Curators of a Minnesota wolf center say the artic wolf pups that arrived in 2016 have reached their challenging years and have been testing behavioral limits this winter.

Axel and Grayson will turn 2 years old this May at the International Wolf Center in Ely, the Mesabi Daily News reported. The pups are troublemakers of sorts and will ride up on shoulders, mount each other and pull each other's tails.

"You may have heard of the phrase, 'terrible twos.' We have a similar concept in wolf management, only it's more like, 'challenging yearlings,'" said Center Curator Lori Schmidt.

Much of the behavior is instinctual and hormone driven, she said. Colder temperatures and snowy weather can also increase pack intensity, she said. Wolves produce more reproductive hormones -- testosterone and estrogen -- in the winter months. In April, they begin producing prolactin, a nurturing hormone.

"There will always be peaks and valleys" when it comes to behavior, Schmidt said. "It's all natural behavior, and it's important not to put human emotion to it."

The challenging pack dynamics should lessen as winter comes to a close, she said.

Young wolves start testing at about 9 months old and will reach full maturity between 18 and 24 months, Schmidt said. Their personality traits and attitudes can intensify during the maturation time, she said.

The pack's dynamic will continue to change as the pups mature, and as the pack's current leader, Aiden, reaches retirement age, Schmidt said.

The center introduces new pups to the pack every four years. The Northwestern subspecies will be added in 2020.