Nickelodeon's Teenage Mutant Ninja Turtles products have grossed more than $475 million in retail sales since the latest animated series premiered in September 2012. The announcement, made this week at the Brand Licensing Europe show in London, comes just four years after the cable network's parent company Viacom bought the property for a reported $60 million.

According to The Hollywood Reporter, about $250 million of those sales come from the United States, with the remainder coming from overseas markets, where the Turtles are just as huge. They're the top action figures in Australia, the United Kingdom, France, Italy and, yes, the United States (where Nickelodeon says they make up 15 percent of the action-figure market).

"Nothing's bigger," Pam Kauffman, the network's president of consumer products, told the trade paper. "We are beating Iron Man, Batman, WWE."

She attributes part of that success to the team aspect of Peter Laird and Kevin Eastman's four characters, Leonardo, Raphael, Michelangelo and Donatello: They come as a package, even in licensing, and there's not one character that outsells the other. "The Turtles are like a boyband," Kauffman said. "They can't be separated."

Debuting in 1984 as a black-and-white self-published comic, Teenage Mutant Ninja Turtles began as a parody of Cerebus, Daredevil, New Mutants and Ronin. However, thanks largely to the efforts of licensing agent Mark Freedman, the property soon spawned animated TV series, movies, video games and endless merchandise.

Laird, who in 2008 completed a buyout of Eastman’s interests in TMNT and Mirage Studios that had begun in 2000, struck the deal with Viacom four years ago, saying, "I never expected to be working on the same thing for this long. And it has worn me down. I am no longer that guy who carries his sketchbook around with him and draws in it every chance he gets. … I miss — I really, really miss — being that guy.”

Teenage Mutant Ninja Turtles launched its second season last week on Nickelodeon; it was renewed for a third in February.