Despite heartfelt pleas, Janice Petrone, a totally lame and uncool zookeeper at the Brookfield Zoo, repeatedly told a group of area sixth graders Monday that they were not allowed to ride on the backs of the western lowland gorillas. 'The silverback's natural instinct is to protect his troop,' said Petrone, who has probably never taken a risk in her entire boring life. 'He would beat his chest, bare his teeth, and then charge at you as soon as you entered the enclosure.' Petrone, a loser who hates fun, also told the kids they were not permitted to swim with the sharks, race the cheetahs, or punch the giraffes. 