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Why animals are
bred but not used
in research

Research laboratories keep records of animals that are not used in ‘regulated’ procedures and the EU has introduced a requirement to publish these numbers every five years. These additional animal statistics provide a record of all animals in the EU which are killed in a research setting whether or not they have undergone a regulated procedure.

Are these animals’ lives wasted if they are bred but not used?

To achieve the highest standards of research that are needed for validated studies many animals will need to be bred but not used. This is due to such things as the current scientific limitations of genetic modification, or the difficulty of predicting the number of animals needed for a particular study.

What are some examples of why animals would be bred but not used?

  • Genetically altered animals
    Animals that do not meet the genotype of a given genetically modified strain and cannot be included in study. There is currently no technology available to ensure that all animals bred will possess the desired genetics required – those bred can sometimes be ‘genetically normal’.


  • Inconsistent and unpredictable demands
    Research programmes are considered in advance and require authorisation through licenses or ethical permits. Individual studies are also planned in advance. However, as research progresses the results may indicate the hypothesis being investigated is not valid (hence the research stops) or the results may lead to different hypotheses requiring investigation. This results in variable demands and can lead inadvertently to a considerable surplus of animals because breeding programmes cannot effectively be switched on and off.


  • Sex bias 
    Requirement by users for an unequal number of male and female animals. This may be driven by the research area e.g: basic research firstly evaluated in one sex and if good results, confirmation in the other sex.


  • User demand
    Trying to meet research needs by breeding animals to supply requirements over a wide range of ages. From a scientific point animal studies will require the use of animals of differing age ranges depending on the research area, body organ, and/or the hypothesis being researched.


  • The number bred
    This may be over and above the numbers needed for the research programme: - Litter sizes and individual mortality rates are unpredictable, which can result in more animals bred than are required if litter sizes (for instance, for mice) were abnormally large, or if more individuals survived than usual.


  • Animals used for the collection of tissues

  • Breeding animals
    These animals undergo no procedures themselves. Animals are also used to sustain inbred colonies (this includes breeding stock and neonatal losses).


  • ‘Sentinel animals'
    Animals from the batch bred which are used for health screening of the other animals in the laboratory

What is being done to reduce the number of animals bred but not used?

The effective management of breeding colonies to reduce surplus is one critical area of focus. An individual establishment’s Animal Welfare Body has a role in overseeing and providing a framework for regular assessment of this. Examples here would include ensuring there are good internal communication processes that allow for accurate forecasts of animal requirements.

In addition, a main way to achieve a reduction in animals bred but not used is to ensure that researchers clearly understand how to implement the 3Rs. This is one of the main priorities of Directive 2010/63 and involves the EU Commission working with the Member States (and the bodies within Member States which are authorised to grant licenses to breed animals for scientific procedures), these in turn will work with individual breeding facilities and researchers. For instance, in the Netherlands the Committee on Animal Testing (CCD), which is solely responsible for granting project licenses, continually monitors and frequently amends codes of practice to minimise the number of animals bred but not used.

CRISPR/Cas9, the genetic edition tool, could be a great opportunity for reduction as it allows the development of more refined lines with genetic alterations which more accurately reflect human diseases, or more specific models for discovery research.

Genetic modification and bred but not used - why has the relative number of animals bred but not used increased?

This is because, in general, the number of procedures using genetically altered animals (such as CRISPR/Cas9) is increasing.

Current technology cannot guarantee that every required genetic alteration actually occurs or occurs in the correct place and in the right cells. The practice is expensive, complicated, and time consuming, and therefore every effort is made to get the right results. As technology improves the science of genetic alteration will improve and the surplus of animals bred but not used will decrease, for example with the new technologies such as Prime editing, and CRISPR/Cas9.