Wednesday, 20 August 2014

Fatally Unfair: The Science of Planet’s Most Venomous Creatures

One of the most exciting things that can happen when you head out into the wilderness is spotting an animal that you wouldn’t see on a typical day. Such sightings, however, often carry with them a risk of bringing you uncomfortably and unintentionally close to the food chain. Depending where you live in the world you could risk being stalked by a tiger, mauled by a bear, or taken down by the bite of a venomous snake.

From the perspective of someone who grew up in a country without many venomous animals, that last option is particularly jarring. Like most Canadians with a penchant for spending time in the woods I have a healthy understanding of what to do if I encounter a certain type of dangerous animal. Given that I am most likely to come across a large mammal like a bear or a mountain lion the options range from playing dead, to climbing a tree (to escape a grizzly), to fighting back and hoping for the best. However, in the face of ambush predators that rely on one quick bite or sting to take you down, all of the tactics I have spent my life learning result in me twitching and dying in the dirt.

It just seems so much less sporting. With a large predator at least you know what got you and can do something to stop it. In that sense, venom is nature’s most unfortunate pull of the slot machine. It's that inherent unfairness that makes venom so fascinating.

First off, it is important to know the difference between venom and poison. Poison is nature’s passive form of toxicity. Poisonous animals will definitely cause you some harm, but you have to bring it on yourself. By contrast, a venomous animal actively seeks to cause trouble. Venomous animals bite and sting and for that reason tend to be predators (ex. snakes, jellyfish, spiders). Poisonous animals are more often prey that use there poison as defense against predators (ex. frogs, beetles, and plants).

One of the more fun questions a person can ask in any discussion of venom is “Which animal has the world’s deadliest venom?” Unfortunately, like all seemingly simple questions in the world of science, this can be a tricky one to answer. The most common method of rating the potency of toxins is determining their LD50 (Lethal Dose 50). This is the dose of the toxin that is lethal for 50% of animals that receive it. Given in milligrams of toxin per kilogram of body mass, LD50 is a good starting point but is far from perfect. Animals react differently depending on where in the world the toxin originated, how it is administered, and a host of other variables.

It may not be foolproof, but it is the best option we have if we want to find the world’s most venomous creature. Now that we have a method we need to narrow down our search geographically. Fortunately this isn’t at all difficult. There is one place that we can go with a reputation for being crawling with the world’s deadliest versions of every poisonous animal. Pack your bags, we’re off to Australia.

We won’t get into why Australian animals are so venomous because that is something even scientists have a hard time answering. Let’s just take it as a fact that Australia is about the last place in the world you want to get bitten or stung by something. Near the top of the list of animals you don’t want to cuddle with is the box jellyfish. Small, unassuming, and nearly transparent but for a faint tint of pale blue these invertebrates pack a serious punch. The venom of a box jellyfish has an LD50 of 0.04 milligrams/kg of body mass. Keep in mind that with LD50, lower numbers equal stronger toxins. For comparison, the LD50 of coral snake venom (one of North America’s most venomous animals) is 1.3mg/kg. Adding insult to injury, the jelly’s venom has the ability to kill skin cells. That means if you don’t end up dead, you can at least count on some pretty gnarly scars.

As troublesome as box jellies may be, however, there is one Australian critter that makes even them nervous (assuming they have emotions which they probably don’t). You may be surprised to learn that the creature most scientists agree is the world’s deadliest is actually the cone snail. This marine snail may look a lot like it’s cousins that kids pick of leaves in their grandparent’s gardens but rest assured, this ain’t your gamgam’s brand of mollusk. Not even close. With a mind-blowing LD50 of 0.012mg/kg, cone snail venom usually kills the snails prey animals (mostly fish and other snails) before their nervous systems can even react to being stung.

Say what you will about my pal the grizzly bear, at least it has the decency to let you hit the ground and have a look at it before finishing you off.