MetallinosTheos is right about the four vocalists credited on the album.
Unfortunately, it doesn't specify which songs each one sings.
But on Mastedon's "It's A Jungle Out There" album it does specify who sings each song.
By comparing the vocals, I'm pretty confident that Dave Amato is the guy singing "Life on the Line" and "When It All Comes Down".
If you have "It's A Jungle Out There," listen to the songs Amato sings. I'm pretty sure it's him.