Marine ecologist Tracey Rogers talks to Jim Al Khalili about her research on one of Antarctica's top predators. This is the leopard seal - a ten foot long killer which glides among the ice floes in search of prey ranging from other seals to penguins to tiny krill. Tracey's research has encompassed the animal's prolific and eerie underwater singing to radical changes in its diet that appear to be linked to climate change.Now a senior researcher at the University of New South Wales in Australia, Tracey first encountered the species as a less than successful seal trainer at a zoo in Sydney. There she met a giant female leopard seal named Astrid. Astrid's singing one Christmas day in the early 1990s set Tracey on the path to become the world's authority on this Antarctic species.Tracey tells Jim how her first expedition to study leopard seals was met with almost universal scepticism until she dropped an underwater microphone into the water. In the following 25 years, she has worked to decode the meanings and qualities of the leopard seal song and explored the changes being forced upon the species by climate change. Tracey describes what made her return to Antarctica again and again and tells the story of how she almost met her end in the perilous shifting world of the pack ice. And then there's the time a leopard seal mistook her for a penguin. There is a longer version of this interview in the podcast of this episode - more on the seal vocalisations and how Tracey saved the life on a young colleague who fell into the freezing sea. Producer: Andrew Luck-Baker.