成都：熊猫少了一个感受器

The two things Chengdu is most famous for are pandas and málà.

They look completely unrelated, but they are about the same thing: what the tongue and the body can actually detect.

Start with the panda.

A panda's digestive system is a carnivore's system: a short gut, without the stomach cattle and sheep have for fermenting plant matter.

But what it eats every day is bamboo.

Processing bamboo with a meat-eater's gut is extremely inefficient: of the energy that goes in, only about 17 per cent can be used by the body.

So the panda has to make it up in volume.

An adult panda eats 12 to 38 kilos of bamboo a day, and spends 12 to 14 hours a day eating.

It spends most of the day paying off that debt.

So why did it switch to bamboo?

Research gives a fairly precise date.

The gene responsible for sensing savoury flavour in the panda lost its function about 4.2 million years ago.

From then on, a panda could eat meat and not taste the savouriness of meat.

That date lines up with the dietary switch inferred from the fossil record.

Put another way: it lost interest in meat first, and walked into the bamboo afterwards.

The Chengdu Research Base of Giant Panda Breeding was built in 1987.

At the start there were only six pandas, all rescued from the wild.

The national survey completed in 2013 put the wild panda count at 1,864.

In 2016, the panda's conservation status was downgraded from "endangered" to "vulnerable".

In the afternoon I went and ate a proper local meal.

First mouthful down, and my lips began to go numb.

A lot of foreigners assume the numbness is a kind of heat. It isn't.

The capsaicin in chilli acts on the receptors that sense high temperature and pain.

Which is to say, heat is the body mistakenly reporting that it has been burned.

Sichuan pepper takes a different route.

The sanshool in Sichuan pepper acts on the receptors responsible for touch.

It makes the little bodies under the skin that detect fine vibration fire continuously.

So the numbness is a touch sensation, manufactured out of nothing.

There are experiments that had subjects compare this feeling with vibrations at different frequencies.

The answers were strikingly consistent: the closest match is a vibration around fifty times a second.

Which is to say, your mouth is reporting a tremor that isn't there.

More interesting still is the gap in time between when these two things reached Sichuan.

Sichuan pepper is native; Chinese people have seasoned with it for more than 2,000 years.

Chilli came from the Americas, and has been in China for only four hundred-odd years.

In 1591, Gao Lian wrote about it for the first time in Eight Discourses on the Art of Living.

The book says its flowers are white, its fruit red, and it is worth looking at.

In other words, when chilli first arrived in China, it was a flower planted in courtyards to be admired.

Entering the food on any scale took another hundred years or so, and it started in Guizhou.

So inside the word "málà", the numbness predates the heat by nearly two thousand years.

The flavour treated today as the soul of Sichuan is half a very old thing and half a very new one.

At dusk I sat by the river thinking about the two of them.

The panda lost one receptor, at a cost of fourteen hours of eating a day.

Humans fooled one extra receptor, and gained an entire cuisine.

Two sides of one thing: on one side a body can't read something that is really there.

On the other, a body reads something that isn't there at all.

17 per cent, 4.2 million years, fifty times a second, four hundred years.

Put those numbers together and you have everything these two Chengdu emblems contain.

How much of what you taste is the world outside, and how much is your own body?