Saber-Tooth Tiger skull - a rare collectible.
There is three skulls: Tiger skull, Fox skull and Mammoth tusk,
that is also a rare collectible. You can found them in Shelter 2.
Where to find[]
Sabertooth skull: Go to Tundra. From there it is easier to go find the tiger skull first because for that there are no special requirements besides dragging all the cubs along. Adviced is to wait till the cubs are older or have left the mother. Go to the spot marked on the picture bellow and you will find the skull inside a cave, best is to stick close to the wall so you won't miss it.
Fox skull: The location for the fox skull is close to the mountains near the Border. There you will find the fox skull. It can be hard to find, but if you stick close to the wall you will get it.
Mammoth tusk: Just like the Tiger skull you have to go the Tundra. The requirements for this collectable is that it has to be winter to get to the mammoth. If you find it before winter you can sit and wait around untill the lake is frozen to get to it. There are no places nearby to jump on to escape wolf packs, and just like with the tiger skull in that area you can have difficulties finding food for the cubs.