Officials said that the man took a risk because hippos actually kill more people in Africa than lions do, KABC reported.
But it was also apparently not safe for Rosie and her mother Mara, the hippos in question.
"Any unauthorized interaction with an animal is unsafe for the animal and potentially unsafe for the patron," April Spurlock, a spokesperson for the zoo, told the Los Angeles Times. "It is never appropriate for anyone to attempt to have contact or interaction with any animal outside of our staff-led animal experiences."
Rosie didn't appear to react when the man slapped her, but her mother Mara did lift her head as the man ran, the Times reported.
Because Rosie didn't have any signs of injury, police are not investigating it as a case of animal cruelty, but the LAPD is investigating it as a trespassing case, the Times reported.
“We seriously feel this was an isolated incident,” Spurlock told the Times. “Most people know not to go in with the animals. It’s common sense.”