How mice are affected by the changing of seasons

What do rodents know about the changing of the seasons? In the case of the most famous prognostic rodent, the groundhog Punxsutawney Phil, the answer is, “not much at all.” You may be surprised to learn that some strains of inbred laboratory mice seem to detect the changing of the seasons more reliably than Phil can.

Some strains like the well-known C57BL/6J were consistently productive throughout the year. On the other hand, A/J mice somehow retain the breeding characteristics of a wild mouse (many pups in the spring and summer months, and a much lower output in the winter), even though they have been inbred for nearly 300 generations. SPRET/EiJ, derived from wild Mus spretus, also shows seasonal lulls in breeding performance. Litter size is not the only reproductive trait that changes with the seasons; gestation length in C57BL/6J does as well. Additionally, bone density and the response to stressful situations can depend on the time of year.

How do mice know when the seasons are changing? It’s probably not because they hear us complain about the lousy winter weather outdoors. Changes in light probably don’t account for this either. Mouse rooms are windowless, and the light cycle we use (14 hours on, 10 hours off) reflects the photoperiod of late spring. Changes in humidity may have something to do with it. We control humidity in our mouse rooms, but mice may still be able to detect changes within the relatively wide range of tolerated humidity levels. This may be one reason why C57BL/6J mice may have higher incidence of hair loss and dermatitis in the winter.

As with so many things related to mouse biology, you may find that your experience is different in your own facility. Some researchers observe seasonal changes that others do not. That is one reason why it is so important to collect detailed information from your own colonies, and evaluate them regularly. You can compensate for a lull in breeding performance by increasing the number of breeders in the months ahead. Who knows—maybe A/J mice could prove to be a more reliable indicator of spring than Phil’s shadow?