A basic principle of animal research is that researchers must always use the most primitive species possible (from the standpoint of sensory physiology) with which the experimental question can be investigated.A number of different animal species have been used for brain research, beginning with insects (flies, locusts, bees) all the way to lancelets, newts and salamanders, birds (zebra finches, pigeons) and rodents (mice, gerbils, rats) and all the way to cats and primates.
The German Animal Welfare Act requires researchers to use the most primitive species possible (from the standpoint of sensory physiology) with which the experimental question can be investigated. But not all animal species are equal when it comes to their suitability for the investigation of certain brain functions. Rats, for example, experience their environment above all through their noses and whiskers, so they are highly suited for experiments about touch and smell. Cats and primates, on the other hand, have very good vision, so they are often used to elucidate the fundamentals of visual perception.
The primates used in brain research are often rhesus monkeys (Macaca mulatta). In more seldom cases cynomolgus monkeys (Macaca fascicularis), night monkeys (Aotus trivirgatur) or Callithrix (marmosets and tamarins) are used. All of these are monkey species rather than apes. Great apes have not been used in research in Germany since 1991.
The reason that rhesus monkeys are used so frequently in research is because they have a very elaborate social behavior and because many of their cognitive abilities are similar to those of humans. In addition, because they have been investigated for so many years, we possess a great deal of detailed knowledge about the structure and function of their brains.
For our experiments at the Max Planck Institute we generally use adult monkeys aged between 5 and 15 years. Most experiments involve quite complex cognitive tasks that the animals are only able to carry out if they are willing to concentrate and cooperate. This only happens if they are healthy and feel at ease in their surroundings.