This file photo shows now-retired U.S. Marine Gen. James N. Mattis, then head of U.S. Central Command, visiting Camp Leatherneck in Afghanistan.

Marine Gen. Jim Mattis, head of U.S. Central Command, is a tough guy. In fact, if you parachuted him unarmed onto an island inhabited by psychotic ninja robots, he would get more kills than famed Scottish warrior William Wallace.

But one reader asked The Rumor Doctor about a story that shows Mattis has a compassionate side to him. The story goes that Mattis stood duty on Christmas back when he was a brigadier general so that a younger Marine could spend the holiday with his family.

Since the privilege of rank makes it extremely rare for general officers to pull duty for their subordinates, The Doctor was intrigued.

Unfortunately, Mattis was unavailable to talk. That’s not surprising considering revolution is sweeping through the CENTCOM theater, putting several vital U.S. allies at risk.

However, The Doctor was able to find out what happened from retired Marine Gen. Charles Krulak, who was commandant when the story took place. Every Christmas during his tenure, Krulak delivered cookies to every Marine duty post around Washington and Quantico, Va.

Back in 1998, he was making his final delivery to Marine Corps Combat Development Command headquarters at Quantico when he asked the Marine on duty who the officer of the day was.

“The young Marine said, ‘Sir, it’s Brigadier General Mattis.’”

Krulak thought the Marine had misunderstood him, so he asked again, but he got the same answer.

“I looked around the duty hut and in the back, there were two cots: One for the officer of the day and one for young Marine. I said, ‘OK, let me cut through all of this: Who was the officer who slept in that bed last night?’

“And the Marine said, ‘Sir, Brigadier General Mattis.’”

At that moment, Mattis walked around the corner.

“So I said to him, ‘Jim, what are you standing the duty for?’ “And he said, ‘Sir, I looked at the duty roster for today and there was a young major who had it who is married and had a family; and so I’m a bachelor, I thought why should the major miss out on the fun of having Christmas with his family, and so I took the duty for him.’ ”

Never before or since has Krulak run into a general officer standing duty on Christmas Day.

“I think it says volumes about Jim Mattis and his leadership style,” Krulak said. “He did it very unobtrusively. He just took the duty.”

THE RUMOR DOCTOR’S DIAGNOSIS: Gen. Mattis once famously said he likes to shoot Taliban, so no one is going to call him a softie. But by standing duty so that a young Marine could spend Christmas with his family, he showed you don’t have to be heartless to be a leader. But don’t expect him to pull that stuff on Presidents Day.