0:00:34 > 0:00:37Morning in the rainforest of Panama.
0:00:43 > 0:00:48Capuchin monkeys are having a rather wasteful breakfast.
0:00:57 > 0:01:02Inside these nuts, there is a kernel that is very, very good eating.
0:01:02 > 0:01:06But the tree has protected them with such a hard shell
0:01:06 > 0:01:11it's almost impossible to get at the kernel inside,
0:01:11 > 0:01:13even if I hit it with a stone.
0:01:18 > 0:01:20Unbroken!
0:01:20 > 0:01:24So it is hard to believe, but nonetheless true,
0:01:24 > 0:01:31that there's an animal in these Central American forests that can open these nuts with its bare teeth.
0:01:32 > 0:01:36But it's certainly NOT these monkeys.
0:01:36 > 0:01:41The best they can do is to scrape off the thin, fleshy coating.
0:01:41 > 0:01:45The nut itself, they simply throw away.
0:01:50 > 0:01:55But another animal is listening for that telltale sound.
0:01:55 > 0:01:58GENTLE THUD AND RUSTLE
0:01:59 > 0:02:01It's an agouti.
0:02:05 > 0:02:11WHISPERS: The agouti is certainly no bigger or stronger than a monkey,
0:02:11 > 0:02:17but it has the right tools to deal with ANY nut...
0:02:19 > 0:02:26..a pair of extremely strong and sharp front teeth that can cut through even the toughest casing.
0:02:30 > 0:02:36Agoutis belong to a large and highly successful group of mammals, the rodents.
0:02:36 > 0:02:42And rodents specialise in being able to chisel their way through almost anything.
0:02:52 > 0:02:57But what is so special about an agouti's front teeth
0:02:57 > 0:03:02that enables them to cut a hole in the toughest of nuts?
0:03:02 > 0:03:06They have a layer of strong enamel, but only on the front surface.
0:03:06 > 0:03:11Behind it, the body of the tooth is made of a softer material - dentine.
0:03:11 > 0:03:16Gnawing something hard wears down the dentine faster than the enamel,
0:03:16 > 0:03:21leaving an edge as sharp as a carpenter's chisel.
0:03:21 > 0:03:27And unlike the front teeth of many other mammals, a rodent's grow continuously,
0:03:27 > 0:03:29replacing what gets worn away.
0:03:32 > 0:03:35Armed with these formidable teeth,
0:03:35 > 0:03:41the rodents have become the most successful and numerous of all mammals.
0:03:41 > 0:03:46The largest group of them are the rats and mice.
0:03:46 > 0:03:54There are 1,300 species of those and they are ALL accomplished stealers of seeds.
0:03:58 > 0:04:03Other rodents use their teeth to cut up vegetation.
0:04:03 > 0:04:05Some even tackle wood.
0:04:12 > 0:04:15The rodent body is very versatile
0:04:15 > 0:04:20and has evolved into an astonishing variety of shapes and sizes
0:04:20 > 0:04:23to suit many different lifestyles.
0:04:28 > 0:04:32So, you might find a rodent almost anywhere.
0:04:34 > 0:04:39Some are amazingly nimble tree-climbers - the squirrels.
0:04:39 > 0:04:44They use their teeth to open cones and collect seeds that few other mammals can reach.
0:04:51 > 0:04:56Abert squirrels use their teeth, not only to cut into cones,
0:04:56 > 0:05:01but to nip off the tips of the branches of ponderosa pines,
0:05:01 > 0:05:06where the underlying bark is particularly nutritious.
0:05:11 > 0:05:18This is a valuable food reserve if the pine cones run out during the long northern winters.
0:05:18 > 0:05:22Abert squirrels are now such specialised feeders
0:05:22 > 0:05:27that they can only survive where there are plenty of ponderosa pines.
0:05:27 > 0:05:34Most squirrels, however, have more general tastes and high on their menus are often acorns.
0:05:41 > 0:05:46Oak trees usually produce a heavy crop of acorns each autumn.
0:05:49 > 0:05:54This is good news for squirrels like these greys,
0:05:54 > 0:05:59because they must eat a great deal to fatten themselves up for winter.
0:06:07 > 0:06:12They must also bury some to eat later on when times get hard.
0:06:18 > 0:06:24But for squirrels here in the woodlands of the eastern United States, in Virginia,
0:06:24 > 0:06:27things are not quite so simple.
0:06:27 > 0:06:31Here there are two kinds of oaks - red oaks and white oaks -
0:06:31 > 0:06:34and their acorns are very similar.
0:06:34 > 0:06:37This is the acorn of a white oak.
0:06:37 > 0:06:42And this, a red oak. This one is just slightly darker.
0:06:42 > 0:06:50But the acorns of the white oak germinate almost immediately, using up their food supply.
0:06:50 > 0:06:55The red oaks, on the other hand, don't germinate until next spring.
0:06:55 > 0:07:01The squirrels recognise the difference between the two and treat them differently.
0:07:04 > 0:07:09When a squirrel finds a white acorn, it always eats it immediately,
0:07:09 > 0:07:13because otherwise it would soon germinate.
0:07:19 > 0:07:26On the other hand, red acorns, like this one, are almost always buried as a store for winter.
0:07:30 > 0:07:37Squirrels are colour-blind, so they can't recognise the difference between the two acorns by sight.
0:07:37 > 0:07:45It's the smell that tells them which should be eaten straightaway and which should be buried for later.
0:07:45 > 0:07:49Some years, the white oaks produce their usual bonanza of acorns,
0:07:49 > 0:07:54while the red oaks provide hardly any at all.
0:07:54 > 0:07:58When this happens - as now - the squirrels change their tactics.
0:07:58 > 0:08:03You might think this one is tucking into a white acorn as usual,
0:08:03 > 0:08:08but after only a quick nibble, it takes it away and buries it.
0:08:08 > 0:08:10What's it up to?
0:08:20 > 0:08:27It has neatly cut away the tip, beneath which lies the embryo of the seed.
0:08:27 > 0:08:31And once that has gone, the acorn will never germinate.
0:08:31 > 0:08:36But its food store will remain fresh throughout winter for the squirrel.
0:08:36 > 0:08:41So, with dexterous paws to hold the acorn
0:08:41 > 0:08:47and sharp front teeth to cut out the embryo with the precision of a surgeon,
0:08:47 > 0:08:53the squirrels get the best possible value out of these apparently well-protected nuts.
0:08:57 > 0:09:02Seed-producing plants grow pretty well everywhere,
0:09:02 > 0:09:07so, potentially, seed-eating rodents can live everywhere, too.
0:09:07 > 0:09:09But there is one problem.
0:09:09 > 0:09:17Although all seeds are full of nutriment, most of them are small, like these from these desert plants.
0:09:17 > 0:09:23You'd need about 500 of these to make the food equivalent of a typical nut like an acorn.
0:09:26 > 0:09:33Any rodent wanting to store the tiny seeds produced by plants like these faces a transport problem.
0:09:33 > 0:09:39It would be hopelessly impractical to carry them away one by one, as squirrels carry acorns.
0:09:41 > 0:09:47Here in California's Mojave Desert, there is a little rodent that has solved that problem,
0:09:47 > 0:09:49and its burrows are all around me.
0:09:53 > 0:09:56Like most desert animals,
0:09:56 > 0:10:02the creature that owns this hole usually doesn't come out until after dark, when it's cooler.
0:10:02 > 0:10:07But if I scatter a few seeds around its entrance,
0:10:07 > 0:10:12maybe that will tempt him to come out a little earlier.
0:10:13 > 0:10:16Well, as it happens, it didn't.
0:10:16 > 0:10:23It wasn't until much later, when the moon was up, that the burrow's owner eventually showed itself.
0:10:25 > 0:10:27A kangaroo rat.
0:10:31 > 0:10:38It can carry hundreds of seeds at one time because it has expandable cheek pouches.
0:10:38 > 0:10:42The pink toes, poking out from the fur,
0:10:42 > 0:10:47are those of its back legs, which are huge and support its whole body.
0:10:47 > 0:10:51That's why it is called a kangaroo rat.
0:10:51 > 0:10:56Its front legs, hidden beneath its chin, are shovelling the seeds into its mouth.
0:10:56 > 0:11:03Normally a kangaroo rat would have to travel quite a long way to find enough seeds to fill its pouches.
0:11:03 > 0:11:06But because of what I have supplied,
0:11:06 > 0:11:10this one's shopping bags are soon full enough to take to its burrow.
0:11:16 > 0:11:19There we are!
0:11:19 > 0:11:23And it covers them over with sand, just to keep them safe.
0:11:31 > 0:11:35Its deep burrow is a relatively safe place
0:11:35 > 0:11:40and the kangaroo rat sleeps here throughout the day and much of the night.
0:11:40 > 0:11:46Even so, the slightest suspicious sound from outside will wake it.
0:11:46 > 0:11:50WHISHING
0:11:50 > 0:11:54A gopher snake is crawling towards the burrow.
0:11:54 > 0:11:59It might find its way inside. It had better be seen off.
0:12:02 > 0:12:04Foot stamping is the first warning.
0:12:10 > 0:12:13That seems to have no effect.
0:12:13 > 0:12:18Perhaps kicking sand in its face will do the trick.
0:12:19 > 0:12:22The snake doesn't like that at all.
0:12:26 > 0:12:29Victory to the kangaroo rat.
0:12:32 > 0:12:37Seeds, of course, are not the only part of a plant that is edible.
0:12:37 > 0:12:44Some rodents, like these marmots that live in the European Alps, eat the entire thing.
0:12:46 > 0:12:48And they fight over grazing rights.
0:12:54 > 0:13:00When Alpine marmots come out of their burrows in late spring, after their winter sleep,
0:13:00 > 0:13:07they immediately set about re-establishing their territorial boundaries.
0:13:07 > 0:13:13Calling and tail-flagging are only the preliminaries. WHISTLING CALL
0:13:13 > 0:13:18If that doesn't work, there is nothing for it but to go to battle.
0:13:23 > 0:13:27The senior males do most of the fighting,
0:13:27 > 0:13:33while their female partners guard the family burrow and watch. CALLING
0:13:43 > 0:13:46When a male wins a territorial scrap,
0:13:46 > 0:13:53he marks the boundaries just established with scent from a gland on his cheek.
0:14:03 > 0:14:10Marmots form strong pair bonds, and the females usually only mate with their one permanent partner.
0:14:10 > 0:14:18By late spring, a marmot family consists of the adult pair together with a few of last year's offspring,
0:14:18 > 0:14:20all of them females.
0:14:20 > 0:14:25Last year's males have already been chased away.
0:14:25 > 0:14:32Although daughters are allowed to stay, their mothers constantly beat them up, particularly in spring.
0:14:33 > 0:14:41They persecute them so severely that, if any of the youngsters have become pregnant, they abort.
0:14:41 > 0:14:48The reason for this strange and merciless behaviour doesn't become apparent until later in the year.
0:14:51 > 0:14:56By midsummer, relative peace has come to the mountainside.
0:14:56 > 0:15:02This season's babies are about to emerge from the maternity burrow for the first time.
0:15:02 > 0:15:07There are usually between four and six babies in a litter
0:15:07 > 0:15:13and they immediately get down to the important business of grazing.
0:15:15 > 0:15:18Alpine summers are short,
0:15:18 > 0:15:23so marmots have to feed as fast as they can, while they can.
0:15:23 > 0:15:29Their incisors slice with ease through the tough stalks of flowers and grasses.
0:15:30 > 0:15:34The babies put on weight rapidly, but even so,
0:15:34 > 0:15:41by the end of the summer, they still do not have fat reserves to match those of their parents.
0:15:41 > 0:15:46In the autumn, the family start on their final harvest -
0:15:46 > 0:15:52hay to line their deep burrow, in which they will sleep through the winter.
0:16:19 > 0:16:23By October, the weather has turned really cold.
0:16:23 > 0:16:30Down in the burrow, the marmots' body temperature has dropped to 2 degrees centigrade
0:16:30 > 0:16:36and their hearts are beating only two or three times a minute. They are hibernating.
0:16:36 > 0:16:43They are sustained only by their fat reserves which will have to last them until the following April.
0:16:46 > 0:16:51They snuggle together to minimise the loss of heat.
0:16:51 > 0:16:55The youngsters are always in the middle of the pile.
0:16:55 > 0:17:02With less body weight, they can't afford to let their temperatures drop as low as the adults.
0:17:02 > 0:17:05The thermal-imaging camera
0:17:05 > 0:17:12shows that a baby, glowing orange, is several degrees warmer than the adults, which show green.
0:17:12 > 0:17:16The burrow, which registers blue, is well below freezing point.
0:17:18 > 0:17:24The adults couldn't keep more than one litter of youngsters warm enough throughout the winter.
0:17:24 > 0:17:31That is why the dominant female made quite sure that none of her youngsters produced any babies.
0:17:34 > 0:17:40But hibernating isn't the only way to survive the winter in the mountains.
0:17:44 > 0:17:51Here in North America, the winters can be just as severe as they are in the high Alps.
0:17:51 > 0:17:57Yet here there is a rodent that manages to find food throughout the winter months,
0:17:57 > 0:18:02and it does so with an extremely ingenious device - a refrigerator!
0:18:02 > 0:18:06A pool of deep, cold water like this.
0:18:06 > 0:18:11And over there is its builder and owner...
0:18:14 > 0:18:18..a beaver, one of a family that lives here in this lake
0:18:18 > 0:18:21at the foot of the Teton Mountains in Wyoming.
0:18:25 > 0:18:32While beavers can get around perfectly well on land, they are most at home in the water,
0:18:32 > 0:18:40where their webbed hind feet and paddle-like tail make them powerful swimmers above and below the surface.
0:18:51 > 0:18:57Like marmots, beavers feed on all kinds of vegetation,
0:18:57 > 0:18:59and eat wood as well as leaves.
0:19:01 > 0:19:07And they are accomplished engineers - this great pond is entirely their own creation.
0:19:10 > 0:19:17Only a few years ago, this shallow, pebbly stream flowed straight down the valley.
0:19:17 > 0:19:22Then a family of beavers moved in and built a dam.
0:19:25 > 0:19:29The main body of it is built of boulders.
0:19:29 > 0:19:35On the downstream side, it has been lined with logs, some of them big and quite heavy.
0:19:35 > 0:19:40And on this side, it has been packed with mud and vegetation.
0:19:40 > 0:19:45It has been built so accurately that it is, to within a few inches,
0:19:45 > 0:19:51horizontal across its entire length of about 150 yards from one side to the other.
0:19:51 > 0:19:56And the lake it has created stretches upstream for almost a mile.
0:20:04 > 0:20:07So important is their dam to them,
0:20:07 > 0:20:14that if they detect the slightest leak - usually by hearing the sound of trickling water -
0:20:14 > 0:20:16they start repair work immediately.
0:20:31 > 0:20:34Mud is needed as well as logs.
0:20:48 > 0:20:53The repair team will labour away until the leak is fully repaired.
0:20:56 > 0:21:02Maintaining the water at a high level brings the beavers several advantages,
0:21:02 > 0:21:06one of which is that it floods the surrounding woodlands
0:21:06 > 0:21:11and so enables them to swim in safety to their main source of food.
0:21:17 > 0:21:24They increase the distance they can swim by digging channels that lead into the very heart of the woodland.
0:21:32 > 0:21:39Here they can use their sharp incisor teeth to strip off the bark from a fallen tree trunk,
0:21:39 > 0:21:46while still being close enough to water to slip away should a bear or a mountain lion turn up.
0:22:17 > 0:22:24Their network of channels also enables them to ferry whole branches back to their pond.
0:22:39 > 0:22:43And there, where the water is deepest,
0:22:43 > 0:22:49they dive down and push each branch firmly into the mud at the bottom.
0:22:51 > 0:22:53This is the beavers' fridge,
0:22:53 > 0:23:00where the vegetation will keep fresh through the long winter when the pond is covered with ice.
0:23:06 > 0:23:10Stocking the fridge takes a lot of work
0:23:10 > 0:23:14and the beavers are at their busiest in autumn.
0:24:05 > 0:24:09At one side of the lake, stands their lodge -
0:24:09 > 0:24:15a fortress built of branches and boulders that is so strong that not even a bear could break into it.
0:24:15 > 0:24:20The only entrance is through tunnels that open underwater
0:24:20 > 0:24:26and the beavers take refuge here whenever they are alarmed.
0:24:34 > 0:24:39That was a warning signal to say that danger was around - that is me.
0:24:39 > 0:24:44And now I may not see the beavers for some time.
0:24:44 > 0:24:49They can stay underwater for five minutes at a time - up to 15, if they need to.
0:24:49 > 0:24:56They can get back to the safety of their lodge without putting their head above water for a single second.
0:25:00 > 0:25:05Most lodges have at least two different entrances.
0:25:11 > 0:25:18By October, winter is well under way, but whereas marmots would now be hibernating,
0:25:18 > 0:25:24the beavers are still active and will remain that way throughout the winter.
0:25:24 > 0:25:28Even when the pond ices over completely,
0:25:28 > 0:25:34they are still able to swim under the ice to get back and forth to their lodge.
0:25:34 > 0:25:39No-one knew exactly what went on inside the lodge during winter,
0:25:39 > 0:25:46so when the beavers were away, we installed a couple of infrared cameras in order to find out.
0:25:50 > 0:25:57A branch from the fridge is being brought back to the lodge for the whole family to feed on.
0:26:00 > 0:26:02And another.
0:26:05 > 0:26:10No wonder they don't need to hibernate, with this ingenious set-up.
0:26:10 > 0:26:14The lodge is warm and safe, even in midwinter,
0:26:14 > 0:26:22and the only sign of activity in the snug home beneath the snow is hot air rising from the vent at the top.
0:26:29 > 0:26:36Inside, our cameras catch a glimpse of what, at first sight, looks like a very small beaver.
0:26:39 > 0:26:43It's a muskrat. There are a pair of them in here.
0:26:43 > 0:26:46This is a new observation.
0:26:46 > 0:26:53Do the beavers actually know, in the pitch blackness, that there are strangers among them?
0:26:59 > 0:27:06We noticed that the muskrats regularly left the lodge to forage under the ice.
0:27:06 > 0:27:12And on several occasions, they returned a few minutes later with a load of fresh reeds.
0:27:12 > 0:27:19Perhaps the muskrats are paying rent by regularly providing fresh bedding for the lodge.
0:27:29 > 0:27:37Maybe that is why the beavers accept them, and even allow them to share their food.
0:27:48 > 0:27:53Our infrared lights, however, are no longer welcome, it seems.
0:27:55 > 0:27:59A lodge makes a very safe home for a beaver.
0:27:59 > 0:28:05Other rodents take more active steps to defend themselves, and one is positively dangerous.
0:28:07 > 0:28:11Darkness provides one of the best forms of defence
0:28:11 > 0:28:13and it is very dark right now.
0:28:13 > 0:28:21The only reason you can see me in the middle of the African night is that we are using a starlight camera.
0:28:21 > 0:28:25So, many rodents don't come out except under cover of darkness -
0:28:25 > 0:28:31and that applies even to one of the most ferocious and well-armed of all rodents.
0:28:31 > 0:28:36I think there could be one around here.
0:28:50 > 0:28:53WHISPERS: There it is.
0:28:54 > 0:28:56And what a formidable sight!
0:28:56 > 0:28:59An African crested porcupine.
0:29:01 > 0:29:06It seems to have found something to eat underneath that spiny bush.
0:29:07 > 0:29:11Most of the time, those quills would be lying flat.
0:29:11 > 0:29:18The fact that they are half erect is a sign that it's not too happy to have me so close.
0:29:29 > 0:29:33Those long quills are VERY sharp.
0:29:33 > 0:29:37And I'm quite sure it knows how to use them.
0:29:39 > 0:29:45It is turning its back on me - but don't be fooled. That's not because it is about to run away.
0:29:45 > 0:29:50If it gets really angry, it will attack by suddenly sprinting backwards.
0:29:50 > 0:29:52And it's off.
0:29:54 > 0:30:01That's not the sort of thing you want to stumble into in the middle of the African night.
0:30:07 > 0:30:13Porcupines feed mainly on roots and tubers, but they are big animals
0:30:13 > 0:30:19and to find enough to eat, they must travel long distances every night across open savanna.
0:30:19 > 0:30:23That means that they have to be prepared to take on all comers.
0:30:24 > 0:30:31This young leopard has probably never been close to a porcupine before,
0:30:31 > 0:30:36so it doesn't realise how dangerous it would be to interfere with it.
0:30:47 > 0:30:49The porcupine, on the other hand,
0:30:49 > 0:30:53seems to have total confidence in its armoury.
0:30:55 > 0:30:57If it's not very careful,
0:30:57 > 0:31:01that naive young leopard will end up with a face full of quills.
0:31:30 > 0:31:35And the porcupine goes back to digging for food.
0:31:35 > 0:31:37There's confidence for you!
0:31:39 > 0:31:44Belding's ground squirrels don't have that kind of nerve.
0:31:44 > 0:31:50They live in the hills of California and make their burrows close to one another
0:31:50 > 0:31:56so there are plenty of watchers to give warning of the slightest danger.
0:31:56 > 0:32:01This is particularly important in summer when there are babies about.
0:32:04 > 0:32:11There are always at least one or two adults on watch nearby so that the young can feed in relative safety.
0:32:11 > 0:32:18Only the females care for the young. They are nearly all related - mothers, sisters and daughters -
0:32:18 > 0:32:25and each looks after her nephews and nieces as carefully as she does her own offspring.
0:32:33 > 0:32:40When one of the guards spots a potential predator, such as an eagle, she sounds a preliminary warning.
0:32:40 > 0:32:46Those trills mean, "Danger seen but evasive action not needed."
0:32:50 > 0:32:53A bobcat - and once again a warning.
0:32:53 > 0:32:56HIGH-PITCHED SQUEAK
0:33:00 > 0:33:02Each female is trying to keep the enemy in view
0:33:02 > 0:33:06to ensure it doesn't sneak up and catch them by surprise.
0:33:11 > 0:33:17But the bobcat is getting closer. Now the warning will change.
0:33:17 > 0:33:22LOUD CHIRRUP That means, "Run for it!"
0:33:44 > 0:33:49Another small rodent that would make a tasty meal for a predator
0:33:49 > 0:33:52lives here in the arid north of Kenya.
0:33:52 > 0:33:56There are dozens within a few yards of me.
0:33:56 > 0:34:00But this one has adopted the safest strategy of all -
0:34:00 > 0:34:03to spend nearly all of its time below ground.
0:34:10 > 0:34:14And this is all that most people will ever see of it.
0:34:14 > 0:34:17A naked mole rat.
0:34:19 > 0:34:22They use their teeth for digging.
0:34:22 > 0:34:28Uniquely, their lips close behind the teeth. That stops earth getting into their mouth.
0:34:28 > 0:34:33With this superb burrowing equipment, they dig a great network of tunnels
0:34:33 > 0:34:39radiating from a central chamber, where the colony gathers and the babies are kept.
0:34:39 > 0:34:45All the babies are produced by one big female, the founder of the colony.
0:34:45 > 0:34:51All the rest of them - and there may be as many as a hundred - are the founder's children,
0:34:51 > 0:34:56and none of them will breed here as long as their mother is alive.
0:34:56 > 0:35:01Their job is to tunnel away, searching for tubers and roots,
0:35:01 > 0:35:08creating a vast network of tunnels that stretches for hundreds of yards under the African plains.
0:35:08 > 0:35:11Above ground, you have to look very hard
0:35:11 > 0:35:17to find any sign of a plant that might provide mole rats with an underground meal.
0:35:17 > 0:35:22Mostly, there is just dry grass and the odd acacia seedling.
0:35:24 > 0:35:29When mole rats meet, they assess one another's status with a sniff,
0:35:29 > 0:35:34and the junior one has to pass underneath the senior one.
0:35:43 > 0:35:50They are blind, and in the darkness they are just as happy travelling backwards as forwards.
0:35:52 > 0:35:59I've now come about a quarter of a mile from where I saw those mole rats digging,
0:35:59 > 0:36:03and this is the plant I have been looking for.
0:36:03 > 0:36:09Now, in the dry season, it's nothing more than a curly stem and a few withered leaves.
0:36:09 > 0:36:14But beneath the ground, there is treasure.
0:36:29 > 0:36:31And here it is.
0:36:31 > 0:36:35Tubers like this are few and far between here
0:36:35 > 0:36:43and it is a matter of pure chance as to whether the mole rats, burrowing underground blindly, bump into one,
0:36:43 > 0:36:47which is why mole rat colonies have to have several dozen workers.
0:36:47 > 0:36:53But if they do find one like this, it could sustain the colony for two or three weeks.
0:37:05 > 0:37:10Their sharp incisors cut into the tough tuber.
0:37:10 > 0:37:15Some of it, the lucky finders will eat on the spot.
0:37:17 > 0:37:24They also drag back lumps to the central chamber to be shared by the rest of the colony.
0:37:27 > 0:37:35This colonial arrangement, where all the workers are sterile and labour away to support their mother,
0:37:35 > 0:37:40is also the basis for the colonies of bees and ants.
0:37:40 > 0:37:47But naked mole rats are the only mammals of any kind that have adopted it.
0:37:51 > 0:37:56Although those long, chisel front teeth are rodent hallmarks,
0:37:56 > 0:38:02rodents are also famous for reproducing with extraordinary rapidity.
0:38:02 > 0:38:05Mole rats certainly do that.
0:38:05 > 0:38:10But the most prolific members of the whole family are the mice.
0:38:12 > 0:38:17Tiny harvest mice produce their babies in a carefully woven nest
0:38:17 > 0:38:21among the stems of reeds or tall grasses.
0:38:27 > 0:38:35Litters of up to eight babies, fed on their mother's rich milk, mature in only a few weeks.
0:38:40 > 0:38:42Each pair can produce three litters,
0:38:42 > 0:38:47that is up to nearly 30 young, in a single season,
0:38:47 > 0:38:51provided that the adults can find enough seeds to feed on.
0:39:00 > 0:39:07The young leave the nest when they are only two weeks old and start searching for food by themselves.
0:39:12 > 0:39:17To begin with, they are not nearly as agile as their parents.
0:39:31 > 0:39:37In fact, sometimes they seem quite unsure about just where they can safely tread.
0:39:45 > 0:39:50These little creatures have been living in our cornfields,
0:39:50 > 0:39:55taking a small proportion of our crops ever since farming began.
0:39:55 > 0:40:02Usually they only became visible when the crops are gathered, which is why they were called harvest mice.
0:40:02 > 0:40:07Even after harvesting, there is still the spilt grain to feed on.
0:40:07 > 0:40:11But in the vast wheat belt of Australia,
0:40:11 > 0:40:15a different mouse takes advantage of this plentiful supply of food,
0:40:15 > 0:40:20digging their burrows along the edge of the sandy fields.
0:40:23 > 0:40:29These are common house mice, the most prolific breeders of them all.
0:40:29 > 0:40:33Each female is capable of becoming pregnant at only five weeks old,
0:40:33 > 0:40:39and if there is a good supply of grain, she will breed every month or so.
0:40:40 > 0:40:45In some years, if all the grain out in the fields gets eaten,
0:40:45 > 0:40:50the mice head for the nearest farm buildings to look for food.
0:40:53 > 0:40:59As they leave the stripped fields, it becomes clear just how many have been feasting out there.
0:41:11 > 0:41:16Once they do find their way into a grain store, they become a plague.
0:41:33 > 0:41:39But eventually, when they finish the grain, their numbers will fall as quickly as they rose.
0:41:41 > 0:41:45On the flat, windswept plains of Patagonia,
0:41:45 > 0:41:53lives a much larger rodent with a more measured system of breeding - the Patagonian cavies or maras.
0:41:53 > 0:42:01Although this litter may appear to be very large, all the young are of slightly different ages.
0:42:01 > 0:42:06That is because they belong to different parents.
0:42:07 > 0:42:12Only one pair is on guard. The others are away grazing.
0:42:12 > 0:42:15This, in fact, is a communal creche.
0:42:15 > 0:42:21It is based around a hole that was often dug initially by burrowing owls.
0:42:21 > 0:42:23The maras have simply enlarged it.
0:42:25 > 0:42:32Although the adults guard each others' offspring, each female only gives her milk to her own babies.
0:42:37 > 0:42:44The others will have to wait until their own mother arrives to take her turn as guard.
0:42:45 > 0:42:48Here comes the next pair.
0:42:48 > 0:42:52The adults are not necessarily related,
0:42:52 > 0:42:57so the changing of the guard can be a somewhat tense business.
0:43:08 > 0:43:13The mara couples behave as though they were rivals rather than friends.
0:43:32 > 0:43:36Galloping around, grazing out on the plains,
0:43:36 > 0:43:41the maras have evolved long legs, so that they look more like antelopes than rodents.
0:43:41 > 0:43:46That may be the reason that they are not particularly good at digging
0:43:46 > 0:43:50and prefer to steal holes from the burrowing owls.
0:43:50 > 0:43:53A plover warns of approaching danger.
0:44:00 > 0:44:05The babies, thanks to the owls, have a hole to go to.
0:44:07 > 0:44:13But it's not big enough for the adults. All THEY can do is run.
0:44:26 > 0:44:28There is yet another kind of rodent
0:44:28 > 0:44:32that gather together in vast numbers to graze.
0:44:45 > 0:44:47They really do remind one
0:44:47 > 0:44:52of the huge herds that you see on the savannahs of East Africa.
0:44:52 > 0:44:54But we are in South America,
0:44:54 > 0:44:58and these are not wildebeeste or antelope.
0:44:58 > 0:45:03These are the biggest rodents in the world - capybara.
0:45:07 > 0:45:12Capybara are, like the mara, related to guinea pigs,
0:45:12 > 0:45:14but these are real giants.
0:45:14 > 0:45:19And they use their rodent teeth to crop grass - lots of it.
0:45:21 > 0:45:23BARKS
0:45:23 > 0:45:28When they want to, they can gallop as fast as a small horse.
0:45:34 > 0:45:39Although they are very agile on land, they are equally at home in water.
0:46:14 > 0:46:17Within these herds, each family sticks together,
0:46:17 > 0:46:19in groups of 20 or so.
0:46:19 > 0:46:23There are mothers and daughters, children and grandchildren.
0:46:26 > 0:46:29And just one large dominant male.
0:46:29 > 0:46:31That's him at the back.
0:46:31 > 0:46:36He marks his territory with scent from a gland on the top of his nose.
0:46:44 > 0:46:47And he will vigorously chase off any rival males
0:46:47 > 0:46:52that venture too close to his family, and particularly to his females.
0:47:17 > 0:47:21Capybara will graze out on the dry savannahs,
0:47:21 > 0:47:24but they're most at home in the water.
0:47:25 > 0:47:29They have oily water-resistant fur, partially webbed feet
0:47:29 > 0:47:34and their eyes and nostrils are on the top of their heads
0:47:34 > 0:47:39so that they can remain almost completely submerged
0:47:39 > 0:47:44but able to hear, see and smell what is happening on dry land nearby.
0:47:44 > 0:47:47Even the male's scent gland, which most other rodents
0:47:47 > 0:47:51carry on their cheeks, is on the top of his nose.
0:47:51 > 0:47:53During the rainy seasons,
0:47:53 > 0:47:58the plains flood and the capybara are truly in their element.
0:47:58 > 0:48:02If the maras are the South American equivalent of antelope,
0:48:02 > 0:48:07then these surely are the rodents' version of hippopotamus.
0:48:07 > 0:48:12Rodents truly are the most adaptable of mammals.
0:48:28 > 0:48:33Rodents really do have the most extraordinary spatial memory.
0:48:33 > 0:48:41I've often wondered, watching grey squirrels pinching all those peanuts that we provide in such quantities
0:48:41 > 0:48:47and burying them in the lawn, how many of those nuts they will, in fact, retrieve.
0:48:47 > 0:48:53A lot of experimental work has been done on that, and the results are astonishing.
0:48:53 > 0:48:55This is a true master of food storage -
0:48:55 > 0:49:00one that even prepares its food to ensure it doesn't germinate.
0:49:00 > 0:49:04But squirrels also need complex survival strategies
0:49:04 > 0:49:06to see them through the winter.
0:49:06 > 0:49:09It's a fine balance between hoarding enough food
0:49:09 > 0:49:14and remembering where it all is. Get it wrong and you face starvation.
0:49:14 > 0:49:18The kangaroo rat has a simpler strategy.
0:49:18 > 0:49:20It crams as much as it can
0:49:20 > 0:49:25into its food pouches then stores the seeds in an underground larder.
0:49:25 > 0:49:30They'll need to be checked from time to time to prevent them going mouldy,
0:49:30 > 0:49:36but the kangaroo rat doesn't have to memorise a set of different locations.
0:49:42 > 0:49:49Likewise, the chipmunks that hibernate through the Siberian winter have a larder next to the bed,
0:49:49 > 0:49:53so there's no need to venture out into the cold.
0:49:53 > 0:50:00If larder hoarding is so successful, why do squirrels scatter their caches around?
0:50:02 > 0:50:06Scatter-caching is related to a risk-averse strategy,
0:50:06 > 0:50:11so that you're not going to suffer any major losses.
0:50:11 > 0:50:16It also seems to be associated with an inability to defend.
0:50:16 > 0:50:24So, if you have lots of species of seed-eater around and you can't defend that larder against them,
0:50:24 > 0:50:28then you could die over the winter if you lose your larder.
0:50:28 > 0:50:35So it might be a better strategy to spread it around. At least you won't lose everything.
0:50:35 > 0:50:39The disadvantage of scatter-caching
0:50:39 > 0:50:46is that the squirrels need to remember where they have buried up to 3,000 acorns.
0:50:46 > 0:50:50Of course, oak trees rely on the squirrels' less-than-perfect recall,
0:50:50 > 0:50:55as some of the acorns they plant will grow into new saplings.
0:50:55 > 0:51:01So, how do squirrels manage to locate at least some of those buried nuts?
0:51:01 > 0:51:07At Exeter University, they tested the squirrels' spatial memory.
0:51:07 > 0:51:09The ring of pegs has some hazelnuts cached inside,
0:51:09 > 0:51:13but only when the red marker is present.
0:51:13 > 0:51:16Within days, the squirrels learn
0:51:16 > 0:51:17that the red marker
0:51:17 > 0:51:20is a landmark that means food
0:51:20 > 0:51:21and the time it takes each one
0:51:21 > 0:51:23to find the nut was carefully logged.
0:51:28 > 0:51:33The experiment was removed for two months,
0:51:33 > 0:51:36then the nuts and the red marker were replaced
0:51:36 > 0:51:39and the squirrels timed again.
0:51:39 > 0:51:40They were just as fast as before.
0:51:40 > 0:51:43They had retained a detailed memory of the set-up,
0:51:43 > 0:51:47including the colour of the marker, for over eight weeks.
0:51:53 > 0:52:00So far, we've seen how well they can remember things over a period of a few weeks.
0:52:00 > 0:52:07But if caching is to help them survive the winter, they'll have to remember things for longer than that.
0:52:07 > 0:52:10How good is their long-term memory?
0:52:11 > 0:52:18Caching animals do need to remember where their caches are over a long period of time.
0:52:19 > 0:52:25It is quite likely that they come back to them and move their caches around,
0:52:25 > 0:52:30so they're just refreshing their memory every so often.
0:52:30 > 0:52:37So, the mechanisms for food caching are quite complex, but they also seem to be quite widespread,
0:52:37 > 0:52:40in birds as well as mammals.
0:52:42 > 0:52:47This is a family of acorn woodpeckers in North America.
0:52:47 > 0:52:53There may be 60,000 acorns in THEIR larder, but it's not underground.
0:52:53 > 0:52:57The woodpeckers manage the larder carefully,
0:52:57 > 0:53:04since the acorns shrink as they dry out, and frequently have to be moved to smaller holes.
0:53:04 > 0:53:10So the acorn woodpecker is the bird equivalent of the kangaroo rat or the chipmunk,
0:53:10 > 0:53:15using a single larder and keeping a close eye on it.
0:53:15 > 0:53:22But there's also a bird which uses the squirrel's strategy, a member of the crow family.
0:53:22 > 0:53:24In terms of current knowledge,
0:53:24 > 0:53:29I think the nutcracker is the champion
0:53:29 > 0:53:32of memory for stored food and retrieval.
0:53:32 > 0:53:39It's an animal that can store thousands of items and retrieve them accurately later on.
0:53:39 > 0:53:42Squirrels, we see them doing this,
0:53:42 > 0:53:49but we don't have a comparable piece of evidence to show that they are indeed capable of doing it.
0:53:49 > 0:53:57We have also seen them rehearsing their memory in a way that is not seen in their avian counterparts.
0:53:57 > 0:54:04So, this memory-refreshing may be essential for the squirrels to match the birds.
0:54:04 > 0:54:08They have been studied for over 30 years,
0:54:08 > 0:54:10and one extraordinary discovery
0:54:10 > 0:54:13was that an area of the brain, the hippocampus,
0:54:13 > 0:54:18gets larger in birds that store food over winter,
0:54:18 > 0:54:22and it's also well developed in some special mammals.
0:54:22 > 0:54:28Just like food-caching animals, London cabbies have to remember hundreds of landmarks and routes.
0:54:28 > 0:54:34And they're constantly refreshing that memory by revisiting locations.
0:54:34 > 0:54:39I'm gonna do the right into Princes Street, go left into Lothbury.
0:54:39 > 0:54:42The traffic might move a bit easier.
0:54:42 > 0:54:47Brain scans of cab drivers show a larger-than-average hippocampus,
0:54:47 > 0:54:52and the longer you've been a cabbie, the bigger it is.
0:54:52 > 0:54:57It's likely that squirrels also use their hippocampus to remember routes,
0:54:57 > 0:55:01even complicated ones devised by humans.
0:55:01 > 0:55:06It's easy to watch them do it, so why is it so difficult to study?
0:55:06 > 0:55:11Well, if you think about the problems that you would encounter
0:55:11 > 0:55:14if you had to follow one squirrel...
0:55:14 > 0:55:16First, they're quite secretive.
0:55:22 > 0:55:30They will pretend to cache a nut and they'll scamper away somewhere else and bury the nut there.
0:55:35 > 0:55:39You'd also need to make a map of the caches,
0:55:39 > 0:55:43because if you put markers by them so YOU know where they are,
0:55:43 > 0:55:46that's a signal to all the other squirrels.
0:55:52 > 0:55:56You'd need to check the caches on a regular basis,
0:55:56 > 0:56:02to see whether they had been pilfered by other squirrels or other animals.
0:56:05 > 0:56:11And you'd also need to follow your squirrel to find out which caches it was recovering,
0:56:11 > 0:56:16whether it was eating them or re-caching them.
0:56:22 > 0:56:25And you couldn't do it on one squirrel,
0:56:25 > 0:56:29you'd need to do it on 10 or 20 squirrels
0:56:29 > 0:56:34to get any reassurance that, in fact, the squirrel you're studying
0:56:34 > 0:56:36is a normal example of a squirrel.
0:56:36 > 0:56:42So, nobody has been able to look at precisely how many caches they're making,
0:56:42 > 0:56:48where they're putting them, how long they last and when they recover them.
0:56:48 > 0:56:52It would be fascinating if we could.
0:56:54 > 0:56:57So, although the sight of a squirrel burying nuts
0:56:57 > 0:57:00is an everyday occurrence,
0:57:00 > 0:57:02it presents a challenge to science.
0:57:02 > 0:57:07That they survive winter is proof of the squirrels' superb memory.
0:57:07 > 0:57:11But understanding how they do what they do
0:57:11 > 0:57:14will remain a test of our intelligence.
0:57:22 > 0:57:28In the next programme of The Life Of Mammals, we meet the hunters,
0:57:28 > 0:57:33the lords of the land, the ultimate in lethal grace and beauty.