Mexican west coast rattlesnake, (Crotalus basiliscus), large, stout, highly venomous pit viper (subfamily Crotalinae, family Viperidae) of dry coastal lands and cactus forests from southern Sonora to Oaxaca in western Mexico. The Mexican west coast rattlesnake is not normally an aggressive snake, preferring to disappear into a rock crevice or animal burrow when disturbed, but its habit of lying alongside paths and roads makes it perilous to humans. It is not at risk of extinction and is listed as a species of “least concern” by the International Union for Conservation of Nature and Natural Resources’ Red List of Threatened Species.
The average adult is 1.5 meters (5 feet) long, though some individuals measure more than 2 meters (6.5 feet) in length. The snake is pale green to gray-olive in color, with large beady scales. It can have a dark, white-bordered diamond pattern along the back similar to that of its northern diamondback relatives (see eastern and western diamondback rattlesnake). The head is unpatterned, though some individuals have a darker green bar extending diagonally from the eye to the back of the jaw. Like those of other rattlesnakes, the hollow fang of the Mexican west coast rattlesnake is similar to a curved hypodermic needle. When the snake’s mouth is closed, the fangs are folded back and lie parallel to the roof of the mouth. Linkages of bones in the upper jaw allow the fangs to be deployed into a vertical position for stabbing and biting. Large venom glands at the base of the jaws are responsible for the distinctly triangular shape of the broad head. The tail’s rattle, presumably a warning device, is composed of horny, loosely connected, hollow segments, one of which is added every time the snake sheds its skin.
The Mexican west coast rattlesnake is active in the cooler parts of the day, becoming nocturnal during the hot summer months. It hunts by waiting for mammals or ground birds to cross its path. Supplementing its vision is a pair of heat sensors—a pit, or depression—one on each side of the snout that detects infrared radiation. Most living organisms give off heat in the form of infrared radiation; hence, any body that is warmer than its surroundings can be detected by these organs. After striking its prey and injecting large amounts of venom, the snake waits for a few moments and then follows the scent trail to its meal. Its venom is especially potent, containing not only hemotoxins, which destroy tissue, but also neurotoxins, which paralyze the nervous system. The bite is very painful and can be lethal to humans, though antivenin is effective if administered quickly in sufficient doses.
Mexican west coast rattlesnakes give birth to live young that develop from eggs retained inside the mother (ovoviviparity). The newborns can be as long as 36 cm (14 inches) and have functioning fangs and venom glands. Their venom is more potent but of lesser quantity than that of their mother, a condition that helps ensure that the young can secure food. Babies are also equipped with a single button on the end of the tail. After the first shedding of their skin (usually within a week of birth), they will have two rattle segments. They can buzz like adults once the third rattle segment has been obtained.