Crotalus unicolor

Geographic Range

Crotalus unicolor is one of the rarest of the rattlesnakes; it was once found on much of Aruba Island and possibly other West Indies Islands. However, this species is now mainly confined to the southern end of Aruba, due largely to habitat destruction tied to tourism industry development.

• Biogeographic Regions
• neotropical neotropical
  • native native

• Other Geographic Terms
• island endemic island endemic

Habitat

Aruba Island rattlesnakes inhabit undisturbed sandy, rocky and arid hillsides of this volcanic island, now totaling only 12 square miles of protected land that includes the Arikok National Park. They are typically found on terraced mountainsides consisting of igneous rock and dry stream beds. The diabase mountains and limestone plateaus have been found to harbor the largest densities of this species, possibly due to the relatively low human presence. They are found from 2 meters above sea level to 188 meters elevation on Mount Jamanota. Oddly, despite the seeming preference for undisturbed areas, Aruba Island rattlesnakes are found within 1 km of the two largest cities on the island.

There is little seasonal variation in temperature in this tropical marine habitat; the average temperature is approximately 81 F (27 C) with daily average highs of 84 F and lows of 78 F. Summers tend to be slightly warmer in temperature – up to average highs of 89 F.

• Habitat Regions
• tropical tropical
• terrestrial terrestrial

• Terrestrial Biomes
• desert or dune desert or dune
• scrub forest scrub forest
• mountains mountains

Physical Description

As with most other rattlesnakes, Aruba Island rattlesnakes are heavy bodied, with a triangular head and a rattle-tipped tail. Dorsal scales are triangular and overlapping, with a slight keel. Diamond-shaped markings are present in a variety of colors ranging from a scarcely discernable white or rust color to an obvious dark brown or blue-gray. These markings blend into a pair of dark lines on either side of the spine which terminate at the head. The body color is also variable from light pink to dark tan, occasionally leaving the markings indistinguishable save for a small band along the spine. There are few markings on the head of this species; at most paired stripes extend from the eye toward the ventral boarder of the jaw. Conspicuous heat-sensing pits are visible below and slightly posterior to the nostrils, one on each side.

• Other Physical Features
• heterothermic heterothermic
• venomous venomous

• Sexual Dimorphism
• male larger

Development

Fertilization in Aruba Island rattlesnakes is internal and occurs during the September through January breeding season. Mated females give birth to 5 to 15 live young after four months of gestation. The autonomous young have already developed fully functional venom sacs at this time and average 20 cm in length and 14 g in weight. After their first shedding, the young begin to hunt for their first meal. They may double or triple in size in the first year. At four years for males and five years for females, they reach reproductive maturity and begin to breed.

Reproduction

When a female is reproductively receptive, she will emit a pheromone attractant to signal males of this state. Attracted males will compete for the attention of females by “wrestling”. In this form of combat, the two snakes will rear up and entwine the front half of their length in attempts to push the opponent down. Venom is never used in these battles. Once a male has gained access to a female, courtship continues in a fashion common to many snake species. The male comes in contact with the female, lines up along side of her, and runs his lower jaw along the female’s spine. The tongue of the male is continually flicking and the lateral body movement is in a jerky manner, presumably to differentiate courtship from incidental contact. If receptive, the female permits access to her cloaca by lifting her tail. The male then wraps his tail around hers and inserts one prong (of one of the paired hemipenes)into her cloaca. The pair may continue to mate for several hours at a time.

• Mating System
• polygynandrous (promiscuous) polygynandrous (promiscuous)

This is an ovoviviparous species; the eggs of the female are retained within the body, and maternal nutrient sharing with the embryos has not been demonstrated. At the end of the four month gestation period, the young emerge from the female as live young, leaving the egg membranes inside the female. Typically 5 to 15 young will emerge during the spring as fully independent snakes. The generation interval has been defined as ten years in all management predictions. Ongoing research will attempt to determine if sperm storing occurs in females.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• sexual sexual
• ovoviviparous ovoviviparous

The only parental investment in this species is pre-fertilization and gestational, as the young are independent upon birth. The female is burdened with the majority of this investment, as producing and yolking eggs is energetically expensive. Carrying the developing young for four months is a physical as well as a nutritional burden on her body; the young increase her body size, probably making hunting and feeding difficult.

Male investment consists of sperm production and the energy necessary for courtship and female defense; it is considered negligible compared to that of the female.

• Parental Investment
• no parental involvement
• pre-fertilization
  • provisioning
  • protecting
    • female
• pre-hatching/birth
  • provisioning
    • female
  • protecting
    • female

Lifespan/Longevity

Expected lifespan in captivity and in the wild is between 12 and 20 years.

Behavior

Aruba Island rattlesnakes inhabit a climate that allows year round activity. Normal daily activity is highest during the early morning hours and the late evening; however increased temperatures in summer months may induce these animals to adopt a nocturnal activity cycle. There are few social interactions, with the exception of reproduction and male-male competition for access to females. These snakes are also unlikely to attack unless provoked.

• Key Behaviors
• terricolous
• nocturnal nocturnal
• crepuscular crepuscular
• sedentary sedentary
• solitary solitary

Home Range

Research to define home range size of Aruba Island rattlesnakes is in progress. There is no evidence of true territorial defense at present.

Communication and Perception

As in all members of the "pitviper" group, subfamily Crotalinae , these rattlesnakes possess a pair of pit organs located below the nostrils. These organs contain infrared receptors that provide information on the body heat, and thus distance, size, and shape, of potential predators and prey. These allow the snakes to hunt during low light conditions, such as during warmer months when they are nocturnal. The directionality of this sense is a result of the physical structure of the pit. Heat information enters the pit much as light enters an eye, and is detected on a curved membrane similar to the way the retina detects the origin of light. It is important to note that this method of detection is more effective in detecting endothermic (mammalian or avian) prey than other ectothermic species and sight is still used in conjunction with infrared sensory detection.

Another form of communication with other species is the series of hollow, keratinized structures that form the rattle at the tip of the tail. The tip of the tail is moved through muscle movement and the vibrations produce a characteristic sound used to warn potential predators.

Pheromones are an important chemical attractant and cue. These rattlesnakes can pick up odors using both the sense of smell and the vomero-nasal (Jacobson’s) organ, where chemicals are supplied by the tongue. The sense of smell is extensive in Aruba Island rattlesnakes, as in most snakes.

• Communication Channels
• visual visual
• tactile tactile
• acoustic acoustic
• chemical chemical

• Other Communication Modes
• pheromones pheromones
• vibrations vibrations

• Perception Channels
• visual visual
• infrared/heat infrared/heat
• tactile tactile
• vibrations vibrations
• chemical chemical

Food Habits

These reptiles hunt with the aid of their heat sensing pits, sight, and smell. Targeted species are mainly rodents, birds, and lizards, especially teiids . After prey is located, these snakes position themselves in a striking stance, characterized by an ‘S’ shaped neck. Launching itself forward, these rattlesnakes opens its mouth, allowing two venom delivering fangs to fold down and sink into the prey animal. The venom contains numerous chemicals, most notably proteolytic enzymes which begin to digest the animal before ingestion while other chemicals are responsible for increasing venom uptake and blocking energy to vital systems. After striking, the animal exhibits a period of increased tongue flicking in order to find the envenomatted prey. Interestingly, this behavior is still present when offering live rodents to captive adult animals raised on frozen or thawed (dead) rodents.

Upon debilitation of the prey, these snakes consume their prey whole by unhinging the lower jaw and stretching the mandibular symphysis at the front of the lower jaw. Only feeding perhaps once to a few times per year, the digestive system of these rattlesnakes is reactivated upon ingestion of the food, increasing blood flow to the digestive organs that are dormant during fasting periods, and probably causing a spike in metabolic rate, as observed in other rattlesnakes.

• Primary Diet
• carnivore carnivore
  • eats terrestrial vertebrates

• Animal Foods
• birds
• mammals
• reptiles

Predation

There are no reported natural predators of Aruba Island rattlesnakes, which are top predators in these habitats. Human encroachment and poaching for rattles and the pet trade are currently the primary threats to the population.

'Crested caracaras Caracara cheriway ', 'merlins Falco columbarius ', and 'osprey Pandion haliaetus ' are possible snake eating species that are found on Aruba, but no specific information about possible predation threats to young rattlesnakes was found. Their cryptic coloration, venom, and habit of warning potential predators of their threat is likely to protect them from much predation.

• Anti-predator Adaptations
• cryptic cryptic

Ecosystem Roles

Aruba Island rattlesnakes are top trophic level predators to many small lizards, mammals, and birds. They lack natural predators as adults, their main threat being humans.

Economic Importance for Humans: Positive

Though illegal, a source of some of the threat to these animals is thought to be capture for the pet trade. More recently, increased education has elevated these animals to national symbols, generating tourism to Arikok National Park. The presence of these snakes was instrumental in spurring the development of this park. Increased research efforts have also occurred in recent years, bringing more money to the island.

• Positive Impacts
• pet trade pet trade
• ecotourism ecotourism
• research and education
• controls pest population

Economic Importance for Humans: Negative

Aruba Island rattlesnakes only attack when provoked, but envenomation can be debilitating or life-threatening to a human who is bitten.

• Negative Impacts
• injures humans
  • bites or stings
  • venomous venomous

Conservation Status

Ecological destruction in the form of charcoal and firewood industry, aloe cultivation, and resort and tourism development has reduced the undisturbed habitat available for Aruba Island rattlesnakes to only twelve square miles. Even in these twelve square miles, there is a growing feral goat population that has been destroying native vegetation. This problem has begun to be addressed with growing support for the preservation of the island's endemic species. Media campaigns and education programs highlight the ecological and economic importance of this native rattlesnake. An improved public image has emerged from articles, interviews, and educational efforts, resulting in the Aruba Island rattlesnake being featured on new currency and postage stamps. Conservation posters are being developed by the Association of Zoos and Aquariums (AZA) to further promote these efforts.

Along with this growing respect for the snakes, the AZA Aruba Island Rattlesnake Species Survival Plan (SSP) helped establish the Arikok National Park. The park consists of 19% of Aruba’s land and includes most of the rattlesnake population. There are also plans underway to remove feral goats and list the species in CITES, Appendix II to ban trade of the snakes into the pet trade and sales of the rattles.

There are currently approximately 230 of these rattlesnakes left in the wild and about 110 managed by the AZA (53 females and 57 males) in zoos. The government donated ten wild caught individuals to help with the breeding program in the SSP and help investigate the details of the breeding physiology and release capabilities. Also, animals not used in breeding are being used to test a vaccine for Paramyxovirus , which has killed several SSP snakes.

Another major issue that is currently being researched is the effects of the introduction of large boas, Boa constrictor , to Aruba.

Other Comments

Aruba Island rattlesnake venom is thought to be among the most toxic of any rattlesnake. Crotalus unicolor was thought to be extinct before efforts were made to investigate "former" habitats.

Additional Links

Encyclopedia of Life

Contributors

Tanya Dewey (editor), Animal Diversity Web.

Sheri Sanders (author), Michigan State University, James Harding (editor, instructor), Michigan State University.

References

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Howard, , K. Einert, L. Auretta, M. Ushar, G. Ian, L. Rocco, M. Goode, R. Odum. 2002. Distribution of the Aruba Island Rattlesnake, Crotalus unicolor, on Aruba, Dutch West Indies. Caribbean Journal of Science , 38 (2): 126-128. Accessed November 05, 2006 at http://scholar.google.com/scholar?hl=en&lr=&safe=off&q=cache:GT2slLuBXxEJ:caribjsci.org/june02/38_126-128.pdf+crotalus+unicolor .

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Walter, M. 1999. "Some Species Get No Respect - protection for the Aruba Island rattlesnake" (On-line). Find Articles.com. Accessed October 03, 2006 at http://www.findarticles.com/p/articles/mi_m0FRO/is_4_132/ai_55198442 .

American Zoos and Aquarium Association. 2000. "AZA - Aruba Island Rattlesnake Fact Sheet" (On-line). Accessed October 03, 2006 at http://www.nagonline.net/Fact%20Sheet%20pdf/AZA%20-%20Aruba%20Island%20Rattlesnake%20Fact%20Sheet.pdf#search=%22aruba%20island%20rattlesnake%22 .

Sedgwick County Zoo. 2006. "Aruba Island Rattlesnake - Crotalus unicolor" (On-line). Sedgwick County Zoo. Accessed October 03, 2006 at http://www.scz.org/animals/r/airsnake.html .

Lincoln Park Zoo. 2006. "Aruba Island Rattlesnake" (On-line). Accessed October 03, 2006 at http://www.lpzoo.com/animals/FACTS/REPTILES/aruba_rattler.html .

Association of Zoos and Aquariums. 2006. Association of Zoos and Aquariums SSP Masterplan for Aruba Island Rattlesnake, Crotalus unicolor. AZA Aruba Island Rattle Snake SSP: 1-46.

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