My dog passed away last Friday by accident. He was 13 years old and I have 4 other dogs. Im wondering if they even realize or know that he has died or just isn't around anymore?

Not in the same way you or I would.

They will know they aren't around... being pack animals they would know if a member of their pack isn't around. Pecking order and all will be effected.

Like smoothy said, dogs are pack animals and realize someone is missing. Even my cats realize when another cat in the group has died. Some animal behaviorists even say to allow the other animals in the group or household to sniff the dead animal, if that's possible. And yes, the hierarchy changes. There are books written about animals in the wild and how they react when a member of their herd/flock/group gets hurt or dies. A good choice, if you are interested, is When Elephants Weep: The Emotional Lives of Animals: Jeffrey Moussaieff Masson, Susan McCarthy: 9780385314282: Amazon.com: Books (http://www.amazon.com/When-Elephants-Weep-Emotional-Animals/dp/0385314280).