• Robb G. Best

Meet the Monkey at the Forefront of Discovery

Science news is full of what certainly sound like promising new studies featuring lab mice or lab rats as the test subjects. New treatments are assessed, and psychological truths are mined. Reading about this research, it’s easy to feel like we are witnessing the first steps of something big, something impactful.

However, you are not a mouse (we assume). If you have ever read a conclusion from a mouse behavior study and wondered at the exact practical implications to your life, it turns out you had a pretty good point. For instance, more than 80% of medical treatments tested on animals—usually mice—fail when tested on people.

Mice are cheap, easy to genetically modify, and we share 99% of our genes, but that one percent holds some crucial differences, especially when it comes to neuroscience research. For example, mice lack a dorsolateral prefrontal cortex, which is involved with human learning, memory, and cognition. It’s hard to model, let’s say, a mental illness, by manipulating a mouse’s brain chemistry; mouse behavior and human behavior are just too different. “Rodents are basically solitary, nocturnal and olfactory and we're social, diurnal and visual,” says Steven Hyman, former director of the US National Institute of Mental Health.

Enter the common marmoset. Originally hailing from northern Brazil, these small monkeys (they generally weigh less than a pound each) are increasingly sought after in the neuroscience world. Why? There’s a number of reasons.

Marmosets display a number of human-like traits. For one thing, they’re extremely social. They communicate with a wide variety of vocal sounds and postures, they raise young in stable family units, and they learn by patterning behavior. While most primates interpret eye contact as a threat, marmosets seem to hold each other’s gazes simply to parse social signals. And, like all primates, they have a dorsolateral cortex.

On a practical level, marmosets present a number of improvements over another possible lab monkey, the rhesus macaque. They’re smaller, easier to maintain, and they both reproduce and grow faster. (There’s also the handy fact that marmosets, unlike the rhesus macaque, are not carriers for hepatitis B.) They typically give birth to twins, which is just plain convenient when it comes to running studies. Also, their brain surface is smoother than a macaque’s, making imaging studies easier.

And in 2009, scientists successfully produced a transgenic marmoset, which glowed green under ultraviolet light due to implanted jellyfish genes. One of the main advantages of mice as a test subject is how easy they are to genetically modify. While marmosets are certainly harder to manipulate on this level, it is possible in at least some circumstances.

Of course, there are some complications. Marmosets are hard to train, and there are certainly still some major differences between them and us. An international agreement with Brazil keeps export rates low. There are also some potential ethical issues when it comes to, say, reproducing the effects of depression in a previously healthy monkey’s brain.

Still, it’s time to take notice of the common marmoset. The next big scientific breakthrough might be waiting in a monkey’s brain.

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