Zoo animals remain hot at the US box office. Singing stars and Adam Sandler are not.

Ben Stiller and his voice co-stars of Madagascar 3: Europe's Most Wanted held on to the number one spot again, with $US35.5 million ($A35.55 million) for the animated sequel's second weekend in domestic theatres.

Studio estimates Sunday put Ridley Scott's sci-fi adventure Prometheus at number two again with $US20.2 million.

Madagascar 3 and Prometheus held off two under-achieving newcomers.

The star-studded musical Rock of Ages, whose cast includes Tom Cruise, Catherine Zeta-Jones and Alec Baldwin, flopped at number three with $US15.1 million.

Sandler's That's My Boy bombed with $US13 million, the worst showing for one of his broad comedies since the mid-1990s.

That's My Boy came in at number five, behind the $US13.8 million for Snow White the Huntsman, a film that's been out for three weekends already.

"Sure, we would have liked to have done more," said Rory Bruer, head of distribution at Sony, which released That's My Boy and most of Sandler's past movies.

"But you've got to give it to Adam. He mixes it up. This is a really off-the-wall comedy," Bruer said.

That's My Boy stars Sandler as a guy who became a fleeting teen celebrity after getting his seventh-grade teacher pregnant and now is a middle-aged loser trying to reconnect with his son (Andy Samberg).

As with most Sandler movies, the reviews were bad, but that usually does not stop his audience of young males from showing up.

The R rating for That's My Boy may have cut into the movie's business, prohibiting those under 17 from seeing it without an adult. Sandler's comedies usually are rated PG-13.

But Sandler has plenty of fans in their late teens and 20s, and that crowd had little interest in That's My Boy.

Among Sandler's mainstream live-action comedies, it was the worst debut since 1996's Happy Gilmore, which played in far fewer theatres than That's My Boy and came as Sandler was just climbing to stardom.

Factoring in today's higher admission prices, That's My Boy sold fewer tickets than Happy Gilmore.

Some of Sandler's handful of comic dramas and other more serious movies did less business than That's My Boy. But even his 2000 bomb Little Nicky opened bigger, with $US16.1 million in its first weekend.

Sandler has been one of Hollywood's steadiest earners since the late 1990s, with hits such as The Longest Yard, Anger Management, Big Daddy and Grown-ups opening with more than $US40 million and becoming $US100 million hits.

Sony remains high on Sandler, backing his upcoming animated comedy Hotel Transylvania, along with Grown-ups 2.

"It's a good business to be in, the Adam Sandler business," Bruer said.

Warner Bros had its own dud with Rock of Ages, adapted from the stage show built around 1980s rock hits and featuring an ensemble that includes Cruise, Zeta-Jones, Baldwin, Julianne Hough and Mary J Blige.

Dan Fellman, the studio's head of distribution, was not ready to pull the plug on the movie, saying he hoped it might hang on in subsequent weekends. Rock of Ages drew mostly older, female crowds, an audience that does not necessarily rush out to a movie over opening weekend.

"Musicals, there have been a couple in the summer like Hairspray and Mamma Mia!, that tend to have good holding power as counter-programming. We're hoping we can get back on track," Fellman said.

Rock of Ages added $US4.1 million in a handful of overseas markets.

Between Rock of Ages and That's My Boy, Hollywood fell back into a pattern that persisted at the start of summer season in May as new movies debuted weakly, unable to dislodge strong holdovers.

The superhero tale The Avengers has dominated so far, remaining in the top-10 after seven weekends, its $US8.8 million take raising its domestic total to $US586.7 million. The film has taken in $US1.4 billion worldwide.

There have been only a few other hits, including Men in Black 3, Madagascar 3 and Snow White the Huntsman.

That's My Boy and Rock of Ages have joined Johnny Depp's Dark Shadows among star-driven releases that fell flat.

"It doesn't always follow that big stars bring in the big bucks," said Paul Dergarabedian, analyst for box-office tracker Hollywood.com. "Madagascar and Prometheus tell you that it's about the concept more than the stars."

DreamWorks Animation's Madagascar 3 raised its domestic total to $US120.5 million after two weekends. The movie added $US53 million overseas, bringing its international receipts to $US157 million and its worldwide haul to $US278 million.

Prometheus, released by 20th Century Fox, lifted its domestic total to $US88.9 million after two weekends. With $US25.5 million overseas, Prometheus pushed its international revenue to $US128.9 million for a worldwide haul of $US218 million.

Estimated ticket sales for Friday through Sunday at US and Canadian theatres, according to Hollywood.com. Where available, latest international numbers are also included. Final domestic figures will be released Monday