00:25:20,046 --> 00:25:24,437
Having taken what they need,
the impala herd moves on.

257
00:25:33,926 --> 00:25:39,125
But the acacia has to withstand
the assault of another attacker.

258
00:25:44,126 --> 00:25:46,515
The gerenuk can crop leaves

259
00:25:46,686 --> 00:25:50,679
that are far beyond the reach
of even an impala.

260
00:25:58,966 --> 00:26:01,355
Its head is small for its height,

261
00:26:01,526 --> 00:26:05,405
so it can get in between the thorny branches,

262
00:26:05,566 --> 00:26:09,559
and its lips and tongue are particularly mobile.

263
00:26:11,686 --> 00:26:15,918
Standing erect demands special adaptations.

264
00:26:16,086 --> 00:26:18,998
The gerenuk's hip joints swivel so far

265
00:26:19,166 --> 00:26:24,160
that its backbone can swing right up
and continue the line of its hind legs.

266
00:26:24,326 --> 00:26:29,446
A feeding group has the grace
of a corps de ballet standing on their points.

267
00:26:58,166 --> 00:27:00,634
Gerenuks have to step aside

268
00:27:00,806 --> 00:27:05,004
when the world's tallest plant predator appears.

269
00:27:10,646 --> 00:27:12,637
The giraffe.

270
00:27:13,886 --> 00:27:16,684
They travel in groups of up to 30

271
00:27:16,846 --> 00:27:21,636
and launch their attacks from necks
that are seven feet long.

272
00:27:24,766 --> 00:27:30,238
The acacia's defences on its upper branches
would deter most browsers...

273
00:27:31,606 --> 00:27:35,599
but the giraffe's weaponry is formidable indeed.

274
00:27:36,926 --> 00:27:39,520
Its tongue is 18 inches long

275
00:27:39,686 --> 00:27:42,439
and so muscular that it has a grasp.

276
00:27:42,606 --> 00:27:47,361
The joint in its neck is so mobile
that its head can tip vertically upwards,

277
00:27:47,526 --> 00:27:52,122
and its lips are so leathery,
they're impervious to thorns.

278
00:27:52,286 --> 00:27:56,325
The acacia is under attack from bottom to top.

279
00:28:00,086 --> 00:28:05,080
With so many predators, you might think
that the march of the acacia

280
00:28:05,246 --> 00:28:09,444
would be held in check,
but the acacia has other plans -

281
00:28:09,606 --> 00:28:13,394
and they're revealed during the dry season.

282
00:28:21,566 --> 00:28:25,400
Throughout the year,
the acacia has tantalised animals

283
00:28:25,566 --> 00:28:29,479
with the chance of eating some,
but not all, of its leaves.

284
00:28:29,646 --> 00:28:33,639
And now that the time has come
to shed its seeds,

285
00:28:33,806 --> 00:28:37,594
that has ensured that
there's a wide range of animals around

286
00:28:37,766 --> 00:28:39,996
to pick them up and disperse them.

287
00:28:41,006 --> 00:28:44,601
Impala and other browsers can crunch the pods.

288
00:28:44,766 --> 00:28:49,840
The indigestible seeds will emerge
with the eater's droppings.

289
00:28:50,006 --> 00:28:53,476
Dik-dik might take them a few hundred yards;

290
00:28:53,646 --> 00:28:56,240
impala for a mile or so.

291
00:28:58,086 --> 00:29:02,284
Giraffes can transport seeds
for 10 miles or even more.

292
00:29:06,006 --> 00:29:10,443
But there is one predator
against which the acacia has no defence.

293
00:29:16,606 --> 00:29:21,077
Even the stoutest, sharpest spines
don't deter an elephant.

294
00:29:21,246 --> 00:29:25,364
It has a simple, devastating way
of getting the branches

295
00:29:25,526 --> 00:29:27,756
even a giraffe can't reach.

296
00:29:38,566 --> 00:29:39,885
(TRUMPETS)

297
00:29:45,646 --> 00:29:49,241
Its reward is a relatively spine-free meal.

298
00:29:49,406 --> 00:29:53,399
The acacia neglects to grow spines
on its topmost branches

299
00:29:53,566 --> 00:29:58,162
since they're beyond the reach
of most browsers.

300
00:29:59,166 --> 00:30:03,762
Elephants have a range of power tools
with which to collect meals.

301
00:30:03,926 --> 00:30:09,125
Tusk and trunk together can cut up
anything their owner fancies.

302
00:30:13,726 --> 00:30:18,038
The woodier a branch,
the more difficult it is to digest.

303
00:30:18,206 --> 00:30:20,800
Elephants have such vast stomachs

304
00:30:20,966 --> 00:30:25,960
that they can allow their meals
to stew there for about three days.

305
00:30:26,966 --> 00:30:32,165
The need for a big stomach may be
one reason they've grown so large,

306
00:30:32,326 --> 00:30:35,159
but being jumbo-sized
brings other advantages.

307
00:30:38,206 --> 00:30:42,518
Wherever there are plant-eaters,
there are meat-eaters.

308
00:30:44,606 --> 00:30:49,600
But even the biggest of them
is not big enough to tackle an elephant.

309
00:30:51,086 --> 00:30:54,078
Smaller plant-eaters are more vulnerable.

310
00:30:54,246 --> 00:30:56,965
How can they defend themselves?

311
00:30:58,846 --> 00:31:01,565
One way is to gather in large numbers,

312
00:31:01,726 --> 00:31:05,321
and that's what grazers do all over the world.

313
00:31:06,446 --> 00:31:11,566
If you live in a herd, many others
help you in detecting danger.

314
00:31:18,686 --> 00:31:23,680
Ears can be rotated to detect
sound from all directions.

315
00:31:31,806 --> 00:31:36,436
Sensitive noses can pick up
the first whiff of an enemy.

316
00:31:42,886 --> 00:31:48,279
Eyes with elongated pupils can keep
watch across the whole horizon.

317
00:31:49,566 --> 00:31:54,276
When heads go down,
eyes swivel in their sockets

318
00:31:54,446 --> 00:31:58,359
to ensure that the pupil stays horizontal.

319
00:31:58,526 --> 00:32:05,955
So even when you're grazing, you can still
keep an eye on what is watching you.

320
00:32:08,046 --> 00:32:10,958
With eyes on the side of your head,

321
00:32:11,126 --> 00:32:15,995
you can see in front and behind
at the same time.

322
00:32:16,766 --> 00:32:19,758
This really is wrap-around vision.

323
00:32:30,846 --> 00:32:33,804
Hunters' eyes, however, point directly ahead,

324
00:32:33,966 --> 00:32:38,323
giving them the ability to assess range.

325
00:32:39,286 --> 00:32:42,835
The targets have to hold their heads sideways

326
00:32:43,006 --> 00:32:46,965
if they're to keep an eye on the hunter.

327
00:32:49,446 --> 00:32:54,395
Sometimes the prey appears
to be stalking the predator.

328
00:33:00,206 --> 00:33:07,078
Grazers even taunt a hunter to make sure
that it can't launch a surprise attack.

329
00:33:07,246 --> 00:33:10,955
At this point, many hunters would give up,

330
00:33:11,126 --> 00:33:13,515
but not always.

331
00:33:13,686 --> 00:33:16,154
An attack is now imminent.

332
00:33:16,326 --> 00:33:18,715
Sound the alarm!

333
00:33:18,886 --> 00:33:21,684
(ANIMALS SHRIEK AND SNORT)

334
00:33:29,886 --> 00:33:32,684
Now the time has come to run.

335
00:33:44,726 --> 00:33:49,675
Herbivores have powerful hind legs,
giving them superlative acceleration.

336
00:33:50,446 --> 00:33:54,280
Invaluable if you're caught unawares.

337
00:34:08,766 --> 00:34:12,315
Once again, numbers bring safety.

338
00:34:20,726 --> 00:34:24,924
The North American pronghorn
is the second-fastest sprinter,

339
00:34:25,086 --> 00:34:29,079
but over long distances, it's the world champion.

340
00:34:35,046 --> 00:34:38,834
But all large herbivores
have to be able to run fast.

341
00:34:39,006 --> 00:34:44,797
They run on tiptoe,
so they cover more ground with each stride.

342
00:34:44,966 --> 00:34:48,242
Muscles are bunched at the top of the legs

343
00:34:48,406 --> 00:34:51,398
so that the limbs are streamlined.

344
00:34:53,286 --> 00:34:56,164
Some grazers flaunt their athleticism,

345
00:34:56,326 --> 00:35:01,719
as if to say ''I'm fit. Save your energy
and pick on someone weaker.''

346
00:35:03,886 --> 00:35:06,480
Cheetahs may be fast sprinters,

347
00:35:06,646 --> 00:35:10,434
but gazelles are better at dodging and jinking.

348
00:35:12,846 --> 00:35:16,805
Slimline legs, however, trip only too easily.

349
00:35:23,806 --> 00:35:27,924
Eyes on the side of your head
so you can't see forward

350
00:35:28,086 --> 00:35:30,475
can be catastrophic.

351
00:35:37,326 --> 00:35:40,921
Even so, herbivores
outmanoeuvre their enemies

352
00:35:41,086 --> 00:35:44,283
more often than you might suppose.

353
00:36:04,966 --> 00:36:10,518
Kicking hooves and thrusting horns
are formidable weapons.

354
00:36:12,486 --> 00:36:17,276
This mother is going to defend
her faun come what may.

355
00:36:30,726 --> 00:36:35,197
A buffalo has incautiously
strayed away from its herd.

356
00:36:36,286 --> 00:36:41,076
It surely can have no defence
against a group of lions.

357
00:36:42,086 --> 00:36:45,078
But the rest of the herd have noticed.

358
00:37:09,286 --> 00:37:13,723
Faced with the threat
of hundreds of tonnes of massed anger,

359
00:37:13,886 --> 00:37:16,354
the lioness turns tail.

360
00:37:18,246 --> 00:37:22,842
The male lion, however,
seems unwilling to give up.

361
00:37:39,086 --> 00:37:44,956
The buffalo, with their heavy
armament, have won this battle,

362
00:37:45,126 --> 00:37:48,198
but the war is a never-ending one.

363
00:38:01,806 --> 00:38:06,800
You might think these weapons are
just a defence against carnivores.

364
00:38:06,966 --> 00:38:12,836
But not so. Their primary use
is to fight one another.

365
00:38:15,766 --> 00:38:19,964
And that's the drawback of living in herds.

366
00:38:20,766 --> 00:38:22,757
The Badlands of North America.

367
00:38:22,926 --> 00:38:26,714
Bull bison are preparing for the annual rut.

368
00:38:30,126 --> 00:38:34,199
Only a few females are on heat
at any one time.

369
00:38:34,366 --> 00:38:39,156
Each male tries to sniff them out
before rivals approach.

370
00:38:40,286 --> 00:38:43,483
Males walk in parallel, assessing one another.

371
00:38:50,006 --> 00:38:52,042
(BISON ROARS)

372
00:38:52,206 --> 00:38:55,004
Pumped up with testosterone,

373
00:38:55,166 --> 00:38:58,317
they paw the ground
to show off their strength.

374
00:38:58,486 --> 00:39:02,081
They spray the earth with urine and roll in it

375
00:39:02,246 --> 00:39:05,238
so they reek of their own hormones.

376
00:39:27,846 --> 00:39:30,155
(BISON ROARS)

377
00:39:31,766 --> 00:39:34,758
This combination of rolling and roaring

378
00:39:34,926 --> 00:39:38,521
is a clear sign that there will be a fight.

379
00:39:43,806 --> 00:39:47,037
Most contests are resolved in seconds.

380
00:39:47,206 --> 00:39:50,198
A few escalate into full-scale battle.

381
00:39:52,406 --> 00:39:57,161
The attack is usually head on.
At full gallop, the impact is titanic -

382
00:39:57,326 --> 00:40:00,477
one tonne moving at 30mph meeting another

383
00:40:00,646 --> 00:40:03,638
coming in the opposite direction.

384
00:40:09,846 --> 00:40:14,283
This male is lucky to escape a fatal stabbing.

385
00:40:18,846 --> 00:40:22,043
The largest horns in proportion to body size

386
00:40:22,206 --> 00:40:25,801
are carried by American bighorn sheep.

387
00:40:27,406 --> 00:40:31,001
When armaments reach this size,
their indiscriminate deployment

388
00:40:31,166 --> 00:40:33,634
could be catastrophic.

389
00:40:33,806 --> 00:40:38,038
Smaller males can be warned off
with a simple kick.

390
00:40:45,566 --> 00:40:48,956
But closely-matched males will have to fight.

391
00:41:03,166 --> 00:41:08,035
The rules are strict.
Contestants must meet head on.

392
00:41:08,206 --> 00:41:12,563
If contact is unbalanced,
both fighters could break their necks.

393
00:41:22,766 --> 00:41:27,556
An impact like that would crush
a human skull like an eggshell.

394
00:41:27,726 --> 00:41:29,921
How does the bighorn survive?

395
00:41:30,086 --> 00:41:34,682
As you might expect, its skull is
reinforced internally with bone.

396
00:41:34,846 --> 00:41:38,156
It also has a number of hairline cracks in it,

397
00:41:38,326 --> 00:41:42,763
and these flex, so acting like shock absorbers.

398
00:41:46,086 --> 00:41:48,884
The bighorn's weapon is a battering ram,

399
00:41:49,046 --> 00:41:52,959
but there are also swords,
scimitars and daggers.

400
00:41:54,366 --> 00:41:57,324
All are ridged and pointed at the tips.

401
00:41:57,486 --> 00:42:01,479
Both those characteristics
have important functions.

402
00:42:03,726 --> 00:42:07,162
Before physical contact is made, the males,

403
00:42:07,326 --> 00:42:11,160
no matter what their species,
size one another up.

404
00:42:17,246 --> 00:42:20,238
If neither retreats, horns will clash.

405
00:42:24,886 --> 00:42:29,437
The gap between the horns
is always narrower than a single horn,

406
00:42:29,606 --> 00:42:35,283
so that it's not possible for a fighter
to strike his opponent on the skull.

407
00:42:37,646 --> 00:42:42,845
Having made contact, they wrestle.
The ridges' function becomes clear.

408
00:42:43,006 --> 00:42:47,796
They prevent slipping. Contestants
can thus test each other's strength.

409
00:42:53,886 --> 00:42:59,483
Now the pointed tips can be used
to stab a rival in the flank or belly.

410
00:43:04,246 --> 00:43:08,922
A competitor won't start a fight
if he's obviously outgunned,

411
00:43:09,086 --> 00:43:12,078
so horns are continually flaunted.

412
00:43:15,846 --> 00:43:19,555
These male topi are even putting on war paint.

413
00:43:25,966 --> 00:43:31,165
By plastering their horns with mud,
they're more intimidating to males

414
00:43:31,326 --> 00:43:34,318
and more attractive to females.

415
00:43:43,126 --> 00:43:48,359
Each in this gathering of several hundred
must establish territory.

416
00:43:50,326 --> 00:43:54,683
Females wander through the pasture,
perhaps sizing up the males.

417
00:43:56,846 --> 00:44:00,725
The males are torn between pursuing females

418
00:44:00,886 --> 00:44:06,483
and battling one another to establish
their individual stamping grounds.

419
00:44:20,326 --> 00:44:23,523
Again and again, a male has to fight.

420
00:44:41,206 --> 00:44:44,004
Eventually, each male has his own patch.

421
00:44:44,166 --> 00:44:47,442
The females decide which they like best

422
00:44:47,606 --> 00:44:51,599
and then present him with his reward.

423
00:45:00,926 --> 00:45:03,394
Mating rituals go on for days.

424
00:45:03,566 --> 00:45:07,400
Males stay in their territories
in case rivals claim them.

425
00:45:07,566 --> 00:45:11,718
They have to fight repeatedly
to maintain their ownership.

426
00:45:11,886 --> 00:45:16,437
As the days pass, they become
more exhausted and, eventually,

427
00:45:16,606 --> 00:45:19,200
they can barely stand.

428
00:45:34,606 --> 00:45:37,803
They're so tired, their defences are down.

429
00:45:37,966 --> 00:45:43,518
The most powerful males have territories
in the centre of the breeding ground.

430
00:45:44,926 --> 00:45:50,717
The weaker must accept those
on the fringes. Not a good place to be.

431
00:45:58,286 --> 00:46:03,076
In spite of the circling hyenas,
the males won't leave their territories.

432
00:46:03,246 --> 00:46:06,443
If they did, they'd have no chance of mating.

433
00:46:06,606 --> 00:46:11,077
They no longer have the strength
to confront the increasingly bold hyenas

434
00:46:11,246 --> 00:46:15,239
unless they themselves are actually attacked.

435
00:46:49,686 --> 00:46:53,474
For most of the year,
when the topi grazed in the herd,

436
00:46:53,646 --> 00:46:55,841
they kept watch for one another,

437
00:46:56,006 --> 00:46:59,999
but the competition to breed
has changed all that.

438
00:47:20,286 --> 00:47:23,881
The dangers of eating grass on the open plain

439
00:47:24,046 --> 00:47:26,844
led the topi to live in herds.

440
00:47:27,006 --> 00:47:30,999
Now the price for doing so is being paid

441
00:47:31,166 --> 00:47:33,555
by the weaker males.

442
00:47:36,806 --> 00:47:41,596
From the topi's battle to breed
to the great migrations of the world,

443
00:47:41,766 --> 00:47:46,157
the underground mines of Mount Elgon
and the shape and size

444
00:47:46,326 --> 00:47:48,920
of the creatures that made them -

445
00:47:49,086 --> 00:47:55,685
all these stem from the apparently
simple act of eating leaves.

446
00:47:55,846 --> 00:47:59,839
As always, in the life of mammals,
what you eat

447
00:48:00,006 --> 00:48:02,076
determines what you are.