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Vincent Munster (researcher at Oxford) states (in the context of testing vaccines for COVID-19):

The rhesus macaque is pretty much the closest thing we have to humans

I thought chimpanzees and bonobos were the two living species most closely related to humans? Why don't we try the vaccines on them instead? Why is the rhesus macaque a better candidate?

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Short answer:

Chimpanzees are indeed closer, but perhaps...too close. Almost all invasive research on non-human great apes has halted for some time over ethical concerns.

Rhesus macaques are the main non-human primate still used in research (some others are also used, like marmosets). "Pretty much the closest" is in reference to which species are available to scientists as a model organism; I assume the researcher quoted included "pretty much" as a bit of a hedge, acknowledging that chimpanzees are closest while not being available for vaccine research. Even then, many countries have limited research using all primates and do not use macaques anymore, either.


Longer answer:

The primates include great apes like gorillas, orangutans, and our closest non-human relatives: chimpanzees. They also include more distant relatives, like Old-world and New-world monkeys, and even more distant lemurs.

Many humans (including influential organizations) have decided that chimpanzees and other apes are too closely related to humans to be eligible for certain types of research for ethical reasons. It has been deemed more ethical to use human subjects, who can consent to participation in studies, or to use other model animals. Apes are also difficult as model organisms: they develop slowly, grow large, and have long life spans with few offspring; all these factors make them expensive research subjects. There is no one biological fact for this distinction between apes and other monkeys, besides their closer relationship to us and the perception that they have more advanced intellectual faculties. Further discussion is outside the scope of this answer.

Old-world monkeys, including rhesus macaques, are the next-closest grouping of primates to the human lineage. Macaque monkeys live and breed fairly well in captivity, and are used in research when other model organisms like mice and rats are unsuitable (like when an aspect of their biology is not sufficiently similar to humans).

Rodents can also be models for vaccine research, but their distance from humans and differing response to certain pathogens may make them less useful for measuring efficacy. Therefore, close relatives like rhesus macaques have been used in past vaccine research.

There is no perfect model organism; even humans aren't always great models for other humans, when they vary by age, sex, pre-existing conditions, etc. There is a balance in risk and ethics in choosing model organisms for research: simpler animals like mice and rats are typically used at earlier stages of research. Primates are used when rodent models don't correlate well enough with humans or when there is a large risk to human trials.


Some references:

https://en.wikipedia.org/wiki/Animal_testing_on_non-human_primates

https://www.nih.gov/about-nih/who-we-are/nih-director/statements/nih-will-no-longer-support-biomedical-research-chimpanzees

https://www.nature.com/subjects/rhesus-macaque

https://www.sciencemag.org/news/2018/11/record-number-monkeys-being-used-us-research

Nath, B. M., Schumann, K. E., & Boyer, J. D. (2000). The chimpanzee and other non-human-primate models in HIV-1 vaccine research. Trends in microbiology, 8(9), 426-431.

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    $\begingroup$ As a non-specialist, this answer doesn't tell me anything. What's the difference between macaques and chimpanzees that make studying chimps too queasy for us? if macaques are farther away, why are they still useful? $\endgroup$ – Azor Ahai -- he him Apr 28 at 20:15
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    $\begingroup$ @AzorAhai "Doesn't tell you anything" seems a bit strong as a criticism, unless you already knew chimp research is banned almost everywhere and macaques are commonly available research organisms - OP didn't seem to know this and I assume a majority of non-biologists don't :) However, I've expanded a bit - does this edit help? $\endgroup$ – Bryan Krause Apr 28 at 20:37
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    $\begingroup$ Fair enough, I was familiar with macaques being common, although I didn't know chimps were completely banned. Thank you for expanding :) $\endgroup$ – Azor Ahai -- he him Apr 28 at 20:40
  • $\begingroup$ Another aspect to this is just sheer size. A gorilla would eat more, need a bigger cage, and be harder to control than a rhesus macaque. So the exclusion of great ape species isn't all on ethical concerns. A basic concern is that the bigger the test animal the more it will cost per animal. $\endgroup$ – MaxW Apr 28 at 20:46
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    $\begingroup$ @MaxW From the answer: "Apes are also difficult as model organisms: they develop slowly and have long life spans with few offspring" - I can add cost to that explicitly but those are all issues that make animals difficult models. A more immediate issue, though, since (as far as I know) every country in the world except one has banned such research on chimps..well, cost isn't much the major factor at this point, even though it was in the past. $\endgroup$ – Bryan Krause Apr 28 at 20:51

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