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It's a good question! Humans as apes are more genetically similar to other apes than tailed monkeys, and that small difference (e.g. 1-2% of dna) has a huge impact on development. When animal research is carried out there is lots of preparation with model cell lines, organoids, tissue cultures, etc., before you get to the actual animal in the cage. With all that effort put into preparation, you want the final animal samples to be as close to human samples as possible.

Other simians also have different diets, microbiomes and are susceptible/resistant to different diseases than humans, which could definitely affect your immunology experiments.

However, the scientific benefits of using our closest relatives (i.e. humans, chimps and bonobos) is outweighed by the ethics of using highly intelligent and emotional organisms for research when alternatives are available. Ultimately, this is a socially constructed limitation but it's one I agree with. Also, it is very very expensive to raise great apes in captivity, which means research budgets can't cover it. Imagine raising a 60kg human with emotional problems that can bench 150kg for years just to cut it open and look at its colon. That's not worth it.

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I mean you can just probe its butt and look at its colon again and again!

Lol, I'm just imagining a chimp that has to deal with scientists sticking things up its bum every day. Like "please not again, I'm still sore"

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The species we use for research are called "model species". We use these species for a combination of reasons. Part of it is because they have some relevant connection to human biology (or other kinds of biology, depending on what we are studying). But just as important, if not more so, are other factors. Things like "how difficult/expensive is this animal to keep in captivity", "how much do we know about this animal in general", "what are the ethics of working with this animal", "how easy is this animal to work with in a lab setting", "how many other scientists work with this animal", and "how easy is it to get these animals".

It's not that rhesus monkeys and baboons are particularly more human-like than other old-world monkeys. But they are widely available, reasonably easy to keep (for a primate), and have had plenty of existing research done on them, which means their care and biology is understood already. All this makes them easier to use than some other species.

You can see similar patterns with lab rats and mice, zebrafish, fruit flies, C. elegans, and other model organisms. It's not that they are especially different from their relatives, but they are widely used, cheap, easy and fast to raise.

Contrasting, say, rhesus monkeys with chimpanzees....chimps are bigger, more intelligent and harder to keep suitably in captivity (so more ethical issues), they reproduce more slowly, and they are endangered. Even though they are more humanlike in their biology, working with them is kind of a nightmare. It's more expensive, you need bigger facilities, you aren't going to get ethics board sign offs as easily, and it's not easy to source chimps. And especially if you are studying the immune system, other animals are pretty close. The marginal benefit of working with chimps just doesn't outweigh the extra difficulty. Really the only time you see research done on chimps is when there's some particular reason you can't use other primates (or non-primates).

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I actually thought use of apes ie chimps is now prohibited by law at least in the us. Ethical reasons mostly and not as “useful” as once thought for all the reasons.

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It would be more like, “I’ve always wondered what human faces taste like and now I’m gonna find out.”

Just the thought of trying to get a chimp to sit still for a syringe when it could suddenly grab it from your hands, bite you, break the syringe, escape and climb away from your reach, or make use of an unpleasant projectile, makes working with pigs and rats seem a lot more appealing.

Currently working with rhesus and pigtail macaques, nothing could get me to work with chimps. Macaques are already hard enough to take care of and are at about the top level of acceptable danger in animals I'm willing to work with. Also I love these guys but they can be huge bastards lol, can't imagine what a chimp could get up to

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You’d be amazed at what you can bribe a monkey to do. One of the research groups we work with lets them watch TV to distract them. They are particularly fond of All My Children.

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