The fellow, approximately 4 feet in length is displaying his/her most aggressive pose as a warning.
Because of its special venom (which contains a neurotoxin) the Mojave Rattlesnake is supposedly the most dangerous snake in the USA. Why is this important ... because in case of a bite a different anti-venom treatment is required for the Mojave as versus the Western Diamondback.
The Mojave Rattlesnake can range through Arizona, New Mexico, and Texas. Additionally the snake comes in a broad variety of colors that allow it to blend into whatever the countryside happens to look like; this color variant issue makes the identification process a very difficult one.
One of the key differentiators between the Mojave and Western Diamondback is the banding near the tail. The width of the Mojave’s white (or lighter colored) stripes are greater than that of the dark stripes (on the other hand the Western Diamondback has dark stripes that are wider than the white stripes). Additionally, again regarding the Mojave, the white band that runs below the eye does not intersect with the mouth.