London, May 1 : Monstrous 'dungeon dad' Josef Fritzl has literally made the children he had with his daughter into 'animals'.
According to Police Inspector Leopold Etz, the two boys plucked from Fritzl's dungeon talk to each other with animal-like growls.
And one walks like a monkey, sometimes crawling on all fours.
The pitiful world created by Stefan Fritzl, 18, and his five-year-old brother Felix during their lifetime of underground captivity was revealed by police on April 30.
It also emerged that the kids' mother Elisabeth- caged with the boys by her rapist father - never told them the truth about their terrible plight.
Instead, she invented a fantasy world for them by filling their heads with wondrous stories of princes and princesses.
"With each other, the boys communicate with noises that are a mixture of growling and cooing. If they want to say something so others understand, they have to focus and really concentrate - which seems to exhaust them," The Sun quoted Etz, as saying.
It is believed that Felix's joints and muscles did not develop properly in the cramped bunker, possibly due to malnutrition. And the little lad walks like a monkey.
Insp Etz said: "He prefers to crawl but can walk upright if he wants. He mostly uses a mixture of the two - half walking, half crawling."
Doctor Berthold Kepplinger, who has examined the boys, said: "They communicate with each other but with a far-from-normal way. Their mother taught them some reading and writing, although Elisabeth herself lost much of her childhood knowledge due to the years of abuse that began when she was 11 and her imprisonment from the age of 18.
"There were no books in the dungeon. The main source of education, over the years, has been the television."
Psychiatry professor Rotraud Perner, who has seen them, said: "It seems they may have created their own illusory world."
Elisabeth, now 42, had seven children through being raped by her 73-year-old father in the dungeon below his house in Amstetten.