The Nassau County Police Department picked its newest group of rookies Monday.

But they didn’t take the civil service test.

An elite group of 11 male German shepherds tried out to join the force Monday, undergoing a day of rigorous testing in which police trainers gauged their reaction to several scenarios, including physical aggression, the sound of gunshots and walking on surfaces such as metal grates.

By day’s end, the department’s police dog trainers chose four new pups — Alex, Gero, Aron II and Aron III, who will get renamed — to undergo seven months of intensive training before joining the department’s K-9 Unit.

Acting Police Commissioner Thomas Krumpter and County Executive Edward Mangano were among the officials who greeted the year-old pooches at a police training facility in Farmingdale, where the dogs fetched balls and a wallet and panted excitedly before a phalanx of news cameras.

“We want the hunt in the dog,” said Lt. Terence Loughlin, commanding officer of the K-9 Unit.

“If the dog is going after somebody that doesn’t want to be arrested, a lot of times those people are going to resist and fight with that dog. So we’re going to see how the dog reacts to physical pressure on him by a handler. We want the alpha dog, the dog that’s going to stand his ground.”

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Gero, a 14-month-old who earned a spot on the Nassau force, bounded onto a grassy field, pulling hard on his black leash.

“Good boy!” Loughlin told the dog as he successfully fetched a ball.

Police Officer Christopher Peters, a head trainer in the K-9 Unit, helped select the dogs.

The father of four sons has three dogs at home, including his own working K-9, Kai, who does bomb detection work and has been on the job for three years. His retired K-9 Duke is 14.

Also at home is his wife’s Maltese, Sparky.

“He’s the boss,” Peters said of the Maltese.

Peters said Kai, who works with the Arson Bomb Squad, is a sweetheart at home.

“They know their place,” he said. “My kids can put him in a head lock and pull his tail and he will just run around and play with them.”

But at work, he’s in “four-paw drive,” Peters said.

The four dogs, who were bred as working dogs in Europe, were purchased from Shallow Creek Kennels Inc. of Sharpsville, Pennsylvania, Loughlin said. Each dog cost $9,000.

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Two of the dogs were paid for with funds donated to the Nassau County Police Foundation, said chairman Eric Blumencranz. Foundation board members Linda Kaplan, of Commack, and Mario Doyle, of Valley Stream, made the donations.

Each will be allowed to name a dog.

Kaplan already has her dog’s moniker selected: Seren, the rank of captain in the Israeli Defense Force.