A review of seasonal/circannual effects of laboratory rodent behavior.

The existence of seasonal or circannual effects on laboratory rodent behavior has been the subject of much debate in recent conversations with colleagues. If such effects are real, they could explain poor replicability or hinder the detection of treatment effects. Here, we review the literature in which seasonal or circannual rhythms were examined under typical 12:12h photoperiods and present our historical data of locomotor activity of male and female Sprague-Dawley rats across several seasons and years. In general, there was little evidence to indicate significant effects on the locomotor activity of rats or mice, while studies of depression behaviors were somewhat inconsistent in their findings. Results of the few anxiety behavior assessments were fairly consistent, at least in rat studies. Two studies of pain-related behaviors indicated decreased responses during spring or summer testing. If such seasonal effects are real, this would imply that laboratory rodents have a type of internal Type 2 circannual clock or endogenous oscillator. However, photoperiod, temperature, or humidity cannot be the zeitgeber. Further, the need for a circannual clock in a short-lived rodent is debatable.