Behavioral Aging Study and Ethical Lifespan Assessment of Hybrid Mice

Researchers analyzed the behavior of hybrid mice and presented a novel method to qualitatively estimate natural lifespan.

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Mice are frequently used as research models in aging studies. In 2019, researchers from the University of GothenburgR&D AstraZenecaHarvard Medical School, and Karolinska Institutet identified logistical and ethical issues with the standard system of handling murine models in aging studies. Historically, researchers have favored using male mouse models instead of females, especially in pharmaceutical drug discovery and testing. However, half of the human population is female, and thus, females are half of the recipients of pharmaceuticals on the market. There is a need to fill this gap in research by emphasizing the assessment of both male and female subjects in research studies. The second logistical problem is the use of inbred mice. Inbred laboratory mice tend to have strain-specific behaviors that can skew study results. Therefore, there is a need to replace inbred mice with hybrid mice, especially in behavioral aging studies.

Lastly, the researchers addressed lifespan assessment in mice. Due to ethical concerns, many institutions do not allow researchers to study lifespan in mice. These concerns arose from researchers allowing mice to pass away naturally, even if some mice are terminally ill and suffering. In a research paper published by Aging (Aging-US) in 2019, the researchers came up with a novel method of ethically assessing lifespan. They also employed male and female F2 hybrid mice in a behavioral aging study. Their paper was entitled, “Conclusions from a behavioral aging study on male and female F2 hybrid mice on age-related behavior, buoyancy in water-based tests, and an ethical method to assess lifespan.”

Behavioral Aging Studies in Mice

“In this study, F2 hybrid female and male mice were assessed for behavioral tests with the aim to investigate sex differences and age-related alterations.”

The team used various behavioral tests in order to gain a better understanding of the behavioral effects of aging in female and male F2 hybrid mice. Behavioral tests included an open-field test in an activity box, the shuttle box passive avoidance test, physiological analyses for behavioral phenotyping at seven, 15 and 22 months of age, and a swim test to measure immobility. Immobility in the swim test was an indicator of depressive-like behavior.

In sum, the researchers demonstrated that decreased exploratory behavior is a robust behavioral marker of aging in both male and female hybrid mice. However, altered learning, memory and depressive-like behavior were not significant markers of aging in these models. To this end, the team did not find sex differences in learning or memory using the passive avoidance test. In females, fat mass accounted for 30-46% of the observed increase in depressive-like behavior compared to males.

“This novel finding emphasizes the need to control for body composition in water-based tests.”

Ethical Murine Lifespan Assessment

The ethical method of lifespan assessment the researchers devised involves using estimates of lifespan. In a separate cohort from the behavioral studies, the researchers created this lifespan estimation by separating mice with signs of pain or severe disease from the healthy aging mice. The ill animals were euthanized and then included in two separate data curves. In one curve, the euthanized animals were counted as if their time of death was from natural causes (an underestimation of their lifespan). The researchers then made a second data curve in which they calculated that the euthanized animals as if they had been as healthy as their littermates (an overestimation of their lifespan since the euthanized animals were terminally ill). These curves created an interval that was used as the minimum and maximum lifespan of this cohort. 

“We think this is a really good method that we hope people will start using in lifespan analysis,” said Malin Hernebring, from the University of Gothenburg and R&D AstraZeneca, in a recent Behind the Study interview with Aging-US


The researchers presented a novel method to estimate natural lifespan in survival studies, in which animals in pain or with severe disease are not left to suffer until the end of their natural lifespan. This new method provides a qualitative estimation of natural lifespan, without the expense of animal welfare. The study also showed that F2 hybrid mice are effective in behavioral aging studies, and that fat mass partially accounts for increased immobility in aging female mice. 

The researchers hope that their findings will lead to changes in the way aging research is conducted. In particular, they hope that more emphasis will be placed on testing both male and female subjects, that inbred mice will be replaced with hybrid mice and that their ethical method of lifespan assessment in mouse models is adopted at scale.

“In summary, this work is the first behavioral phenotypic aging study to use hybrid mice and include analyses of both sexes.”

Click here to read the full research paper published by Aging (Aging-US).

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Aging (Aging-US) is an open-access journal that publishes research papers bi-monthly in all fields of aging research. These papers are available to read at no cost to readers on Open-access journals offer information that has the potential to benefit our societies from the inside out and may be shared with friends, neighbors, colleagues, and other researchers, far and wide.

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