When asked to name substances that cause sickness or death, many people would mention poisons that are harmful to humans – chemical elements such as arsenic, plants such as hemlock, or death cap mushrooms. In fact, nearly every major group of organisms makes natural poisons, which come in two flavours: toxin and venom.
What’s the difference between a toxin and a venom?
They’re distinguished by delivery route. A toxin passively enters a recipient’s body after being swallowed, inhaled or absorbed through contact. By contrast, a venom is actively delivered into a victim via a bite, spray, stab or sting. But you can’t simply say that toxins are ingested and venoms are injected. When the toxin tetrodotoxin is delivered through the bite of a blue-ringed octopus, it’s then technically venom!
The two poisons also differ by function. Toxins are typically used for defence, to deter predators or competitors, venoms are used for offence, to catch prey. They have different compositions, too. A toxin is typically a single molecule and a venom is a mixture that will include toxins. Marine cone-snail venoms can contain over 100 distinct toxins to target a variety of prey.
Where do toxins and poisons come from?
Some living things synthesise a toxin themselves, others collect (or modify) toxins from sources in the environment, including other organisms. And while a venomous animal might produce its own venom, the ingredients in that chemical cocktail can originate from toxic products made by bacteria, plants or fungi.
Both toxins and venoms are often created and stored in dedicated structures or glands, so poisonous creatures must be resistant to the poisons they carry. In venomous animals, a gland will connect to a delivery device or ‘venom apparatus’ such as the hypodermic needle of a viper’s fang or a wasp’s lance. Members of a species rarely use venoms against one another, but slow lorises combine armpit secretions with saliva to deliver a flesh-melting bite.
How do toxins work?
The physiological effects depend on their function. A defensive toxin might block the activity of a predator’s digestive enzymes, for instance, or it might simply taste so unpleasant that it prompts a would-be consumer to spit-out its food.
Some toxins even manipulate behaviour, enabling prey to escape. Toxins in the mucus that surrounds a Xenopus frog will trigger uncontrollable yawning in a snake, which allows the amphibian to crawl out of the reptile’s gaping mouth. What about venoms? Many venoms work in a similar way because they’re used for the same objective: to quickly immobilise prey.
Venoms stop a victim escaping and prevent injury to the predator. As speed is of the essence, they commonly include a neurotoxin to disrupt the nervous system and muscles in order to cause pain, paralysis or convulsions.
Some venoms (notably snake) contain digestive enzymes that liquify soft tissue and dissolve the walls of blood vessels, causing massive internal haemorrhaging. Sounds pleasant. Which is worst? Hard to say!
The worst may be the toxin produced by the bacterium Clostridium botulinum: one-millionth of a gram will kill a human. Toxicity can be measured by the lethal dose that would kill 50 per cent of a mouse population in the lab (LD50).
Vultures and other carrion-eaters can tolerate botulinum toxin, however, which shows that metrics like LD50 are misleading because they don’t reflect a poison’s impact on its intended target species in the wild.
Why are some toxins so deadly?
Why make lethal doses if you only need enough to survive an encounter? One theory is that toxicity is driven by a coevolutionary arms race between predators and prey. That’s seen in rough-skinned newts, whose skin has varying toxicities based on whether local garter snakes resist tetrodotoxin. Tetrodotoxin is 10,000 times more poisonous than cyanide (to people) and occurs in about 140 species. Discovered in puffer fish, it’s famous for needing careful preparation so the Japanese delicacy fugu doesn’t become a person’s last meal.
Main image: Green and Black Poison Dart Frog (Dendrobates auratus) in Costa Rica. © Getty Images
