Tratz also worked with Heinz Heck on a comparative study examining chimpanzees and bonobos, most of the work for which was done during the Second World War at Munich's Hellabrunn Zoo.
The Heck horse aimed to recreate the Tarpan (the true European Wild Horse, ancestor of all European domestic horse breeds), and the Heck cattle, aimed to recreate the aurochs, the wild cattle of the European forest.
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