The answer lies in a tiny molecule, called capsaicin, that naturally occurs in chilli peppers,

and which has the ability to trigger an intense burning sensation.

For how that works, you'll have to watch Greg's video, but I'm really interested in why some plants go to the effort of making such a sadistic chemical.

You see, chilli peppers are a type of fleshy fruit that plants develop after being fertilised – just like apples, bananas, and raspberries.

The point of these fruits is to be enticing to passing animals, in the hope that they'll spot them,

chow down on them, and then travel far away before the resistant seeds find their way back out through their poo.

With any luck, the seeds will survive and germinate in their new home, far away from

the mother plant, giving them more than an even chance to survive and thrive.

And chilli peppers are no exception to the eat-travel-poop method of seed dispersal.

The bright reds, yellows and oranges of the ripe fruit are just what's needed to attract passing creatures.

But they hide a vicious secret. Most fruits of the Capsicum genus – that's chilli

peppers to you and me - contain the burn-inducing chemical capsaicin, lying in wait for unsuspecting snackers.

What benefit can there be to advertising sugar but only delivering spice?

You might think that it's because other animals don't feel the same burn we do when they munch on a scotch bonnet.

But researchers have found that both mammals and insects also experience the heat of capsaicin

when it comes into contact with the insides of their mouths or even with their skin, making

them avoid the deceptive fruits of the Capsicum genus.

Which is lucky, because it's these animals that have the ability to throw a spanner in the works of the seed dispersal plan.

Even though the seeds, like bits of sweetcorn, are able to survive the journey through an

animal's gut, the small mammals that eat the chilli fruits do so by grinding the tough

bits with their teeth, damaging the seeds so that when they pop out the other end, they won't germinate.

Likewise, insects feed on chilli peppers by puncturing the flesh and sucking out the juice

– but these puncture wounds are perfect breeding grounds for fungi that can also damage

the seeds, and the plant's hopes of reproducing.

So with spicy capsaicin deterring these dangerous munchers, the seeds have a decent chance of surviving to germinate.

But how do they get dispersed?

Well, there is one group of animals that capsaicin doesn't affect – and that's birds.

With a different configuration of nerves, the birds take no notice of even scorchingly

high concentrations of spice, and gobble chilli peppers down like a kid with sweets. And the

crucial thing is that they don't chew or grind them – the seeds go straight in, and

come straight back out again, hopefully after a nice long flight.

So by using a simple chemical to drive away the grinders, and attract the gobblers, Capsicum

plants get only the best dispersers, making sure none of their precious reproductive resources are wasted.

But this does raise another question – if capsaicin is so good at ensuring successful

seed dispersal, why don't all fruiting plants produce it?

The answer, is that chilli spice isn't such a simple chemical after all, and it takes

a lot of the plant's precious nutritional resources – like nitrogen – to produce it.

The Capsicum plants took a big risk when they put their eggs in the chemistry basket, as

they're relying on an uninterrupted supply of water and nutrients to even start to make the manipulative chilli pepper.

And when there's a dry spell, it's the chillies that suffer, while the traditionalists come out on top.

But the chilli's peculiar evolutionary gamble has paid off in more ways than one. Since

their discovery by native American human populations thousands of years ago, who found they could

provide just the zing their cuisine needed, the plants have been nurtured and cultured and selectively bred to suite every palate.

As a result, they've spread further than even a bird could carry them. Even here, to the Alnwick Garden in Northumberland!

So, that's the secret to spicy plants.

Click here to check out the Earth Lab video on how they tingle our tastebuds,

and don't forget to like and subscribe for more fascinating nature videos.