While studying the impact of cage size on inbred and outbred mice and rats, researchers from Charles River stumbled on an unusual behavioral pattern that suggests even laboratory rodents may need a break from the kids.

From time to time, the adult mice and rats, primarily the moms but also the dads, wedged themselves upright into the corner of their cages, and appeared to briefly zone out, though why they felt compelled to turn their backs on their offspring is unclear, said Brianna Gaskill, an assistant professor of animal welfare at Purdue University who worked on the study as a post-doc at Charles River.

Gaskill presented findings from the study during the annual Charles River Short Course, four days of talks on laboratory animal science that end on Thursday. Gaskill said the behavior, dubbed “corner inactivity” for obvious reasons, had never been documented in the scientific literature before as an indicator of stress. It was more evident in two common outbred strains, the CD-1 mouse and CD rat that are about twice the size of the two inbred strains—the C56BL/6 mouse and Brown Norway rat—also included in the study. In addition to being larger, outbred strains also have much larger litters.

The corner inactivity surfaced as part of a much larger study looking at the impact of cage size on the welfare of laboratory rodents that was triggered nearly three years ago by significant changes in industry standards. New recommendations that appeared in the Guide for the Care and Use of Laboratory Animals increased the minimum number of square inches of floor space that should be used to house female rodents and their babies, and in 2011 the US National Institutes of Health ordered facilities that received federal research funds to comply with the guide. (Since then, facilities have been allowed to apply for exemptions provided they show that the cage sizes they were using previously were not impacting the welfare of the animals.)

The Charles River study set out to compare a range of biological and behavioral factors—from reproductive rates and nest building to playing—among  the four strains of inbred and outbred mice and rats that had been assigned to cages of different sizes and social structures. The mouse cages ranged in size from 330-800 square centimeters; the rat cages from 580-1,365 square centimeters. The study didn’t uncover anything remarkable in the breeding patterns of the mice and rats that might have been attributed to cage size, underscoring how difficult it is to interrupt the reproductive habits of mice and rats.

But in the process of reviewing hours of videotape, Gaskill and her colleagues did notice the corner inactivity. Along with it being more apparent in outbred strains, it also seemed to increase as the pups got closer to weaning age. One reason the moms may have resorted to corner inactivity was to get some rest, though the activity was also seen in the male rodents.

Gaskill said one solution to the corner inactivity may be a penthouse effect—more vertical space that the mom can jump up into to take a break. This was actually studied, she said, by Washington State University animal scientist Sylvie Cloutier. Using a two-level cage whose top shelf was inaccessible to the pups, Cloutier found the moms spent a great deal of time on the lower level when their offspring were between 5 and 10 days old, but when they reached about three weeks of age went to the upper level more. “She found the quality of the space may be more important than the quantity of space,” said Gaskill.

The Short Course also included talks on using exhaust air dust samples to replace rodent sentinels and what we can and can’t learn from Central Nervous System trials in rodents, which were also the subject of Eureka blogs here and here. Cindy Buckmaster, Director of the Center for Comparative Medicine at Baylor College of Medicine in Texas and a Vice President for the American Association for Laboratory Animal Science (AALAS), rallied the audience to rise up and begin engaging the public about the value of what they do and significant contributions that are made every day by animal research.