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Venomous

How Earth's Deadliest Creatures Mastered Biochemistry

Scientific Amerian Farrar, Straus and Giroux

MASTERS OF PHYSIOLOGY

Venoms are not accidents, poisons may be.

— ROGER CARAS

If you decided to create a list of the most improbable animals on the planet, the platypus is an easy first pick. The platypus is so peculiar that even the great naturalist George Shaw, who provided the first scientific description of the animal in 1799, could hardly believe it was real. "A degree of skepticism is not only pardonable, but laudable," he wrote in the tenth volume of his Naturalist's Miscellany, "and I ought perhaps to acknowledge that I almost doubt the testimony of my own eyes." It is a sentiment I understand. As I sat staring at a large male platypus at the Lone Pine Koala Sanctuary in Melbourne, Australia, I could hardly believe the creature in front of me was real. Even up close, it looked like some kind of masterful puppet, Jim Henson's greatest feat.

Rebecca Bain, known as Beck, the head mammal keeper and one of the people responsible for Lone Pine's two male platypuses, was kind enough to let me in behind the scenes to indulge my interest in the animal. As Beck wrestled the older male from his nest box, I was surprised by his beaveresque tail, duck-like bill, and ottery feet. But while these traits are all fantastically unthinkable, there is one feature of the platypus that stands out among these oddities. It was the feature that drew me to Australia, the reason I came to see the bizarre creatures in person. Beware the male platypus: of the 5,416 currently recognized species of mammals, he alone possesses a venomous sting, using toxic ankle spurs to fight over females.

We know of twelve venomous mammals; all except for the platypus deliver a venomous bite. There are four species of shrew, three vampire bats, two solenodons (long-snouted, rodent-like burrowing mammals), one mole, the slow loris, and the platypus. There's some evidence that the slow loris may actually be four species of slow lorises, which would bump the total to fifteen, but even so, that's still just three handfuls of venomous mammals.

Of the animal lineages, there are venomous representatives in the phyla Cnidaria, Echinodermata, Annelida, Arthropoda, Mollusca, and Chordata — the phylum that includes humans. Compared with other groups of animals, the mammals boast very few venomous members; the Cnidaria, including jellyfishes, anemones, and corals, are an entire phylum — more than nine thousand species — of venomous animals, though if we want to talk sheer numbers, the venomous arthropods, including spiders, bees and wasps, centipedes, and scorpions, undoubtedly reign supreme. There are venomous snails, venomous worms, and venomous urchins. And that's not even including the rest of the venomous vertebrates in the Chordata. There are venomous fishes, frogs, snakes, and lizards.

The term venomous carries with it an explicit set of requirements. Many species are toxic: they possess substances that cause a substantial degree of harm in small doses (a toxin). We used to think of the terms toxic, poisonous, and venomous as interchangeable; now modern scientists distinguish between them. Both poisonous and venomous species are indeed toxic, for they produce or store toxins in their tissues. You may have heard that everything is a toxin in the right dose, but that's not quite true. A large enough dose can make something toxic, but if it takes a lot to kill you, then a substance isn't a toxin. Sure, you can drink enough cans of Coke for it to be fatal, but sodas are not considered toxins because the amount it takes for them to be toxic is huge (you'd have to chug liters at a time). The secretion of the anthrax bacterium, on the other hand, is a toxin because even a teeny bit can be deadly.

We can furthµer classify species that are toxic based on how those toxins arrive in a victim. Any toxin that causes harm through ingestion, inhalation, or absorption is considered a poison. Poisonous species, like dart frogs or pufferfishes, must wait for other species to make a mistake before inflicting their toxins. Some scientists would argue there is a third subcategory of toxic, in addition to poisonous and venomous — the toxungenous animals — which are essentially poisonous with purpose: toxungenous animals are equipped with poisons, but they're more impatient. Animals like the poison-squirting cane toads or the spitting cobras actively aim their poisons at offenders when they're annoyed, refusing to wait to be touched or bitten, like other poisonous animals, to transmit their toxins.

To earn the prestigious descriptor of "venomous," an organism must be more than just toxic; it must also have a specific means of delivering its dangerous goods into another animal. It has to be proactive about its toxicity. Snakes have fangs. Lionfish have spines. Jellyfish have stinging cells. Male platypuses have spurs.

The venomous spurs on the platypus aren't hard to spot. As Beck described the animals and their care at Lone Pine, I stared at the yellow toothlike points jutting from the hind legs. At about an inch long, they are much larger than I had expected. There's no doubt that any wound created by such impressive spurs would be terribly painful even without the venom. As I placed my hands within inches of the spurs to get a closeup photograph, I shuddered at the thought of how much it would hurt to be stung by the animal in front of me.

Platypuses are really awfully, terribly venomous. From what I've heard, being stung by a platypus is a life-changing experience, as any deeply traumatic event shapes who you are. Their venom causes excruciating pain for several hours, even days. In one recorded case, a fifty-seven-year-old war veteran was stung in his right hand when he stumbled on what seemed like a wounded or sick platypus while he was out hunting and, concerned for the little guy, picked it up. For his kindness, he was hospitalized for six days in excruciating agony. Over the first half hour of his treatment, doctors administered a total of 30 milligrams of morphine (the standard for patients in pain is usually 1 milligram per hour), but it had almost no effect. The veteran said the pain was far worse than the pain from the shrapnel wounds he'd gotten as a soldier. Only when the medics numbed all feeling in his hand with a nerve-blocking agent did he finally feel relief.

Even more bizarre is that the venom the platypus delivers is very different from the venoms of its mammalian relatives. Similar to the animal's outward appearance, with its collection of body parts seemingly taken from other species, it is as if the platypus's venom is composed of a random spattering of proteins stolen from other animals. There are eighty-three different toxin genes expressed in the platypus venom gland, some of whose products closely resemble proteins from spiders, sea stars, anemones, snakes, fish, and lizards, as if someone cut and pasted genes from the entire diversity of venomous life into the platypus's genome. Both externally and internally, the platypus is a testament to the power of convergent evolution, the phenomenon in which similar selective pressures can lead to strikingly similar results in very different lineages. Yet they are also wonderfully unique animals, the only ones we know of that use venom primarily for masculine combat rather than for feeding or defense.

Before she placed him back in his nest box, Beck allowed the platypus to release his rage. She pulled out a towel and dangled it behind him. The animal quickly and gleefully grabbed the towel with his hind legs and began writhing vigorously. The fervor with which he envenomated the cloth was adorable and terrifying. I silently thanked the awkward animal for accommodating my presence, however unwillingly. I'm pretty sure he imagined it was my arm and not the towel he clung to.

Unlike the platypus, many species use needle-like teeth to deliver potent toxins from modified salivary glands, the method preferred by the snakes and most of the mammals. The slow loris, however, has its own way of delivering a venomous bite. The small nocturnal primate, a contender with the platypus for most bizarre venomous animal on the planet, uses grooved teeth called tooth combs to deliver its painful venom. But before it can do so, it has to collect venom from glands on its elbows. Spiders, centipedes, and many other arthropods also inflict a venomous bite with the aid of fangs or other modified mouthparts. You could even say that some snails deliver a venomous "bite": they strike their food with a harpoon-like structure called a radula, which I think of as a kind of hardened tongue.

Then there are the other stingers. Bees, wasps, ants, and scorpions are the most well-known for their stings, as are the venomous rays (or sting rays). A wide array of spiky armaments are employed by caterpillars, urchins, and plants to deliver a potent sting. The Cnidaria possess a unique mechanism — the "stinging cells" (or cnidocytes) exclusive to the phylum. They are found along the tentacles of these jellies, corals, and anemones, and can be readily triggered to launch a tube-tethered microscopic needle into whatever comes too close. While we tend to think of them as a venom delivery system, cnidocytes are diverse in form and function, with only some serving to deliver venom. Others discharge glue-like substances or simple hooks to ensnare potential prey.

The two main categories of wounding implements reflect the two main uses for venom: to aid in acquiring or eating prey, or to defend oneself against potential predators. The different uses lead to different selective pressures, and, often, to different venom activities. Those that bite generally use venom predominantly for offense. The stingers, instead, are defensive adaptations. Of course, there are exceptions to each. The scorpion and jellyfish sting to kill prey, and the slow loris bites in defense. And often, species will use their venoms for both, switching from offense to defense as necessary.

Offensive venoms tend to be more physically disastrous. They're often packed with potent neurotoxins to paralyze the intended food or awful cytotoxins that help digest the meal. But they can also be the mildest venoms as far as humans are concerned: if the venoms are intended for an insect or some other species dissimilar to our own, the venom components might not cause the same effects in our tissues as they do in the animal's prey. Or the delivery system may not be tough enough to get through our skin: many species of anemones, for example, are harmless to us because their nematocysts — the most common type of cnida, the "firing" organ inside each cnidocyte — can't penetrate our dermal layers. Meanwhile, the defensive venoms generally contain different neurotoxins — ones meant to induce horrific, inescapable pain, a warning to select a different dinner. Because they're meant as a warning, most defensive venoms aren't lethal.

One thing is true of all venoms: they're expensive. I don't mean they cost a lot of money on the black market (though some do fetch a pretty penny) — I mean they cost a lot of energy to produce. An animal has to devote hard-earned calories to producing and maintaining its toxic weaponry rather than to other important uses, such as growth or reproduction.

Scientists know that venoms are costly from several kinds of evidence. Perhaps the simplest clue is that even within venomous branches (what scientists refer to as clades) of an evolutionary tree, there are often species that have lost their toxic touch. If venoms are so damned useful evolutionarily, why would any species give up the advantage unless it cost more than it was worth? A shift in diet from active to passive prey, for example, might make a predatory venom far less useful to possess. That's why scientists believe that when the marbled sea snakes switched to eating eggs, they lost their potent venom.

In many venomous groups, there are similar key examples of reduced or lost toxicity. The constrictor snakes could be one such example: some scientists believe that the origins of venomous reptiles date back to before snakes split from their lizard relatives, but the ones that could catch enough prey with constriction had no further need of their venomous bite. The venomous fish lineages are scattered among nonvenomous groups, suggesting that the gain and loss of venom is frequent in fishes. Being toxic just wasn't worth it from an evolutionary perspective.

Something else is true of all venomous animals: we're fascinated by them. Detailed descriptions of venomous animals and how their bites and stings torment our bodies can be found in some of the earliest medical texts known, and have been pondered at length by the likes of Aristotle and Cleopatra. Mithridates VI of Pontus, a formidable enemy of Rome, was so obsessed with venoms and poisons that he became known as "the poison king." His father was murdered by poison when he was only twelve, so from a young age, Mithridates sought a universal cure to any toxin. He began ingesting small amounts of toxins on a daily basis, believing that he could build an immunity to all poisons over time.

Following on the heels of the poison king were the physicians Nicander (roughly 185–135 B.C.) and Galen (A.D. 131–201), both of whom wrote extensively about venomous animals and treatments for injuries inflicted by their diverse toxins. These physicians were considered some of the best authorities on venom and medicine in general, and well into the fifteenth and sixteenth centuries, their writings were still read and translated into Latin and other languages.

Though many physicians and writers would talk about venomous animals, it would take until the seventeenth century for scientists to begin systematic studies of these dangerous creatures. Francesco Redi (1621?–1697) was among the first to compile what was known about snake venoms at the time, and to demonstrate that they were in fact venoms and not poisons — that many were harmless if they were ingested, but deadly if injected under the skin. In the nineteenth century, taxonomy as we know it emerged, and scientists began classifying and sorting venomous animals.

Strangely enough, though the platypus's spurs were noted in some of the first specimens — indeed, the first documented sting was in 1816 — scientists would debate for decades whether the animals were actually venomous. Henri de Blainville (1777–1850), chair of anatomy and zoology at the University of Paris, created one of the first detailed descriptions of the spur and its associated glands, concluding that the spurs were venom organs, intended to inject toxins "comme cela a lieu dans les serpens venimeux" ("as occurs in the venomous snakes"). Yet in 1823, an anonymous medical commenter assured The Sydney Gazette that "I have dissected this animal particularly, to ascertain this much controverted point, and have not been able to trace, either in the living or dead animal, the virus supposed to be contained in the sac; and I am not solitary in my opinion, that there is no poison; nor is it, properly speaking, a gland which the spur is conjoined to."

"It is my firm conviction that the animal has not the power of instilling poison by its spur," wrote the lawyer Thomas Axford in 1829. He even went so far as to say, "I am so convinced that the spur is harmless, that I should not fear a scratch from one."

The view that the platypus was harmless would hold through the nineteenth century despite reliable reports of envenomations. Even in 1883, the English naturalist Arthur Nicols scoffed at the idea that the platypus was venomous, condescendingly dismissing those wary of the animals: "On seeing me handle my specimen with perfect indifference to the supposed weapon, the black fellow expressed very decided apprehension, and pointed to the spur with gestures of alarm. Here, then, was another example of the ignorance of practical natural history among the Australian natives." The platypus was considered remarkable for its placement as an evolutionary bridge between mammals and reptiles — a mammal that lays eggs! — not because of its potent venom. Scientists were far more focused on its reproduction than its toxins. But, as the nineteenth century came to a close, a growing group of scientists would take interest in venoms, spurring advances in technology that became the foundation of modern studies. The science of venoms was about to take off, just in time to end the debate about the venomousness of the platypus.

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Excerpted from Venomous by Christie Wilcox. Copyright © 2016 Christie Wilcox. Excerpted by permission of Scientific Amerian Farrar, Straus and Giroux.
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