Dog goes woofCat goes meowBird goes tweetAnd mouse goes sque . . .

Dog goes woof
Cat goes meow
Bird goes tweet
And mouse goes squeek

Cow goes moo
Frog goes croak
And the elephant goes toot

Ducks say quack
And fish go blub
And the seal goes ow ow ow

But there’s one sound
That no one knows
What does the fox say?
. . .

In the song "The Fox" by the Norwegian duo "Ylvis", which animals are not mammals?