The London Zoo is pushing back against critics who don't like its late-night parties. The adult-only affairs are advertised as an event where people can enjoy seeing the animals after hours but activists say noisy visitors and too much alcohol put some of the zoo's 19,000 animals at risk.

"Zoo Nights," as they are called, made a comeback this summer after being cancelled in 2015 following reports of wasted visitors harassing animals in enclosures. The zoo's chief curator, Brian Zimmerman, said the idea that there are lots of drunk people "doing things that they shouldn't be doing is really exaggerated."

"It's been a few isolated incidents from several years ago. They were dealt with at the time and we haven't had anything for the last few years," Zimmerman told CBS News' Imtiaz Tyab.

Areas have been closed off to protect the more sensitive animals and extra staff are on hand in case anyone gets out of control.

It's not all fun and fauna for visitors who are greeted by protesters as they arrive. Those critics say the zoo is putting profits over animal protection.

"There is scientific evidence that shows that loud noises affect animals in captivity a lot," said PETA campaigns manager Jordi Casamitjana. "They get fearful, they get stressful."

Zoo Nights is expected to make over a million dollars this summer with most of the cash going to conservation projects but more than 150,000 people have already signed a petition demanding the event be shut down. They say party animals shouldn't be anywhere near wild ones in captivity.

