"faces a continuous struggle." "Its prey is heavily armoured, often indigestible, sometimes even poisonous." "What makes this struggle between predator and prey more surprising is that the predators are elephants and the prey are plants." "These herds are the task force in a war that has been fought for millions of years and has produced some of the most complex and highly evolved relationships in the whole of the natural world." "In this tree, there is one of the most extraordinary plant predators." "It's one animal I don't need to sneak up on." "Boo!" "This extraordinary creature is half blind, half deaf, and this is just about as fast as it can move." "That's what can happen to you if you live on nothing but leaves." "It's a sloth." "It's not exactly an enthusiastic leaf-eater." "A couple of half-hearted chews and the leaves go straight down to its stomach." "Leaves, however, are not easily digested." "The sloth's technique is to give them time." "Then, eventually, this mobile compost heap pulls itself together and starts on a long, dangerous journey." "This is a very unusual sight." "A sloth in a hurry." "It wants to defecate." "The only place it's happy doing that is down on the ground." "It only does it about once a week, but why does it come down to do it?" "And why does it nearly always do so in exactly the same place?" "Whatever the reason, it must be important, for a sloth on the ground is almost helpless." "Any predator could attack it, and it doesn't have the speed to escape." "Why it comes down in this way is a mystery." "Nobody knows." "Now it's finished and back it goes up to the safety of the canopy." "Leaves are not very nutritious." "The sloth's way of compensating is not to eat more, but to do less." "Its claws hook over the branches so the sloth can hang without any effort of its thin muscles." "To save energy, it spends most of its time hanging around half asleep in the tree tops." "So, with little muscle and a reaction time only a quarter as fast as ours, how does a sloth's day compare with our day?" "While I write a few letters, the sloth manages to groom itself." "While we have our lunch, the sloth nibbles a few leaves." "Then, as we film a sequence for the series, it's time for another nap." "Not surprisingly, many mammals are dependent upon plants." "We live, after all, on a green planet." "Plants capture the energy they need from the sun, turning much of the Earth's surface into a vast and varied salad bowl." "The leaf's nutriment is locked away within a mesh of cellulose walls." "No mammal can digest cellulose." "Leaf-eaters rely on bacteria in their stomachs to break through this dense lattice." "The first broad-leaved trees appeared 100 million years ago." "They spread and, eventually, they formed lush rainforests" "like this one in South America." "And it was in places like this that early mammals first started to eat leaves in a wholesale way." "One of those plant predators, very little changed, still survives here today." "There's its track." "The prints are very fresh, so it could be quite close." "The animal I'm following is said to be as difficult to see as a jaguar." "I must be careful, because it's also said to be quite dangerous." "There it is." "This is the largest animal in the whole of the South American rainforests." "It's a tapir." "There's a female on the left and a half-grown calf on the right." "With a calf there, she could be a bit aggressive." "Better not get too close." "They're feeding on leaves." "In fact, most of their meals are made on leaves." "You'd think here, they've got more than enough to choose from, but they're extremely selective about which leaves they choose." "And you can see why." "Many of the leaves are protected by spines." "Branches and trunks are armoured, and spikes can inflict real damage." "Even plants which appear harmless may have defences if you look close enough." "Their tissues are loaded with poison, some of which are really powerful, such as strychnine." "Tapirs have ways of dealing with that problem." "They eat only a little of any one kind of leaf, then move on to another so they don't get a lethal dose of any one." "And they have another defence." "This riverbank is a special place that has been visited by tapirs over many generations." "It's eating earth, kaolin, a special kind of clay that binds to poisons, neutralising them before they cause any harm." "The kaolin is a medicine, one that we ourselves use for the same purpose when we have stomach ache." "So, in spite of all the defences plants have evolved, tapirs manage to find all the food they need in these forests." "This struggle between mammals and the plants they feed on is waged all over the world." "The Canadian Rocky Mountains." "The beginning of an autumn day." "A pika, a member of a small community that lives among the boulders bordering a mountain meadow where they all feed." "That's a warning call, telling other pikas that this patch is now taken." "Pikas start their foraging early." "They eat all of a plant - not just leaves, but the flowers as well." "Grazing in the open is dangerous." "There are eagles around, so pikas never stray very far from the safety of the rocks." "There seems to be plenty of food now, but soon snow will come, flowers will die back, and winter will be upon us." "What happens then when little is growing?" "Watch what happens to these if I leave them just there." "It's not eating my flowers, at least not yet." "It's stacking them in a special larder, creating a store that will last it through the hard days when this valley's covered in snow." "It'll need a stack several feet thick if it's to survive the winter." "Strangely, many of these leaves are extremely poisonous." "Why does it collect them?" "Well, the poison acts as a natural preservative and the leaves remain fresh until mid-winter." "The poison works to the pika's advantage." "But the pika's preparations are very subtle." "It takes care to collect a variety of plants." "Those with a little poison become edible quickly, whereas those with a lot will remain fresh until almost the end of winter." "Each pika makes several hundred trips a day," "Iiterally making hay while the sun shines." "Sometimes the problem is not what's in your food, but what is not." "Dealing with dietary deficiencies has had a dramatic outcome here on the flanks of Mount Elgon in East Africa." "The first Europeans to visit these caves noticed marks like these in the walls." "They imagined they'd been made by ancient Egyptians who came here to mine for gold and precious stones." "These grooves do look like the marks made by a pickaxe." "But to discover what actually made them, you have to wait till nightfall." "We've set up infrared lights that the animals can't see, but cameras can." "I'll be able to keep watch from the safety of a side chamber." "The bats are stirring, preparing to leave to search for their food in the night skies outside." "In a few minutes' time, it'll be as dark outside as it is in here." "Something is moving." "Bushbuck." "They're looking extremely nervous." "That's why." "There's a buffalo close by." "They're only a few feet apart, but they can't see one another." "Remember, as far as it is concerned, it's in pitch blackness." "It seems to be searching for something." "It's eating." "I can see its throat as it swallows." "It's understandably very nervous and apprehensive." "It's licking salt." "The bushbuck has heard something." "It sounds like distant thunder." "It's an elephant." "Every foot's being placed very carefully." "He bumped his head." "Well, no one's perfect." "This deep rumble, this resonating noise that's coming, is probably a signal to others waiting outside the cave." "He's by himself at the moment." "That's the picture at the cave mouth." "The rest are climbing up to the entrance." "How they're managing this steep slope, I don't know." "There's even a young calf among them." "Maybe the male's rumbles were messages to say that all is safe." "They're following exactly the same path that the male took." "The female's using her trunk to guide her calf over the cave floor." "Has she detected one of our cameras?" "Maybe not, but they clearly know where they're going." "This passage is so narrow, the big male can only just squeeze through." "And now I can hear that noise." "He's using his tusks to gouge out the salt." "Of course, it's falling to the ground." "So what he does now is use his trunk to sniff it up." "And then blow it into his mouth." "You can hear that." "Elephants must have come here for centuries, each generation deepening the cave and passing to the next its knowledge of the route through the darkness to the precious salt." "So the marks near the entrance were not made by ancient Egyptians, but by elephants." "Could this cavern have been created by them?" "It's surely an extraordinary thing that elephants choose to come to a cave, go into its depths, then travel for hundreds of yards through total blackness." "You couldn't have a more dramatic demonstration of how important a mineral can be to an animal." "So the demands of diet have had the extraordinary effect of turning elephants into salt miners." "Plants make themselves indigestible, defend themselves with spines and poisons and are so poor in nutriment that predators go to great lengths to get dietary supplements." "Yet, despite all this, plant-eating mammals are a success story, nowhere more spectacularly so than here on the open plains of Africa." "Here plant predators gather in unparalleled numbers, the greatest concentration of mammals found on Earth." "The leaves they seek are those of one particular kind of plant, grass." "The relationship between them and their prey is very complex." "Grass is not as passive as it might appear." "The edges of its leaves are armoured with tiny spines." "Inside its tissues, there are needles of silica." "The grazers, in response, have developed countermeasures." "They have teeth that grow continuously up just as fast as they're worn down... and they digest everything twice." "Each mouthful goes down into a multi-chambered stomach for processing, and is then brought up again for further mastication." "This second chewing can be done at leisure and in relative safety." "Instead of having your head down to graze, you can now keep it up, watching out for danger." "The leaves then go back for a final treatment in a different chamber." "What nutriment is left refertilises the plants from which it came." "But there is a season when the rains stop and the grass shrivels." "The grazers have to find food elsewhere." "The annual migration has started." "Such yearly compulsions grip grazers all over the world." "In Alaska, caribou also have to move to escape the privations of the Arctic winter." "But wherever the plant predators travel, they are beset by animal predators." "Only from the air can you get a real impression of the vast scale of these annual upheavals." "Every year, millions of animals travel hundreds of miles across burning hot plains and freezing cold tundra, but what is the real reason for these risky journeys?" "Speed up the herds' movements and a pattern appears." "The wildebeest, for example, are following special trails in the grass." "Grass may look the same, but it varies in one particular component that we now know is essential for the survival of the wildebeest." "Phosphorus." "Wildebeest can detect which patch of grass is rich in phosphorus, so they graze some parts and ignore others." "They time their migration to arrive on the short grass plains of the Serengeti just as phosphorus-rich grasses are beginning to sprout." "Before long, this grass will also dry out, and the herds will be forced to move again." "Although wildebeest eat the leaves, they don't damage the stems, so grass continues to sprout." "A greater threat to its survival comes from another plant - small acacia bushes." "In due course, it may grow into a big tree." "If it does, it will compete so effectively with grass for natural resources that grass - and therefore grazers - are driven away and the trees will extend their territory." "But every plant has its predator." "The dik-dik is the smallest antelope on the plains, and it browses on the acacia's lowest leaves." "Its pointed muzzle enables it to avoid the spines that protect the acacia's branches from more wholesale browsers." "The dik-dik is so small, it can't reach leaves more than a couple of feet above ground." "Others attack the higher branches." "The impala, with its larger muzzle and longer neck, can reach three times higher than the dik-dik." "Having taken what they need, the impala herd moves on." "But the acacia has to withstand the assault of another attacker." "The gerenuk can crop leaves that are far beyond the reach of even an impala." "Its head is small for its height, so it can get in between the thorny branches, and its lips and tongue are particularly mobile." "Standing erect demands special adaptations." "The gerenuk's hip joints swivel so far that its backbone can swing right up and continue the line of its hind legs." "A feeding group has the grace of a corps de ballet standing on their points." "Gerenuks have to step aside when the world's tallest plant predator appears." "The giraffe." "They travel in groups of up to 30 and launch their attacks from necks that are seven feet long." "The acacia's defences on its upper branches would deter most browsers... but the giraffe's weaponry is formidable indeed." "Its tongue is 18 inches long and so muscular that it has a grasp." "The joint in its neck is so mobile that its head can tip vertically upwards, and its lips are so leathery, they're impervious to thorns." "The acacia is under attack from bottom to top." "With so many predators, you might think that the march of the acacia would be held in check, but the acacia has other plans - and they're revealed during the dry season." "Throughout the year, the acacia has tantalised animals with the chance of eating some, but not all, of its leaves." "And now that the time has come to shed its seeds, that has ensured that there's a wide range of animals around to pick them up and disperse them." "Impala and other browsers can crunch the pods." "The indigestible seeds will emerge with the eater's droppings." "Dik-dik might take them a few hundred yards;" "impala for a mile or so." "Giraffes can transport seeds for 10 miles or even more." "But there is one predator against which the acacia has no defence." "Even the stoutest, sharpest spines don't deter an elephant." "It has a simple, devastating way of getting the branches even a giraffe can't reach." "Its reward is a relatively spine-free meal." "The acacia neglects to grow spines on its topmost branches since they're beyond the reach of most browsers." "Elephants have a range of power tools with which to collect meals." "Tusk and trunk together can cut up anything their owner fancies." "The woodier a branch, the more difficult it is to digest." "Elephants have such vast stomachs that they can allow their meals to stew there for about three days." "The need for a big stomach may be one reason they've grown so large, but being jumbo-sized brings other advantages." "Wherever there are plant-eaters, there are meat-eaters." "But even the biggest of them is not big enough to tackle an elephant." "Smaller plant-eaters are more vulnerable." "How can they defend themselves?" "One way is to gather in large numbers, and that's what grazers do all over the world." "If you live in a herd, many others help you in detecting danger." "Ears can be rotated to detect sound from all directions." "Sensitive noses can pick up the first whiff of an enemy." "Eyes with elongated pupils can keep watch across the whole horizon." "When heads go down, eyes swivel in their sockets to ensure that the pupil stays horizontal." "So even when you're grazing, you can still keep an eye on what is watching you." "With eyes on the side of your head, you can see in front and behind at the same time." "This really is wrap-around vision." "Hunters' eyes, however, point directly ahead, giving them the ability to assess range." "The targets have to hold their heads sideways if they're to keep an eye on the hunter." "Sometimes the prey appears to be stalking the predator." "Grazers even taunt a hunter to make sure that it can't launch a surprise attack." "At this point, many hunters would give up, but not always." "An attack is now imminent." "Sound the alarm!" "Now the time has come to run." "Herbivores have powerful hind legs, giving them superlative acceleration." "Invaluable if you're caught unawares." "Once again, numbers bring safety." "The North American pronghorn is the second-fastest sprinter, but over long distances, it's the world champion." "But all large herbivores have to be able to run fast." "They run on tiptoe, so they cover more ground with each stride." "Muscles are bunched at the top of the legs so that the limbs are streamlined." "Some grazers flaunt their athleticism, as if to say "I'm fit." "Save your energy and pick on someone weaker."" "Cheetahs may be fast sprinters, but gazelles are better at dodging and jinking." "Slimline legs, however, trip only too easily." "Eyes on the side of your head so you can't see forward can be catastrophic." "Even so, herbivores outmanoeuvre their enemies more often than you might suppose." "Kicking hooves and thrusting horns are formidable weapons." "This mother is going to defend her faun come what may." "A buffalo has incautiously strayed away from its herd." "It surely can have no defence against a group of lions." "But the rest of the herd have noticed." "Faced with the threat of hundreds of tonnes of massed anger, the lioness turns tail." "The male lion, however, seems unwilling to give up." "The buffalo, with their heavy armament, have won this battle, but the war is a never-ending one." "You might think these weapons are just a defence against carnivores." "But not so." "Their primary use is to fight one another." "And that's the drawback of living in herds." "The Badlands of North America." "Bull bison are preparing for the annual rut." "Only a few females are on heat at any one time." "Each male tries to sniff them out before rivals approach." "Males walk in parallel, assessing one another." "Pumped up with testosterone, they paw the ground to show off their strength." "They spray the earth with urine and roll in it so they reek of their own hormones." "This combination of rolling and roaring is a clear sign that there will be a fight." "Most contests are resolved in seconds." "A few escalate into full-scale battle." "The attack is usually head on." "At full gallop, the impact is titanic - one tonne moving at 30mph meeting another coming in the opposite direction." "This male is lucky to escape a fatal stabbing." "The largest horns in proportion to body size are carried by American bighorn sheep." "When armaments reach this size, their indiscriminate deployment could be catastrophic." "Smaller males can be warned off with a simple kick." "But closely-matched males will have to fight." "The rules are strict." "Contestants must meet head on." "If contact is unbalanced, both fighters could break their necks." "An impact like that would crush a human skull like an eggshell." "How does the bighorn survive?" "As you might expect, its skull is reinforced internally with bone." "It also has a number of hairline cracks in it, and these flex, so acting like shock absorbers." "The bighorn's weapon is a battering ram, but there are also swords, scimitars and daggers." "All are ridged and pointed at the tips." "Both those characteristics have important functions." "Before physical contact is made, the males, no matter what their species, size one another up." "If neither retreats, horns will clash." "The gap between the horns is always narrower than a single horn, so that it's not possible for a fighter to strike his opponent on the skull." "Having made contact, they wrestle." "The ridges' function becomes clear." "They prevent slipping." "Contestants can thus test each other's strength." "Now the pointed tips can be used to stab a rival in the flank or belly." "A competitor won't start a fight if he's obviously outgunned, so horns are continually flaunted." "These male topi are even putting on war paint." "By plastering their horns with mud, they're more intimidating to males and more attractive to females." "Each in this gathering of several hundred must establish territory." "Females wander through the pasture, perhaps sizing up the males." "The males are torn between pursuing females and battling one another to establish their individual stamping grounds." "Again and again, a male has to fight." "Eventually, each male has his own patch." "The females decide which they like best and then present him with his reward." "Mating rituals go on for days." "Males stay in their territories in case rivals claim them." "They have to fight repeatedly to maintain their ownership." "As the days pass, they become more exhausted and, eventually, they can barely stand." "They're so tired, their defences are down." "The most powerful males have territories in the centre of the breeding ground." "The weaker must accept those on the fringes." "Not a good place to be." "In spite of the circling hyenas, the males won't leave their territories." "If they did, they'd have no chance of mating." "They no longer have the strength to confront the increasingly bold hyenas unless they themselves are actually attacked." "For most of the year, when the topi grazed in the herd, they kept watch for one another, but the competition to breed has changed all that." "The dangers of eating grass on the open plain" "led the topi to live in herds." "Now the price for doing so is being paid by the weaker males." "From the topi's battle to breed to the great migrations of the world, the underground mines of Mount Elgon and the shape and size of the creatures that made them - all these stem from the apparently simple act of eating leaves." "As always, in the life of mammals, what you eat determines what you are."