The first video is about how baby barnacle geese have to jump off the cliff face to get food at bottom. First the father jumps down so that the babies follow. But they don't follow. After the mother jumps they jump. The babies need hollow bones so that they don't break them when they hit the rocks. Their softness also helps because they don't hit the wall hard. Only the ones with better bones and more softness survive. The others don't survive because the are not so strong. They had to leave 2 because they wanted to get away from predators.
The second video is about how a three month old arctic has to survive alone through 6 months of harsh winter. To survive they have to have soft paws to walk for a long time and to dig for food in the snow. Also they had to have strong legs to jump into lemming holes for food. If they didn't have these traits they couldn't survive. This natural selection makes sure that only the strongest foxes survive the winter.