There are mice running around in labs that are smarter than their peers. This distinction goes beyond an above-average aptitude for mouse-intelligence tests: These mice are smarter because half the cells in their brain are actually human cells.

A new study, published in The Journal of Neuroscience, explores the consequences of creating mice that have mousy neurons but human glial cells—cells that support nerve cells and strengthen connections between them. Steve Goldman, a researcher at the University of Rochester Medical Center in New York, led a research team that took immature glial cells from human fetuses (donated to research) and injected them into the brains of mouse pups, reports Andy Coghlan for New Scientist. Since the glial cells were immature, they continued to grow and divide in the mouse brain, crowding out the mouse cells.

"It’s still a mouse brain, not a human brain," Goldman told Coghlan. "But all the non-neuronal cells are human." Glial cells can mature into the star-shaped glial cells called astrocytes that help connect neurons. But the human astrocytes have more tendrils and are larger than mouse astrocytes. That difference gives the mice with these human-mouse chimera brains an advantage. "It’s like ramping up the power of your computer," Goldman says.

The altered mice scored higher on a bunch of tests of mouse memory and cognition, including those mentioned above.

This isn’t the first so-called humanized mouse that researchers have created; it just is the most striking because it involves brain cells. There are mice that are mostly mice but that have a human immune system, for instance. Scientists are interested in creating these kind of animals because, although mice make good study animals and are close enough to humans, they aren’t a perfect match. Humanized mice close the gap a bit more and can give us better insight to human diseases like cancer, Alzheimer’s, heart disease and more.

But in creating mice that are more like us, we are blurring the line between human research and mouse research. These mice with "half-human" brains are still very much mice. There are also plans to create rats with human brain cells. But Goldman told New Scientist that he and his colleagues think putting human cells into monkeys brings up more tricky ethical issues.

Jamais Cascio, a writer and futurist, outlines the difficulties of drawing the line between humans, research animals and humanized research animals in his blog, Open the Future. This research group doesn’t intend to add human brain cells to monkeys, but that doesn’t mean that another group won’t. He writes:

[We’re] making these non-human animals demonstrably smarter. We are, in a very limited fashion, uplifting them (to use David Brin's terminology). They will be able to understand the world a bit (or even a lot) better than others of their kind. And at some point, we may well even end up with test subjects significantly smarter than typical and able to demonstrate behaviors unsettlingly close to our own.

What rights should any of these types of uplifted animals have? Do we need to spell out a greater set of rights for the human chimera mice in the news report? Or as we create increasingly more-intelligent-than-typical animals, will there a point at which they could no longer be limited to the rights given to all scientific research animals? At what point would it become a crime to kill them, no matter how humanely or in accordance with ethical standards?

These questions may seem to be well into the realm of science fiction, but just several years ago a mouse carrying human brain cells would have seemed just as outlandish. And already, our growing understanding of the intelligence of great apes and cetaceans has ignited debate about what rights those animals have. These are questions that someone needs to ask.