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Why animals are bred but not used in research

Research laboratories in the EU keep records of animals that are not used in regulated procedures (those recorded in the statistics on animal use) and the European Commission has introduced a requirement to publish these numbers every five years.


These additional animal numbers, categorised as 'bred but not used', provide a record of all animals in the EU which are killed in a research setting, regardless of whether they have undergone a regulated procedure or not.

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 animals will need to be bred, but then not used in experiments or testing. This is due to such things as the current scientific limitations of breeding genetically modified animals, or the difficulty of predicting the number of animals needed for a particular study. While these animals have not been used in testing they still play an important and necessary part in the overall biomedical research process. 

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

  • Genetically altered animals
    Animals that do not meet the genotype of a given genetically modified strain that is required, for instance in Covid-19 research, and cannot be included in a study. There is currently no technology available to ensure that all animals bred will possess the desired genetics required – those bred can sometimes actually 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. At this stage the process of breeding the required animals begins. However, as research progresses the results may indicate the hypotheses being investigated are not valid (hence the research stops), or the results may lead to different theories that require additional investigation. This can lead inadvertently to a considerable surplus of animals being bred because the breeding programmes cannot effectively be switched on and off.


  • Sex bias 
    The requirement of researchers for an unequal number of male and female animals, leading to a surplus of one sex. This may be driven by the research area e.g: basic research first evaluated in one sex, only followed by confirmation in the other sex if there are good results.


  • User demand
    Trying to meet research needs by breeding animals to requirements over a wide range of ages. Sometitmes 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 can be unpredictable (for instance, litter sizes
    for mice being abnormally large, or if more individuals survived than usual), which can result in more animals being bred than are required.

  • Animals used for the collection of tissues

  • Breeding animals
    Animals used for the purpose of breeding alone are likely to undergo no procedures themselves. Animals are also used to sustain inbred colonies.


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

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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 that is being focused on. An individual research 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 the animals required for a study.

In addition, a main way to achieve a reduction in animals that are 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.

The rapidly developing CRISPR/Cas9 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.

Why has the relative number of animals bred but not used that are genetically altered 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.

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