- Common names: tiger rattlesnake, tiger rattler.
Crotalus tigris is a highly venomous pit viper species found in the southwestern United States and northwestern Mexico. No subspecies are currently recognized. The specific name, tigris, Latin for "tiger", refers to the many narrow dorsal crossbands, which create a pattern of vertical stripes when viewed from the side.
- 1 Physical Description
- 2 Distribution and Geographic range
- 3 Habitat and Ecology
- 4 Behavior and Habits
- 5 Diet and Food Habits
- 6 Predators
- 7 Reproduction
- 8 Development
- 9 Conservation status
- 10 Relevance to Humans and Ecosystems
- 11 Venom
- 12 See also
- 13 References
- 14 Further reading
- 15 External links
Tiger rattlesnakes are easily identified by their small, spade shaped head, which is about 1/25 of their total body length. They have the smallest head of any rattlesnake and a large rattle. The color pattern consists of a gray, lavender, blue-gray, pink, or buff ground color that usually turns to pink, pale orange or cream on the sides. Tiger rattlesnakes are the only rattlesnake with crossbands on the anterior portion of the body, with a series of 35 to 52 gray, olive, or brown bands across the dorsum consisting mainly of heavy punctations. These crossbands have vague borders and are wider dorsally than laterally. Also mid-dorsally, the crossbands become wider than the spaces that separate them. Posteriorly, the crossbands become darker and more clearly defined. They have 6 to 10 posterior rings. The markings on the head are mostly vague and irregular, although towards the rear of the head, a few dark markings may be arranged as paired occipital blotches and upper temporal streaks. The most distinguishable mark on the head is a dark cheek strip. Dorsal scales are keeled and in 21 to 27 rows. A relatively small species, individuals can weigh as much as 454 g (16.0 oz) and can range in length from 460 mm (18 in) to 910 mm (36 in), with an average length of 609 mm (24.0 in). The largest specimen on record measured 88.5 cm (34.8 in) (Klauber, 1956), until H.M. Smith and Brodie (1982) reported a maximum length of 91.2 cm (35.9 in). Females have 164 to 177 ventral scales, and males have 158 to 172 ventral scales. Females have 16 to 21 caudal scales, and males have 23 to 27 caudal scales and are typically larger than females. They have relatively small eyes with an elliptical pupil.
|Range Mass||Range Length||Average Length||Other Physical Features||Sexual Dimorphism|
|454 g (16.0 oz)||460 mm (46 cm; 18 in) – 910 mm (91 cm; 36 in)||609 mm (60.9 cm; 24.0 in)||ectothermic; heterothermic; bilateral symmetry; venomous||Males larger than females|
Distribution and Geographic range
The species' range extends from central Arizona south through south-central Arizona in the United States, to southern Sonora, Mexico, including Isla Tiburón in the Gulf of California and was recently discovered in the southern Peloncillo Mountains of Arizona. The type locality is described as "Sierra Verde and Pozo Verde". The latter is a spring located on the Sonora side of the US-Mexico border, near Sasabe. According to Stejneger (1893), this spring is on the western slope of the southern Sierra Verde, which is also known as the Sierra del Pozo Verde. In these areas, the Tiger rattlesnake is observed at elevations from sea level to about 1,465 metres (4,806 ft) Reported occurrences at higher elevations have not been confirmed.
Habitat and Ecology
The Tiger rattlesnakes habitats include rocky desert canyons, foothills, and bajadas, in vegetation zones ranging from thornscrub, ocotillo-mesquite-creosote bush, saguaro-paloverde, mesquite grassland, and chaparral to tropical deciduous forest (southern Sonora) and the lower edge of oak woodland. In southeastern Arizona, this snake occurs strictly in rocky areas in winter and spring but uses edges of arroyos in summer. It is a terrestrial species but may climb into low vegetation.
Tiger rattlesnakes have also often been observed in the foothills, rocky canyons, and ravines of deserts or mesquite grasslands from 1,000 feet (300 m) to 5,000 feet (1,500 m) in elevation, throughout their geographic range. Tiger rattlesnakes also inhabit escarpments, outcroppings and cliff-faces in thorny scrub desert habitat. In southeastern Arizona, mean home range size was 3.48 hectares (0.0134 sq mi; 0.0348 km2), and individuals moved an average of 33 metres (108 ft) per day during the active season.
Little information is available concerning the average home range size of tiger rattlesnakes. One study reported an observed home range of approximately 3.5 km2 (1.4 sq mi).
Behavior and Habits
Tiger rattlesnakes are terrestrial (ground-dwelling) snakes and are nocturnal during the hot summer months (June–August), but become diurnal and crepuscular in during the fall season. It hibernates during the cold months of late fall and winter in rock crevices or animal burrows. In spite of being a ground-dwelling inhabitant of the desert, its activity is not restricted to the ground. It swims readily and also has been found in bushes 60 cm (24 in) above the floor. There is little information available concerning communication and perception among tiger rattlesnakes. However, like other pit-vipers, tiger rattlesnakes have heat sensing pits (located on each side of the face between the eye and nostril) to detect warm-blooded predators and prey.
This snake is inactive in cold temperatures (December/January) and extreme heat (July/August). It may be active both day and night; daytime activity consists mainly of basking on cool days. These snakes are often observed being active after warm rains. In southeastern Arizona, they are active mainly from March to October.
Diet and Food Habits
Tiger rattlesnakes generally feed on mammals such as rodents, but they have also been known to prey on lizards. The tiger rattlesnake ambushes much of its prey but also actively forages small rodents and lizards, juveniles relying heavily on lizards and adults depending more on rodents. In addition, these small rattlesnakes have been known to eat fairly large prey, including kangaroo rats, deer mice, packrats, and even spiny lizard. This species' venom is considered the most toxic of all rattlesnakes and contains a myotoxin known to cause muscle necrosis and a neurotoxin similar to Mojave toxin. Like all venomous snakes, tiger rattlesnakes inject venom into prey through long, hollow, retractable fangs. If envenomated prey crawl into a small crevice, this species is especially suited for extracting them due to its unusually small head.
There is little to no information available regarding predators specific to tiger rattlesnakes. However, likely predators include hawks, eagles, coyotes, and other snakes. Their cryptic coloration helps camouflage them from potential predators and helps reduce risk of predation. If disturbed, they rapidly shake their rattle and may strike in defense. Tiger rattlesnakes are known to have an irascible temperament and are easily excitable and can be quite aggressive.
Tiger rattlesnakes are polygynandrous, and either the male, female, or both have more than one mate within a single breeding season. Little else is known of the reproductive behavior of this species. The reproductive behavior of this species is thought to be similar to the that of Crotalus atrox and Crotalus scutulatus. Copulation in viperids can take minutes, hours, or days and can occur multiple times within a couple of days.
Tiger rattlesnake females follow a biennial reproductive cycle. Males follow a seasonal reproductive cycle, where sperm is stored in the vasa deferentia during winter. Breeding occurs from late May to mid August, during the summer monsoon season. Like the majority of rattlesnakes, tiger rattlesnakes are ovoviviparous. Mean clutch size for is 4 to 6 young. The smallest known sexually reproducing female measured 541 mm (21.3 in; 54.1 cm) snout-vent length (SVL), while the smallest mature male measured 512 mm (20.2 in; 51.2 cm) SVL.
Generally, rattlesnakes invest little in offspring following birth. However, like other viperids, female tiger rattlesnakes invest in provisioning resources for developing embryos. She eats early in the pregnancy and then find a safe place to hide while providing the optimal thermal environment for development.
|Breeding interval||Breeding season||Range number of offspring||Average number of offspring|
|Tiger rattlesnake females mate biennially, and males mate annually||Late May to mid August, during the summer monsoon season||1 to 6||4 to 6|
Tiger rattlesnake embryos are retained inside the female in a transparent, membranous sac, where some materials and gasses are exchanged between embryo and mother. Embryos receive fluids and sustenance from the yolk mass. Once born, neonates break through the embryonic sac and travel a short distance to a safe nook with its siblings. Young tiger rattlesnakes are not born with a rattle. Neonates have a skin cap at the tip of the tail and after every molt, a new rattle segment is added. Like all rattlesnakes, tiger rattlesnakes are ovoviviparous and thus, are well developed at birth.
This species is classified as Least Concern on the IUCN Red List of Threatened Species (v3.1, 2001). Species are listed as such due to their wide distribution, presumed large population, or because they are unlikely to be declining fast enough to qualify for listing in a more threatened category. The population trend was stable when assessed in 2007.
The population trend of the tiger rattlesnake is considered to be stable. On a range-wide scale, Campbell and Lamar (2004) mapped 33 collection sites. Lowe et al. (1986) stated that the species is known from approximately 100 localities throughout the range. The adult population size is unknown but presumably exceeds 10,000. This snake is fairly common in some areas, but "some local populations seem small" (Ernst and Ernst 2003). Its extent of occurrence, area of occupancy, number of subpopulations, and population size are probably relatively stable or declining at a rate of less than 10% over 10 years or three generations.
Southern populations on the flatter areas of the coastal plain of Sonora are probably losing habitat due to the intensification of agriculture. Overall, however, this species is not seriously threatened.
Relevance to Humans and Ecosystems
Although tiger rattlesnakes are reluctant to strike, they are highly venomous, cantankerous and as a result pose a potential threat to humans. Their venom contains a neurotoxin called Mojave toxin and a myotoxin known to cause muscle necrosis. Although venom production is low compared to other rattlesnakes, their venom is the most toxic of any snake in the Western Hemisphere. The combination of neuro- and mytoxins in their venom makes them extremely dangerous to humans.
In general, rattlesnake skin and tail rattles are often considered valuable and are often sold as souvenirs throughout the American Southwest. Rattlesnake venom is often used in biomedical research investigating neurological diseases. Finally, tiger rattlesnakes prey upon a number of rodent species considered pests by humans throughout their geographic range. Tiger rattlesnakes feed on a number of small vertebrate species and likely help regulate their abundance and distribution. There is no information regarding parasites specific to this species.
Although it has a comparatively low venom yield, its toxicity is considered to be the highest of all rattlesnake venoms, and the highest of all snakes in the Western Hemisphere. It has a high neurotoxic fraction that is antigenically related to Mojave toxin (see Crotalus scutulatus, venom A), and includes another component immunologically identical to crotamine, which is a myotoxin also found in tropical rattlesnakes (see Crotalus durissus). A low but significant protease activity is in the venom, although there does not seem to be any hemolytic activity.
Brown (1973) lists an average venom yield of 11 mg (dried venom) and an LD50 value of 0.6 mg/kg IP for toxicity. Other studies give LD50 values of 0.07 mg/kg IP, 0.056 mg/kg IV, and 0.21 mg/kg SC. Minton and Weinstein (1984) list an average venom yield of 6.4 mg (based on two specimens). Weinstein and Smith (1990) list a venom yield of 10 mg.
There is very little information available for bite symptoms. Human bites by the tiger rattlesnake are infrequent, and literature available on bites by this snake is scarce. The several recorded human envenomations by tiger rattlesnakes produced little local pain, swelling, or other reaction following the bite, and despite the toxicity of its venom no significant systemic symptoms. The comparatively low venom yield (6.4–11 mg dried venom) and short 4.0 mm (0.40 cm) to 4.6 mm (0.46 cm) fangs of the tiger rattlesnake possibly prevent severe envenoming in adult humans. However, the clinical picture could be much more serious if the person bitten was a child or a slight build individual. The early therapeutic use of antivenom is important if significant envenomation is suspected. Despite the low venom yield, a bite by this rattlesnake should be considered a life-threatening medical emergency. Untreated mortality rate is unknown but this snake has a very high venom toxicity and its bites are capable of producing major envenomation.
- List of crotaline species and subspecies
- Crotalus by common name
- Crotalus by taxonomic synonyms
- Crotalinae by common name
- Crotalinae by taxonomic synonyms
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