Masking and temporal niche switches in spiny mice.

Activity patterns are the product of interactions between an internal circadian clock and direct responses to photic and nonphotic features of the environment that are said to "mask" the influence of that clock. Evolutionary transitions between nocturnality and diurnality involve changes in mechanisms underlying both of these processes. Here, the authors examined how masking influences activity patterns of golden spiny mice ( Acomys russatus), which can be either nocturnal or diurnal, and common spiny mice (Acomys cahirinus), which are strictly nocturnal. Animals kept on a 12:12 LD cycle were exposed to 3-h dark pulses starting at ZT 2, light pulses of varying intensities (50, 100, 700, or 1500 lux) at ZT 14, and a 3.5:3.5-h LD cycle. In common spiny mice, activity increased by 379% during the dark pulse and decreased during light pulses to 23% of baseline levels. Golden spiny mice also increased their activity in response to the dark pulse (by 345%), but there was extreme inter-and intraindividual variability and no significant response to light pulses at night. In the 3.5:3.5 LD cycle, common spiny mice showed a preference for the dark phase with 86% +/- 0.01% of activity occurring then, whereas golden spiny mice showed a pronounced circadian rhythm but no evidence of masking. Masking responses to light and dark were thus unsurprising in common spiny mice but were highly unusual in golden spiny mice. Patterns seen in the latter species may reflect mechanisms enabling these animals to occupy either a diurnal or a nocturnal niche in their natural habitat.