Adorable Tiger Cubs Turn Into Fearsome Big Cats Over Course Of 1 Year

They grow up so fast!

My, how they've grown!

Yorkshire Wildlife Park marked a trio of Amur tigers' first birthday in style in northern England on Tuesday.

The park celebrated Hector, Harley and Hope's landmark occasion with the release of adorable video showing how much they've grown over the past 12 months:

The footage shows their transformation from helpless newborns entirely dependent on their mom, Tschuna, into fearsome, big cats, who prowl and play fight as if they're going in for the kill.

The park posted the video to YouTube and Facebook on Tuesday with the message, "Happy 1st birthday to our very special (not so little anymore) tiger cubs, Harley, Hector and Hope!"

The tigers have since been a great hit with visitors since their March 2015 birth, as "they are always up to mischief," the park said on its website.

The cubs live at the park with their mom and dad, Vladimir, and another tiger called Sayan.

It's believed there are only around 540 of the endangered big cats, also known as Siberian tigers, living in the wild in their native forests of the Russian Far East.

They are at risk from poachers, the illegal wildlife trade and loss of habitat from illegal logging, according to the conservation nonprofit World Wildlife Fund for Nature.