Mice were housed in standard laboratory plastic cages (20x20x40cm), each equipped with a running-wheel available to the mice at all times. Wheel turns were monitored with a physical switch and the activity stored on a computer (Effects of LL: DAM System, Trikinetics, Waltham, MA or Effects of RF: ClockLab, Actimetrics, Evanston, IL) for later analysis. Groups of six cages were held in light-tight wooden chambers (60x51x186cm) and lighting was provided by two four-foot fluorescent bulbs of 34 Watts producing 1400 Lux at cage height (Luna-Pro light meter; Gossen, Germany) in each chamber. The chambers were continuously ventilated using fans with temperature maintained at 24°C (±3°C) and humidity of 54% (± 5%). Each cage was always equipped with a water bottle that was refilled on a regular basis. When they were allowed to feed, the mice were given IAMS Chunks dog food.

The effects of food restriction paradigms (RF) in LD and LL conditions on running-wheel activity of four mice are represented in Figure 1. When the mice were exposed to 12:12 LD ad libitum conditions, all mice (11/11) individuals showed significant nocturnal activity (Figure 1A). When the LD entrained mice were then exposed to a two-hour period of RF during the daytime, 10/11 (91%) individuals still exhibited significantly more activity during the nighttime – i.e. – they were still generally nocturnally active. However, all (11/11) mice also showed a significant increase in daytime activity during the 1 hour prior to food availability. This increase in diurnal activity persisted in 4/11 animals when food restriction was discontinued and ad libitum conditions reinstated (Figure 1 - left panels).

When exposed to LL, the main activity bouts of most (9/11) animals exhibited no coordination with the restricted feeding (Figures 1B - right panel) while 2/11 animals appeared to entrain to the RF (Figure 1B - left panel). Neither activity levels nor taus were significantly different in either LL food restriction or LL ad libitum (t = 0.12; p > 0.05).

The effects of LD and DD conditions on alpha, tau and running-wheel activity are shown in Figure 2. All animals (10/10) entrained to the LD and these rhythms persisted in DD with periods less than 24 hours. Alpha showed a significant lengthening over time (F (2,18) = 7.19; p < 0.02) in DD conditions (Figure 2B). Neither tau (F (2,18) = 1.53; p < 0.25) nor activity levels (F (2,18) = 0.38; p < 0.69) changed significantly over the 50+ days in DD.

The effects of an intervening LL (175 lux) on DD are presented in Figure 3. When animals (N = 12) were exposed to LL after DD₁, a significant decrease in alpha was observed (F (2,22) = 11.18; p < 0.003; Figure 3B). When LL was subsequently changed to DD₂ alpha increased significantly, but not to its original DD₁ value. Tau was less than 24 hours in DD₁ but became significantly longer (F (2,22) = 21.46; p < 0.0001) upon LL exposure. When the photic conditions reverted back to DD (DD₂), there was a significant shortening in tau compared to both LL and DD₁taus. In addition, activity significantly diminished from LL to DD₂ (F (2,22) = 8.43; p < 0.003).

The effects of increasing LL intensities (ranging from 66 to 1400 lux) on alpha, tau and activity levels are presented in Figure 4. As LL intensity increased, a significant decrease in both alpha (F (3,33) = 8.20; p < 0.0009) and running-wheel activity (F (3,33) = 18.86; p < 0.0001) occurred. However, tau was not significantly affected (Figure 4B).

Our study is the first to provide evidence of food anticipatory activity (FAA) in the common spiny mouse. In LD, all animals significantly increased their wheel-running activity during the one hour prior to food availability and thus exhibited FAA in LD (Figure 1A). Similar responses to restricted food access have been seen in a wide variety of animals. Syrian hamsters [10], rats [8,17], rabbits [18], mice [19], predatory marsupials [20,21], and birds [6,7] all have been shown to exhibit FAA (but not squirrel monkeys, [22]).

Our study is also the first to provide evidence of food entrainable oscillator (FEO) in the common spiny mouse. When RF was discontinued and food became freely available in LD, diurnal activity persisted for several cycles in 4/11 animals (Figure 1A - left panel), indicating the presence of a separate FEO in this species. Persistence of this activity demonstrates entrainment of an underlying clock and has been observed in rats [23] and some strains of mice but not others [24].

RF also effectively entrained the main activity bout in a small percentage of animals. In LL, two animals clearly synchronized to the restricted feeding as their activity increased during the one hour that would have been prior to food availability (Figure 1B - left panel). The remaining nine animals exhibited free-running activity and showed no coordination with the period of restricted feeding (Figure 1B - right panel). Rats [25] and Syrian hamsters – [26] have also demonstrated synchronization to feeding times in LL. RF is also an effective entraining signal for fetal spiny mice [14]. However, in stark contrast, food restriction paradigms failed to entrain activity rhythms in squirrel monkeys [27] and hamsters [28]. Likewise in some small carnivores, there is little evidence of an endogenous timing mechanism to synchronize feeding behavior [4].

Overall, in common spiny mice, restricted feeding was less effective in eliciting FAA in LL than in LD. These findings are similar to those in rats [8]. In addition, nearly all shoals of shiners showed FAA in LD whereas only 22% exhibited this behavior in constant conditions (DD) [29]. These authors suggested that while shiners may have an FEO, their RF activity may also be tied to an LEO. This hypothesis has been suggested for a number of animals [30] and could be the case for the common spiny mouse.

Daytime food availability also significantly increased daytime activity in this nocturnal animal; Figure 2). Interestingly, working for food apparently causes a switch from nocturnal to diurnal activity in mice [31]. In other nocturnal species, diurnal bouts of daytime activity has been shown to entrain a variety of tissues to the daily RF schedule [32-35]. This type of biphasic activity pattern has also been reported in field mice [19]. Interestingly another member of this genus, A. russatus, can completely shift its activity from nocturnal to diurnal activity in response to coexistence with A. cahirinus[36,37]. Shkolnik [37] reported that the two species compete for the same food and this temporal partitioning is a means to allow allopatry. Haim & Fluxman [38] have suggested that both the switch from nocturnal to diurnal activity and reduced activity of A. russatus is due to chemical signals released by A. cahirinus. Interestingly, they further report that this chemical induced shift in activity only occurs in a photoperiodic environment, not in constant conditions.