A reasonable solution to this ethical (and in Germany legal 2 ) problem would be the feeding of these animals to carnivores in human custody, such as in falconries, zoos, and private homes (e.g., snakes). Currently, millions of animals, especially mice and rats, are bred and killed solely for the purpose of feeding larger animals. Husbandry conditions for research animals are of a considerably higher standard than those of feeder animals, reducing welfare concerns and risk of zoonosis 5,6,7,8 .

HoHlbaum et al. Conflict of interest TH and HH are named inventors on a patent by Johns HopkinsUniversity on the production of mini-brains (also called Brain-Spheres), which is licensed to AxoSim, New Orleans, LA, USA. TH, LS and HH consult AxoSim, and TH is a shareholder.

In order to assess the extent to which the legally prescribed training for the acquisition of animal experimentation expertise provides scientific personnel with the necessary competence and expertise to carry out a correct harm-benefit analysis in the context of animal experimentation applications, we conducted an interactive stress assessment concerning the basic animal experimentation expertise course. First, before the practical part of the course and then, after the practical part, the participants assessed images and video material of healthy and stressed animals. The results were assessed comparatively and showed a significant increase in performance in all categories (p-value < 0.001). In addition, the results were comparatively assessed against those of scientists already experienced in animal experiments and experienced animal caretakers in research and clinics. In all groups, the vast majority of participants were able to recognise stress in laboratory animals. A significant proportion of the participants were also able to rate the level of stress correctly according to three degrees of severity: mild, moderate and severe. Nevertheless, a small number of participants were unable to distinguish between healthy and stressed animals and thus, the stress in the individual groups was assigned very differently from the different degrees of severity. The results of this study illustrate, on the one hand, the high significance that training must have in order to acquire the expertise, and, on the other hand, how strongly the assessment of stress is influenced by subjectivity.