Serious Survival Mode
     To unlock Serious Survival Mode, where you try to survive as long
as possible without a partner, beat the game in regular survival mode.

Another house location:
     Go far in the jungle to find a grassy place. You can make another
house there.

Getting food and firewood:
     You can ask your partner for food and firewood. When you have 40%
or less, tell her or him to get food or firewood.

The ruins: Hidden area:
     If  you  look  closely on the walls of the ruins, you will notice
what  appear  to  be old style fire pits. Light these with your torch.
The  wall will open up and you will gain access to a new room. One way
will  be  a  new exit to the swamplands. Two openings will lead you to
rooms  that  have huge pedestals that must be lit with your torch. You
must do these in the order as they appear one after another. They will
lead you all over the island.

Sleep:
     You  can  only  go  to  sleep if you do not have 0% for water and
food.  However,  that  only  applies  to your character. The game will
still let you and your partner go to sleep if you at least have 1% for
food  and  water,  even if your partner is at 0%. You can both rest to
fill  up the stamina meter as long as the character you're playing has
food/water. Hold the partner's hand to rest.