Brian Truitt

USA TODAY

After ruling the box office for three weeks, the fabulous fish of Finding Dory were deep-sixed by another pack of talking animals.

The animated comedy The Secret Life of Pets chomped at the heels of its competition with an enormous $103.2 million debut, according to studio estimates from comScore. Featuring the voices of Louis C.K. and Eric Stonestreet as a pair of domesticated dogs unleashed in the big city, the movie played mostly well for critics (with 76% approval on review aggregate site RottenTomatoes.com) and definitively so for moviegoers (an A- at CinemaScore).

Pets secured the largest opening ever for an original film that wasn't based on previous source material (such as a book or comic), surpassing Inside Out's $90.4 million last year.

"If any pets were wondering what their owners were doing this weekend, they were seeing this movie," says comScore senior media analyst Paul Dergarabedian, adding that Secret Life "overperformed massively," thanks to a great title and an irresistible concept. "This proves if you have a truly original idea — not a sequel, not a franchise, not something already known — that you can still create a massive hit."

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The Legend of Tarzan came in second place with $20.6 million, edging out Finding Dory, which earned $20.4 million. More impressively, though, the Finding Nemo sequel surpassed Captain America: Civil War as the biggest movie of the year thus far in just 24 days. With a $422.6 million domestic haul, Finding Dory looks poised to soon become the highest-grossing animated movie of all time, when it breaks the North American record held by Shrek 2's $436.7 million.

Dory was a critical success perfectly timed in a year with strong family-friendly performers (such as The Jungle Book and Zootopia), but being part of Pixar is its real "magic dust," Dergarabedian says. "For it to make this much money shows you how powerful the brand is."

Fourth place went to the raunchy new comedy Mike and Dave Need Wedding Dates with $16.6 million. Reviews (41% positive on Rotten Tomatoes) were mixed for the Zac Efron and Adam Devine vehicle, and a B on CinemaScore means audiences weren’t totally approving, either. Still, it exceeded expectations in the low teens, Dergarabedian says.

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Tossing out a bawdy film to go against family fare was "a classic counterprogramming strategy," he says. Plus, Mike and Dave contributed to a solid box-office weekend "that could prove to be a real turning point for a summer movie season that has been trending downward."

Rounding out the top five was the horror sequel The Purge: Election Year with $11.7 million.

Art-house offering Captain Fantastic lived up to the name: The family drama starring Viggo Mortensen made $98,500 in its initial limited-run opening — averaging $24,600 per screen.

Final figures are expected Monday.