A look at what would cause a ferocious tiger shark to cuddle up with a diver and why pigs are swimming in the Caribbean; and what mysterious blobs have washed up by the thousands on the shores of Oregon.
Begins in northern Norway, 500 kilometres north of the Arctic Circle. Here, there is only just enough light for the pine trees to survive, but it is extremely cold during the winter. Pine cone seeds provide one of the few foods available at this time of year, and large herbivores such as the moose must also rely on their fat reserves.
The fifth programme explores the alliances formed between the animal and plant worlds. It examines the ways in which plants live together and rely on each other. Whether living together in harmony, relying on each other for homes, protection or food, or living off each other, by strangling or otherwise destroying each other in a bid to survive.
Mammals dominate the planet. They do it through having warm blood and by the care they lavish on their young. Weeks of filming in the bitter Antarctic winter reveal how a mother Weddell seal wears her teeth down keeping open a hole in the ice so she can catch fish for her pup. A powered hot air balloon produces stunning images of millions of migrating bats as they converge on fruiting trees in Zambia, and slow-motion cameras reveal how a mother rufous sengi exhausts a chasing lizard. A gyroscopically stabilised camera moves alongside migrating caribou, and a diving team swim among the planet's biggest fight as male humpback whales battle for a female.
How and why dogs have become such good working companions, including Midge, the world's first police Chihuahua, and Joey, a sheepdog from the Cotswolds. Mother-of-four Toni Curtis gives her remarkable account of the fateful day when she was swept out to sea off the north coast of Wales and how she owes her life to the Newfoundland dog that saved her life..
In southern Africa, a pride of lions has rewritten the rules - by learning to take down elephants. In this follow up to Africa's Giant Killers, we join the pride at the start of the rainy season. As the elephants depart, a catalogue of dramatic events unfolds. The pride males turn against each other, an inexperienced mum puts her new born cubs in mortal danger, a rival group of lions challenge the pride for its territory and, when lightning strikes, fires burn day and night. When the dust eventually settles, the pride is left with only one choice - to face their old foe the elephants or risk starvation. The final showdown awaits.