0:00:03 > 0:00:05Planet Earth.
0:00:06 > 0:00:08Millions of species.
0:00:11 > 0:00:13But a few are special.
0:00:16 > 0:00:17Born to thrive.
0:00:27 > 0:00:29These are the opportunists.
0:00:32 > 0:00:34The collaborators.
0:00:38 > 0:00:39The survivors.
0:00:44 > 0:00:47What makes these animals so successful?
0:00:49 > 0:00:53In this series, we'll delve deep beneath the skin,
0:00:53 > 0:00:57to reveal the unique features that set some species apart.
0:01:02 > 0:01:06New behaviour and the very latest scientific discoveries
0:01:06 > 0:01:11will offer fresh insight into the wonder of animals.
0:01:27 > 0:01:30Across the planet, carnivores are struggling
0:01:30 > 0:01:33to compete in a world with a rocketing human population.
0:01:38 > 0:01:41But one predator is bucking the trend.
0:01:43 > 0:01:46Its numbers appear to be increasing,
0:01:46 > 0:01:49and its geographical range, expanding.
0:01:51 > 0:01:55It's now the most widespread of all carnivores,
0:01:55 > 0:01:56the fox.
0:02:02 > 0:02:04Resourceful...
0:02:06 > 0:02:07..opportunistic...
0:02:09 > 0:02:13..and capable of exploiting the most extreme habitats on Earth.
0:02:15 > 0:02:18Whatever the circumstances,
0:02:18 > 0:02:19the fox can adapt.
0:02:27 > 0:02:30Our planet is constantly changing,
0:02:30 > 0:02:34but foxes are simply taking it in their stride.
0:02:39 > 0:02:43Their super-sharp senses mean that they can hunt
0:02:43 > 0:02:46during the day, and also at night.
0:02:48 > 0:02:53Their intelligence allows them to cope with seasonal fluctuations,
0:02:53 > 0:02:58and their flexibility means that foxes can thrive
0:02:58 > 0:03:03in spite of some of the most dramatic human-induced changes.
0:03:06 > 0:03:10In this episode, we're going to find out exactly what it is
0:03:10 > 0:03:12that makes foxes so adaptable...
0:03:14 > 0:03:17..starting with the senses.
0:03:23 > 0:03:26Most predators have evolved to specialise
0:03:26 > 0:03:29either nocturnally or diurnally.
0:03:31 > 0:03:34But foxes are different.
0:03:38 > 0:03:41They can hunt both day and night.
0:03:44 > 0:03:48And that's mainly down to their impressive eyes.
0:03:56 > 0:04:01The fennec fox lives in the Sahara Desert
0:04:01 > 0:04:05and like all foxes, it can operate under the cover of darkness.
0:04:07 > 0:04:13Its pupils dilate, allowing as much light as possible to enter the eye.
0:04:16 > 0:04:20At least 97% of the photoreceptors in their retina
0:04:20 > 0:04:22are rods, rather than cones.
0:04:22 > 0:04:25Cone cells detect colour,
0:04:25 > 0:04:30whereas rods are much more sensitive in low light levels.
0:04:30 > 0:04:32Having such a high proportion of these
0:04:32 > 0:04:35gives the fox acute night-vision.
0:04:38 > 0:04:42But in the day, these light-sensitive rod cells
0:04:42 > 0:04:44serve a completely different purpose.
0:04:52 > 0:04:54This is the Tibetan sand fox.
0:05:00 > 0:05:03Rabbit-like creatures, known as pikas, are their prey.
0:05:06 > 0:05:10But the fox must first spot them, far away in the distance.
0:05:13 > 0:05:15This is where the rods switch function.
0:05:20 > 0:05:23They instantly become motion sensors,
0:05:23 > 0:05:26allowing the fox to pinpoint the tiny pikas...
0:05:29 > 0:05:33..even when they are almost a kilometre away.
0:05:40 > 0:05:43The moment their prey moves, the densely packed rods
0:05:43 > 0:05:45pick up the change in light...
0:05:52 > 0:05:55..triggering the fox to lock on to its target.
0:06:06 > 0:06:08Once it's in striking distance,
0:06:08 > 0:06:10the attack begins.
0:06:31 > 0:06:35But it's not just the retina that helps foxes to hunt in the day,
0:06:35 > 0:06:38the shape of the pupil is also key.
0:06:43 > 0:06:48Wolves and other members of the dog family have round pupils.
0:06:51 > 0:06:55But foxes eyes are different to the rest of the canids.
0:06:56 > 0:07:00Their vertically-slit pupils are more similar to a cat's.
0:07:04 > 0:07:08Both animals have evolved this physiology separately,
0:07:08 > 0:07:11but they have the same effect.
0:07:14 > 0:07:17For a long time, it was thought that this slit pupil
0:07:17 > 0:07:20gave both animals better night vision.
0:07:22 > 0:07:25But new research from the University of Sydney
0:07:25 > 0:07:29suggests this shape actually helps them to hunt in the day.
0:07:29 > 0:07:33The fennec fox must hunt in daylight to find enough food,
0:07:33 > 0:07:36but sunlight can be harmful.
0:07:38 > 0:07:43In the UK, the UV index is classified at a moderate 6,
0:07:43 > 0:07:45but here in the Sahara,
0:07:45 > 0:07:48it reaches the maximum strength of 11.
0:07:50 > 0:07:54When combined with light reflection from the sand and rock,
0:07:54 > 0:07:59the sun's rays have the potential to do serious damage to the eye.
0:08:01 > 0:08:06And this is where the slit pupil comes into its own.
0:08:08 > 0:08:11The coloured rings show the areas on the lens
0:08:11 > 0:08:14which are used to focus on different colours.
0:08:15 > 0:08:19In bright light, round pupils contract to protect the eyes
0:08:19 > 0:08:24against harmful rays, blocking the outer colour completely.
0:08:26 > 0:08:30This means that particular colour can't be focused,
0:08:30 > 0:08:32blurring the overall image.
0:08:34 > 0:08:36But when a slit pupil contracts,
0:08:36 > 0:08:39all colours remain focused.
0:08:42 > 0:08:45This means that even in the most intense light,
0:08:45 > 0:08:48foxes can protect the sensitive night-vision areas
0:08:48 > 0:08:49at the back of the eye
0:08:49 > 0:08:54whilst also maintaining a sharp, full-colour image.
0:08:57 > 0:09:00The result is a remarkable ability
0:09:00 > 0:09:04to hunt at any time of day or night.
0:09:10 > 0:09:13But when it's the seasons that change
0:09:13 > 0:09:17foxes have to rely on other senses.
0:09:33 > 0:09:36Superior eyesight is obviously no good
0:09:36 > 0:09:39when prey is hidden deep beneath the snow.
0:09:43 > 0:09:46For this, the fox has an extra sense,
0:09:46 > 0:09:51and it's one that no other animal on Earth has harnessed for hunting.
0:09:56 > 0:10:00Winter in the Yellowstone National Park.
0:10:00 > 0:10:04The snow here is regularly more than two metres deep.
0:10:05 > 0:10:09A red fox can survive here all winter,
0:10:09 > 0:10:11but only if it can find enough food.
0:10:16 > 0:10:19Their ability to pinpoint rodents beneath the thick layer of snow
0:10:19 > 0:10:24has always been attributed to their exceptional hearing,
0:10:24 > 0:10:27and that is...part of the process.
0:10:30 > 0:10:34Foxes can move each ear independently,
0:10:34 > 0:10:37rotating them up to 150 degrees.
0:10:44 > 0:10:47More than a dozen separate muscles finely tune
0:10:47 > 0:10:51the position of the ear canal so the fox can identify a sound
0:10:51 > 0:10:53and locate it more accurately.
0:10:57 > 0:11:03But in 2010, scientists uncovered something astonishing.
0:11:03 > 0:11:06It's long been known that to reach the prey beneath the snow,
0:11:06 > 0:11:12or even in thick grass, foxes use a technique called a "mouse pounce".
0:11:14 > 0:11:17What this recent survey found
0:11:17 > 0:11:21was that the overall hit rate was just 18%,
0:11:21 > 0:11:26but when the fox faced in a north-easterly direction
0:11:26 > 0:11:31the hit rate rose to a staggering 73%.
0:11:31 > 0:11:36Unbelievably, the foxes seemed to be aligning their pounces
0:11:36 > 0:11:40to the Earth's magnetic field,
0:11:40 > 0:11:44which tilts downward in the northern hemisphere.
0:11:44 > 0:11:48It's thought that the fox can detect this magnetism.
0:11:54 > 0:11:57As the fox creeps forward,
0:11:57 > 0:12:01it listens for the sound of a mouse,
0:12:01 > 0:12:03searching for that sweet spot
0:12:03 > 0:12:07where the angle of the sound hitting its ears
0:12:07 > 0:12:11matches the slope of the Earth's magnetic field.
0:12:13 > 0:12:14When it finds that spot,
0:12:14 > 0:12:18the fox knows that the prey is a fixed distance away,
0:12:18 > 0:12:24and it can calculate exactly how far to jump to land right on top of it.
0:12:31 > 0:12:35Scientists think that the secret behind this talent
0:12:35 > 0:12:37might be a protein in the fox's eye
0:12:37 > 0:12:40called crypto-chrome, which is sensitive
0:12:40 > 0:12:44to the Earth's natural magnetism.
0:12:44 > 0:12:48What's more, they speculate that this might actually allow them
0:12:48 > 0:12:53to see the magnetic field as a patch in their vision.
0:12:55 > 0:12:57If the scientists are correct,
0:12:57 > 0:13:00the red fox would be the first animal known
0:13:00 > 0:13:03to use the Earth's magnetic field to hunt.
0:13:08 > 0:13:11Whatever the circumstances,
0:13:11 > 0:13:12the fox adapts.
0:13:16 > 0:13:19Acute senses are important,
0:13:19 > 0:13:21but sometimes they're not enough
0:13:21 > 0:13:23and intelligence is what is needed.
0:13:25 > 0:13:28This is the Arctic...
0:13:29 > 0:13:31..in the heart of winter.
0:13:39 > 0:13:44At this time of year, the Arctic fox is a scavenger.
0:13:54 > 0:13:58They have an extremely complex sense of smell.
0:13:59 > 0:14:03They can detect a frozen carcass buried beneath a metre of snow.
0:14:07 > 0:14:09But foxes are less effective
0:14:09 > 0:14:13at picking up smells over long distances. So, to find food,
0:14:13 > 0:14:18the fox has come up with an ingenious method.
0:14:19 > 0:14:22It borrows one of the most highly specialised noses
0:14:22 > 0:14:25in the animal kingdom.
0:14:25 > 0:14:28SNUFFLING
0:14:33 > 0:14:37Polar bears are able to sniff out a seal carcass
0:14:37 > 0:14:40from up to 32km away.
0:14:44 > 0:14:48In winter, a polar bear is much more likely to track down fresh food.
0:14:51 > 0:14:55Trailing one is a cunning way of guaranteeing a meal.
0:15:06 > 0:15:08The Arctic fox has learned
0:15:08 > 0:15:11that as long as it follows at a safe distance,
0:15:11 > 0:15:13this tactic will pay off.
0:15:15 > 0:15:21As the bears generally only eat the blubber of their prey...
0:15:24 > 0:15:28..a fresh kill means that there are plenty of leftovers.
0:15:34 > 0:15:40This intelligent strategy is key to surviving the harsh winter.
0:15:50 > 0:15:54When the summer finally returns, the Arctic fox moults.
0:15:54 > 0:15:57Insulating properties are lost,
0:15:57 > 0:16:00but their colouration is perfect for camouflage.
0:16:03 > 0:16:07Millions of birds have arrived in the Arctic to breed.
0:16:09 > 0:16:12There's a sudden boom in the food supply.
0:16:20 > 0:16:25But knowing how to deal with this bounty takes forethought.
0:16:28 > 0:16:33Five million snow geese are here to lay their eggs.
0:16:35 > 0:16:41And this Arctic fox family has chosen a prime position for its den.
0:16:46 > 0:16:49With so many geese around, sneaking up unnoticed
0:16:49 > 0:16:52would be absolutely impossible.
0:16:56 > 0:17:00Instead, the fox heads straight in amongst them.
0:17:03 > 0:17:07Now, this might look like chaos, but there is a plan here.
0:17:10 > 0:17:14It's seeking out nests with just one protector.
0:17:18 > 0:17:20And causing a commotion,
0:17:20 > 0:17:24the fox forces that goose away from its nest.
0:17:29 > 0:17:33As soon as it's far enough away, it can make its move.
0:17:42 > 0:17:44But with too many eggs to eat now,
0:17:44 > 0:17:46the fox stashes them for later.
0:17:54 > 0:17:58In fact, 97% of the eggs taken by Arctic foxes are cached in this way.
0:18:00 > 0:18:06A single fox can take over 1,500 eggs in one nesting period.
0:18:11 > 0:18:13They keep much better than meat,
0:18:13 > 0:18:17as the protective shell reduces their decomposition rate.
0:18:20 > 0:18:23Even if it's in the ground for two months,
0:18:23 > 0:18:27the egg will only lose 8% of its nutritional mass.
0:18:29 > 0:18:31And by scattering them across their territory,
0:18:31 > 0:18:34they lower the chances of rival foxes finding them
0:18:34 > 0:18:38and ensure that they have plenty to eat throughout the winter.
0:18:43 > 0:18:45Arctic foxes stash most of the eggs,
0:18:45 > 0:18:48because in a month,
0:18:48 > 0:18:50another opportunity presents itself.
0:18:54 > 0:18:59Any eggs they didn't steal are now hatching,
0:18:59 > 0:19:03and these goslings contain twice as much nutritional value.
0:19:39 > 0:19:43A gosling is a high-energy food source,
0:19:43 > 0:19:49just one could sustain this fox for the whole day.
0:19:49 > 0:19:54But there's no shortage here, so this one is for the cubs.
0:19:59 > 0:20:02The Arctic fox is capable of intelligently adapting
0:20:02 > 0:20:05its hunting behaviour to the time of year.
0:20:07 > 0:20:09Scavenging in the winter...
0:20:12 > 0:20:14..and hoarding in the summer.
0:20:16 > 0:20:20But foxes have also been able to cope
0:20:20 > 0:20:23with a far greater change than the seasons.
0:20:28 > 0:20:30Over the last couple of centuries,
0:20:30 > 0:20:34the natural world has been disappearing fast.
0:20:34 > 0:20:36Now, you might not think that urbanisation
0:20:36 > 0:20:38offers a fabulous opportunity,
0:20:38 > 0:20:43but I've got to tell you that even here, foxes are making a success.
0:20:47 > 0:20:50And this is all down to their remarkable ability
0:20:50 > 0:20:54to be flexible in terms of their behaviour.
0:20:59 > 0:21:02The birth of suburbia in 1930s Britain
0:21:02 > 0:21:07brought with it low-density housing and relatively large gardens,
0:21:07 > 0:21:12providing an ideal habitat for the red fox.
0:21:15 > 0:21:18Over the next decade they colonised the green belt.
0:21:22 > 0:21:23And by the 1960s,
0:21:23 > 0:21:27they'd established themselves in the heart of Britain's cities.
0:21:32 > 0:21:36No other mammal has thrived in this new urban environment
0:21:36 > 0:21:38quite like the red fox.
0:21:41 > 0:21:46The rural and urban red fox are exactly the same species,
0:21:46 > 0:21:52so how have they adjusted to live in such different environments?
0:21:55 > 0:21:59Firstly, they've adapted their denning behaviour.
0:22:02 > 0:22:06In the countryside, foxes will often dig their own dens.
0:22:08 > 0:22:11In the city, they rarely need to.
0:22:14 > 0:22:17Urban areas provide a huge range of options.
0:22:19 > 0:22:23And it's much more energy-efficient to use these man-made dens.
0:22:31 > 0:22:34And foxes have also adapted their diet.
0:22:37 > 0:22:41Food is readily available in the form of human leftovers.
0:22:43 > 0:22:45And this is the reliable,
0:22:45 > 0:22:49non-seasonal resource that the urban fox thrives on.
0:22:52 > 0:22:56More food means not having to travel so far in search of it.
0:23:00 > 0:23:05In the city, they can survive in just a tenth of a square kilometre.
0:23:07 > 0:23:09Whereas in the countryside,
0:23:09 > 0:23:12they'd need an area 500 times larger.
0:23:16 > 0:23:22So, urban foxes can live at much higher densities,
0:23:22 > 0:23:24but they therefore come into contact
0:23:24 > 0:23:28with competing neighbours much more frequently.
0:23:34 > 0:23:36But rather than fight,
0:23:36 > 0:23:41scientists believe the urban foxes have learned to adapt.
0:23:47 > 0:23:51After all, why waste energy in conflict
0:23:51 > 0:23:54when there's plenty of food to go round?
0:24:04 > 0:24:09Scavenging on leftovers can have its downsides.
0:24:11 > 0:24:15But even this has been turned to the fox's advantage.
0:24:20 > 0:24:25Eating rotten meat exposes them to a high dose of different pathogens.
0:24:27 > 0:24:32In theory, this should mean that the fox frequently becomes ill.
0:24:35 > 0:24:40But over time, the regular exposure to "off" meat has contributed
0:24:40 > 0:24:44to the red fox developing a more sophisticated immune system...
0:24:47 > 0:24:50..enabling it to combat a much wider variety of diseases.
0:24:56 > 0:24:58It's early days in the research,
0:24:58 > 0:25:01but scientists think that this is what's helped them
0:25:01 > 0:25:04to colonise urban areas so effectively.
0:25:09 > 0:25:12But there's something even more surprising
0:25:12 > 0:25:13about the urban foxes' diet.
0:25:17 > 0:25:21Only 60% of it is made up of human leftovers.
0:25:24 > 0:25:27They'll adapt to whatever food source is available.
0:25:31 > 0:25:34On this rubbish tip, foxes aren't the only animals
0:25:34 > 0:25:36to seize the chance to indulge.
0:25:40 > 0:25:43Gulls have also come to feed on the bounty.
0:25:44 > 0:25:47But the fox is one step ahead.
0:25:52 > 0:25:56It knows how to turn competition into opportunity.
0:26:01 > 0:26:04Although rural foxes survive primarily
0:26:04 > 0:26:06on small rodents and birds,
0:26:06 > 0:26:10one might expect urban foxes to have left that sort of hunting behind.
0:26:20 > 0:26:23But up to 40% of an urban fox's diet
0:26:23 > 0:26:28is still made up of birds, insects, rodents and fruit.
0:26:32 > 0:26:36This really is adaptable behaviour at its best.
0:26:40 > 0:26:46For a long time, the urban fox was regarded as a British phenomenon
0:26:46 > 0:26:48but significant numbers of red foxes
0:26:48 > 0:26:52now live in cities right across the globe,
0:26:52 > 0:26:55including New York, Sydney and Moscow.
0:26:58 > 0:27:01And they haven't just spread into cities.
0:27:01 > 0:27:04Wherever humans have changed the landscape,
0:27:04 > 0:27:06the red fox has found a way to fit in.
0:27:09 > 0:27:12In the USA, when forests were cleared for agriculture,
0:27:12 > 0:27:18both native and introduced red foxes spread across the farmland.
0:27:18 > 0:27:21In Australia, after a few dozen were introduced for sport,
0:27:21 > 0:27:25the red fox came to colonise most of the country.
0:27:31 > 0:27:35And in Arctic Canada, when human settlements first appeared,
0:27:35 > 0:27:38so did the red fox.
0:27:44 > 0:27:46Until recently, the grey wolf
0:27:46 > 0:27:50was the world's most widely distributed land mammal,
0:27:50 > 0:27:55but now, that crown has been passed to the ubiquitous red fox.
0:28:06 > 0:28:11Foxes have an incredible ability to adapt to any change.
0:28:15 > 0:28:18Their complex senses can operate effectively
0:28:18 > 0:28:22both during the day and at night.
0:28:23 > 0:28:28Their intelligence allows them to choose to scavenge or to hoard,
0:28:28 > 0:28:30depending on the season.
0:28:36 > 0:28:38And their remarkable ability to adapt
0:28:38 > 0:28:43means that they can cope with the greatest change of all,
0:28:43 > 0:28:49thriving in perhaps the most hostile environment on Earth
0:28:49 > 0:28:54and THAT is the wonder of foxes.