What kind of professor turns his back on hard-earned tenure so he can hang out with reptiles and amphibians at a city zoo? If you're Joseph Mendelson, and you know that city zoo has a good reputation for research, then you jump at the chance and you ignore those warning of career suicide. (Besides, the zoo also offers an adjunct appointment at a nearby world-class academic institution in Midtown.) That's what Mendelson did 14 years ago, and he's never looked back. Mendelson, an adjunct associate professor in the School of Biological Sciences, is director of herpetological research at Zoo Atlanta.