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. "His condition has progressed much faster in the last few months than any of us could have anticipated," the statement read. "Sadly, Dad is no longer able to participate in public life as he used to, where people all over the world watched, learned and laughed alongside him."

Jack Hanna's visits to the David Letterman show were legendary and this particular segment with baby tigers, scorpions and tiny snakes writhing all over Dave's desk is one of the best.