Organism of the week #22 – Faking it and making it

Nettles have a rather unhappy reputation as bringer of painful welts, and – at this time of year – dribbling noses too. The welts are probably caused by histamine, and the pain by oxalic and tartaric acids, which the nettle injects into your skin through the tiny brittle hairs that cover its stems and leaves. If you’re stung badly, the pain can last for several hours.

If you are stung by nettles at some point, you’ll probably avoid trampling barefoot on them in future. If getting trampled by humans is a big ecological problem for nettles, then a less stingy nettle stands a poor chance of growing up to make baby nettles. Stingless nettles will therefore go extinct, and their stingier competitors will inherit the earth. Praise be to Darwin.

Unfortunately for you (and fortunately for those who make a living from it), it’s almost always true that “I think you’ll find it’s a bit more complicated than that” in biology. If humans can be persuaded to avoid meddling with nettles through a single painful experience, there is a lot of opportunity for cheats to exploit your fear. In the case of nettles, one well-known group of cheats are the so-called dead nettles:

The leaves of dead nettles look remarkably like those of stinging nettles, but the dead nettles neither sting, nor are they even close relatives of stinging nettles: stinging nettles are related to hops and cannabis; dead nettles to mint and sage. The flowers give the game away at this time of year, but in spring, the two plants are really very similar. You need to get quite close to spot the missing stings on the dead nettles, and if you’ve had a bad experience with the real thing in the past, getting quite close is probably something you – or a fluffy wuffy bunny, or whatever – would think twice about.

Good biologists should always be skeptical of plausible stories, so I should add that I’ve not actually been able to track down any experimental studies seeing whether bunnies who have learnt to avoid stinging nettles also avoid dead nettles, let alone any that show dead nettles are more successful at making seeds when real nettles are in the same area. Assuming this actually is the case, dead nettles would be “Batesian mimics” of stinging nettles, or – if you’d rather – fakers. They don’t have to waste energy making histamine and oxalic acid and hypodermic needles; they merely have to look somewhat similar to stinging nettles to receive all the benefits of having bunnies avoid them, with fewer of the costs.

But what would happen if dead nettles were such good fakers that they became very common? The bunnies would rarely meet the real thing, and would probably never learn to avoid nettle-like plants of any sort. Even if the bunnies did occasionally meet stinging nettles, those reckless bunnies that threw caution to the wind and ate things that looked like nettles would still tend to get more to eat than more cautious bunnies. In either case, the dead nettles would get nibbled back into relative rarity. And then the more reckless bunnies would get stung more often, as they’d meet real stinging nettles more frequently, and this would – in its turn – favour bunnies that were more cautious again, leading to a resurgence of the dead nettles. And so on, and so on.

The relative rarity of dead nettles and stinging nettles wouldn’t necessarily roller-coaster up and down like this: the cycles could be quite small. However, it’s interesting that neither a field of dead nettles on their own, nor of stinging nettles on their own, is stable. A field of nothing but stinging nettles is prone to invasion by fake dead nettles; but if the number of dead nettles gets too high, the bunnies will never meet the real thing, and won’t learn to avoid nettle-like plants in the first place. There is likely to be some ratio of real to fake nettles (and of cautious to reckless rabbits) that is stable in the long term, but it won’t be 0% or 100%.

These sorts of ‘game’ between mimics – the dead nettle “fakers” – and their models – the stinging nettle pain “makers” – are very common in biology, and are an important part of the ecology of many organisms. Wherever an organism has made some sort of ‘effort’, there is likely to be a living made scrounging off them, or mimicking their appearance.

But of course, it’s always a bit more complicated in biology. Not all mimics are fakes. Some mimics benefit from looking dangerous because they really are dangerous.

The honeybee and bumblebee in the image above both have black and yellow striped bodies. Both are able to sting, and both seem to have similar colours. Is one mimicking the other, and if so, why?

As I said earlier, you should be skeptical of plausible stories. Bumblebees and honeybees are quite closely related, so perhaps the black-and-yellow is just a colour-scheme they’ve inherited from their common ancestor that has nothing to do with mimicry. We need more evidence.

As it turns out, there is very good evidence that black-and-yellow is meaningful mimicry, not accidental similarity. For example, the cinnabar moth caterpillar in the third image is not closely related to the bees, so it is likely that this caterpillar’s colours have evolved independently from those of the bees. Can it sting? Not exactly, but it is poisonous, because it mostly eats ragwort, and it steals the ragwort’s poisons for its own defence. Any bird that has learnt to avoid black-and-yellow insects through unhappy run-ins with bees is likely to avoid this caterpillar too. Importantly, this works both ways: any bird that’s had a bad experience with cinnabar moth caterpillars is also likely to avoid bees (and wasps, and other similar insects).

This sort of mimicry, where makers – the animals and plants that can back up their threats – all come to have similar warning colours is called Müllerian mimicry. If you need any more convincing, it’s telling that there are also many Batesian fakers of the black-and-yellow “warning” colour-scheme too, like the harmless hoverfly shown in the fourth image.

The natural world if full of liars and cheats; except when it isn’t.