成都：百分之十七，和每秒五十次

Chengdu exports exactly two images of itself: pandas, and málà.

They are usually treated as unrelated, one a matter of conservation, the other of food culture.

But follow either one to the bottom and they ask the same question: what do your receptors decide for you?

Take the panda first.

A panda's digestive system is still a textbook carnivore's: a short gut, none of the multi-chambered fermenting stomach ruminants have, and no enzyme for breaking down cellulose.

And what it takes in every day is almost entirely bamboo.

Processing high-fibre bamboo with a carnivore's gut costs it dearly in efficiency: of the energy taken in, only about 17 per cent is actually absorbed.

The panda's solution to that deficit is extremely plain: eat more.

An adult panda eats 12 to 38 kilos of bamboo a day, with feeding time running to 12 to 14 hours.

It looks indolent; in fact it spends all day paying back that energy shortfall.

So how did it end up on this path.

Molecular biology has offered a fairly precise date.

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

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

That date broadly matches the dietary switch inferred from the fossil record.

Put differently: it lost the perception of meat first, and only then walked into the bamboo.

One receptor failing rewrote a species' daily schedule for the next four million years.

The Chengdu Research Base of Giant Panda Breeding was built in 1987, starting with just six animals rescued from the wild.

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

In 2016, the IUCN downgraded the giant panda from "endangered" to "vulnerable".

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

The first mouthful went in and my lips immediately began to go numb.

Outsiders often read the numbness as a variant of the heat; physiologically that reading doesn't hold up at all.

The capsaicin in chilli acts on the class of receptors that register high temperature and pain.

So heat is a false alarm: the body reports being burned while the actual temperature hasn't moved.

Sichuan pepper runs down an entirely different channel.

The sanshool in Sichuan pepper acts on the touch system, making the corpuscles under the skin that detect fine vibration fire continuously.

So the numbness is a touch sensation manufactured out of nothing, unrelated to taste or to temperature.

There is research that had subjects match this oral sensation against mechanical vibration at different frequencies.

The results clustered tightly: the closest match is a vibration around fifty times per second.

Your mouth is faithfully reporting a tremor that does not physically exist.

More worth noting is the gap in time between when these two flavours reached Sichuan.

Sichuan pepper is a native species; Chinese cooks have seasoned with it for more than 2,000 years, and it appears in pre-Qin texts.

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

In 1591, Gao Lian described it for the first time in Eight Discourses on the Art of Living, calling its flowers white, its fruit red, and fit to be admired.

Which is to say, chilli in China began as a flowering plant in a courtyard.

It took another hundred years or so to enter daily cooking, and the earliest records come from Guizhou, where salt was scarce and chilli stood in for it.

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

The flavour label treated today as the soul of Sichuan is half a pre-Qin inheritance and half a Ming-dynasty arrival that was not originally meant to be eaten at all.

At dusk I sat by the river and set the two side by side.

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

Humans fooled one extra receptor, and the return was a whole cuisine.

On one side, a body cannot read what is really there; on the other, a body reads what is not there.

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

Set those numbers side by side and you have what Chengdu's two calling cards actually contain.

If a berry can trick your senses into reporting a vibration that doesn't exist, then how much of the world you taste comes from outside, and how much from your own nerves?