Specimens over 7 feet (213 cm) are rare, but well documented. Klauber (1997) includes a letter he received from E. Ross Allen in 1953, in which Allen explains how for years he offered a reward of $100, and later $200, for an 8-foot (243.8 cm) specimen, dead or alive. The reward was never claimed. He did receive a number of specimens and some skins, but said that such skins can be taken from a 6-foot (182 cm) snake.
The average size is much less: lengths of 3.5-5.5 feet (106-167 cm), 84-183 cm are given. One study found an average length of 170 cm based on 31 males and 43 females.
The scalation includes 25-31 (usually 29) rows of dorsal scales at midbody, 165-176/170-187 ventral scales in males/females and 27-33/20-26 subcaudal scales in males/females. On the head, the rostral scale is higher than it is wide and contacts 2 internasal scales. There are 10-21 scales in the internasal-prefrontal region and 5-11 (usually 7-8) intersupraocular scales. Usually there are 2 loreal scales between preoculars and the postnasal. There are 12-17 (usually 14-15) supralabial scales, the first of which is in broad contact with the prenasal, and 15-21 (usually 17-18) sublabial scales.
The color pattern consists of a brownish, brownish yellow, brownish gray or olive ground color, overlaid with a series of 24-35 dark brown to black diamonds with slightly lighter centres. Each of these diamond-shaped blotches is outlined with a row of cream or yellowish scales. Posteriorly the diamond shapes become more like cross-bands and are followed by 5-10 bands around the tail. The belly is a yellowish or cream color with diffused dark mottling along the sides. The head has a dark postocular stripe that extends from behind the eye backwards and downwards to the lip; the back of the stripe touches the angle of the mouth. Anteriorly and posteriorly, the postocular stripe is bordered by distinct white or yellow stripes.
This species has been declining for years due to habitat destruction, hunting and persecution. It has been suggested that, because of the decline of these snakes, the rabbit population in Florida is on the rise.
Like most rattlesnakes, this species is terrestrial and not adept at climbing. However, they have on occasion been reported in bushes and trees, apparently in search of prey. Even large specimens have been spotted as much as 10 m above the ground.
In contrast, they are well known to be excellent swimmers. Specimens have often been spotted crossing stretches of water between barrier islands and the mainland off the Georgia coast, in the Gulf of Mexico and in the Florida Keys, sometimes miles from land.
Individual disposition varies, with some allowing close approach while remaining silent, and others starting to rattle at a distance of 6-9 m. The rattle is well developed and can be heard from relatively far away. When threatened they raise the anterior half of the body off the ground in an S-shaped coil and strike to a distance of at least a third of their body length. Many will stand their ground and may strike repeatedly, but if given the opportunity they will usually retreat while facing the intruder and moving backwards towards shelter, after which they disappear.
One popular myth is that these snakes must rattle before striking. They are, of course, quite capable of striking while remaining completely silent. In fact, according to one hypothesis, individuals that remain silent are less likely to be heard, seen and killed, and therefore more likely to pass on their genes to the next generation, leading to the idea that we are selecting for rattlesnakes that do not rattle.
Hawks, eagles, and other snakes have been known to prey upon young and adolescent specimens.
Because of their large size, the adults have no problem eating prey as large as fully-grown cottontail rabbits. As the juveniles are capable of swallowing adult mice, even they do not often resort to eating slimmer prey, such as lizards. In fact, eastern cottontails and marsh rabbits (Sylvilagus) form the bulk of their diet in most parts of Florida. Squirrels, rats, and mice are also on the menu, along with birds such as towhees and quail. Other prey that have been reported include a king rail, bobwhites, a young wild turkey, and a mother woodpecker along with four of her eggs.
Females give birth to between 7 and 21 young at a time, usually doing so between July to early October. Neonates are 30-36 cm in length and are similar in appearance to the adults, except for having only a small button instead of a rattle on the tip of the tail.
Adult wild-caught specimens are often difficult to maintain in captivity, but captive-born individuals do quite well and feed readily on pre-killed laboratory rodents. They require a dry and well-ventilated cage with a hide-box, maintained a temperature of 23-27°C for normal activity.
This species has the reputation of being the most dangerous venomous snake in North America. While not usually aggressive, they are large and powerful. Wright and Wright (1957) mention a mortality rate of 30% and that some victims have died within a matter of hours.
In proportion to its length, it has the longest fangs of any rattlesnake species, with calculations leading us to expect that an 8-foot (240 cm) specimen would have fangs with a total length of 27 mm (over one inch). For comparison, a 160 cm (5 ft. 3 in.) specimen had fangs with a length of 17 mm. It has a very high venom yield: an average of 400-450 mg, with a maximum of 858-1,000 mg. Brown (1973) gives an average venom yield of 410 mg (dried venom), along with LD50 values of 1.3-2.4 mg/kg IV, 1.7-3.0 mg/kg IP and 14.5-10 mg/kg SC for toxicity. The estimated human lethal dose is 100-150 mg.
The venom contains a thrombin-like enzyme (TLE), called crotalase, that is capable of clotting fibrinogen, leading to the secondary activation of plasminogen from endothelial cells. Although the venom does not activate platelets, the production of fibrin strands can result in a reduced platelet count, as well as the hemolysis of red blood cells. Even with this defibrination, however, clinically significant bleeding is uncommon (Hasiba et al., 1975). Nevertheless, the venom does exhibit high hemorrhagic activity (Minton, 1974). It also contains a low-molecular-weight basic peptide that impedes neuromuscular transmission (Lee, 1972) and can in theory lead to cardiac failure. This peptide is similar to crotamine from C. durrisus terrificus and makes up 2-8% of the protein found in the venom. In general the venom can be described as highly necrotizing, mildly proteolytic and containing a large phosphodiesterase fraction. It stimulates the release of bradykinin that can result in severe pain, as well as profound, transient hypotension.
Klauber (1997) describes one case in which the symptoms included instant pain "like two hot hypodermic needles," spontaneous bleeding from the bite site, intense internal pain, bleeding from the mouth, hypotension and a weak pulse, swelling and discoloration of the affected limb and associated severe pain. The symptoms were further described as strongly hemolytic and hemorrhagic.
CroFab and Wyeth's ACP are effective antivenins against bites from this species, although massive doses may be needed to manage severe cases of envenomation. Generally, ACP is very effective at countering the defibrination syndrome that is often seen, but may do little for low platelet counts.