Animal experiments are indispensable in biomedical research [1, 2]. The use of animals for the sake of further development of medical knowledge, treatment of currently uncurable diseases as well as safeguarding of a healthy and prospering human population is warranted by the freedom of research guaranteed in the constitution of most European countries, the Charta of Europe, and the covenant on social rights of the UN [3, 4]. However, this is not a free pass allowing disregard of ethical, scientific as well as sociopolitical aspects and standards – and these are continuously rising due to the ethical dilemma posed by the use of animals as well as the general validity crisis in research [5]. Therefore, using animals in research brings an enormous responsibility for the scientist to meet the scientific and animal welfare standards [6]. The latter is captured formally in the EU directive on the use of animals in science [2] and scientifically in the 3R principle, which calls upon the reduction in the number of animals, the refinement of methods used to ensure the least burden, and, ultimately, strive to replace animals with nonanimal alternatives. The 3Rs were first defined by Russell & Burch in 1959 [7] and have since been widely adopted by the scientific community as a valid framework for the treatment and use of animals in research. It has also found its way into legislation (e.g., the abovementioned EU directive 2010/63/EU and thereby to all national legislation in the EU) and has sparked a vivid research community, focusing on a wide variety of aspects related to the 3R principles [8].

With implementation of the 3Rs into regulation, the animal's perspective has been taken to a higher level of consideration for scientists than it had ever been before. In case animal use is inevitable, it is the 3R refinement which is of ultimate importance. It calls for the least harm, pain, and distress inflicted upon the animals. Naturally, fulfillment of this statutory principle requires methods and techniques that enable an assessment of the animal’s well-being as well as the burden inflicted on the animals by experimental procedures and settings. While regulation clearly depicts a categorical scale to be used for this assessment (non-recovery, mild, moderate, and severe) and lists examples for these categories, the way to achieve the underlying assessment requires scientific evidence [9, 10]. However, within different animal models, different species, different procedures on the one and the different aspects that can inflict a burden to the animals on the other hand, this is a complex task requiring ongoing research [11]. Without filling the gap between regulatory requirements and scientific evidence, scientists face unavoidable legal uncertainty, and, importantly, the animal´s perspective is not covered sufficiently.

Within this Topic Article Package, the reader will find relevant information covering severity assessment aspects in different species (NHP, pigs, sheep, rats, mice) and various biomedical disciplines (general surgery, neurosurgery, orthopedic surgery, behavioral science, experimental radiology, oncology, neurology, pharmacology, epilepsy research). The information can be used to implement techniques within research protocols in small as well as in large animal models up to NHP to better assess and subsequently minimize the burden for them to improve animal welfare, an important step in the refinement and thereby in the implementation of the 3R principle of Russel and Burch in a given project. We do hope that the reader will benefit from this Topical Article Package and that this will help improve animal welfare and the science.

The authors and issue editors thank all contributors to this special issue.

The authors have no conflicts of interest to declare.

Funding of the DFG (FOR 2591; AB: BL953/10-1 and 10-2, BL953/11-1 and 11-2; BV: VO 450/15-1, VO 450/15-2; RT: TO 542/5-1 and 5-2, TO 542/6-1 and 6-2) enabled establishment of a research unit to target relevant questions in severity assessment in animal-based research. This funding gave rise to initiate this special issue.

Andre Bleich, Brigitte Vollmar, and Rene H. Tolba contributed to writing, editing, and proofreading of this editorial.

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