Of Mice And Men: Translating Mouse Age To Human Age

The mouse is the go-to model organism for biomedical research. Mice are relatively easy to look after, are physiologically quite similar to humans, and share most of the same genes. Yet there is a very important difference between our two species, especially when it comes to studying the ageing process: humans live a lot longer than mice. When translating findings in mouse models to humans, it helps to understand how mice age relates to human age – that is to say, we should be able to take a mouse that is X months old and say: this mouse is equivalent to a Y year-old human.

How many ‘human years’ is one ‘mouse year’?

The average lifespan of a laboratory mouse is about two years, while the average human lives about 80 years. This would make one mouse day roughly equivalent to 40 human days, while one human year equates to about 9 mouse days. Things aren’t quite that simple though, as biological age isn’t just the proportion of one’s average lifespan that has been lived, but rather the extent to which the biological changes related to ageing have progressed. As it happens, biological ageing in mice doesn’t follow the same pattern as it does in humans, meaning that 1 mouse day only equates to 40 human days on average. The true number will vary depending on which stage of life is being considered.

Translating mouse days to human days at different stages of life

There are many different metrics that could be used to translate the biological age of a mouse to a human, one of which is to look at developmental milestones:

• Newborn mice are weaned after around 28 days, while humans take approximately 180 days. Based on speed of development, this makes one mouse day equivalent to about 6.5 human days during this period.
• Hormonal changes and secondary sexual characteristics associated with adolescence develop after around 42 days in mice and 11.5 years in humans, making 1 mouse day equivalent to about 100 human days up until adolescence.
• This relationship is maintained until adulthood, following which mouse ageing slows relative to human ageing. In female mice, reproductive function is lost at around 15 months, while in women, menopause occurs at an average age of 50, making one mouse day equivalent to around 41 human days.
• Finally, the period of life prior to death during which biomarkers of old age (markers that relate to the biological age of an organism’s tissues) are prominent lasts about 60 days in mice and around 29 years in humans, making one mouse day roughly equal to 176 human days during this period.

An alternative approach is to define stages of life with a focus on the presence of biomarkers of ageing. The graph below divides life into three stages as follows:

• Mature adult: maturational development has stopped and biomarkers of old age are undetected.
• Middle-aged: some, but not all biomarkers of old age can be detected.
• Old: almost all biomarkers of old age can be detected.

Clearly, different approaches to assessing biological age will produce different estimates as to how mouse age equates to human age. Regardless of what method is used, mice evidently age much more rapidly relative to humans in the early stages of life. When considering only the proportion of total lifespan lived, a one month-old mouse should be roughly equivalent to a 3-4 year-old human, yet in terms of maturational ageing, they are closer to a 12 year-old human. For most of their lives, however (beyond 6 months of age), mice mature at a slower rate (relative to their total lifespans) than do humans.