Here an Eastern Cottonmouth rattles its tail to warn of its presence and desire to be left alone.
Rattlesnakes have evolved an amplification method for their tail twitching. Some dry skin remains at the end of their tail each time they shed skin, which allows for a rattle louder than a tail simply moving in dry leaves. Tail twitching is a method of announcing a snake's presence and the rattle is a highly effective design. However, rattling the tail is generally an action of last resort, so many snakes will not rattle even if a human is close.
There are stories suggesting that rattlesnakes are evolving back to quieter snakes without rattles, because non-rattling snakes go undetected and survive. However, there are not any studies that show snakes are rattling less frequently than in the past. Snakes obviously do not want to reveal their location to prey they are trying to eat and unnecessarily revealing their location to non-prey animals may result in death or injury (see the Internet for numerous images of snakes shot or chopped before they could do any "harm"). Remaining motionless and allowing camouflage to work likely allows snakes to go unnoticed and unharmed. Conversely, if a snake feels that death or injury is imminent, rattling the tail may cause the source of potential danger to move away. It always works with me.
Besides the encounter shown in the video above, other Eastern Cottonmouth snakes have alerted me to their presence when they felt I was too close. While patrolling in the swamp, I saw snake tracks in the mud, but could not find the snake. I did not want to step any farther without knowing exactly what was sharing my close surroundings. The polite cottonmouth, a few feet to my right, rattled its tail against the dead log atop which it rested to let me know its location and that it preferred to be left alone. I obliged and safely moved away. It was possessive of space, but not aggressive.
|Eastern Cottonmouth - Mark Musselman|
|Timber Rattlesnake - Mark Musselman|