What quality defines the snake's use of its rattle when sufficiently alarmed?

Answer

It is an active warning requiring conscious effort

The act of rattling is an active warning; the snake must make a conscious effort to shake the rattle, signaling awareness of the threat before striking.

What quality defines the snake's use of its rattle when sufficiently alarmed?

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Species spotlight: Northern Pacific Rattlesnake (Crotalus oreganus)

venomhabitatanimalserpentRattlesnake