i dont know
What does Half-Life have to do with a walrus?
"air-breathing vertebrate animals characterized by the possession of hair, three middle ear bones, and mammary glands functional in mothers with young."
from the online internerds wikipedia.org.
Gabe newell is a walrus, he breaths air, he has some hair and functional mammary glands.
So yeah I beileve that walrus are mammals.