Composer Brian Tyler, was hired to the score for the film. The film's main theme track is a clash of two main themes, one consisting of the Predator type theme (bongos and basses) and the second of the Aliens (high pitched violins, violas and flutes). The directors Colin and Greg Strause wanted to take a new direction from Harald Kloser's Alien vs. Predator score and wanted Tyler to use some reference to the three films' original score pieces, such as the horrific violas and percussion from James Horner's Aliens and the primitive tribal percussion from Alan Silvestri's Predator and Predator 2. Tyler also referenced composer John Frizzel's Alien: Resurrection into the score, in the tracks of "National Guard" and "Taking Sides". The soundtrack album was released on December 11, 2007 by Varèse Sarabande. "Grim Discovery" and "The Chopper", the two tracks from Predator was listed in the film's end credits, both written and composed by Silvestri. The tracks are not included in the film's soundtrack and the Predator soundtrack, but can be found on YouTube. The score tracks ("National Guard", "Power Struggle" and "Predator Arrival"), were also used again in the 2010 film Unstoppable.