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Mojave Rattlesnake - Crotalus scutulatus
Scalesofanubis Wrote:Mojave Rattlesnake - Crotalus scutulatus

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Scientific classification
Species:Crotalus scutulatus

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Crotalus scutulatus (common names: Mohave rattlesnake,  Mojave green, more) is a venomous pitviper species found in the deserts of the southwestern United States and central Mexico. It is perhaps best known for its potent neurotoxic venom. Two subspecies are recognized, including the nominate subspecies described here.

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This species grows to an average of less than 100 centimetres (3.3 ft) in length, with a maximum of 121centimetres (3.9 ft) Other size records, such as the 1.3m+ animals, are deemed unreliable as there is no voucher specimen and the fact this very well could be an error. The color varies from shades of brown to pale green depending on the surroundings. The green hue found among Mojave rattlesnakes has led to them being known as "Mojave greens" in some areas. Like C. atrox (the Western Diamondback rattlesnake), which it closely resembles, the C. scutulatus has a dark diamond pattern along its back. With C. scutulatus the white bands on the tail tend to be wider than the black, while the band width is usually more equal in C. atrox. Additionally, C. scutulatus has enlarged scales on top of the head between the supraoculars, and the light post-ocular stripe passes behind the corner of the mouth. In C. atrox, the crown is covered in small scales, and the light post-ocular stripe intersects the mouth.

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Geographic Range
Found in the southwestern United States in southern California, southern Nevada, extreme southwestern Utah, most of Arizona, southern New Mexico and western Texas. Also ranges southward through much of Mexico to southern Puebla. It is found in deserts and other areas with xeric vegetation from near sea level to about 2500 m altitude. No type locality is given. Smith and Taylor (1950) proposed "Wickenburg, Maricopa county, Arizona" (USA), while Schmidt (1953) listed the type locality as "Mojave Desert, California" (USA).

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Primarily a snake of high desert or lower mountain slopes, they are often found near scrub brush such as mesquite and creosote, but may also reside in lowland areas of sparse vegetation, among cacti, Joshua tree forests, or grassy plains. They tend to avoid densely vegetated and rocky areas, preferring open arid habitats.

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This species is classified as Least Concern (LC) 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 it is unlikely to be declining fast enough to qualify for listing in a more threatened category. The population trend is stable. Year assessed: 2007.

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Most active from April to September, and Brumate alone or in small groups during the winter. Ambush predators, they eat mostly small rodents and lizards. Females bear live young, from two to seventeen (average about eight), from July through September. Although they have a reputation for being aggressive towards people, such behavior is not described in the scientific literature. Like other rattlesnakes they will defend themselves vigorously when disturbed.

The most common subspecies of Mohave Green rattlesnake (type A) has venom that is considered to be the most debilitating and potentially deadly of all North American snakes, although chances for survival are very good if medical attention is sought as soon as possible after a bite. In people bitten by Venom A Mojave rattlesnakes (those outside the relatively small Venom B area in south-central Arizona), the onset of serious signs and symptoms can be delayed, sometimes leading to an initial underestimation of the severity of the bite. Significant envenomations (as with all snakebites, the quantity of venom injected is highly variable and unpredictable) can produce vision abnormalities and difficulty swallowing and speaking. In severe cases, skeletal muscle weakness can lead to difficulty breathing and even respiratory failure. Contrary to popular belief, fatalities are uncommon. This is largely due to the wide availability of antivenom, as any untreated rattlesnake bite is often fatal, especially from larger species or those with more potent venom.
Unlike the rattlesnake antivenom used in the United States over the previous fifty years, CroFab antivenom (approved by the U.S. Food and Drug Administration (FDA) in October 2001) uses Mojave rattlesnake Venom A (in addition to venom from three other species) in its manufacture,[16] making it particularly effective for treatment of Venom A Mojave rattlesnake bites. Antibodies in CroFab produced by the other three species' venoms effectively neutralize Mojave rattlesnake Venom B.
All rattlesnake venoms are complex cocktails of enzymes and other proteins that vary greatly in composition and effects, not only between species, but also between geographic populations within the same species. C. scutulatus is widely regarded as producing one of the most toxic snake venoms in the New World, based on LD50 studies in laboratory mice. Their potent venom is the result of a presynaptic neurotoxin composed of two distinct peptide subunits.  The basic subunit (a phospholipase A2) is mildly toxic and apparently rather common in North American rattlesnake venoms.[19] The less common acidic subunit is not toxic by itself but, in combination with the basic subunit, produces the potent neurotoxin called “Mojave toxin.” Nearly identical neurotoxins have been discovered in five North American rattlesnake species besides C. scutulatus. However, not all populations express both subunits. The venom of many Mojave rattlesnakes from south-central Arizona lacks the acidic subunit and has been designated “Venom B,” while Mojave rattlesnakes tested from all other areas express both subunits and have been designated “Venom A” populations.
Based on median LD50 values in lab mice, a Venom A from subspecies A Mojave rattlesnakes is more than ten times as toxic as Venom B, from type B Mohave Green rattlesnakes which lacks Mojave toxin. Medical treatment as soon as possible after a bite is critical to a positive outcome, dramatically increasing chances for survival. However Venom B from the type B subspecies of Mohave Green rattlesnakes causes pronounced proteolytic and hemorrhagic effects, similar to the bites of other rattlesnake species; these effects are significantly reduced or absent from bites by Venom A snakes. Risk to life and limb is still significant, as with all rattlesnakes, if not treated as soon as possible after a bite.

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Ceratodromeus Wrote:Geographic Diet Variation of Mojave Rattlesnake (Crotalus scutulatus)
The Mojave Rattlesnake (Crotalus scutulatus) is widely distributed in western North America, occurring from the Mojave Desert through the Sonoran and Chihuahuan Deserts to central Mexico (Stebbins, 1985).  It can be distinguished from other rattlesnake species by features of color pattern and scalation, and especially by the presence of enlarged plates (scutes) on the head between the eyes (Degenhardt, et al. 1996). There are two distinct venom types known to occur in this species (Wilkinson et al. 1991).  Snakes with type A venom have Mojave toxin, a powerful neurotoxin.  Snakes with type B venom lack the Mojave toxin, but have a hemorrhagic toxin instead.  Additionally type A +  B snakes occur that contain both the Mojave and hemorrhagic toxin.  There are no morphological differences between type A and type B snakes (Glenn et al. 1983), and the two types apparently belong to the same gene pool (Wilkinson et al.1991). Type B snakes have a restricted geographic distribution; they are found only in central Arizona.  Type A snakes are found throughout the remainder of the United States range.  Type A+B snakes occur at the contact zone between type A and type B snakes in Arizona (Ernst, 1992).  Little is known about the species in Mexico, although Glenn et al. (1983) report three specimens of type A snakes from that country. The known diet is typical of most-medium sized rattlesnakes: mammals are the primary prey items, with other vertebrates occasionally taken (Ernst, 1992).   The most thorough study of mammalian prey in Mojave rattlesnakes is by Reynolds and Scott (1982).  These authors examined the stomach contents of mostly road-killed snakes of this and several other species on a transect between Ojinaga and Aldama in Chihuahua, Mexico.  They compared the abundance of different rodent species along this transect with the frequency of prey species in snake stomachs.  They demonstrated preferences by snakes for particular mammalian prey species.  Their results showed that the most abundant rodent along the transect was  Dipodomys merriami; this kangaroo rat was also the most prevalent food item in Crotalus scutulatus.  After that, however, these snakes showed preference for prey items that were not necessarily abundant.  These included the rodent species Perognathus flavus, Spermophilis spilosoma, and Dipodomys spectabilis; these were over-represented in the snake stomachs compared with their abundance along the transect.  In an arena study, C. scutulatus rejected prey items that were too large, too small, or those that were potentially dangerous.   Some snake ecologist have proposed the natural selection influences snake venom composition with respect to the kinds of prey species utilized (e.g. Daltry et al. 1996; da Silva and Aird 2001).  This hypothesis predicts that C. scutulatus of different venom types would be feeding on different prey species.  Since Type A snakes have a neurotoxic venom that is 30 to 40 times more toxic than B (Glenn et al. 1983), dietary differences between the types could well be profound.   This preliminary study will investigate the diet in C. scutulatus in different parts of its geographic range, including the parts that have different venom types.

This paper is a rather interesting read - here's the citation for a full read:
Salazar, Jennifer D., and Carl S. Lieb. Geographic diet variation of Mojave rattlesnake (Crotalus scutulatus). Diss. BS thesis, University of Texas at El Paso, El Paso, 2003.

I personally find the idea of this species' venom varying in different parts of its geographic distribution particularly fascinating..
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  • Claudiu Constantin Nicolaescu, theGrackle
How rattlesnakes got, and lost, their venom

Date: September 16, 2016
Source: University of Wisconsin-Madison

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Western Diamondback Rattlesnake (stock image). Different species of snakes kept the genes for different types of toxins and shed others, new research shows.
Credit: © Steve Byland / Fotolia

Millions of years ago, as the snake family tree grew new branches, the ancestor of modern rattlesnakes was endowed with a genetic arsenal of toxic weaponry, including genes for toxins that poison the blood, toxins that damage muscle and toxins that affect the nervous system, a research team headed by Sean B. Carroll at the University of Wisconsin-Madison has learned.

But in a relatively short period of evolutionary time, as that limb branched further, rattlesnakes like the Eastern and Western Diamondback of North America shed their neurotoxin genes altogether, keeping instead those for toxins that damage the muscles and blood vessels of their prey. Meanwhile, the Mojave rattlesnake retained the neurotoxin and lost certain other genes. Their study is published in the journal Current Biology.

"We were mining the DNA record for information about how evolution works," says Carroll, professor of molecular biology and genetics at UW-Madison and vice president for science education at the Howard Hughes Medical Institute (HHMI). "To peer into these snakes, which are relatively young in terms of evolutionary time, and to see such dramatic differences in who's-got-what genetically is really surprising. These kinds of genetic changes don't usually happen on this time scale, to this extent."

With Elda Sanchez, a collaborator at the National Natural Toxins Research Center and chemistry department at Texas A&M University-Kingsville, and colleagues at HHMI, UW-Madison postdoctoral researchers Noah Dowell and Matt Giorgianni got to work tracing the genetic origin and evolution of rattlesnake toxins.

To do so, they examined the genetic code of rattlesnake family members and reconstructed their evolutionary history. They found that neurotoxin genes evolved about 22 million years ago, before the first rattlesnakes appeared, beginning 12 to 14 million years ago.

"Snakes presented this really interesting problem in that you have all these different species of rattlesnakes that came into the New World relatively recently, and they have expanded and diversified greatly," says Giorgianni. "For evolutionary biologists, that's really interesting. We were curious how the components of their venom have changed so quickly over time."

So, they studied the individual branches on the rattlesnake family tree. What they found surprised them and challenged all of their initial hypotheses: Rattlesnakes have quickly evolved a great variety of differences through the loss of genes, resulting in varying venom gene numbers and types.

Each rattlesnake lineage has deleted two to four entire venom genes compared to their common ancestor, while retaining the genes for only a subset of venom types. The subset of genes each snake species retained varies. Further, only two of the original seven full-length venom genes are shared between the Mojave rattlesnake, the Western Diamondback and the Eastern Diamondback.

"A lot of the genes we've worked on in the lab are genes that are incredibly conserved through history and have changed very little in half-a-billion years, either in number or in character," says Carroll. "This wholesale loss is unusual ... it's not just run-of-the-mill, ordinary variation."

In most species studied, genes that are no longer necessary usually linger a long time in the genome, eventually degrading. For instance, in the human genome we still see the remnants of the large olfactory receptor gene family that gave our evolutionary ancestors a keen sense of smell, even though humans no longer rely on them.

It left the researchers wondering: "How do you have these very different weapons and how did they evolve so quickly and so differently?" says Dowell. Why did snakes delete completely the genes for a variety of toxins?

Giorgianni and Dowell worked tirelessly to determine what they call the "birth order" of the toxin genes to learn just when they first showed up, and when each rattlesnake deleted them. If the genes were a long train, it would be akin to determining the order in which the train's cars were originally linked and when each rattlesnake lost individual cars in the train.

They learned that Western and Eastern Diamondbacks independently deleted the neurotoxin genes roughly 6 million years ago, while the Mojave rattlesnake lost its muscle toxin gene about 4 million years ago.

Importantly, the researchers also learned how this happened.

The genes that make the toxin proteins sit within a complex that has embedded within it a type of genomic sequence called a transposable element. Transposable elements are made up of the same nucleotide letters that define all genetic material, but only sometimes code for the genes that lead to proteins. However, they make it easier for genes to be duplicated within the complex, and for genes to be deleted.

"You can envision a real quick and dynamic process in rattlesnakes, where this whole locus (stretch of DNA) is sort of breathing -- expanding and contracting," says Giorgianni. "It really highlights how dynamic this genomic region is and helps put into perspective how quickly these things could happen."

Not only does this appear to have led to the unusual venom differences between species, but the researchers found variety in the genes within species as well. Dowell and Giorgianni, assisted by Sanchez, examined four Western Diamondback snakes and looked at their venom gene complexes. One of the snakes had two additional venom genes that the other three lacked, as well as other changes in the complex.

"Aficionados in snake venom have appreciated this variation in venom types within a single species for a long time," says Dowell. "No one had provided a genetic explanation at this level."

That genetic explanation was finally made possible because of technology that enabled the lab to perform high-quality sequencing of specific genome regions. But the researchers also looked in places that biologists rarely do: at regions of the genome that do not code for proteins. It enabled them to peer into an evolutionary window most biologists ignore, Carroll says.

"There are so many opportunities now for understanding what's going on and extending this work outside of snakes to ask: 'How do genomes work in general?'" says Dowell. The researchers can't say for certain why snakes got rid of some of their weapons, but ecologically, they say it's likely related to the individual circumstances each species found itself in over time. Perhaps their prey was more susceptible to one type of venom or another, or evolved defenses against one type but not another.

"I think there's good evidence in nature that there is an arms race going on that generally exists between predators and prey," says Carroll. "Those arms races can be pretty intense and not dissimilar to things like antibiotics and bacteria, where you have a really strong, kind of do-or-die selective pressure that can accelerate the pace of evolution and intensify the changes that take place over time."

"It's the ecological theater in the evolutionary play and we're watching the drama unfolding," he adds.

He is also optimistic the snake genome will continue to tell interesting stories. "We're interested in generality in biology," Carroll says. "You want to discover new phenomena, new rules, new insights. The gamble was that snakes, because of their lifestyle, because they make this concoction of toxins, might have some evolutionary tricks we haven't seen before … There are other chapters to unfold here."

Story Source: University of Wisconsin-Madison. "How rattlesnakes got, and lost, their venom." ScienceDaily. (accessed September 16, 2016).

Journal Reference:
Noah L. Dowell, Matt W. Giorgianni, Victoria A. Kassner, Jane E. Selegue, Elda E. Sanchez, Sean B. Carroll. The Deep Origin and Recent Loss of Venom Toxin Genes in Rattlesnakes. Current Biology, 2016; DOI: 10.1016/j.cub.2016.07.038

•Contrary to assumptions, the most recent common rattlesnake ancestor was neurotoxic
•Gene number in the phospholipase A2 complex is evolutionarily dynamic
•Neurotoxin and myotoxin genes were lost independently in different lineages
•Venom diversity is due to both gene duplication and gene loss

The genetic origin of novel traits is a central but challenging puzzle in evolutionary biology. Among snakes, phospholipase A2 (PLA2)-related toxins have evolved in different lineages to function as potent neurotoxins, myotoxins, or hemotoxins. Here, we traced the genomic origin and evolution of PLA2 toxins by examining PLA2 gene number, organization, and expression in both neurotoxic and non-neurotoxic rattlesnakes. We found that even though most North American rattlesnakes do not produce neurotoxins, the genes of a specialized heterodimeric neurotoxin predate the origin of rattlesnakes and were present in their last common ancestor (∼22 mya). The neurotoxin genes were then deleted independently in the lineages leading to the Western Diamondback (Crotalus atrox) and Eastern Diamondback (C. adamanteus) rattlesnakes (∼6 mya), while a PLA2 myotoxin gene retained in C. atrox was deleted from the neurotoxic Mojave rattlesnake (C. scutulatus; ∼4 mya). The rapid evolution of PLA2 gene number appears to be due to transposon invasion that provided a template for non-allelic homologous recombination.
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  • Claudiu Constantin Nicolaescu, theGrackle
Viper's strike quantified in nature for the first time
Research aims to understand factors that determine the success/failure of a strike or escape in predator-prey interactions

Date: January 13, 2017
Source: University of California - Riverside
The antagonistic predator-prey relationship is of interest to evolutionary biologists because it often leads to extreme adaptations in both the predator and prey. One such relationship is seen in the rattlesnake-kangaroo rat system. Now researchers have captured in high speed (500 frames per second) a rattlesnake trying to capture a kangaroo rat.

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What factors determine the success/failure of a strike or escape in predator-prey relationships?
Credit: Higham lab, UC Riverside

Feeding is paramount to the survival of almost every animal, and just about every living organism is eaten by another. Not surprisingly, the animal kingdom shows many examples of extreme specialization -- the chameleon's tongue, fox diving into snow, cheetah sprinting -- for capturing prey or escaping predators.

The antagonistic predator-prey relationship is of interest to evolutionary biologists because it often leads to extreme adaptations in both the predator and prey. One such relationship is seen in the rattlesnake-kangaroo rat system -- a model system for studying the dynamics of high-power predator-prey interactions that can be observed under completely natural conditions.

Curiously, however, very little is known about the strike performance of rattlesnakes under natural conditions. But that is now about to change because technological advances in portable high-speed cameras have made it possible for biologists like Timothy Higham at the University of California, Riverside to capture three-dimensional video in the field of a rattlesnake preying on a kangaroo rat.

"Predator-prey interactions are naturally variable -- much more so than we would ever observe in a controlled laboratory setting," said Higham, an associate professor of biology, who led the research project. "Technology is now allowing us to understand what defines successful capture and evasion under natural conditions. It is under these conditions in which the predator and prey evolve. It's therefore absolutely critical to observe animals in their natural habitat before making too many conclusions from laboratory studies alone."

A question Higham and his team are exploring in predator-prey relationships is: What factors determine the success/failure of a strike or escape? In the case of the rattlesnake and kangaroo rat, the outcome, they note, appears to depend on both the snake's accuracy and the ability of the kangaroo rat to detect and evade the viper before being struck.

"We obtained some incredible footage of Mohave rattlesnakes striking in the middle of the night, under infrared lighting, in New Mexico during the summer of 2015," Higham said. "The results are quite interesting in that strikes are very rapid and highly variable. The snakes also appear to miss quite dramatically -- either because the snake simply misses or the kangaroo rat moves out of the way in time."

Many studies have examined snake strikes, but the new work is the first study to quantify strikes using high-speed video (500 frames per second) in the wild.

Study results appear in Scientific Reports.

In the paper, Higham and his coauthors conclude that rattlesnakes in nature can greatly exceed the defensive strike speeds and accelerations observed in the lab. Their results also suggest that kangaroo rats might amplify their power when under attack by rattlesnakes via "elastic energy storage."

"Elastic energy storage is when the muscle stretches a tendon and then relaxes, allowing the tendon to recoil like an elastic band being released from the stretched position," Higham explained. "It's equivalent to a sling shot -- you can pull the sling shot slowly and it can be released very quickly. The kangaroo rat is likely using the tendons in its lower leg -- similar to our Achilles tendon -- to store energy and release it quickly, allowing it to jump quickly and evade the strike."

To collect data, the team radio-tracked rattlesnakes by implanting transmitters. Once the rattlesnake was in striking position, the team carried the filming equipment to the location of the rattlesnake (in the middle of the night) and set up the cameras around the snake. The team then waited (sometimes all night) for a kangaroo rat to come by for the snake to strike.

"We would watch the live view through a laptop quite far away and trigger the cameras when a strike occurred," Higham said.

Next, the researchers plan to expand the current work to other species of rattlesnake and kangaroo rat to explore the differences among species.


Story Source: University of California - Riverside. "Viper's strike quantified in nature for the first time: Research aims to understand factors that determine the success/failure of a strike or escape in predator-prey interactions." ScienceDaily. (accessed January 13, 2017).

Journal Reference:
Timothy E. Higham, Rulon W. Clark, Clint E. Collins, Malachi D. Whitford, Grace A. Freymiller. Rattlesnakes are extremely fast and variable when striking at kangaroo rats in nature: Three-dimensional high-speed kinematics at night. Scientific Reports, 2017; 7: 40412 DOI: 10.1038/srep40412

Predation plays a central role in the lives of most organisms. Predators must find and subdue prey to survive and reproduce, whereas prey must avoid predators to do the same. The resultant antagonistic coevolution often leads to extreme adaptations in both parties. Few examples capture the imagination like a rapid strike from a venomous snake. However, almost nothing is known about strike performance of viperid snakes under natural conditions. We obtained high-speed (500 fps) three-dimensional video in the field (at night using infrared lights) of Mohave rattlesnakes (Crotalus scutulatus) attempting to capture Merriam’s kangaroo rats (Dipodomys merriami). Strikes occurred from a range of distances (4.6 to 20.6 cm), and rattlesnake performance was highly variable. Missed capture attempts resulted from both rapid escape maneuvers and poor strike accuracy. Maximum velocity and acceleration of some rattlesnake strikes fell within the range of reported laboratory values, but some far exceeded most observations. Thus, quantifying rapid predator-prey interactions in the wild will propel our understanding of animal performance.

Attached to this post:[Image: attach.png] Rattlesnakes_are_extremely_fast_and_variable_when_striking_at_kangaroo_rats_in_nature.pdf (620.1 KB)
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  • Claudiu Constantin Nicolaescu, theGrackle
Mojave rattlesnakes' life-threatening venom is more widespread than expected

January 15, 2019, Clemson University

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Strickland milks a Mojave rattlesnake for its venom. Credit: Gregory Territo

The Mojave rattlesnake, living in the deserts of the southwestern United States and central Mexico, is characterized by its lethal venom that can either shut down your body or tenderize your insides. Clemson University researchers say which one depends on where you're located.

Based on snakebite documentation from Mojave rattlesnakes as far back as the 1920s, it was thought that these feared pit vipers only had neurotoxic venom, a cocktail of enzymes and peptides that disrupts neurons and paralyzes the nervous system. However, a few cases were curiously different, with patients displaying symptoms such as tissue damage, disorientation and difficulty clotting blood. After several decades of inquiry, scientists later credited these symptoms to a second venom type in the Mojave rattlesnake: hemorrhagic, which acts by destroying tissues in the body.

To discover which venom type occurs where, herpetologists—the amphibian-and-reptile-loving scientists—have been collecting data on Mojave rattlesnakes in the Southwest. However, it wasn't until a recent publication by Clemson College of Science postdoctoral researcher Jason Strickland and professor of biological sciences Christopher Parkinson that the extent of the distribution was better understood. Their findings show a peculiar variability in the species.

"Prior to this paper, the 'herp' community thought that hemorrhagic venom was really rare and was only in one location in Arizona and one location in Mexico, but we show that it's found in several places throughout the snake's distribution," Strickland said. "There were a few instances when individuals in our sample had both types of venom, which our data suggest are hybrids."

This preservation of multiple venom types within one species defies science's expectations. The principles of natural selection—survival of the fittest" as it's colloquially known—would predict that one of the venom types would fix and the other would slowly diminish over the course of several generations. The venom that wins out is dependent on which type—hemorrhagic or neurotoxic—best suits the Mojave rattlesnake as it hunts for prey in the arid desert. Yet this is not what's happening.

The finding is made more peculiar by the results of a summer 2018 study that discovered four genetically distinct lineages of the Mojave rattlesnakes throughout the southwestern U.S. and central Mexico along with evidence that the lineages are breeding with one another. Diving deeper into natural selection, this swapping of genes among Mojave rattlesnakes should have reduced the genetic diversity between their lineages, effectively homogenizing venom types until the fittest rules.

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Mojave rattlesnake venom contains a number of harmful toxins that damage cells, neurons and tissues. Credit: Gregory Territo

"From an evolutionary standpoint, this is abnormal. This is not what you'd expect," Strickland said. "Gene flow should prevent this much variation. But even with the amount of gene flow we find in these populations, the selection is strong enough to maintain these specific venom types in a very local environment."

"What we've been able to show is that there are local optima. In Texas, the neurotoxic venom type seems to be the optimum for this species. But if we go south into Mexico or over near Phoenix, something in nature is changing, where the local optimum requires a different venom type," said Parkinson, who holds a joint appointment at Clemson University in College of Agriculture, Forestry and Life Science's forestry and environmental conservation department.

The public's role in the scientific process

The discovery required a large research collaboration between the principal investigators and students in five labs across the U.S. and Mexico, in addition to almost 100 citizen-scientists whom Strickland and colleagues assembled through social media and networking. In sum, the researchers collected 216 Mojave rattlesnakes for their study in California, Arizona, New Mexico, Texas and Mexico. The "herp" community, it turns out, is a strong one, with everyday snake enthusiasts eager to help researchers for the betterment of the snake species.

The team met some of those "herpers" at an annual west Texas conference called SnakeDays—directed by enthusiast Jeff Adams—that unites citizens who are interested in reptiles, as well as academics, fish and wildlife law enforcement and "herp" photographers to celebrate and raise money for wildlife diversity conservation.

"By relying on the people at SnakeDays, what would have taken Jason and Chris 10-20 years' time to study was then completed in just a few years," Adams said. "It's much more efficient for scientific professionals to rely on citizens outside of their research boundaries, because citizens know more about finding the local flora and fauna. It's also cost efficient. Citizen science reduces the cost of research projects so that researchers like Jason and Chris can better utilize their funding."

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Mojave rattlesnakes are a pit viper species living in the deserts of the Southwest US and Mexico. Credit: Travis Fisher

It's the citizen science aspect, as well as "the educational aspect of graduate students and postdocs working across borders," that Parkinson takes away from this study.

"The science is really important, no question, but collaborative research and citizen science are so important to move science forward. It's not just about some geeky academic sitting in a room anymore, it's about how the research affects people," Parkinson said.

A venomous species with clinical importance

Less than 1 percent of the U.S. population is bitten by a venomous snake each year, and even fewer die from their symptoms. For the few unlucky people who are bitten, knowing the Mojave rattlesnake distribution can lead to better treatment outcomes. If doctors know what venom type persists in their region, they can administer the proper treatment more strategically.

"These authors have published the most extensive study of one of the most medically important snakes in North America, delivering provocative novel perspectives and surprising new discoveries," said Sean Bush, a clinical herpetologist at Eastern Carolina University. "This work provides an understanding of how and why venom varies, which translates clinically into a foundation for anti-venom development, drug choice and the tailored medical management of snakebite."

For scientists, the team's study points toward the Mojave rattlesnake as being a fascinating model species for population genetics and evolutionary studies. Whether related to prey, the environment or a dietary shift over the course of development, why is the Mojave rattlesnake challenging science's predictions? What is the mechanism?

Future research in the Parkinson lab intends to consider these questions through studies of venom evolution in New World snake species.

Journal Reference:
Jason L. Strickland et al, Evidence for divergent patterns of local selection driving venom variation in Mojave Rattlesnakes (Crotalus scutulatus), Scientific Reports (2018). DOI: 10.1038/s41598-018-35810-9

Snake venoms represent an enriched system for investigating the evolutionary processes that lead to complex and dynamic trophic adaptations. It has long been hypothesized that natural selection may drive geographic variation in venom composition, yet previous studies have lacked the population genetic context to examine these patterns. We leverage range-wide sampling of Mojave Rattlesnakes (Crotalus scutulatus) and use a combination of venom, morphological, phylogenetic, population genetic, and environmental data to characterize the striking dichotomy of neurotoxic (Type A) and hemorrhagic (Type B) venoms throughout the range of this species. We find that three of the four previously identified major lineages within C. scutulatus possess a combination of Type A, Type B, and a ‘mixed’ Type A + B venom phenotypes, and that fixation of the two main venom phenotypes occurs on a more fine geographic scale than previously appreciated. We also find that Type A + B individuals occur in regions of inferred introgression, and that this mixed phenotype is comparatively rare. Our results support strong directional local selection leading to fixation of alternative venom phenotypes on a fine geographic scale, and are inconsistent with balancing selection to maintain both phenotypes within a single population. Our comparisons to biotic and abiotic factors further indicate that venom phenotype correlates with fang morphology and climatic variables. We hypothesize that links to fang morphology may be indicative of co-evolution of venom and other trophic adaptations, and that climatic variables may be linked to prey distributions and/or physiology, which in turn impose selection pressures on snake venoms.
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Uncoupling the link between snake venom and prey

March 13, 2019, Bangor University

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What was fast-becoming received wisdom among herpetologists, namely that snake venom composition normally reflects the variety of their prey, has been disproved in one common species of North American rattlesnake.

Many recent studies had identified links between the type of prey and the type of venom that had evolved in venomous snake species world-wide. This was thought to reflect natural selection to optimise venom for different prey, and sometimes evolutionary 'arms- races' between snake and prey species.

New research by an international team led by Bangor scientists Dr. Giulia Zancolli (now at the Swiss Institute of Bioinformatics) and Dr. Wolfgang Wüster of the University's School of Natural Sciences, and published today in Proceedings of the Royal Society B, has disproved this assumption in the North American Mojave rattlesnake, Crotalus scutulatus, and has reopened the debate on what drives the evolution of diverse venoms in snakes.

Across their range, different populations of Mojave rattlesnake display two distinct venom types: neurotoxic venoms, termed "Venom A", which cause paralysis, or haemotoxic venoms, termed "Venom B", which cause local tissue damage and bleeding. The neurotoxic venoms can be as much as ten times more lethal than the haemotoxic venoms, despite populations with varying venom types living within fairly close proximities to each other.

While seeking to understand in greater depth the reasons behind this spectacular venom variation within this one common species, what the researchers found, or rather, did not find in this case, was the expected correlation between diet and snake venom types. Neither did they find a correlation between venom variety and the genetic divergence of different populations within the same species.

"We expected to find that the distribution of the different venom types would reflect what prey the snakes eat in different places. Instead, what we found was that the venom variation matched environmental factors such as climate and vegetation much more closely." explains Dr. Wolfgang Wüster of Bangor University's School of Natural Sciences.

He continues: "This challenges the current widespread assumption that diet composition is the key driver of venom composition. These results reopen the entire discussion on what really drives the evolution of venom composition in snakes."

"The reason we study snake venom evolution is that animal venom systems provide us with a good model to better understand the forces at play within evolutionary adaptions. Clearly, the exact way in which natural selection acts on venom composition can be a lot subtler and more complex than is often assumed."

Giulia Zancolli added: "What I found particularly fascinating is the way our study was able to bring to light how in nature there are barriers that impose strong selective pressures, but which are not obvious to our eyes. It is only with comprehensive sampling and looking at many different factors, like diet, genetics and environment at the same time that we can start to capture them".

Journal Reference:
Giulia Zancolli et al. When one phenotype is not enough: divergent evolutionary trajectories govern venom variation in a widespread rattlesnake species, Proceedings of the Royal Society B: Biological Sciences (2019). DOI: 10.1098/rspb.2018.2735

Understanding the origin and maintenance of phenotypic variation, particularly across a continuous spatial distribution, represents a key challenge in evolutionary biology. For this, animal venoms represent ideal study systems: they are complex, variable, yet easily quantifiable molecular phenotypes with a clear function. Rattlesnakes display tremendous variation in their venom composition, mostly through strongly dichotomous venom strategies, which may even coexist within a single species. Here, through dense, widespread population-level sampling of the Mojave rattlesnake, Crotalus scutulatus, we show that genomic structural variation at multiple loci underlies extreme geographical variation in venom composition, which is maintained despite extensive gene flow. Unexpectedly, neither diet composition nor neutral population structure explain venom variation. Instead, venom divergence is strongly correlated with environmental conditions. Individual toxin genes correlate with distinct environmental factors, suggesting that different selective pressures can act on individual loci independently of their co-expression patterns or genomic proximity. Our results challenge common assumptions about diet composition as the key selective driver of snake venom evolution and emphasize how the interplay between genomic architecture and local-scale spatial heterogeneity in selective pressures may facilitate the retention of adaptive functional polymorphisms across a continuous space.
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