| 1 | Western hognose snake |
Maximum length: 100.6cm.
The western hognose snake is a species of the US Midwest, ranging from southern Canada to extreme northern Mexico. They measure 50-90cm, and possess a strong resemblance to rattlesnakes, causing predators to have second thoughts when they approach them. Another of their skills is resistance to some of the most toxic toads in North America.
Western hognose snakes already have adaptions in the form of enlarged rear teeth, which grip toads when they inflate into their slippery balloon form. Their second, equally important adaption, is resistance to the lethal bufotoxins toads contain.
Bufotoxins act by stopping a snake’s and indeed a human’s heart. For most animals, toads are a completely inaccessible food source, but the western hognose snake is the exception, as it has massively enlarged adrenal glands. After swallowing a western toad, for example, they immediately release a large flood of adrenalin, restoring their heart rate, until the body metabolises the bufotoxins and the danger passes.
Without this adaption, western hognose snakes would be far less successful, and far more likely to go extinct, or be pushed back to a small corner of the USA where turtle eggs (another of their prey) are constantly available. A 2016 study from Illinois found that western hognose snakes ate 51% turtle eggs and 23% toads, including the American toad, followed by lizards, but no mammals.
| 2 | Common keelback |
Maximum length: 100cm.
Officially the luckiest snake in Australia. The cane toad (Rhinella marina) is an invasive species native to Brazil, which was introduced to Australia in the 1930s to control beetle infestations in sugar plantations.
This plan didn’t work, mainly because the bugs stayed high up on the stalks, while the cane toads stuck to the ground. What did occur was a large fall in the local snake population, as cane toad skins are coated with a deadly mixture of bufotoxins and bufadenolides. The likes of red-bellied black snakes prey on native amphibians heavily, but have no natural resistance to the cane toad originating in a foreign country.
The lone exception was the common keelback (Tropidonophis mairii). This is a harmless species of lakes and swamps in Queensland, which normally flees far across the water when humans approach. Most keelbacks in the Tropidonophis genus live in southeast Asia, where toads with bufotoxins are plentiful. The group as a whole is resistant, and when common keelbacks made the jump to Australia millions of years ago, they retained these resistant genes, despite Australia containing no native toads with bufotoxins.
Cane toads might think they’re in complete control of Australia, but they’ll get a shock if they hop into a lake containing a common keelback. This species is also resistant to toxic native frogs, like Dahl’s aquatic frog.
| 3 | Blue-necked keelback |
Maximum length: 75cm.
The blue-necked keelback can not only survive swallowing toad toxins, but repurpose them as weapons. This shy species is found in extreme southern Thailand, Malaysia, and parts of Indonesia. Its venom causes neurotoxic symptoms such as slurred words, dizziness and an unsteady gait, but must be chewed in repeatedly to cause serious damage.
This species is the cousin of the red-necked keelback, but far rarer and more elusive, with little detailed research conducted. What’s known is that like their relative, blue-necked keelbacks prey mainly on frogs and toads, and are completely resistant to their deadly bufotoxins.
One poisonous species they overlap with in Malaysia is the Asian common toad (Duttaphrynus melanostictus). After consuming a toad, blue-necked keelbacks store the white-coloured bufotoxins in nuchal glands in their neck, which burst when touched by a predator, e.g. bird talons. The red-necked keelback can even fire stored bufotoxins at humans from a distance, and it’s assumed that blue-necked keelbacks can do the same.
Blue-necked keelbacks are mainly found in lowland rainforests far from humanity. Because of their love for toads, they occur near water sources like streams, flooded fields and standing forest pools. Finding this snake is a lucky experience for reptile enthusiasts. Getting sprayed in the face is even luckier (or unlucky, depending on how you view it).
| 4 | False habu |
Maximum length: 121.7cm.
A 60-100cm species of southeast China and Taiwan. False habus (Pseudagkistrodon rudis) are typically found in moist meadows and forests, often on the edges of streams. This snake is a viper mimic, with subtle beige-brown patterns and a wide triangular head. This is one of many subtle advantages that keeps the species alive, and another is bufotoxin resistance.
This species is known to prey primarily on amphibians. In a 2012 study, the false habu flecked its tongue most aggressively when exposed to toad scents, with two frogs enticing the second most flecks, with mammals, reptiles and birds far behind. The specific toad tested was the Japanese common toad (Bufo japonicus), a poisonous species riddled with bufotoxins, which is even toxic in its egg stage.
The false habu can swallow toads without its heart stopping, similarly to its distant cousin the red-necked keelback. Scientists theorise that they store toad toxins in their liver, as a defence against large predators.
Though few westerners have heard of this species, Pseudagkistrodon rudis is commonly encountered in eastern Asia, with a “least concern” conservation tag.
| 5 | Wagler’s snake |
Maximum length: 127.5cm.
A highly bufophagous (toad-eating) snake of eastern and southern Brazil. Wagler’s snake mainly appears in open areas, avoiding the Amazon rainforest, but is common in sweeping cerrado savannah and fields near dark Atlantic forests.
Despite a lack of venom or savage fangs, Wagler’s snake (Xenodon merremi) has managed to become extremely widespread, extending deep into Argentina, and reaching the easternmost point of Brazil further north. An advantage they possess is resistance to bufotoxins found in toads, granting them access to a food source which other snakes can’t begin to contemplate.
One sighting happened in Rio Grande do Norte, eastern Brazil, when a Wagler’s snake seized a common lesser toad (Rhinella granulosa) on a sunny trail. It grabbed the toad by its hind legs, before swallowing without any hesitation. This species belongs to the same genus as the infamously toxic cane toad.
Like the US hognose snakes, Xenodon meremmi possesses enlarged adrenal glands, which keep its heart rate steady after consuming bufotoxins. Wagler’s snake possesses no venom of its own, but is an extremely close mimic of its neighbour the crossed pitviper. When threatened, Wagler’s snake rears its chest up and hisses loudly, to create a false impression of deadliness.
| 6 | Terrestrial garter snake |
Maximum length: 109.2cm.
Resistance to toad bufotoxins is widespread in the garter snake (Thamnophis) genus. Common garter snakes and aquatic garter snakes all boast a helpful armour, and even in species that don’t eat toads, such as Butler’s garter snakes, the genetic mutations are still detectable.
The terrestrial garter snake is the most land-dwelling garter snake, eating more mammals than usual, while keeping a healthy serving of toads as well. A 2017 study confirmed that they have elevated levels of ATP1a3, a gene which prevents the important NKA enzyme from being disabled. Most tellingly, levels of this gene were elevated in heart tissue, which bufotoxins preferentially target, compared to lung, kidney and gut tissue.
This allows terrestrial garter snakes to swallow toxic toads without any concern. They may slow down for several days, becoming lethargic and still while their body metabolises the toxins, but they’ll soon be on their way again, with their calorie stores nicely replenished.
Toxic toads in the USA include the native American toad (Anaxyrus americanus) and western toad (Anaxyrus boreas), as well as the invasive cane toad, which is found mainly in Florida.
| 7 | Diamondback watersnake |
Maximum length: 175.3cm.
The diamond watersnake (Nerodia rhombifer) lives in central states of the USA, particularly eastern Texas, Arkansas and Louisiana. They’re extremely common in slow-moving rivers, streams and ponds, and are non-venomous, but with an aggressive personality if picked up. One of their special skills is the ability to swallow toxic toads without their heart stopping.
The diamondback watersnake was once the subject of a great experiment. Members of this species were fed doses of the bufadienolide Gamabufotalin in comparison with gopher snakes, which have no resistance.
The gopher snakes rolled over and died rapidly, but it took 20 times the quantity of bufotoxins to cause harm in diamondback watersnakes. Nerodia rhombifer is actually wasting its resistance somewhat, as its main food supply is fish, with a side helping of amphibians like American bullfrogs. Toads are only an occasional meal, yet this species can still swallow them without harm.
Elsewhere, their brown watersnake cousin has been witnessed swallowing cane toads without harm, both in the wilds of Florida and in a laboratory. This isn’t a species with enlarged adrenal glands, so it’s likely that the Nerodia watersnake genus has resistance at the genetic level.
| 8 | Spotted night adder |
Maximum length: 70cm.
For our next snake, we cross the Atlantic, travel thousands of miles and reach the wilds of West Africa. The spotted night adder (Causus maculatus) averages at 30-60cm, and is a patient venomous species rather than a charging maniac. It inhabits a wide variety of habitats, including woodlands and savannahs, but particularly gravitates towards moist areas with streams.
The reason – this viper is heavily reliant on toads and frogs. One of their favourite prey is the flat-backed toad, AKA Bufo maculatus. Resistance to bufadenolides/bufotoxins is confirmed at the genetic level.
Bufadenliodes work by binding to an enzyme called Na+/K+-ATPase (simplified to NKA) in cell membranes. Without this enzyme, calcium ions surge, increasing the contraction time of the heart and slowing the heart rate overall.
It’s a simple yet deadly mechanism, and most resistant snakes have similar mutations in the NKA enzyme which prevent bufotoxins from binding to them. This is accomplished with just a couple of amino acid substitutes. The spotted night adder is another member of this club, joining the likes of US garter snakes.
Bufotoxin resistance is rare for vipers, but Causus maculatus is weird in many ways, as it’s a rare viper to lay eggs. Spotted night adders are found in western and central Africa, particularly Nigeria, Togo and Ghana.
| 9 | Brazilian smooth snake |
Maximum length: 3 metres.
The Brazilian smooth snake is a large, thuggish species which patrols Amazon rainforest rivers and swallows up anything that gets in its way. It’s most common in Paraguay, northern Argentina and southern Brazil, and is non-venomous, but with a dangerous set of fangs which it doesn’t mind using.
Despite this, a cocky attitude won’t save you if toad toxins are binding to enzymes in your heart and preventing it from beating. Even an Indian cobra would grind to a halt if it swallowed a small cane toad. Fortunately, the Brazilian smooth snake is another species with bufotoxin resistance, conferred by the same mutations in the NKA enzyme as others.
A study from Argentina found that their diet includes Rhinella schneideri, or the Roroco toad (formerly known as Bufo paracnemi). Most importantly, the Brazilian smooth snake being observed didn’t roll over and die. This toad is confirmed to secrete poisonous steroids and alkaloids in its skin, and is even believed to have potential as an insecticide.
The Brazilian smooth snake relies heavily on amphibians overall. A study from northeast Argentina found a dietary makeup of 41.4% amphibians, 20% reptiles, 15.7% fish and 14.3% mammals, so it makes sense to hang onto the resistance genes.
| 10 | Grass snake |
Maximum length: 205cm.
A non-venomous snake of Europe, which is easily the most commonly observed snake in the UK. While adders and asp vipers would get sick and die slowly if they ate a European toad, the grass snake (Natrix natrix) seems to be immune. Living in the leafy English countryside, there’s an enormous amount of observations of this species, and eating deadly toads is one behaviour people have noticed.
Grass snakes eat over 80% amphibians, and their physiology has evolved to make this possible. They’re proven to have mutations in the genes for NKA enzymes, similarly to the US garter snakes to which they’re distantly related. Again, these prevent bufotoxins from binding to receptors in heart cell membranes, and fatally altering the heartbeat.
Other Natrix members possess these mutations too: the dice snake of eastern Europe (Natrix tesselata), which mainly eats fish and occasionally toads, and the viperine watersnake (Natrix maura), which almost never eats toads. Of the three, the grass snake is the species which uses this resistance most in its daily life.
The grass snake is a semi-aquatic species, which is commonly found in ponds amid floating lilypads and tadpoles. In Spain, their poisonous prey includes the Iberian spiny toad (Bufo spinosus). European green toads (Bufotes viridis) are another confirmed prey.