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If you give them any thought at all, you probably associate them with sewers, cargo ships and maybe animated movies about animals that want to become French chefs. But for almost 200 years, tens of millions of rats have played a central role in science, performing a job they did not volunteer for, and one that almost always results in a premature death.

Here at SciShow we explore the big questions like, "Why the rat?".

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If you give them any thought at all you probably associate them with sewers, cargo ships, maybe animated movies about animals that want to become French chefs, but for almost 200 years, tens of millions of rats have played essential role in science performing a job that they did not volunteer for and one that almost always results in a premature death.

There are lots of critters big and small out there. My question is, why the rat?

The truth is, rats no longer hold the unlucky distinction of being the most widely used scientific test animal. Over the past 20 years they have been surpassed by mice. Very rough estimate suggest that about 100 million vertebrates are used in research every year. The vast majority of these, like 95 percent, are either rats or mice.

Today, generations of selective breeding have essentially turned these rodents into standardized lab tools. And while only about 25 percent of rodents used in research today are rats, their story is nonetheless a fascinating one.

 History (01:00)

When we say lab rat, we're almost always talking about a species known as Rattus norvegicus, otherwise known as Norwegian brown rat. The animal most likely originated in northern China, but it ended up in Europe by the 18th century. Here, it became one of the most prevalent urban pests. Rat catchers would trap the rodents, selling them for food or sport. That sport was rat baiting, and it involved filling a pit or ring with brown rats and then timing how long it took a terrier to kill them all.

Beginning in the early 1800s, rats were kept and bred for these specific activity, with the exception of albino rats, which were removed from the groups and used at special rat shows. Eventually those albino specimens ended up in laboratories and no one is exactly sure why. Brown rats are valuable for research because they grow quickly to sexual maturity and are easy to keep and breed in captivity. It could be, that albino rats were initially just easier to tell apart, making it easier to start a strain.
In any case, R. norvegicus became the first mammal to be domesticated for scientific research. Hooded rats, which are distinctive for their white bellies, were also popular.

 Scientific Use of Rats (02:05)

Scientists first used the brown rats to study physiology, with some early experiments in the 1820s focusing on the effects of food deprivation. After Gregor Mendel's famous laws of inheritance, an 1866 scientist Hugo Crampe became the first to confirm their validity using more than 15,000 white, grey and black rats to study the inheritance of coat color.

Selective breeding began in the early 20th century. In 1906, scientists at Wistar Institute in Pennsylvania wanted to standardize the albino rats so that reproducible studies could be conducted on the growth and development of its nervous system. The Wistar rat became the first animal to be bred solely for scientific research. The strain prove so successful it's still sold today more than a hundred years later.

The Wistar rat and other strains that followed proved to be the perfect test animal for experiments that required animals of the same genetic makeup and, unlike in other species, interbreeding generally doesn't have a negative effect on rats.

In order to get animals that are as similar as possible so the test results are consistent, labs often breed brother and sister rats.
This can be done for up to 300 generations and still gives scientists test subjects that are more than 99 percent genetically identical. Labs don't even consider a line to be inbred until the 20th generation.

 Lab Mice vs. Lab Rats (03:16)

The surging popularity of lab mice, that happened because the species is easier to genetically modify and provide models for an array of diseases that affect humans. Mice have about the same number of genes as humans and they're about 85 percent genetically identical.

But almost all the human genes known to be connected to diseases are shared with rats, so the rat remains important for things like cancer research and because rat brains are more complex than mice, the species is more likely to display behaviors associated with Parkinson's and Alzheimer's disease, as well as autism.

So it is not a glamorous life, but it is an important one.
And using lab rats for science seems much more worthwhile than throwing them in a pit and seeing how fast a dog can kill them.

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