Photos: A fighter who will fight anyone above his weight class

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Justin Zellweger fights Gabriel Duckworth at Anchorage's Egan Convention Center on Thursday, January 29, 2015. Zellweger, known in the ring as "Tire Man," is known for taking on anyone who will challenge him, regardless of their weight. Duckworth is 48 pounds heavier and more experienced.

Why fight?

It's a question that harkens back to perhaps the animal kingdom's longest-held instinct, the life-or-death choice between running or standing your ground. And for those who choose to step between the ropes and into the Thursday Night at the Fights spotlight each week at the Egan Convention Center, it's a question with many answers.

Some fight to get into shape, others for the challenge or experience or maybe because they just like punching people – and getting punched – in the face. Before one recent bout, first-time boxer William Keller Jr., who said he carries logs through the woods for a living, had a bluntly honest answer.

"To get tough," he said before getting knocked down but not out in a three-round loss to fellow newcomer Junior Faumuina.

Read more: Fighting for a cause