No One Likes to Live Alone: Social Housing of Lab Animals


Sara Capdevila del Estabulari del PRBB a ALN Magazine


Certain research circumstances call for a mouse—typically a male mouse—to live alone. In view of the considerable and wide-ranging effect solo living has on this naturally social individual, any more than a brief period alone is regrettable: the animal suffers unnecessarily and the physiological disturbance that follows from extreme stress might affect the study results. Nevertheless, for a variety of reasons—some being reasons of convenience—mice are housed alone much more often than strict adherence to the research design would require.

So we would like to persuade you that reducing the number of female and male mice in individual housing—and wherever possible shortening the time in solitary—is in your best interests.

To explain why we are going to such lengths to make a case for reducing this source of stress and distress, we will spell out some ways sole occupancy can kick-start a profound system-wide cellular and behavioral downward spiral. To weight the argument, we add the words of three deeply respected biomedical research thought leaders. And to show that a decision to limit individual housing can be a practical one, we offer details of some steps PRBB has taken to reduce the time male mice spend alone.

There is evidence that dorsal raphe dopamine neurons represent the experience of social isolation in mice and that rats’ experience of social isolation produces similar results. The activity of these neurons changes depending on the style of living, socially or isolated, and the level of aversion to the experience was measured, revealing that isolation after the experience of living socially is even more stressful, and that dominant male mice suffer more from loneliness than submissive male mice.

Furthermore, with three-month old male C57BL/6 mice that were socially isolated by individual housing for another three months, D. Siuda and colleagues1 found significant changes in brain metabolism. Social isolation of male adult mice led to an increased global DNA methylation, as well as Histone methylation. Gene-specific effects were observed for Hdac1, Hdac3, and the serotonin transporter Slc6a4.

Additionally, Pyter2 demonstrated the journal, Stress, that a period of individual housing of three weeks could model chronic stress as well as the model of restraint (12 hours a day for eight days). Furthermore, healing was delayed due to metabolic and hormonal changes.

At this point, you are thinking you know that social isolation affects the well-being of mice and also causes stress, as well as physiological changes that could affect research results. We’d like to suggest that this parameter is important to be reported in research studies, and you always avoid it when you can. We’d also like to suggest that it might be possible to reduce further or greatly reduce the need to house a mouse alone and/or the amount of time a mouse spends alone, and to mitigate the effects of solitary existence.

A male mouse, housed individually.

Aggression in Male Mice
While mice are housed individually for many reasons including segregation by genotype, gender or age, arranging matings, and surgery, the most oft-cited reason for segregating a male mouse is male mouse fighting. This may appear to be as stressful for the animal as segregation but the mechanisms and effects are very different, because fighting is natural and living alone is not. Aggression begins with sexual maturity, or at about 10 weeks of age and continues throughout life. Any social change in male grouping can prompt an aggressive response.

Vera Baumens

“To me, fighting is the only acceptable reasons to individually house mice,” says Vera Baumans, one of Europe’s most respected biomedical research veterinarians and animal welfare experts, “as specially BALB/c males may fight fiercely. Still,” Vera continues, “you should first try to reduce fighting by reducing group size to three, providing shelters/nesting material, and to the extent possible avoid multiple handling for procedures.”

Javier Guillén, DVM
Javier Guillén, DVM

“Programs we evaluate,” says Javier Guillén, DVM, Senior Director for Europe and Latin America, AAALAC International, “ensure that single housing is limited to the minimum period necessary and, where possible, visual, auditory, olfactory and, depending on the species, protected tactile contact with compatible conspecifics are provided. However, there are places where tradition and old practices, including individual housing, are still present. Sometimes it is difficult to challenge tradition. Research on the subject and dissemination of good practices are essential. At the institutional level, the Ethics Committee/IACUC/Animal Welfare Body should be the body reviewing the cases of male mice individually housed and promoting strategies to address these situations.”

Strategies for Reducing Male Mouse Time Alone
Some strategies for reducing overall time mice of both genders spend alone range from the simple to the more costly.

Pascalle van Loo, a European expert on male mouse aggression, offers some suggestions about how to manage male mouse fights. The most important one is the group size, from three to five, though a group of three seems to mitigate fighting.

“Isolation is on the minds of most technicians but may not be in the forefront of a researcher’s thinking. “Training for animal caretakers focuses on enrichment strategies and that includes for individually housed animals. More emphasis on this training for researchers would be welcome,” Javier Guillén points out.

“However, there is room for improvement. The ARRIVE Guidelines are very helpful and useful, yet too often they and other clear guidelines are not followed. Still, with requirements for researchers to specify housing conditions in the research reports, a majority of researchers do report the conditions and so are considering them.”

From Theory into Practice: PRBB Puts a Plan in Place
Even though it is sometimes necessary to house mice individually—for example when senile mice up to 18 months have Alzheimer’s Disease—there is much facilities can do. Here is some information about our progress at PRBB.

Juan Martin Caballero DVM, Ph.D.
Juan Martin Caballero DVM, Ph.D.

In 2014, Juan Martin Caballero DVM, Ph.D., and Director of PCB-PRBB Animal Facilities, PCB-PRBB Animal Facility Alliance, set the avoidance of mouse isolation as a goal for the Facilities. He explained why. “In the 1990s, I read several articles about chronic stress caused by placing rodents in individual housing. Not only was there evidence from behavioral and neuroscience research generally, but also in the study of tumor behavior and immunology. The systemic effect of living alone was deleterious. But most shocking still were reports of the anxiety and anguish the animals suffered in the absence of psychosocial stimuli provided by peers. These isolated animals were nervous and extremely difficult to manage, and at night, using infrared cameras, you could see the stereotypies emerge.

“I took a very close look at our practices and realized that we individually house many more animals than we had to. I developed a plan for reducing individual housing numbers and time, and convinced our Ethics Committee/IACUC, to agree that approving individual housing or total isolation for any animal should be presented, defended and scientifically justified before such an action would gain approval.

“Were we successful? Yes, though slowly. Though it made good common sense, and for a time the entire facility and our research groups within it were focused on this issues, it was not simple to implement because we had a comprehensive—read costly—plan.

“First, we decided to create a module within our computer software system so we could document the incidence of individual housing and the welfare implications. This software produces an e-mail to keep the researcher informed at all times when an animal is isolated and about the isolated animal’s activities and condition. Sometimes a female animal may have to be regrouped; if an adult male has an issue, he cannot be regrouped, so the researcher must decide if the isolation of that individual could affect the experimental outcome.

“We trained our caregivers and technicians to mark and group females instead of removing them to individual housing; and, motivated greatly by the 3Rs, we also checked all our breeding strategies aiming to ensure that breeding males or females would not live alone for more than two weeks.

“The most efficient strategy for genetically modified males has been to keep them always with a peer. If, due to genotype, there is only one mouse in the cage, another male of the established social group is placed in the same cage. This visitor mouse is identified in the software as ‘companion mouse’ and will remain with his friend throughout the study mouse’s time in the animal facility.

“If a mouse must be individually housed, of course we enrich the cages with cardboard tunnel and cellulose. The solo mouse seems very happy with these distractions.

“I am delighted to say we have had a very big return on our investment in reducing the number of mice in individual housing and the length of time they stay alone. There is more uniformity in experimental results in behavioral tests, tumor induction, and immunization. And, I am pleased to report, the rodents are happier at night, the technicians see that their efforts have yielded good results for the animals, and experiments are providing a better return on the investment.”

The fact of male mouse fighting insists on a great deal of solo living and we will continue to investigate ways to make their lives less stressful while alone and to reduce the incidence of solo living. But there is a long road ahead. “I encourage professionals on the ground making efforts to address male mouse isolation in solo housing to present their work at scientific meetings and submit them for publication,” Javier Guillén says. “And not only successful strategies and tests should be shared through publications, scientific meetings, and other professional fora; also those which are not successful, from which everyone can learn and upon which we can build.”


  1. Siuda D, Wu Z, Chen Y, et al. Social isolation-induced epigenetic changes in midbrain of adult mice. J Physiol Pharmacol. 2014 Apr;65(2):247-55.
  2. Pyter LM, Yang L, McKenzie C. et al. Stress. Contrasting mechanisms by which social isolation and restraint impair healing in male mice. 2014 May;17(3):256-65.

Additional Reading

  • Baumans. V and Van Loo, P. L.P. How to improve housing conditions of laboratory animals: The possibilities of environmental refinement The Veterinary Journal 195 (2013) 24–32.
  • Matthews, Gillian A., Nieh, Edward H. et al. Dorsal Raphe Dopamine Neurons Represent the Experience of Social Isolation Cell. 2016 Feb 11; 164(4): 617–631.
  • Van Loo, Pascalle L.P., Molb, Jan A. et al. Modulation of aggression in male mice: influence of group size and cage size. Physiology & Behavior 72 (2001) 675± 683
  • Van Loo P.L.P., de Groot A.C., Zutphen B.F.M.V., Baumans V. Do Male Mice Prefer or Avoid Each Other’s Company? Influence of Hierarchy, Kinship, and Familiarity. J. Appl. Anim. Welf. Sci.2001;4:91–103.

Sara Capdevila, DVM, is Deputy Director of the Animal Facility, Barcelona Biomedical Research Park. She holds a Master’s Degree in Laboratory Animal Science and Welfare. Write to Sara at

Helen Kelly is a freelance research writer and ALN Magazine’s International Editor. Helen is always pleased to hear from readers.

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