Rocky Mountain wood ticks are mostly found in arid areas of the Rocky Mountain region (Nebraska, South Dakota, Arizona, New Mexico, etc.) including California, where it is only found east of the Sierra Nevada mountains. They can also be found in various parts of southwestern Canada, including British Columbia, Alberta, and Saskatchewan. ("The Genera Dermacentor and Otocentor (Ixodidae) in the United States, with Studies in Variation", 1938; "Dermacentor andersoni", 2012; "Geographic distribution of arthropod-borne diseases and their principal vectors", 1989; Yee, 2006)
Adult Rocky Mountain wood ticks are generally found in elevations of 2100 m to 2500 m. Temperature may be important in determining its geographic range as these ticks show a preference for lower elevations (1600 m to 2200 m) at higher temperatures and higher elevations (2350 m to 2500 m) at lower temperatures. This species also prefers brushy areas of foothills and mountains as these areas host small mammals and other host organisms. (Eisen, 2007)
Rocky Mountain wood ticks are generally brown or reddish brown in color. Females have a distinct dorsal silver-gray ornamentation (known as the 'shield') which turns more gray when the tick feeds, while males are spotted gray and white with no distinctive shield marking. Both sexes are mottled in this fashion over portions of their legs and mouthparts (basis capitulum). Their bodies are flat and pear-shaped, ranging anywhere from 2 to 5.3 mm in length, though fully engorged females may reach up to 16.5 mm. This species is sexually dimorphic; in addition to the marking differences mentioned above, females range from 2.8-5.4 mm long (13.8 to 16.5 mm when fully engorged) while males range from 2.1 to 6.1 mm. Body mass ranges from .005 to .7 g. This species is also polymorphic, with a lot of physical variation between individuals. Features that distinguish this species from other ticks include the number and size of goblets (used in aeration/respiration) on its spiracular plates, with this species having 100-200 goblets on average. Females produce a neurotoxin in their oral secretions that prevents the release of acetylcholine, a neurotransmitter necessary for muscle contractions, often causing paralysis even in larger mammals such as dogs and humans. (Furman and Loomis, 1984; Lysyk, 2010; Richard and Farrow, 1991; "Dermacentor andersoni", 2012; "Dermacentor andersoni", 2009; Yee, 2006)
The life cycle of Rocky Mountain wood ticks is characterized by three stages: larval, nymph, and adult. Progression through these stages can take 1-3 years. Individuals usually take a single blood meal from a mammalian host before progressing to the next stage and the size of hosts increases along with the size of ticks. Host availability may cause variation in timing and duration of mating, ovipositing, egg hatching, molting, and overall growth. The life cycle begins when eggs are deposited on vegetation by engorged females during early summer. Larvae remain inactive until outside stimulation (i.e. temperature, light, humidity) cause them to seek their first mammalian host (usually some sort of rodent). Following a feeding period ranging from 2 to 6 days, larvae drop off and form nymphs via a molting process. Nymphs display behavior similar to larvae, remaining dormant until stimulated by the presence of a potential host. Following attachment to the second host, males and females demonstrate different behaviors. In preparation for mating, females feed on the host for long periods (5 to 15 days) while males feed for shorter periods and spend more time copulating with partially fed females. After females are fully engorged, they drop off their hosts and seek a place to deposit their eggs. Males and females die shortly after completing reproduction. ("Rocky Mountain Wood Ticks", 2012; "Dermacentor andersoni", 2012; "Dermacentor andersoni", 2009; Yee, 2006)
This species has a polygynous mating system, where males disrupt their adult stage feeding period to begin copulation with partially fed females. Mating occurs in the third and final life stage. Females release 2,6-dichlorophenol during feeding, allowing males to identify them as potential mates. Once females have consumed enough blood, they release cholesteryl oleate, signaling that they are ready to mate. These two chemicals are common to hard ticks (Family Ixodidae). (Sonenshine, et al., 1982; Sonenshine, 2004; Sonenshine, et al., 1988; Sonenshine, et al., 1989; Sonenshine, et al., 1986)
These ticks are reproductively active from May through June, with some variation possible due to host availability, temperature, humidity, and other environmental factors. This species is known to have a single breeding season in late spring with adults dying soon after copulation. When grown in ideal conditions in a lab, maturation and reproduction have come to completion in approximately 68 days; in the wild, it takes both males and females approximately a year to reach sexual maturity. Females may lay anywhere between 2500 and 7400 eggs, depending on their nutrition levels, over the course of 10 to 33 days. ("Scorpions Spiders Mites and Ticks: Arachnida - Rocky Mountain Wood Tick (Dermacentor andersoni): Species Account", 2012; "Dermacentor andersoni", 2012; "Dermacentor andersoni", 2009; Yee, 2006)
The primary determinant of lifespan for Rocky Mountain wood ticks is the availability of the blood meal required to reach each life stage. Larvae can survive up to a month without a blood meal; however, with a single meal they are able to molt into nymphs, which can then survive upwards of a year before needing another blood meal. With yet another single feeding, nymphs are able to morph into adults, which can survive up to 2 years without feeding. Members of this species will typically live 1-3 years total in the wild. The longest recorded lifespan in captivity for this species is 4 years. Variation in life span may be influenced by external factors such as humidity, temperature, and host availability, which may cause these ticks to be dormant for longer periods of time between life stages. ("Life Span", 2012; "Scorpions Spiders Mites and Ticks: Arachnida - Rocky Mountain Wood Tick (Dermacentor andersoni): Species Account", 2012; "Dermacentor andersoni", 2012; "Dermacentor andersoni", 2009; Yee, 2006)
Rocky Mountain wood ticks are parasitic ticks with a 3-stage life cycle, each of which is associated with a different host. They are mostly sessile for the first two life stages, only moving when seeking a host. Adults are highly mobile as they scramble to find mates, which they encounter on what is known as the definitive host. Once attached to a host, the ticks remain there, dropping off only once they have consumed a sufficient amount of blood to transition into the next life stage or complete breeding. To engage in feeding, these animals must first be stimulated by changes in the external environment, sensed via a front-leg structure known as Haller's organ, which senses changes in stimuli such as humidity, temperature and carbon dioxide levels (produced by potential hosts). Once they sense a prospective host, these ticks attach using cushioned pads on their feet known as pulvilli, which secrete an adhesive substance. Rocky Mountain wood ticks use their palpus to feed, which they inject into an exposed area of their hosts' dermis, proceeding to draw blood. This species does not have social hierarchies and does not display any complexity in its social system. ("Dermacentor andersoni", 2012; "Dermacentor andersoni", 2009)
In the adult stage, home range is typically limited to the host. ("Rocky Mountain Spotted Fever (RMSF)", 2011; "Dermacentor andersoni", 2012; "Geographic distribution of arthropod-borne diseases and their principal vectors", 1989)
The primary sense organ, Haller's organ, is common to both hard and soft-bodied ticks. There are slight differences in the organ from species to species, but it is generally located on the dorsal surface of the tarsi (leg segments closest to head of the tick). Haller's organ plays a key role in finding hosts as setae on the organ respond to changes in humidity, carbon dioxide levels, and olfactory stimuli. This organ is also important in pheromone detection during mating season. In addition to Haller's organ, these ticks also have many setae on their scutum and legs. Longer, curved setae on the ventral surface aid in chemosensory and tactile responses while shorter, straighter setae act as temperature receptors. Rocky mountain wood ticks have a pair of simple eyes that are more or less parallel to the second pair of legs, just past the head. (Sonenshine, et al., 1986; "Dermacentor andersoni", 2012; "Dermacentor andersoni", 2009; Woolley, 1972)
Rocky Mountain wood ticks are sanguivores (most typically of mammals) who take a single blood meal from their hosts at each of their three life stages. ("The Genera Dermacentor and Otocentor (Ixodidae) in the United States, with Studies in Variation", 1938; "Dermacentor andersoni", 2012; "Dermacentor andersoni", 2009; Woolley, 1972; "Geographic distribution of arthropod-borne diseases and their principal vectors", 1989)
The main defensive mechanism of this species against typical tick predators such as ants, beetles and spiders is the secretion of allomones, a special set of substances which influence predator behavior in favor of the tick (i.e. discouraging pursuit if following scent). The main component (nearly 25% of mass) of these allomones is squalene, a biochemical precursor to steroids. These allomones are secreted by large wax glands on the underside of the tick's legs. Once depleted, these wax reservoirs take up to 10 days to replenish, a period during which the tick has little defense against predation. (Sonenshine, et al., 1993; Sonenshine, 2004; Yoder, et al., 1993)
As an obligate parasite, Rocky Mountain wood ticks rely on mammalian blood to provide the nutrition necessary to transition through their various life stages. During their lifespan, these ticks also play an important role as host to various types of bacteria, two of which are Arsenophonus and Wolbachia. Of the two, Wolbachia has a special relationship with many other arthropods and is noted for its ability to alter the reproductive abilities of its hosts by infecting the testes and ovaries. Such infections may result in "male killing" where males are killed during larval development, "feminization" where infected males develop as females or (infertile) pseudo-females, or sometimes leading to the development of parthenogenesis, where females reproduce without coming into contact with a male. The most important effect of hosting Wolbachia, however, is "cytoplasmic incompatibility", where Wolbachia-infected males are unable to reproduce with uninfected females. This has been suspected as a potential cause of speciation. Rocky Mountain wood ticks are also known hosts of Rickettsia bacteria, specifically R. prowazeki and R. rickettsii; infection with these bacteria is most often deadly for Rocky Mountain wood ticks and can be passed to host species as well. Rickettsia rickettsii, for example, is known as a cause of Rocky Mountain Spotted fever, the most common fatal tick-borne disease in humans found in the United States. (Burgdorfer, et al., 1973; Dergousoff and Chilton, 2010; Niebyiski, et al., 1999; Samish and Rehacek, 1999)
Rocky Mountain wood ticks provide no known economic benefit to humans.
One of the important ecosystem roles carried out by Rocky Mountain wood ticks is that they serve as a vector for the tick-borne disease known as Rocky Mountain Spotted Fever (RMSF). Upon biting a host, the tick passes on the disease-causing gram-negative coccobacillus, Rickettsia rickettsii, which is an obligate intracellular parasite and is the true disease-causing agent. Symptoms characteristic of this disease include a rash beginning in the palms of the hands and soles of the feet, fever, nausea, emesis, severe headache, muscle pain, and loss of appetite. Although lethal in some cases, the disease is treatable. Taking antibiotics upon infection, for example, greatly reduces the mortality rate from an alarming 20% chance to a relatively low 5%. Although not known from this particular species, species in the genus Dermacentor are also known to serve as a vector to Tularemia, also known as Pahvant Valley plague or rabbit fever. As with RMSF, this disease is caused by a bacterial endosymbiont (Francisella tularensis), which is also a non-motile gram-negative coccobacillus. What makes this different from RMSF, however, is that it does not rely on direct contact in order to be spread. It can be waterborne, inhaled, or ingested (i.e. poorly-cooked meat). ("Rocky Mountain Spotted Fever (RMSF)", 2011; Burgdorfer, et al., 1973; Cunha, 2011; Dergousoff and Chilton, 2010; Kim, et al., 2010; Kwaik, et al., 2007; "Geographic distribution of arthropod-borne diseases and their principal vectors", 1989; Yee, 2006)
This species has no special conservation status. (Furman and Loomis, 1984; "Scorpions Spiders Mites and Ticks: Arachnida - Rocky Mountain Wood Tick (Dermacentor andersoni): Species Account", 2012; "Geographic distribution of arthropod-borne diseases and their principal vectors", 1989)
Jean-Claude Michel Munyarubuga (author), University of Michigan-Ann Arbor, Jeremy Wright (editor), University of Michigan-Ann Arbor.
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
an animal that mainly eats meat
an animal which directly causes disease in humans. For example, diseases caused by infection of filarial nematodes (elephantiasis and river blindness).
either directly causes, or indirectly transmits, a disease to a domestic animal
uses smells or other chemicals to communicate
a period of time when growth or development is suspended in insects and other invertebrates, it can usually only be ended the appropriate environmental stimulus.
animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.
parental care is carried out by females
union of egg and spermatozoan
forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.
(as keyword in perception channel section) This animal has a special ability to detect heat from other organisms in its environment.
fertilization takes place within the female's body
A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.
having the capacity to move from one place to another.
This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.
the area in which the animal is naturally found, the region in which it is endemic.
reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.
an organism that obtains nutrients from other organisms in a harmful way that doesn't cause immediate death
chemicals released into air or water that are detected by and responded to by other animals of the same species
having more than one female as a mate at one time
"many forms." A species is polymorphic if its individuals can be divided into two or more easily recognized groups, based on structure, color, or other similar characteristics. The term only applies when the distinct groups can be found in the same area; graded or clinal variation throughout the range of a species (e.g. a north-to-south decrease in size) is not polymorphism. Polymorphic characteristics may be inherited because the differences have a genetic basis, or they may be the result of environmental influences. We do not consider sexual differences (i.e. sexual dimorphism), seasonal changes (e.g. change in fur color), or age-related changes to be polymorphic. Polymorphism in a local population can be an adaptation to prevent density-dependent predation, where predators preferentially prey on the most common morph.
an animal that mainly eats blood
scrub forests develop in areas that experience dry seasons.
breeding is confined to a particular season
remains in the same area
offspring are all produced in a single group (litter, clutch, etc.), after which the parent usually dies. Semelparous organisms often only live through a single season/year (or other periodic change in conditions) but may live for many seasons. In both cases reproduction occurs as a single investment of energy in offspring, with no future chance for investment in reproduction.
non-motile; permanently attached at the base.
Attached to substratum and moving little or not at all. Synapomorphy of the Anthozoa
reproduction that includes combining the genetic contribution of two individuals, a male and a female
uses touch to communicate
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
Living on the ground.
uses sight to communicate
University of Alberta E.H. Strickland Entomological Museum. 2012. "Dermacentor andersoni" (On-line). Accessed February 03, 2012 at http://www.entomology.ualberta.ca/searching_species_details.php?b=Acari&c=7&s=31507.
University of California, Davis. 2009. "Dermacentor andersoni" (On-line). Accessed February 03, 2012 at http://vetpda.ucdavis.edu/parasitolog/Parasite.cfm?ID=40.
World Health organization. 1989. "Geographic distribution of arthropod-borne diseases and their principal vectors" (On-line). Accessed February 24, 2012 at http://www.ciesin.org/docs/001-613/001-613.html.
Encyclopedia Britannica. 2012. "Life Span" (On-line). Accessed March 27, 2012 at http://www.britannica.com/EBchecked/topic/340297/life-span/63861/Perennials.
2011. "Rocky Mountain Spotted Fever (RMSF)" (On-line). Center for Disease Control and Prevention. Accessed February 26, 2012 at http://www.cdc.gov/rmsf/#whatis.
City and County of Denver. 2012. "Rocky Mountain Wood Ticks" (On-line). Denver Animal Shelter: Disease Control. Accessed September 21, 2012 at http://www.denvergov.org/denveranimalshelter/DenverAnimalShelter/HealthVaccinations/DiseaseControl/RockyMountainWoodTicks/tabid/434845/Default.aspx.
Net Industries. 2012. "Scorpions Spiders Mites and Ticks: Arachnida - Rocky Mountain Wood Tick (Dermacentor andersoni): Species Account" (On-line). Accessed March 27, 2012 at http://animals.jrank.org/pages/2268/Spiders-Scorpions-Mites-Ticks-Arachnida-ROCKY-MOUNTAIN-WOOD-TICK-Dermacentor-andersoni-SPECIES-ACCOUNTS.html.
U.S. Treasury Department. The Genera Dermacentor and Otocentor (Ixodidae) in the United States, with Studies in Variation. 171. Washington D.C.: National Institute of Health. 1938. Accessed February 27, 2012 at http://www.afpmb.org/sites/default/files/pubs/techguides/tg26/References/Cooley_1938.pdf.
Burgdorfer, W., L. Brinton, L. Hughes. 1973. Isolation and characterization of symbiotes from the Rocky Mountain wood tick, Dermacentor andersoni. Journal of Invertebrate Pathology, 22/3: 424-434. Accessed February 24, 2012 at http://www.sciencedirect.com/science/article/pii/0022201173901730.
Cunha, B. 2011. "Rocky Mountain Spotted Fever" (On-line). Drugs, Diseases, & Procedures. Accessed February 03, 2012 at http://emedicine.medscape.com/article/228042-overview.
Dergousoff, S., N. Chilton. 2010. Detection of a new Arsenophus-type bacterium in Canadian populations of the Rocky Mountain wood tick, Dermacentor andersoni. Experimental and Applied Acarology, 52: 85-91. Accessed February 28, 2012 at http://www.springerlink.com.proxy.lib.umich.edu/content/1q686461g827t145/fulltext.pdf.
Eisen, L. 2007. Climate change and tick-borne diseases: A research field in need of long-term empirical field studies. International Journal of Medicinal Microbiology, 298: 12-18. Accessed February 25, 2012 at http://www.sciencedirect.com/science/article/pii/S1438422107001919.
Furman, D., E. Loomis. 1984. The Ticks of California (Acari: Ixodida). United States of America: University of California Publications. Accessed March 25, 2012 at http://books.google.com/books?id=d79aj0GdE8UC&pg=PA3&lpg=PA3&dq=d+andersoni+morphology&source=bl&ots=_49j087j9L&sig=z20pPTXSnb4DGsC5m6OG1zc8dsc&hl=en&sa=X&ei=ZgZxT-2BMcm8tweLh_i8Dw&ved=0CEsQ6AEwBg#v=onepage&q=d%20andersoni%20morphology&f=true.
Gregson, J. 1967. Observations on the movement of fluids in the vicinity of the mouthparts of naturally feeding Dermacentor andersoni Stiles. Parasitology, 57: 1-8. Accessed February 03, 2012 at http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=4212616.
Kim, D., T. Reilly, S. Schommer, S. Spagnoli. 2010. Rabbit Tularemia and Hepatic Coccidiosis in Wild Rabbit. Emerging and Infectious Diseases, 12: 2016-2017. Accessed February 28, 2012 at http://apps.webofknowledge.com.proxy.lib.umich.edu/full_record.do?product=BIOSIS&search_mode=GeneralSearch&qid=1&SID=2FiIaCOCigC6H@a5h9m&page=1&doc=8.
Kwaik, Y., D. Metzger, A. Sjostedt, R. Titball. 2007. Tularemia: History, Epidemiology, Pathogen Physiology, and Clinical Manifestations. Annals of the New York Academy of Sciences, 1105: 1-29. Accessed February 28, 2012 at http://dl2af5jf3e.search.serialssolutions.com/?ctx_ver=Z39.88-2004&ctx_enc=info%3Aofi%2Fenc%3AUTF-8&rfr_id=info:sid/summon.serialssolutions.com&rft_val_fmt=info:ofi/fmt:kev:mtx:journal&rft.genre=article&rft.atitle=PREFACE&rft.jtitle=Annals+of+the+New+York+Academy+of+Sciences&rft.au=KWAIK%2C+Y.+A&rft.au=METZGER%2C+D&rft.au=NANO%2C+F&rft.au=SJOSTEDT%2C+A&rft.au=TITBALL%2C+R&rft.date=2007-03-29&rft.issn=0077-8923&rft.volume=1105&rft.issue=1&rft.spage=ix&rft.epage=x&rft_id=info:doi/10.1196%2Fannals.1409.018&rft.externalDBID=n%2Fa&rft.externalDocID=10_1196_annals_1409_018.
Lysyk, T. 2010. Tick paralysis caused by Dermacentor andersoni (Acari: Ixodidae) is a heritable trait. Journal of Medical Entomology, 47: 210-214. Accessed February 03, 2012 at http://www.ncbi.nlm.nih.gov/pubmed/20380302.
Masters, E., G. Olson, S. Weiner, C. Paddock. 2003. Rocky Mountain Fever: A Clinician's Dilemma. Archives of Internal Medicine, 163: 769-774. Accessed February 24, 2012 at http://archinte.ama-assn.org/cgi/content/full/163/7/769.
Niebyiski, M., M. Peacock, T. Schwan. 1999. Lethal Effect of Rickettsia ricketsii on Its Tick Vector (Dermacentor andersoni). Applied and Environmental Microbiology, 78: 773-778. Accessed February 05, 2012 at http://aem.asm.org/content/65/2/773.full.pdf+html.
Richard, M., B. Farrow. 1991. Tick Paralysis in North America and Australia. Veterinary Clinics of North America: Small Animal Practice, 21: 157-171.
Samish, M., J. Rehacek. 1999. Pathogens and Predators of Ticks and Their Potential in Biological Control. Entomology, 44: 159-182. Accessed March 27, 2012 at http://www.annualreviews.org/doi/abs/10.1146/annurev.ento.44.1.159?journalCode=ento.
Sonenshine, D. 2004. Pheromones and other semiochemicals of ticks and their use in tick control. Parasitology, 129: S405-S425. Accessed March 26, 2012 at http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=290850.
Sonenshine, D., J. Gordon, C. Hamilton. 1988. Evidence for occurrence of mounting sex hormone on body surface of female Dermacentor variabilis (Say) and Dermacentor andersoni (Stiles) (Acari: Ixodidae). Journal of Chemical Ecology, 14: 401-410. Accessed March 25, 2012 at http://www.springerlink.com/content/u625016775701380/.
Sonenshine, D., J. Hamilton, W. Lusby. 1989. Cholesteryl oleate: Mounting sex pheromone of the hard tick Dermacentor variabilis (Say) (Acari: Ixodidae). Journal of Insect Physiology, 35: 873-879. Accessed March 26, 2012 at http://www.sciencedirect.com/science/article/pii/0022191089901030.
Sonenshine, D., G. Khalil, P. Hornsher, S. Mason. 1982. Dermacentor variabilis and Dermacentor andersoni: Genital sex pheromones. Experimental Parasitology, 54: 317-330. Accessed March 25, 2012 at http://www.sciencedirect.com/science/article/pii/0014489482900418.
Sonenshine, D., D. Taylor, K. Carson. 1986. Chemically mediated behavior in Acari: Adaptations for finding hosts and mates. Journal of Chemical Ecology, 12: 1091-1108. Accessed March 25, 2012 at http://www.springerlink.com/content/x26rn3253605257q/.
Sonenshine, D., J. Yoder, A. Spielman, D. Johnstons, R. Pollack. 1993. Secretion of Squalene by Ticks. Journal of Insect Physiology, 39: 291-296. Accessed March 27, 2012 at http://www.sciencedirect.com/science/article/pii/002219109390059Z.
Woolley, T. 1972. Some Sense Organs of Ticks as Seen by Scanning Electron Microscopy. Transactions of the American Microscopical Society, 91: 35-47. Accessed March 27, 2012 at http://www.jstor.org.proxy.lib.umich.edu/stable/3224855?seq=12&Search=yes&searchText=ticks&searchText=electron&searchText=microscopy&searchText=organs&searchText=sense&list=hide&searchUri=%2Faction%2FdoBasicSearch%3FQuery%3Dsome%2Bsense%2Borgans%2Bof%2Bticks%2Bas%2Bseen%2Bby%2Belectron%2Bmicroscopy%26acc%3Don%26wc%3Don&prevSearch=&item=1&ttl=78&returnArticleService=showFullText&resultsServiceName=null.
Yee, A. 2006. "Dermacentor andersoni: Female" (On-line). Accessed February 03, 2012 at http://www.microscopy-uk.org.uk/mag/artnov06macro/ay-macro.html.
Yoder, J., R. Pollack, A. Spielman. 1993. An ant-diversionary secretion of ticks: First demonstration of an acarine allomone. Journal of Insect Physiology, 39: 429-435. Accessed March 27, 2012 at http://www.sciencedirect.com/science/article/pii/002219109390031L.