I can find no reference to what is meant by this quote. HOWEVER -- Peter De Vries (February 27, 1910 - September 28, 1993) was an American editor and novelist known for his satiric wit. Also, the quote is listed included in "Aphorisms, Quotations, & Wisecracks."
With that in mind, I would suggest that De Vries was not waxing deeply "philosophical" when he made the statement.
That coupled with the following definitions of "zoo" and "jungle," and I'm inclined to think the meaning of the comment leans more towards life being a state of confusion or chaos within a semi-hostile environment. The comment dripping with the sarcasm that De Vries is so well known for.
Zoo -- A place or situation marked by confusion or disorder.
Jungle -- A dense, confused mass; a jumble. Something made up of many confused elements; a bewildering complex or maze: sorting through the jungle of regulations.
A place or milieu characterized by intense, often ruthless competition or struggle for survival: the corporate jungle.
https://en.wikipedia.org/wiki/Peter_De_Vries
http://www.ask.com/web?qsrc=167&o=10429&l=dir&q=humorous+quotes+life+is+a+zoo+in+a+jungle&search=
