The next programme deals with flying insects. It begins in Central Europe, where the Körös River plays host to millions of giant mayflies as they rise from their larval skins to mate. — the climax of their lives. Mayflies and dragonflies were among the first to take to the air about 320 million years ago, and fossils reveal that some were similar in size to a seagull. Damselflies are also looked at in detail.
The Cave of Swallows in Mexico is a 400m vertical shaft, deep enough to engulf the Empire State Building. The Lechuguilla cave system in the USA is 193km long and 500m deep with astonishing crystal formations hanging from its chambers. Although often overlooked, caves are remarkable habitats with equally bizarre wildlife. Cave angel fish cling to the walls behind cave waterfalls with microscopic hooks on their flattened fins. Cave swiftlets navigate by echo-location and build nests out of saliva. The Texas cave salamander has neither eyes nor pigment. Unique access to a hidden world of stalactites, stalagmites, snotites and troglodytes brings a wealth of surprises.
Liz Bonnin presents a controversial and provocative episode of Horizon, investigating how new scientific research is raising hard questions about zoos - the film explores how and why zoos keep animals, and whether they need to change to keep up with modern science, or ultimately be consigned to history. Should zoos cull their animals to manage populations? Liz travels to Copenhagen Zoo, who killed a giraffe and fed it to the lions, to witness their culling process first hand. They think it is a natural part of zoo keeping that is often swept under the carpet. Should some animals never be kept in captivity? In a world exclusive, Liz visits SeaWorld in Florida and asks if captivity drove one of their orcas to kill his trainer. But could zoos be the answer to conserving endangered species? Liz examines their record, from helping breed pandas for the wild to efforts to save the rhinos. She meets one of the last surviving northern white rhinos and discovers the future of this species now lies in a multimillion-dollar programme to engineer them for stem cells. Veteran conservation scientist Dr Sarah Bexell tells Liz the science of captive breeding is giving humanity false hope.
Patrick follows the grizzly bears that are taking a risk with the weather by leaving their winter dens early. Hungry wolves are struggling to bring down their elk prey in the unusually shallow snow. And for great grey owls, it is the iciness of the snow that is hampering their hunts. Yellowstone's winter is always one of the most brutal on the planet. But 2016 saw weather records broken, and the wildlife was forced to adapt to survive. Kate Humble gets to grips with the science behind this remarkable season, from understanding the importance of the snowpack's structure as the melt begins to uncovering why Yellowstone's unique geology poses problems for some grazer's teeth.
At first glance, they look like pigs, but they're actually white-lipped peccaries from the Brazilian wetlands of the Pantanal, 10 times the size of the Everglades. Follow scientists as they track these mysterious mammals in their daily quest for food, while keeping a watchful eye for their main predator: jaguars.