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Circadian activity rhythm and potential predation risk of the prairie vole, Microtus ochrogaster.

Although population density of the prairie vole, Microtus ochrogaster, is positively correlated with vegetative cover (Birney et al., 1976), populations also achieve high densities in habitats providing sparse cover (Getz, 1985; Getz et al., 2001, 2005). Low risk of predation, especially to avian predators, has been proposed to be a reason for high densities of M. ochrogaster in habitats providing sparse cover (Lin and Batzli, 2001; Getz, 2005; Getz et al., 2005). Ability of individuals to perceive and avoid specific types of predators may result in reduction in risk of predation (Glickman and Morrison, 1969; Muller-Schwarze and Muller-Schwarze, 1971; Spiegel et al., 1974; Derting and Crawford, 1989). Such behavior, however, has not been attributed to M. ochrogaster.

By limiting the amount of time active during daylight hours, M. ochrogaster occupying habitats with sparse cover would lower vulnerability to diurnally feeding predators, especially hawks. In east-central Illinois, about one-half of the potential avian predators of M. ochrogaster are diurnally feeding hawks (Lin and Batzli, 1995). Because of high energy requirements, however, voles must feed every 2-6 h (Madison, 1985). Microtus ochrogaster uses underground nest chambers and does not cache food (Mankin and Getz, 1994). Although individuals must move about the surface to feed, they may restrict the amount of time feeding to a minimum so as to reduce risk from avian predators. Non-feeding activity (e.g., territorial defense, construction of runways, dispersal) may be restricted to night so as to reduce risk of avian predation.

The few reports of circadian activity rhythms of M. ochrogaster are inconclusive in respect to the time of major activity. Calhoun (1945) and Dewsbury (1980) reported laboratory raised M. ochrogaster to be nocturnal in laboratory studies. Calhoun measured responses to varying photoperiods (reversed photoperiod and continuous dark), but tested only one animal in non-normal photoperiods. Glass and Slade (1980) concluded from live-trapping data that M. ochrogaster was mainly nocturnal, whereas Martin (1956) suggested from live-trapping evidence and Carley et al. (1970) from photorecorders that M. ochrogaster was diurnal. Barbour (1963) also concluded from field records of time in the nest that M. ochrogaster was diurnal, but provided data for only one animal. Harper and Batzli (1996) indicated most activity occurred at dawn and dusk.

I here describe the circadian activity rhythm of M. ochrogaster in response to photoperiod. Animals were taken directly from the field and tested under normal and reversed photoperiods, as well as in constant dark and light.

MATERIALS AND METHODS--Apparatus--Experiments were conducted in a 4 by 5-m basement room in an unoccupied building. There was no human activity within the building that would trigger activity bouts by voles. Room temperatures were ca. 20[degrees]C and were relatively stable from day to night. The two windows in the room were covered with a thick layer of black plastic; no outside light penetrated into the room. The door to the room excluded all other light.

Experimental cages consisted of 50 by 20 by 20 cm, 0.5-cm mesh, hardware-cloth cages balanced in the middle on a 3-mm, round metal rod. A 5 by 10-cm cylindrical nest chamber was on the floor of the cage at the pivot point. A water bottle was placed on one side of the cage at the pivot point and a feeding chute on the opposite side; food consisted of Purina rabbit chow (No. 5321; Purina Mills, Saint Louis, Missouri). A perpendicular metal arm was attached at the middle of one side of the cage and extended 10 cm above the top of the cage. A small mercury switch was attached to the top of the arm. Each time the cage moved ca. 1 cm off center (the animal had to leave the nest chamber to cause the cage to move this distance), the switch was activated or deactivated. Movements to feed and drink activated the switch, as did those within the cage. The switch was connected to an Esterline Angus event recorder (Esterline Angus Instrument Company, Indianapolis, Indiana); each time the switch was activated or deactivated a mark was made on the recorder. In each set of trials, eight activity cages were employed. The cages were placed 1 m apart on a bench along one wall of the room.

An exposed 100-watt incandescent light bulb was suspended 2 m above the center of the array of experimental chambers. The light was connected to a 24-h timer that set the light:dark periods. The light: dark period used for each set of trials was based on the current daylight sunset times when the animals were captured. The room was inspected once a day to ensure all cage systems were working and to refill food chutes and water bottles, if needed. A note was made on the recorder chart as to when the room was entered. Records for the 15 min following the inspection were deleted from the compilations. Except for continuous dark trials, the room was entered during the light phase. During constant dark trials, a small penlight was used to check the cages and add food and water.

Subject--Microtus ochrogaster used in trials were captured 10 km NE Urbana, Champaign Co., Illinois (40[degrees]15'N, 88[degrees]28'W) and placed directly into experimental cages, 1/cage. Only adult ([greater than or equal to]30 g) males and non-reproductive females were used. The first set of trials began in late December, with a 0700-1700-h light period, four each of males and females. The second set of trials, three males and five females, began mid June, with a 0500-1900-h light period. At the end of trials, all voles were released at the site where they had been trapped. All procedures were approved by the University of Illinois Laboratory Animal Care Committee and meet the guidelines recommended by the American Society of Mammalogists (Animal Care and Use Committee, 1998).

Procedure--Voles were allowed a 3-day cage familiarization period before data were recorded. The sequence of each set of trials was: 1) normal photoperiod, 2) reversed photoperiod, 3) constant dark, 4) constant light. Data for analysis were taken beginning at midnight (0000 h) and continuing 10 days for each trial. In the 0500-1900-h light trials, following the continuous light trial, a trial using a 0000-1200-h light period was established, followed by a reversed 1200-2400 h light trial.

Data Analysis--Number of movements were totaled for each vole for each hour. From these, mean number of movements per hour for each of the eight animals in the trial was determined for each hour of the 24-h period. Thus, there was a total of 80 h of data for each hour of a trial. There was no difference in activity pattern of males and females; data for the sexes were combined for analyses. One-way ANOVA (Zar, 1999) was used to test for differences between light and dark periods and between original light:dark periods when in constant dark and constant light. SPSS 10.0.7 for Macintosh (SPSS, Inc., 2001) was used for statistical analyses.

RESULTS--Inspection of recorder charts indicated no evidence of entrainment of activity, i.e., one animal becoming active, followed by animals in adjacent cages. Activity bouts and amount of activity of the eight animals in a trial appeared independent. For example, amount of activity recorded in each of the 8, 15-min intervals of the first 2 h of darkness was compared over a 5-day period for the 0700-1700-h light-period trials. On average, less than one-half, 45.7%, of individuals were active during each 15-min period.

For both groups of voles, activity (mean number of movements/h [+ or -] SE) was significantly greater during the dark than the light period, whether in a normal photoperiod (0700-1700-h light group: 9.9 [+ or -] 1.0 and 3.0 [+ or -] 0.5, respectively, [F.sub.15,176] = 11.128, P < 0.001; 0500-1900-h light group: 23.4 [+ or -] 1.9 and 3.5 [+ or -] 1.4, respectively, [F.sub.15,176] = 26.305, P < 0.001; Fig. 1) or in a reversed photoperiod (0700-1700-h light group: 7.0 [+ or -] 1.0 and 1.3 [+ or -] 0.5, respectively, [F.sub.15,176] = 11.056, P < 0.001; 0500-1900-h light group: 7.6 [+ or -] 1.6 and 2.4 [+ or -] 0.5, respectively, [F.sub.15,176] = 2.810, P = 0.001; Fig. 1).

The two groups differed slightly in respect to circadian activity when in constant dark and constant light regimes. The 0700-1700-h light group displayed no residual activity pattern when in constant dark (5.8 [+ or -] 0.9 and 6.8 [+ or -] 0.5, respectively, [F.sub.15,176] = 0.548, P = 0.280; Fig. 1), whereas the 0500-1900-h light group displayed significantly more activity during the original dark than the original light period (11.2 [+ or -] 1.1 and 4.4 [+ or -] 0.7, respectively, [F.sub.15,176] = 9.653, P < 0.001; Fig. 1). This occurred although the reversed-photoperiod trial immediately preceded the constant-dark trial.

[FIGURE 1 OMITTED]

When in constant light, the 0700-1700-h light group displayed significantly more activity during the original dark than the original light period (1.9 [+ or -] 0.2 and 0.4 [+ or -] 0.1, respectively, F1 5,176 = 9570, P< 0.001; Fig. 1). However, level of activity throughout the 24 h was low with respect to activity when in a dark:light regime. When in constant light, the 0500-1900-h light group displayed no difference in activity between the original dark and light periods (4.6 [+ or -] 0.9 and 5.4 [+ or -] 0.9, respectively, [F.sub.15,176] = 0.224, P = 0.999; Fig. 1).

For both groups, mean number of movements/h during constant dark was significantly greater than that during constant light (0700-1700-h light group: 6.4 [+ or -] 0.4 and 1.3 [+ or -] 0.2, [F.sub.15,176] = 37.950, P < 0.001; 0500-1900-h light group: 7.2 [+ or -] 0.9 and 5.1 [+ or -] 0.6, [F.sub.15,176] = 6.065, P < 0.001; Fig. 1). When the 0500-1900-h, light-group photoperiod was set for light during 0000-1200 h and dark during 1200-2400 h, activity was greater during the dark period (10.8 [+ or -] 1.1 and 3.5 [+ or -] 0.3, respectively, [F.sub.15,176] = 2.553, P = 0.002; Fig. 1). When photoperiod was reversed, activity also was greater during the dark period (10.5 [+ or -] 1.8 and 5.5 [+ or -] 1.1, respectively, [F.sub.15,176] = 34.314, P < 0.001; Fig. 1).

DISCUSSION AND CONCLUSIONS--Prairie voles displayed a basic nocturnal circadian activity rhythm. Although there was some activity during the light period, as would be expected so as to meet energy demands (Madison, 1985), such activity was less than one-third of that recorded during the dark period. When placed on a reversed photoperiod, the pattern of activity immediately switched, with major activity during the dark period. Further, when in constant light, level of activity was low throughout the 24-h period, and less than that when in constant darkness.

Korslund (2006) has demonstrated social entrainment of activity in the root vole, Microtus oeconomus. There was no evidence of entrainment of activity of the eight voles tested in each trial. Sounds of one vole moving within its experimental cage did not appear to have triggered activity in the others. Although M. ochrogaster is an extended-family, communal-nesting, species (Getz et al., 1993), owing to high nestling mortality, much of the time male-female pairs form most of the social groups. During periods of activity, one member of the pair remains in the nest with young while the other forages (cruder-Adams and Getz, 1985). Thus, social entrainment may not be a major factor in activity of M. ochrogaster under natural conditions. Comparison of daily activity records of the eight animals in each trial in the present study revealed no synchronization of activity bouts.

Results of this study suggest that circadian activity rhythm of M. ochrogaster results in little activity during daylight. This would reduce risk of predation by avian predators, potentially permitting relatively high population densities to be achieved in habitats providing sparse cover. Most species of voles occupying habitats with dense cover also display mainly a nocturnal activity pattern (Madison, 1985). The meadow vole, Microtus pennsylvanicus, is sympatric with M. ochrogaster in various habitats in the vicinity of Urbana, Illinois. This species occurs in low densities in habitats with sparse cover (Getz, 1985; Getz et al., 2001, 2005; Lin and Batzli, 2001), presumably because of high risk of predation in such habitats. Microtus pennsylvanicus appears to be equally active during dark and light periods or displays a crepuscular activity pattern (Webster and Brooks, 1981; Harper and Batzli, 1996). The nocturnal activity pattern of M. ochrogaster may not be the only reason for greater survival in sparse cover habitats; it may, however, be a contributing factor for higher densities in such habitats.

This research was supported in part by a grant from the National Science Foundation (GB 6203) and by funds from the University of Illinois School of Life Sciences.

Submitted 26,July 2006 Accepted I September 2008. Associate Editor was Philip D. Sudman.

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Lowell L. Getz

Department of Animal Biology, University of Illinois, Urbana-Champaign, 2113 Lynwood Drive, Champaign, IL 61821

Correspondent. l-getz@life.uiuc.edu
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Author:Getz, Lowell L.
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Date:Jun 1, 2009
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