Celebrated zookeeper and Letterman guest Jack Hanna retiring due to dementia

One of David Letterman's greatest guests, Jack Hanna, has to retire, according to his family. The animal expert has been stricken with dementia.

via CNN:

'Doctors have diagnosed our dad, Jack Hanna, with dementia, now believed to be Alzheimer's disease,' Hanna's family wrote in a statement posted to his verified Twitter account. — Read the rest