Tiger rattlesnake (Crotalus tigris)

Description: The Tiger rattlesnake, scientifically known as Crotalus tigris, boasts an array of distinctive characteristics that make it readily distinguishable from other rattlesnake species:

Head: A notable feature of the tiger rattlesnake is its diminutive, spade-shaped head, which accounts for roughly 1/25 of its total body length. In fact, it possesses the smallest head among all known rattlesnakes. Rattle: At the end of its tail, the tiger rattlesnake possesses a relatively large rattle, which it employs as a warning instrument when feeling threatened.

Coloration and Pattern: These rattlesnakes exhibit a captivating color pattern. Their dorsal, or back, color can range from hues of gray, lavender, blue-gray, pink, to buff, often transitioning to pink, pale orange, or cream on the sides. However, what truly distinguishes them is the presence of crossbands on the anterior (front) part of their bodies. These crossbands, numbering between 35 to 52, are typically gray, olive, or brown, featuring prominent heavy punctations. These crossbands tend to have indistinct borders and are broader dorsally (towards the back) than laterally (on the sides). Towards the mid-dorsal region, these crossbands become progressively wider than the spaces that separate them. 

Head Markings: While the markings on the head of the tiger rattlesnake are generally vague and irregular, towards the rear of the head, one may discern a few dark markings arranged as paired occipital blotches and upper temporal streaks. The most prominent head marking is a distinctive dark cheek strip.

Scales: Dorsal scales are keeled and arranged in 21 to 27 rows.

Size: Tiger rattlesnakes are relatively small in size when compared to other rattlesnake species. Individuals can reach weights of up to 454 grams (16.0 ounces) and vary in length from 460 to 910 mm (18 to 36 inches), with an average length of approximately 609 mm (24.0 inches). The largest recorded specimen, as documented by Klauber in 1956, measured 88.5 cm (34.8 inches) until a subsequent report by H.M. Smith and Brodie in 1982, which documented a maximum length of 91.2 cm (35.9 inches).

Sexual Dimorphism: Females typically exhibit 164 to 177 ventral scales, while males typically have 158 to 172 ventral scales. Caudal scale counts also differ between sexes, with females having 16 to 21 caudal scales and males having 23 to 27. Males are generally larger than females.

Eyes: Tiger rattlesnakes have relatively small eyes characterized by elliptical pupils. 

Distribution:

The distribution and habitat of the tiger rattlesnake are defined by its adaptation to the challenging desert environments of Arizona and southern Mexico:

Geographic Range: The tiger rattlesnake is primarily found in central, south-central, and extreme southeastern Arizona in the United States. It occupies elevations ranging from approximately 1,000 feet to 5,000 feet.

Habitat:

Habitat Preferences: Tiger rattlesnakes inhabit a variety of ecosystems, including Arizona Upland Sonoran Desertscrub, Chihuahuan Desertscrub, Interior Chaparral, and Madrean Evergreen Woodland communities. They tend to favor rocky slopes, washes within rocky mountains and foothills, and occasionally desert flatlands. While they may venture into the desert flatlands, they typically remain within a mile of foothills, mountains, or rocky habitats.

Diverse Habitats: The species demonstrates adaptability to diverse habitats, including rocky desert canyons, foothills, and bajadas. They can be found in different vegetation zones, ranging from thornscrub, ocotillo-mesquite-creosote bush, saguaro-paloverde, mesquite grassland, chaparral, and even tropical deciduous forests in southern Sonora. They also occupy the lower edge of oak woodlands. In southeastern Arizona, tiger rattlesnakes are commonly found in rocky areas during winter and spring, transitioning to the edges of arroyos in the summer. While primarily terrestrial, some individuals may occasionally climb into low vegetation.

Altitudinal Range: Tiger rattlesnakes have a significant altitudinal range within their distribution area, from near sea level along the Colorado River to about 2,800 feet.

Specific Habitats: They are chiefly associated with Mohave Desertscrub and Lower Colorado River Valley Sonoran Desertscrub but may also enter relatively level terrain in the lower reaches of Arizona Upland Sonoran Desertscrub. They are frequently observed in open, flat desert areas with creosotebush growth and sandy or loamy soils. Additionally, they are common in dune fields in southwestern and western Arizona.

Despite their limited geographic range, tiger rattlesnakes showcase their adaptability to various elevations and habitats within the arid landscapes of Arizona and southern Mexico. However, the major threat to their survival remains human expansion into their territory.

Behavior:

The behavior of the tiger rattlesnake is characterized by a range of adaptations to its desert habitat and seasonal variations:

Seasonal Activity: Tiger rattlesnakes exhibit distinct activity patterns that vary with the changing seasons. During the hot summer months of June to August, they are primarily nocturnal, coming out at night to avoid the scorching daytime heat. However, as fall arrives, they transition to a diurnal and crepuscular schedule, becoming active during daylight and twilight hours.

Hibernation: When late fall and winter bring cold temperatures, tiger rattlesnakes enter a period of hibernation. They seek refuge in rock crevices or animal burrows to escape the chilly weather, conserving energy until warmer conditions return.

Versatile Habitat Use: While they are predominantly ground-dwelling, tiger rattlesnakes are not confined to the ground alone. They are adept swimmers and have been found in bushes approximately 60 cm (24 inches) above the ground. This versatility in habitat use allows them to navigate their environment effectively.

Sensory Adaptations: Tiger rattlesnakes, like other pit vipers, possess specialized heat-sensing pits located on each side of their face between the eye and nostril. These heat-sensing pits enable them to detect warm-blooded predators and prey, enhancing their ability to locate food sources and avoid potential threats.

Inactivity in Extreme Conditions: Inactivity is observed during extreme temperature conditions. Tiger rattlesnakes are inactive during the cold months of December and January when temperatures drop significantly and during the peak of heat in July and August when daytime temperatures become excessively high. On cooler days, they may engage in basking behavior, which involves warming themselves in the sun.

Response to Rainfall: Tiger rattlesnakes are often observed becoming active after warm rains. These rains likely provide moisture and increase the activity of their prey, making it an opportune time for hunting.

Seasonal Activity in Southeastern Arizona: In southeastern Arizona, their primary activity period spans from March to October, aligning with the milder temperatures of spring, summer, and early fall.

Diet:

The diet of the tiger rattlesnake primarily consists of a variety of small mammals and lizards. This versatile predator exhibits different hunting strategies and prey preferences throughout its life stages:

Prey Species: Tiger rattlesnakes primarily target mice and other small mammals as their main source of food. They are also known to prey on lizards, particularly in their juvenile stages.

Hunting Methods: These rattlesnakes employ a combination of ambush and active hunting strategies. They often lie in wait, ambushing their prey when it comes within striking distance. However, they are not solely reliant on ambush tactics and actively hunt for small rodents and lizards.

Age-Related Diet: Juvenile tiger rattlesnakes tend to rely heavily on lizards as a significant portion of their diet. As they mature into adults, their diet shifts more towards rodents.

Diverse Prey Selection: Tiger rattlesnakes have demonstrated the ability to consume a wide range of prey, including pocket mice, kangaroo rats, deer mice, packrats, and even spiny lizards. This adaptability in prey selection highlights their ability to thrive in varied desert ecosystems.

Venomous Predation: To subdue and begin digesting their prey, tiger rattlesnakes use venom delivered through their long, hollow, retractable fangs. Their venom is particularly potent, considered the most toxic among all rattlesnake species. It contains a myotoxin that can cause muscle necrosis and a neurotoxin similar to Mojave toxin.

Efficient Extraction: In cases where envenomated prey seeks refuge in small crevices, the tiger rattlesnake's unusually small head allows it to be especially suited for extracting prey from tight spaces. The tiger rattlesnake's diet is a testament to its adaptability and effectiveness as a carnivorous predator in desert ecosystems, where it plays a crucial role in controlling small mammal and lizard populations.

Reproduction:

The reproduction and life cycle of the tiger rattlesnake are characterized by specific behaviors and adaptations to its desert environment:

Mating Season: Mating among tiger rattlesnakes occurs during the summer monsoon months of July and August when desert conditions are relatively more favorable.

Reproductive Behavior: Tiger rattlesnakes exhibit polygynandrous mating behavior, where both males and females may have multiple mates within a single breeding season. However, detailed information about their specific reproductive behavior is limited. It is believed to be similar to that of closely related rattlesnake species such as C. atrox and C. scutulatus.

Reproductive Cycles: Female tiger rattlesnakes follow a biennial reproductive cycle, while males have a seasonal reproductive cycle. During the winter, males store sperm in their vasa deferentia in preparation for breeding. Breeding typically occurs from late May to mid-August, coinciding with the summer monsoon season in the desert.

Reproductive Mode: Tiger rattlesnakes are ovoviviparous, which means that they give birth to live young instead of laying eggs. The average clutch size is around five offspring.

Female Investment: Female tiger rattlesnakes invest in their offspring by providing resources for developing embryos. They consume food early in pregnancy and then seek out a secure location to create an optimal thermal environment for embryo development.

Embryo Development: Tiger rattlesnake embryos are retained inside the female in a transparent, membranous sac. Within this sac, certain materials and gases are exchanged between the embryo and the mother. Embryos receive sustenance from the yolk mass, which nourishes them during development.

Birth and Rattles: Once fully developed, neonates (young snakes) break through the embryonic sac and are born. It's important to note that they are not born with a rattle. Instead, they have a skin cap at the tip of their tail. With each molt, a new rattle segment is added to their tail. This is a characteristic feature of all rattlesnake species.

The tiger rattlesnake's reproductive strategy is well adapted to its desert habitat. They give birth to live young, invest in their offspring's development, and exhibit behaviors that align with the seasonal patterns of their environment. The addition of rattle segments after birth is a unique aspect of their life cycle, contributing to their distinctive identity among rattlesnake species.

Conservation Status:

The conservation status of the tiger rattlesnake is currently assessed as "Least Concern" on the IUCN Red List of Threatened Species. This classification is attributed to several factors, including its wide distribution, presumed large population size, and the absence of significant evidence indicating a rapid decline in numbers. Assessments made in 2007 indicated a stable population trend.

Although specific population numbers are not precisely known, there is a reasonable belief that the adult tiger rattlesnake population exceeds 10,000 individuals. While the species can be relatively common in certain areas, there are also reports of smaller, more localized populations. Nevertheless, the overall extent of occurrence, area of occupancy, number of subpopulations, and population size are thought to be relatively stable, with any decline occurring at a rate of less than 10% over a period of 10 years or three generations.

As for potential threats to the tiger rattlesnake, some southern populations located in the flatter coastal plain areas of Sonora, Mexico, may face habitat loss due to increased agricultural activities. However, it's important to note that the species, as a whole, is not considered to be seriously threatened at this time.

While certain localized populations may face habitat challenges, the tiger rattlesnake, as a species, is currently assessed as having a stable population and is categorized as "Least Concern" in terms of its conservation status. Conservation efforts should continue to monitor and protect the species, particularly in regions where habitat loss is a concern.

Venom:

The tiger rattlesnake possesses venom that is both potent and dangerous to humans. While these rattlesnakes are typically reluctant to strike and should be left undisturbed if encountered, they are highly venomous and can exhibit aggressive behavior when provoked.

The venom of the tiger rattlesnake contains two significant components: a neurotoxin and a myotoxin, both of which are known to have adverse effects on the body. This venomous combination is responsible for the snake's high toxicity and the severe potential danger it poses to humans.

Despite having a relatively low venom yield compared to some other rattlesnake species, the venom of the tiger rattlesnake ranks as the second most toxic among rattlesnake venoms, surpassed only by the Mojave rattlesnake. In fact, among all snakes in the Western Hemisphere, the venom of the tiger rattlesnake is also the second most toxic.

The neurotoxic fraction of their venom is related to Mojave toxin, while another component shares immunological similarity with crotamine, a myotoxin found in tropical rattlesnakes. While the venom does possess some protease activity, it is not associated with hemolytic effects.

In terms of venom yield, various studies have reported slightly different figures, ranging from 6.4 mg to 11 mg of dried venom. Additionally, LD50 values, which indicate the venom's lethality, have been reported differently in studies, with values ranging from 0.07 mg/kg IP, 0.056 mg/kg IV, to 0.21 mg/kg SC.

The variation in these values may be due to factors such as individual snake variability. Despite the limited information available on bite symptoms from the tiger rattlesnake, it is clear that human envenomations by this species can result in local pain and swelling. However, the clinical picture may vary, with some reported cases showing limited local reactions. Notably, the comparatively low venom yield and relatively short fangs of the tiger rattlesnake might mitigate severe envenomation in adult humans. Nevertheless, bites by this species should be treated as life-threatening medical emergencies, especially for children or individuals with a slight build. Early administration of antivenom is crucial if significant envenomation is suspected, as this snake's bites are capable of causing severe harm.

References:

Brennan, T. C. (2023b). Tiger rattlesnake (Crotalus tigris). The reptiles and amphibians of Arizona. https://reptilesofaz.org/snakes-subpages/h-c-tigris/

Tiger rattlesnake (Crotalus tigris). iNaturalist. (n.d.). https://www.inaturalist.org/taxa/30758-Crotalus-tigris