In the fourth episode, Dromaeosaurs stalk a hadrosaur herd as they cross a freezing river. Male Ornithomimus raid rival nests to bolster their own. Olorotitan raise their offspring on fertile volcanic fields but contend with biting mosquitoes.
Troodontids hunt mammals flushed out by a forest fire. A juvenile Antarctopelta scours the forest for a new winter den. Within the snow-covered forest, a tense standoff develops between ancient rivals, Pachyrhinosaurus and Nanuqsaurus.

Destiny is beyond our control, our fate already set, perhaps from the onset of disease or being born with a special ability. You may think our lives are determined by the genes we're born with, but that is not the case. In the non-coding 98% of our DNA, we have countless switches to promote or suppress the physiological reactions of our bodies. Interestingly, we can change the states of these switches through our own efforts and even can affect the DNA conditions of our offspring before their birth.
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Each year more than one million wildebeest and zebra invade the Serengeti grasslands, making it a paradise for the predators that live there. But what happens when the herds move off again? We follow the moving story of one lion family's struggle to survive until the return of the great migration. Nature's Great Events tells the story of the epic trek of herds that follow the rains to fresh pastures, and the tale of the predators they leave behind.
The crew captures the desperate plight of a single pride of lions, revealing a different side to the Serengeti. Rather than being a predators' paradise, it is a land in constant change, with wildebeest following the rains and leaving the lions to tough it out. The Ntudu pride has seven cubs, and is already suffering as the wildebeest leave to find fresh pastures. The four pride females struggle to find enough food for their hungry offspring. As weeks turn to months, the pride members become more emaciated and frailer, and the number of cubs dwindles to just two. As the herds begin to return, the plains reveal one final secret. For the first time since 1967 the Serengeti's only active volcano, Ol Doinyo Lengai, begins to billow ash and smoke. Filmed from the air, the team captures the exciting action. Fertilised by the volcanic ash over millions of years, these short grass plains are among the most productive grasslands in the world. After months of hardship, the pride's tragic story, through sickness, drought and fire, is over as the herds return, providing plentiful food.