Instinct (1999)

Within the brilliant mind of primatologist Ethan Powell lies an ominous secret--a deep-seated mystery unsolved by his estranged family and the law enforcement officials who have tracked him down and jailed him for murders that occured in the jungles of Rawanda. Highly educated and sophisticated Powell's truth was derived from years of studying mountain gorillas--to the point where he was living among them in the wild, and accepted as one of their own. Now, held captive in a brutal prison for the criminally insane, Powell who has not spoken in years, is remanded to psychiatrist Theo Caulder. Caulder's unbridled ambition drives him to risk everything and put his life on the line in order to understand the actions of this headline-gripping madman. Eventually both men embark on an extraordinary journey fueled by their insatiable hunger for truth, regardless of the cost.