I once became obsessed with following the Shibuya River as far as I could through central Tokyo. It’s hard to explain the fascination, as the river is merely a concrete channel — little more than an ugly drain — and is mostly built over. But that was the key to my interest: The idea that there was an almost-secret river flowing through and under this most urban of Japanese cityscapes. I wanted to explore it.

The waterway probably gave the area — now the youth mecca of the metropolis — its name. The phrase commonly used to describe it in the olden days — “shibu-iro no kawa (iron-ore-colored river)” — combines with the character for valley (“ya”) and results in Shibuya.

Anyone who knows the route of the river will know that it flows under the mighty edifice of Shibuya Station — and it was there where my quest was thwarted. I balked at the idea of disappearing alone into the Tokyo underworld. With my video camera I went a little way in, finding on the broadening river banks signs of subterranean human life: mainly beer cans and the wrappings of food snacks. Very quickly the light disappeared — and there were rats. I’d seen them even in daylight around the famous statue of the loyal dog Hachiko outside the station, and if they weren’t scared of running around in daylight above ground, how much more confident would they be in their home, their nerve center?

Rats — both the black and brown varieties — have long been a problem in Tokyo. Growing numbers have been reported of late and the health implications are obvious. Best to go away and come back better prepared, with lights and protective clothing, I told myself.

Now I’ve read — and at first this seemed like something out of a horror novel — that scientists at Tokai University in Tokyo have bred “genius rats.”

Tetsu Watanabe and his team at Tokai’s public health department have been working on a rat-breeding project for more than 30 years. The animals were given a cleverness test, and those that passed it were allowed to breed.

Eventually — after breeding 95 generations of selectively brainy rodents — the researchers have created a line of super-smart rats.

The test goes like this.

Rats were given a mild electric shock. The researchers installed a lever in the rat cage which, if pressed once every 30 seconds, prevents the electric shock. After a training period each day, most rats still didn’t learn to press the lever — but some did, and these ones were allowed to breed.

To avoid getting an electric shock, a rat has to press the lever regularly throughout the 30-minute testing periods when the cage becomes “live.” An average rat usually fails to press the lever for 80 percent of the time the cage is live, but the genius rats only fail a handful of times on average.

The higher intelligence of these individuals is apparently tranferable to other skills, say the researchers: The smart-bred rats are also better at memory tests that require them to swim to reach a goal.

No, the scientists at Tokai aren’t breeding genius rats with some sort of evil biowarfare plan in mind. One reason they are doing it is to help in the testing of chemicals used in agriculture.

There are fears that such chemicals can have adverse effects on intelligence, so health officials need ways to monitor and test any potential risk. If they use normal rats, as is usually the case in testing labs, then they need hundreds of animals to produce meaningful results, because rats vary so widely in intelligence. If they instead use the line of genius rats, far fewer animals need be tested to see if the chemicals are affecting intelligence.

Since the line of genius rats has been found to breed true (meaning smart rats beget smart rats), the effects of chemicals on the fetus and offspring of the rats can also be tested.

Let’s put aside the controversial issues of how many chemicals should be used in food production and the ethics of animal testing. What I’m interested in is whether the highly complex environment in Shibuya has itself been selecting for genius rats.

The rodents don’t just live in the sewers under Shibuya. They live in high-rise buildings. They cross the streets by running along power lines. They climb up apartment blocks in elevator shafts. The brown rats more common at street level compete with the black rats that colonize the elevated parts of the city. Both species of rat compete with the even more intelligent jungle crows that are the other main pest in urban Japan.

Complex environments are known to stimulate development, and are perhaps key in the evolution of intelligence. How is natural selection working on these animals in extreme urban environments?

I wonder how a wild-caught Shibuya River rat would compare in intelligence tests with Watanabe’s genius lab rats?

I never did return to explore subterranean Tokyo. I’d still love to do it, and to document the wildlife living and evolving beneath the city. If anyone from NHK or another broadcaster is reading this and wants to help explore, contact me via Twitter. I doubt we’ll find Godzilla under Tokyo, but I bet we’d find some interesting, if gruesome surprises.

Follow Rowan Hooper on Twitter at twitter.com/rowanns. The second volume of Natural Selections columns translated into Japanese is published by Shinchosha at ¥1,500. The title is “Hito wa Ima mo Shinka Shiteru (The Evolving Human).”

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