Mohave Rattlesnakes aka: "Mohave Greens"
First: Mohave or Mojave?
The spelling of Mohave (Mojave) can generate some surprisingly heated discussions. I use "Mohave" for the rattlesnake and most other names because that's the spelling suggested by the major herpetological societies in the U.S. and Canada (Crother et al., 2017:64-65). Crother's 2017 nomenclature committee states, "According to linguistic experts on Native American languages, either spelling is correct, but using either the "j" or "h" is based on whether the word is used in a Spanish or English context." However, you will find that I use the "j" spelling when that form was originally used to describe something; e.g., "Mojave Toxin" (Bieber, Tu and Tu, 1975).
For those interested in more information, check out Lorraine Sherer's "The Name Mojave, Mohave: A History of Its Origin and Meaning" (Sherer 1967). You can also find additional information in my book, The Mohave Rattlesnake and How It Became an Urban Legend (Cardwell, 2020).
How to Identify Mohave Rattlesnakes
Rattlesnake species can be difficult to tell apart. Mohaves and western diamondbacks, in particular, look very much alike and live together in Arizona, southwestern New Mexico, western Texas and mainland Mexico. These species may also overlap slightly in California's southeastern San Bernardino County, as well. I, along with several colleagues, have published a detailed description of how to tell these rattlesnakes apart. You can find our paper in the journal Wilderness and Environmental Medicine (Cardwell, Massey, Smelski and Wüster, 2020). Or click here for a PDF.
Myth: Mohave Rattlesnakes are the deadliest of rattlesnakes
Mohave rattlesnakes (Crotalus scutulatus) are notorious animals, particularly in the southwestern United States where they are often known as "Mohave greens." Mohaves are often described as the most dangerous, most aggressive, and the deadliest. Other common claims suggest that they are hybrids or have some mysterious recent origin, that their bites are always fatal, and that antivenom is ineffective. I will address these claims and more below.
How deadly are Mohave Rattlesnakes?
Nobody knows exactly how many venomous snakebites occur in the United States each year because (1) there is no mandated centralized system to record them and (2) we know that some snake-bitten people never seek medical care. But the estimate that has been widely accepted for many years is about 8,000 venomous bites each year. Fatalities, however, are better counted and recorded.
According to the U.S. Centers for Disease Control and Prevention's "Wide-Ranging Online Data for Epidemiologic Research" database, 48 people were killed in the United States by "Venomous snakes and lizards" during the eight years 2008--2015. By the way, since Gila monsters are no longer considered deadly, I count all cases in this category as snakebites. That's an average of six snakebite deaths per year. If we divide 8,000 by six, we find that roughly one death occurs from every 1,300 venomous bites, on average. Put another way, less than 1/10 of 1% (<0.01%) of venomous snakebite victims die in the United States. By comparison, "Hornets, wasps, and bees" killed 60 people/year and "Dogs" killed 34 people/year during the same period. (Forrester, Weiser and Forrester, 2018)
So how do Mohave rattlesnakes figure into these six annual deaths? While there are some location data buried in the CDC's fatality statistics, I don't know of a recent map. But most experts agree that the map published by Dr. Henry Parrish in 1980 probably still approximates the state-by-state incidence of snakebite deaths in the United States (Parrish 1980).
Geographic distribution of deaths from snakebites in the United States, 1950-1959. (Parrish, 1980:91)
This is Dr. Parrish's map. I have added the Mohave rattlesnake's distribution in red.
The reason so many experts still rely heavily on Parrish's estimates of treated snakebites in the United States is that he did what nobody has done since: at one point he mailed questionnaires to 5,535 hospitals and to 36,627 practicing physicians, sampling every state except Alaska and Hawaii. In some cases, second and third requests were sent. Eventually, 85% of hospitals and 75% of physicians participated. Although there were more snakebite deaths sixty years ago than today, the regional distribution of deaths has changed little, if at all.
Parrish's data documented 138 snakebite deaths in ten years, for an average of about 14 deaths/year... a little over double the annual deaths in recent decades. The decline is likely due to better antivenoms and a robust national poison center system that can quickly connect a local physician to snakebite experts, when necessary.
We know that Mohave rattlesnakes bite dozens of people every year in southern California and Arizona, and probably a few more in the Big Bend region of Texas. Within their range, they tend to be common animals in the flat desert where most people live, work and play. They are outnumbered only by western diamondbacks in Arizona and they are the most commonly-encountered rattlesnake throughout most of California's Mohave Desert where diamondbacks are absent. Yet most of the six or so annual fatalities over the last few decades happen where there are no Mohaves (see map above).
Several published snakebite studies from southern Arizona are worth noting. The late Dr. David Hardy Sr. was a Tucson anesthesiologist with a keen interest in rattlesnakes and snakebites. He was also an expert herpetologist. In 1983, Dr. Hardy reported on 15 Mohave rattlesnake bites where he was able to personally identify the snake, resulting in no fatalities (Hardy, 1983). In 1988, he reported on 159 Tucson area rattlesnake bites where he estimated that 30% were by Mohave rattlesnakes, yet none were fatal (Hardy, 1988). And in 2012, Daniel Massey and his colleagues reported on 516 rattlesnake bites in southeastern Arizona between 2002 and 2009, looking for evidence of neurotoxic bites by Mohaves. While there was no way to tell exactly how many of these bites were from Mohaves, clearly they represented a significant portion of the bites. This sample contained one fatality (Massey et al., 2012).
In the last few decades before I left California, the handful of rattlesnake fatalities occurred in the state's mountains and valleys, well removed from the Mohave Desert and its Mohave rattlesnakes. But don't get me wrong, Mohave rattlesnakes can be deadly. The most recent death of which I am aware was a woman in Yavapi County, Arizona, bitten on the toe in her yard in October 2007. It is interesting to note, however, that Dr. Hardy reviewed Arizona death certificates between 1969 and 1984 for cases where rattlesnake envenomation was listed as the cause of death. Of the nine cases he found, two specified Mohave rattlesnakes and, in both cases, the victims never sought treatment and died at home (Hardy 1985).
This is a portion of the Prescott News article from 09 October 2007 by Lynne LaMaster, reporting the fatal Yavapai County Mohave rattlesnake bite I mentioned above. The photograph allows for positive identification of the snake.
So how did this myth get started?
Toxicologists and physicians have been testing rattlesnake venoms for many decades. The standard test determines the "median lethal dose" and is usually called an LD test (LD = lethal dose; 50 = 50% of the test animals). Mice are commonly used but other animals like pigeons have been used in the past. Various doses of venom, scaled to the body mass of each test animal (i.e., measured in units of venom per unit of body weight), are administered to groups of test animals until the dose that kills half of the test animals is determined - the median lethal dose.
Median lethal dose data, determined using mice, taken from Glenn and Straight (1978). "Venom-A" and "Venom-B" refer to the different venom types produced by Mohave rattlesnakes. Venom-A is found throughout most of the Mohave's range and contains a potent pre-synaptic neurotoxin called "Mojave Toxin." Venom-B is very similar to the venom of other rattlesnakes and does not contain the neurotoxin. Numbers on the right indicate the amount of venom needed to kill mice, so smaller values equal more deadly venom.
This is the common method used to compare the lethality of one venom to another. That is, a venom with a smaller median lethal dose compared to another means that it takes less of the first venom to produce death, suggesting that the venom with the smaller median lethal dose is more deadly. During similar tests over many years, Mohave rattlesnake venom has consistently produced one of the smallest median lethal doses compared to other North American pitvipers (summarized by Glenn and Straight, 1982 ). But do these animal tests always translate well to humans?
Maybe the best example of how animal tests can fail to predict human clinical experience concerns the early evaluation of the first new rattlesnake antivenom in half a century: CroFab, first licensed for use by the FDA in October 2000. Based on mouse studies, the original package insert warned that "a very high dose" of CroFab is required to neutralize the venom of the southern Pacific rattlesnake (Crotalus viridis helleri; now reclassified as Crotalus oreganus helleri or just Crotalus helleri) (Protherics 2000). Yet clinical experience with southern Pacific rattlesnake bites in southern California soon demonstrated CroFab's effectiveness against their bites, similar to bites by other rattlesnake species. Exceptionally high doses of CroFab were unnecessary (Bush et al. 2002).
So, while Mohave rattlesnake bites can certainly produce human deaths, they seem to be the deadliest of rattlesnakes only to lab mice!
Myth: Antivenom doesn't work on Mohave bites
Since 2000, antivenoms licensed for use in the United States have contained antibodies against Mojave toxin, the neurotoxin produced by venom-A Mohave rattlesnakes. They also neutralize the many other toxins produced by other species (and venom-B Mohaves) that destroy cell membranes and skeletal muscle, as well as affecting blood clotting, cardiac rhythm, and other essential bodily functions.
There are currently two antivenoms licensed in the United States for the treatment of rattlesnake bites. CroFab contains antibodies raised in sheep against the venoms of four snakes: the western diamondback rattlesnake (Crotalus atrox), the eastern diamondback rattlesnake (C. adamanteus), venom-A Mohave rattlesnakes (C. scutulatus), and the cottonmouth moccasin (Agkistrodon piscivorus). So, while CroFab is a polyvalent product licensed for use against the bites of all pitvipers in the United States, it is also specific to neurotoxic Mohave bites.
The other FDA-licensed antivenom, Anavip, contains antibodies raised in horses against the venoms of two Central American snake species: the fer-de-lance (Bothrops asper) and the tropical rattlesnake (Crotalus durissus). While Mohave rattlesnake venom is not used to make Anavip, the venom of the tropical rattlesnake contains a pre-synaptic PLA2 neurotoxin that's very similar to Mojave toxin and the antibodies cross-react with Mojave toxin very well. Anavip performed well in clinical trials and is licensed for use against bites by all rattlesnakes in the United States.
What about before 2000? Well, the standard for half a century was Wyeth Laboratories' "Antivenin (Crotalidae) Polyvalent" or ACP. Wyeth's ACP used the venoms of four species: the western diamondback rattlesnake (Crotalus atrox), the eastern diamondback rattlesnake (C. adamanteus), the tropical rattlesnake (C. terrificus), and the common South American lancehead (Bothrops atrox). For these purposes, the rattlesnakes used in Anavip (C. durissus) and in ACP (C. terrificus) are essentially the same animal. Over time, biologists have considered them the same animal, called terrificus a subspecies of durissus, and, more recently, split these tropical American rattlesnakes into more than two species.
In any event, the lack of numerous Mohave rattlesnake fatalities prior to 2000 is likely due to the tropical rattlesnake antibodies in ACP being efficacious against Mojave toxin. Another factor may be that the onset of the paralytic effects of Mojave toxin are not immediate (often several hours) and are manageable with supportive therapy, including mechanical ventilation in severe cases.
Available since October 2018
Available since October 2000
Available from 1947 to 1999
Myth: Mohave rattlesnakes have a mysterious origin
One of the most interesting claims associated with the "Mohave green" legend is the idea that Mohave rattlesnakes only recently appeared and have no history in the biological literature. I have heard multiple explanations suggested for this idea but they all draw upon the discovery in 1928 by well known rattlesnake expert Laurence Klauber that the original or "type" specimen described in a 1900 Smithsonian Institution report is not a Mohave rattlesnake (Klauber, 1956:42).
The original description of the Mohave rattlesnake was published in 1861 (Kennicott, 1861) but did not identify the preserved museum specimen (aka: type specimen) used for the description. By the time Edward Cope identified the "type" specimen in 1900 (Cope, 1900), Kennicott was unavailable to assist, having died in an arctic expedition years before. In any event, Cope got it wrong.
Ninety-two years after Cope's error was published, herpetologists at the Smithsonian Institution and at the Academy of Natural Sciences of Philadelphia worked together to determine that the specimen numbered ANSP 7069 in the Philadelphia museum had originally been Smithsonian number USNM 5027 before being given to Philadelphia. And they concluded that this specimen is the animal described by Kennicott in 1861.
Astonishingly, as I discovered more than a century later, Laurence Klauber had examined ANSP 7069 in 1934 and wrote in large red letters on his data sheet, "may be type of scutulatus." Yet, although well known for his analytical skills and attention to detail, Klauber never published that observation, including in 1956 when he wrote that the type specimen designated by Cope in 1900 (USNM 5021) was not the correct species.
This is the top of Klauber's data sheet for ANSP 7069 (aka: PANS; Philadelphia Academy of Natural Sciences), dated 09 September 1934. Klauber's data sheets, notes, and diaries are archived in the San Diego Natural History Museum's Research Library. Many thanks to Library Director Margaret Dykens for her help and hospitality during my multiple trips to SDNHM to search through the Klauber material.
When I became interested in the type specimen puzzle in 2008, I discovered that Cope had included a detailed drawing of the arrangement of head scales on the animal he declared to be the type specimen of Crotalus scutulatus. Working with colleagues at the Smithsonian Institution, we determined that the drawing in Cope's 1900 publication, labeled "USNM 5021" was actually a drawing of USNM 5027. Then my Smithsonian colleagues discovered two very old specimen tags in the jar with 5027. The older one bore barely detectable writing with the number 5027, while the newer one was numbered 5021. That discovery and other evidence, including early records listing 5021 twice while omitting 5027, led us to conclude that the tagging error had occurred in the late 1800s. Our efforts resulted in a publication titled, "Type specimens of Crotalus scutulatus (Chordata: Reptilia: Squamata: Viperidae) re-examined, with new evidence after more than a century of confusion" (Cardwell et al., 2013). A more detailed account can be found in my Mohave rattlesnake book (Cardwell, 2020).
Hybrid Mohave rattlesnakes
Hybridization often figures into the legend of the "Mohave green." I have been told that Mohave greens are the result of the government breeding green mambas with some other kind of snake during the Viet Nam War and turning the surplus animals loose at Fort Irwin (near Barstow, CA) after the war. In the July/August 2000 issue of Natural History magazine, the cover story claimed that rattlesnake venom is rapidly becoming more toxic, making human bites more difficult to treat. It goes on to suggest that the cause is neurotoxins from Mohave rattlesnakes spreading to other species via hybridization (Grenard 2000). These assertions were roundly debunked by Dr. Andrew Holycross (2000) and by Drs. William Hayes and Stephen Mackessy (2010). Nonetheless, the hybridization idea had a life of its own.
This clipping from a California newspaper would be laughable, if it were not describing what "experts" allegedly told a young father about the Arizona rattlesnake that had bitten and critically injured his toddler son.
The "coontail rattlesnake" is undoubtedly a reference to the western diamondback, which is by far the most common rattlesnake in southern Arizona but is found from southeastern California all the way to central Texas and Oklahoma. Diamondbacks and Mohaves live together in Arizona and west Texas and can be difficult to tell apart. The tails of both species are ringed in black and white, a distinctive trait that is not due to interbreeding.. Multiple efforts by geneticists to find wild Mohave/diamondback hybrids, have been unsuccessful.
In a remarkable effort to pack as many Mohave green myths as possible into one story, the "expert" also states that these hybrids have only appeared in the past year. Wow!
The hybrid Mohave garnered some apparent validity fromy well respected herpetologist and author, Dr. Robert Stebbins, who wrote in his popular book, A Field Guide to Western Reptiles and Amphibians, that Mohave rattlesnakes hybridize with southern Pacific rattlesnakes (Crotalus oreganus helleri or now just C. helleri) in the western Antelope Valley of Los Angeles County (Stebbins 1985:232 and 2003:416). Having lived nearby for decades and never seeing an apparent hybrid, I corresponded with Dr. Stebbins several times and eventually spoke with him in person about the claim. Although he could not recall the source of the information, he graciously suggested that I search his field notes and correspondence archived in the Grinnell Library of the Museum of Vertebrate Zoology at UC Berkeley. I have great respect for Dr. Stebbins (who died in 2013) and have no doubt that there must have been some apparently convincing information, but I have searched through his archived material twice without finding anything relevant. Over nearly a century (starting with Laurence Klauber), several competent herpetologists have searched the Antelope Valley for evidence of such hybrids without success. You'll find more details in my Mohave rattlesnake book (Cardwell 2020).
However, in recent years, one population of wild hybrid Mohaves has been identified genetically. Lead author Giulia Zancolli and her colleagues, including her Bangor University advisor Wolfgang Wüster, have documented a hybridization zone between Mohave rattlesnakes and prairie rattlesnakes (Crotalus viridis) in western New Mexico. There is ample genetic evidence that this hybridization has been going on for a long time, with many "back crosses" (hybrids reproducing with genetically pure animals of both species), thus thoroughly mixing the genes. And while the hybrids produce venom containing the neurotoxin from the Mohaves and the tissue-destroying metalloproteinase from the prairie rattlers, neither venom type seems to be spreading into the other species away from the hybrid zone (Zancolli et al., 2016).
I should point out that there are a handful of well-documented captive hybrids between Mohaves and several other rattlesnakes which occasionally occur when animals are crowded together over time in unnatural conditions. But such hybrid offspring are usually not healthy enough to thrive and/or they are infertile. An obvious example is a mule, the first generation offspring produced by a horse and a donkey. The parents are related closely enough to be able to produce offspring but genetic mismatches between their chromosomes produces sterile kids.
Multiple recent analyses of the "relatedness" between rattlesnake species (i.e., how long ago two species shared a common ancestor) have found that Mohaves and prairie rattlesnakes are "sister taxa," meaning that they shared a common ancestor more recently than with any other living rattlesnake species. Thus, if there is a wild population of hybrid Mohaves, it's not surprising that Zancolli and her colleagues (2016) found one with prairie rattlesnakes. The same genetic analyses have consistently found that diamondbacks and other rattlesnakes shared a common ancestor with Mohaves long before Mohaves and prairie rattlesnakes began to evolve separately (a great example of such analyses is Reyes-Velasco et al., 2013).