Complex sentence: Despite no longer encountering ancestral predators while living in captivity, while experimenting with exposures to images and stimuli relating to these big cats, the deer seemed to instinctively react with a cautious predator-prey response typical of wild deer.
Split into simpler sentences: Despite no longer encountering ancestral predators while living in captivity. While experimenting with exposures to images and stimuli relating to these big cats, the deer seemed to instinctively react with a cautious predator-prey response typical of wild deer.