Ylvis, of Viral Video 'The Fox' Fame, Sign to Warner Music, Play iHeartRadio Fest

Ylvis performs onstage during the iHeartRadio Music Festival at the MGM Grand Garden Arena on September 20, 2013 in Las Vegas, Nevada.

Photo by Christopher Polk/Getty Images for Clear Channel

Norwegian comedy duo Ylvis made their live U.S. debut at Las Vegas' iHeartRadio festival on Friday, Sept. 20 as the night's surprise guests.

The two musician brothers, Vegard and Bard Ylvisaaker, performed their YouTube hit "The Fox" at the MGM Grand Arena in between sets by Chris Brown and Katy Perry, wearing the signature squirrel and fox costumes from their music video, which went viral in recent weeks, garnering some 43 million hits thus far.

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"We rented the costumes from the Norwegian Film Institute," Vegard revealed during a sit-down with The Hollywood Reporter on the Ylvis trailer backstage at iHeartRadio. Added Bard: “They wouldn’t let us buy them, so we had to rent them for $50 each."

Watch Ylvis' iHeartRadio performance:

The duo also confirmed that, on the heels of the song's runaway success, Warner Music has signed them to a worldwide deal.

A source with knowledge of the negotiations says the label coughed up "huge" money to lock down the act following its success on YouTube. The label is presently promoting the single, helmed by Swedish producing team Stargate, to Top 40 radio in the States. And perhaps in a first showing of its recently announced partnership with radio giant Clear Channel, secured a slot for Ylvis on the coveted bill only one day ahead of the fest's Friday start.

The pair flew into Vegas from Los Angeles on Friday afternoon following an appearance on The Ellen DeGeneres Show. They'll next appear on Late Night with Jimmy Fallon on Oct. 9.

Still, Bard told THR, for Ylvis, it's laughs first, music second: “We’re comedians. And we like to sing.”

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"We'll never pursue pure music as a career," added Vegard. “This is very new for us and everyone is asking, ‘What are you going to do now?’ We don’t know. We’re busy trying to figure out what to do the next day.”

For now, that means recording more songs from their show, both in English and Norwegian, and flying back to Norway for the series' third season, which runs through mid-November.

"The Fox," meanwhile, continues to surge on the U.S. charts. The single is currently No. 25 on the Billboard Hot 100 and just cracked the Top 20 on iTunes.

What's at the heart of the song, you wonder? Vegard broke it down for THR: “I think it came from a genuine wonder of what the fox says. Because we didn’t know. There's not a clear answer to that. If you check out all the YouTube videos with real fox sounds, each is individual. It seems like they decide by themselves. They’re born with a generic voice and they can decide.”