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 a central 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. A very rough estimate suggests that about 100 million vertebrates are used for research every year. But the vast majority of these, like
95% are either rats or mice. Today, generations of selective breeding have essentially turned these rodents into standardized lab tools. And while
only about 25% of rodents used in research today are rats, their story is none the less a fascinating one. When we say lab rat, we're almost always
talking about a species known as Rattus Norvegicus, otherwise known as the 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 bathing. It involved filling up a pit or ring with brown rats and then timing how long it took a terrrior
to kill them all... Beginning in the early 1800s, rats were kept inbred for this 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, our norvegicus became the first mammal to be domesticated for scientific
research. Hooded rats, which are distinctive for their white bellies were also popular. 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, in 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 the Wistar Institute in pennsylvania, wanted to standardize the albino rat so that reproducible studies could
be conducted on the growth and development of it's nervous system. The Wistar rat became the first animal to be bred solely for scientific research. The
strain proved so successful it's still sold today, more than 100 years later. The Wistar rat and other strains that followed, proved to be the perfect test animal for
experiments that required animals with the same genetic makeup and unlike 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 give scientists test subjects that are more than 99% geneticaly identical. Labs don't even consider a line to be inbred until the
20th generation. The surging population 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% genetically identical, but almost
all of 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 behavior 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 worth while than throwing them in a
pit and seeing how fast a dog can kill them. Thank you for watching this episode of SciShow. If you have any questions, comments, or ideas for us,
you can leave them on facebook or twitter, or down in the comments below. And if you want to keep getting smarter, you can go to youtube.com/SciShow and subscribe.