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MICROHABITAT USE OF DEER MICE: EFFECTS OF INTERSPECIFIC INTERACTION RISKS JILL C. FALKENBERG AND JENNIFER A. CLARKE Department of Biological Sciences, University of Northern Colorado, Greeley, CO 80631 Microhabitat use by deer mice (Peromyscus maniculatus) was tested under five different simulated moonlight intensities, using an indoor habitat chamber with shrub covered areas and open areas, and food sources. Tests were run in the presence and absence of a potential competitor, Ord's kangaroo rat (Dipodomys ordii). From tracks in the sand, we determined that, when alone, deer mice decreased total activity as moonlight increased and were more active in areas with cover than in open areas. However, deer mice did not vary proportions of activity allocated to open areas (ca. 20%) and cover/edge areas (ca. 80%) with variations in moonlight. Deer mice consumed more seeds in areas with cover but also did not vary the proportions of seeds eaten in the open (ca. 43%) or cover (ca. 57%) with variations in moonlight. Using infrared video filming, we determined that deer mice increased use of cover to nearly 100% in the presence of Ord's kangaroo rats. Aggression (active chasing and locking fights) by kangaroo rats towards deer mice caused this shift. Our experiments substantiate field observations of variable microhabitat use by deer mice in areas with and without kangaroo rats and identify behavioral interactions involved. Key words: Peromyscus, Dipidomys, moonlight, interspecific competition, microhabitat Microhabitat use by animals is influenced by many factors, including risks of encountering predators and interspecific competitors. Numerous rodent species reduce use of microhabitats that lack cover on bright moonlit nights, which has been attributed to decreasing risk of predation (e.g., Peromyscus maniculatus-Blair, 1943; Clarke, 1983; Dipodomys-Bowers, 1982; Daly et aI., 1992; Kaufman and Kaufman, 1982; Kotler, 1984a, 1984b; Lockard and Owings, 1974a, 1974b; Long1and and Price, 1991; Onychomys leucogaster-Jahoda, 1973; Microdipodops-Kotler, 1984a, 1984b; Price et aI., 1984). In addition to influences of predation risks on rodents' use of areas with cover, interspecific interactions with competitors also may affect use of structurally different microhabitats. Many sympatric rodent species use microhabitats that differ in vegetative cover, potentially reducing competitive interactions (Kenagy, 1973; Price, 1978; Randall, 1993). Differences in sympatric species' uses of open areas and Journal of Mamma/ogv. 79(2):558-565. 1998 areas with cover are often associated with differences in locomotory patterns. Bipedal kangaroo rats (Dipodomys) preferentially use open habitats, but quadrupedal pocket mice (Chaetodipus and Perognathus) use habitats with cover (Randall, 1993). Additionally, aggressive interactions on the part of a competitor may restrict another species' habitat use (Bowers and Brown, 1993; Bowers et aI., 1987; Brown and Munger, 1985; Frye, 1983; Grant, 1972; Valone and Brown, 1995). For example, in kangaroo rats large individuals are typically dominant and can display high levels of aggression (Blaustein and Risser, 1976; Bleich and Price, 1995; Eisenberg, 1963), Deer mice (P. maniculatus) and Ord's kangaroo rats (Dipodomys ordii) live sympatrically in desert and dry grassland areas and have many of the same resource requirements (Koehler and Anderson, 1991). Further, these two species have been found to be active during the same nocturnal hours in winter, and activity of both is af558 May 1998 FALKENBERG AND CLARKE-DEER MOUSE MICROHABITAT USE fected by moonlight (O'Parrell, 1974), The role of behavioral interactions between these species in determining microhabitat use is unknown. We investigated microhabitat use by deer mice and evaluated effects of increased predation risk in conjunction with presence and absence a larger and potentially aggressive competitor, Ord's kangaroo rat. Activity of deer mice in open areas and areas with cover was compared in tests with different moonlight intensities, as moonlight is associated with predation risk, with and without the presence of a kangaroo rat. We tested hypotheses that deer mice would decrease activity and foraging in open areas as moonlight increased, because moonlight represents increased predation risk, and that deer mice would further decrease their use of open areas as moonlight increased when an Ord's kangaroo rat was present, because the latter represents an aggressive bipedal competitor that prefers open areas. The role of behavioral interactions between these species in producing microhabitat associations and the summative effect of interactions between risks of predation and an encounter with an aggressive competitior on habitat use by deer mice are unknown. MATERIALS AND METHODS Study animals.-Because many factors can affect activity of animals in the wild, we performed our study in a large indoor chamber with semi-natural habitat and where the majority of variables influencing activity patterns could be controlled. Adult deer mice (11 females, 12 males) and adult Ord's kangaroo rats (four females, three males) were live trapped in an area dominated by sand sagebrush (Artemesia filifolia) in Weld Co., Colorado. Animals were maintained and tested in the University of Northern Colorado Animal Care Facility under conditions of 16°-1SoC and a photoperiod of 12L:12D. Animals were housed individually in 46 by 23 by 20-cm plastic boxes with stainless steel wire tops. Each deer-mouse box had hardwood shavings, cotton, and a 455-ml metal can in which to construct a nest. Kangaroo-rat boxes had sand, cotton, and a 1.3-L metal can in which to 559 construct a nest (J. Randall, pers. comm.). All animals were provided food and water ad lib. Deer mice were fed Purina Rodent Chow supplemented with sunflower seed, millet seed, oats, meal worms, crickets, apples, and lettuce. Kangaroo rats were fed Purina Rodent Chow, oats, and a commercial gerbil diet. Tests were conducted from 1 September to 15 April 19921993. No individual was kept more than S months and all animals were released at their original capture site after testing. Test conditions.-In the first phase of our study, we recorded total activity levels, activity allocation to open and cover-edge areas, and foraging patterns in open and cover-edge areas of the deer mice under different simulated moonlight intensities in the absence of kangaroo rats, and in the second phase of our study, a kangaroo rat was present. Tests were conducted in a chamber with semi-natural habitat. The chamber measured 3.5 by 2 by 2 m and was constructed of wood and glass. The floor of the chamber was divided into 25.5 by 25.5 cm squares with a wooden grid covered with fine-grain sand. Four shrubs were placed in the chamber, one in each of the corners. One-half of the chamber's grid squares were in the open, and one-half of the squares were shaded by the shrubs or were along the walls of the chamber (cover-edge areas). Light bulbs (105 bulbs, 0.5 W each) on rheostat control were distributed evenly over the ceiling and were used to simulate five different moonlight intensities. We obtained moonlight reflectance measurements on clear nights using a standard 90% white card and a Luna-Pro light meter (Gossen, Germany) in an agrarian area of Colorado that lacked any unnatural light sources. Moonlight-reflectance levels were new moon (0.05 lux), quarter moon (0.17 lux), half moon (0.35 lux), three-quarters moon (0.93 lux), and full moon (2.2 lux). We reproduced these light levels in the chamber using the standard card, light meter, and the 0.5 W bulbs on rheostat CODtrol. Two infrared lights in the chamber's ceiling provided illumination for infrared camera recording in the second phase of our experiment. Experimental procedure.-In the first phase of the study, we tested 12 deer mice (six of each sex) under the following protocol: 1) ca. 1.5 h after the onset of night, nocturnal illumination was set to one of the five simulated moonlight intensities (hereafter referred to as simply "moonlight intensity") selected from a random- 560 JOURNAL OF MAMMALOGY ized schedule; 2) fOUT seed dishes, each with 20 shelled sunflower seeds of about equal sizes, were placed in the chamber-two dishes were placed in locations randomly selected in areas of cover edge and two were in the open (water and a nest refuge were also provided); 3) a deer mouse was selected from a randomized schedule and placed in the chamber for 12 b; 4) seed dishes were removed the following morning, and the number of seeds eaten in the open and coveredge areas were recorded to determine where the individual focused its foraging activity; 5) the mouse was allowed to remain in the chamber for the remaining 12-h daytime period (daytime light reflectance = 300 lux) and during this time, each mouse remained in the nest refuge; 6) 1 h after the night commenced (observed to be the onset of the peak activity period), the mouse was removed from the chamber and the sand was redistributed to prevent scent trailing and was swept smooth to determine tracking; and 7) the mouse was reintroduced into the chamber for 12 min and then removed, and its activity for the 12-min period was measured by determining the amount of tracking in the sand. We used the sand-tracking method to estimate the amount of tracking in each grid square based on the following scale: 0 = 0% tracked, 1 = 110% tracked, 2 = 11-20% tracked, 3 = 21-30% tracked. and so on to 10 = 91-100% tracked. We summed tracking scores for all grid squares to obtain an index of activity for each test (the maximum possible index was 980 if all 98 grid squares were 100% tracked) and calculated indices and relative proportions of activity in open and in cover-edge areas. Before the next test commenced with a different mouse and different moonlight intensity, we redistributed and smoothed the sand. This cycle was repeated until all deer mice were tested under all of the five moonlight intensities. In the second phase of our study, we used 11 deer mice (five females, six males) and seven Ord's kangaroo rats. As in the first phase, each individual was allowed to become familiar with the test chamber, alone and overnight with seed dishes. Then we placed two individuals (one deer mouse and one kangaroo rat, randomly paired) in separate comers of the chamber 1.5 h after sunset under one of the five moonlight intensities selected from a randomized schedule. Because individual tracking could not be distinguished with more than one animal in the cham- Vol. 79, No.2 ber, we filmed activity levels and individuals' interactions using a Sanyo VDC9212 infrared camera and a Sanyo lLS900 VCR for 1 min. After 12 min, the two animals were removed from the chamber, and the film was analyzed to determine activity patterns of the deer mouse and use of open and cover-edge areas (filming and tracking methods produced identical results). Using a transparent acetate sheet with a grid matching that of the chamber's that was positioned on the video screen, the rodents' locations were marked every 5 s for 12 min. The cycle was repeated until all deer mice had been tested under all five moonlight intensities with one of the seven kangaroo rats. We recorded number and proportions of observations that occurred in open areas and cover-edge areas and behavioral interactions of aggression and avoidance. Aggression was defined as rushing or locking fights (Eisenberg, 1963), and avoidance was defined as one individual moving to another area of the chamber when the other animal moved in its general direction. We recorded aggression and avoidance as separate events. We conducted 50 additional tests with a different set of 10 mice to determine if tracking and filming yielded statistically comparable results regarding mouse use of open and coveredge areas. One observer analyzed mouse activity using the tracking method, and a second observer analyzed activity using the video tape. Results from the two methods did not differ regarding deer mouse use of open areas (MarmWhitney U test z = ~0.63, P = 0.50) and areas with cover (z = -0.62. P = 0.51). Thus, activity data regarding use of open and cover-edge areas using tracking and filming techniques were statistically identical and comparable. Analyses.-Due to the small samples and non-normal distribution of the data, nonparametric statistical analyses were performed using SAS (SAS Institute, Inc., 1988). Spearman rank correlation (r,) was used to examine relationships between moonlight intensity and rodent activity. Kruskal-Wallis (H) one way analysis of variance was used to determine if activity differed significantly between more than two moonlight intensities. Mann-Whitney U tests with the z statistic (SAS Institute, Inc., 1988) were used to determine if activity differed significantly between two moonlight intensities, and for comparisons between activity levels in May 1998 FALKENBERG AND CLARKE-DEER MOUSE MICROHABITAT USE TABLE I.-Average number of seeds eaten by deer mice in an enclosure under five simulated moonlight intensities in the absence of knngaroo rats and the range and average percentage of seeds eaten in the open and cover-edge areas (12 trials in each moonlight level). X Range Percentage Moonlight seeds eaten Open Cover Open Cover New Quarter Half Three-quarters Full 37.5 31.0 36.8 32.5 39.7 2-38 0-40 2-33 0-29 2-40 4-40 1-40 9-40 1-40 6-36 46.8 39.5 45.1 38.8 43.3 53.2 60.5 54.9 61.2 56.7 open areas and cover-edge areas. Significance was set at P < 0.05 for all tests. RESULTS Deer mice alone in various moonlight intensities. -Total number of seeds consumed did not differ between moonlight intensities and averaged 35.5 seeds eaten/trial (n = 60 trails, H ~ 1.844, P ~ 0.764; Table I). Deer mice consumed seeds in open areas and cover-edge areas in each moonlight intensity and ate fewer seeds in the open than in cover-edge areas (z = 2.485, P = 0.013). On average, deer mice ate 42.7% (17/40) of the seeds in the open and 57.3% (23/40 seeds) of the seeds in cover-edge areas. Deer mice did not vary that pattern with changes in moonlight intensity, and no differences existed between moonlight intensities in number of seeds consumed in open areas (H = 2.422, P = 0.909) or cover-edge areas (H ~ 1.962, P ~ 0.743). Deer mice never ate all seeds available in either area. When deer mice were alone in the chamber, their overall activity level was correlated negatively with simulated moonlight intensity (r, ~ 0.4522, P ~ 0.001). Mean activity indices were 75.8, 118.8, 179.1, 145.3 and 233.8 in full, three-quarters, onehalf, quarter and new moonlight, respectively. Activity levels varied between moonlight intensities (H = 12.07, P = 0.016). Pairwise comparisons revealed that activity in new moonlight was higher than 561 in three-quarters or full moonlights (z = -2.325, P ~ 0.020 and z ~ -2.811, P ~ 0.005, respectively), and activity in half moonlight was higher than that in full moonlight (z ~ 2.295, P ~ 0.022). Deer mice were more active in coveredge areas than open areas (z = -8.323, P = 0.0001; Fig. la), but their proportional allocation of activity (use of open versus cover edge areas) did not vary between moonlight levels (H ~ 3.\85, P ~ 0.527). On average, deer mice allocated 21.2% of their activity to open areas and 78.8% to cover-edge areas, regardless of moonlight intensity. Deer mice with Ord's kangaroo rats.In the presence of Ord's kangaroo rats, deer mice exhibited an increase in their use of cover-edge areas compared with their use of cover-edge when alone (z = -8.09, P = 0.0001). Cover-edge areas of the chamber were used almost exclusively (2:95%) at all moonlight intensities (Fig. Ib). Aggression toward deer mice by kangaroo fats and avoidance by deer mice of kangaroo rats contributed to this shift in microhabitat use by deer mice. In the 55 trials, aggression directed by kangaroo rats toward deer mice was observed 74 times. The average number of cases of aggression was 1.3 casesl trial. Deer mice never reciprocated with aggressive behavior; rather they fled the rushing kangaroo rat. In two cases, we observed physical contact; the kangaroo rat chased the deer mOUSe, caught it, and the two rolled in the sand together. The fight ended with the deer mouse on its back and the kangaroo rat backing off for a moment only to chase the mouse again when it fled. In one case, the mouse escaped into a shrub; in the other case, one researcher interfered with the fight because injury to the deer mouse appeared imminent. In another case, after a kangaroo rat had chased a mouse up into a shrub, it began pulling branches down on the shrub, seemingly in an attempt to reach the deer mouse. Each of the kangaroo rats displayed about equal numbers of aggressive actions toward deer mice. 562 JOURNAL OF MAMMALOGY "'" ., a A voidance of kangaroo rats by deer mice was observed in every trial (68 instances during the 55 trials). In all cases of avoidance, deer mice moved away from a kangaroo rat but remained near cover-edge areas. All deer mice displayed about equal numbers of avoidance behaviors during our study (one to two cases of avoidance per trial). Cases of avoidance increased over the course of our study from one case to two cases per trial but cases of aggression decreased from two or three cases to no cases per trial. C .~ .:du -< "' " ~ ;;:" " 0 0 "'~ 0.D3 Vol. 79, No.2 0.17 0.35 0.93 2.20 Moonlight (lux) • Open • Cover • Moonlight (lux) Open • Cover c '" > u -< "' "0 ;;:" " ~ ~ FIG. I.-Average proportions of activity of deer mice in the open and under cover in five simulated moonlight intensities in an enclosure: a) in the absence of kangaroo rats, ± 1 SD (60 trials, 12 trials per moonlight intensity); b) in the presence of kangaroo rats, SD < 1.5 (55 trials, 11 trials/moonlight intensity). The lux values of 0.05, 0.17, 0.35, 0.93, and 2.21 correspond to new, quarter, one-half, three-quarters. and full moonlight. respectively. DISCUSSION Our experiments revealed that deer mice decreased overall activity as moonlight increased, consistent with findings of prior studies (Clarke, 1983), and they were more active in areas with cover under all levels of nocturnal illumination. However, contrary to our expectations, deer mice maintained the same relative proportions of activity in the open (ca. 20%) and in cover (ca. 80%) in all moonlight intensities; thus, microhabitat use remained comparatively unchanged. The open-cover activity allocation may be related, in part, to predation costs. Activity in the open would likely increase a mouse's exposure to avian and mammalian predators, but remaining strictly under cover on bright nights may also increase a rodent's chances of encountering a snake predator (Randall, 1993) because rattlesnakes (Crotalus viridis) also avoid bright moonlight by remaining near cover (Clarke et aI., 1996). Due to the low metabolic requirements of the snake and its highly efficient digestive systems (Pough, 1978), days may pass before a meal is required. Thus, risk of predation to the deer mouse by an endotherm in the open may exceed the risk of snake predation under a shrub. The observed activity allocation of 20% open to 80% cover by deer mice also may be related to foraging benefits. Deer mice ate about the same number of seeds in each moonlight intensity and maintained the Muy 1998 FALKENBERG AND CLARKE-DEER MOUSE MICROHABITAT USE same relative proportions of seeds eaten in the open (ca. 43%) and in cover (ca. 57%) in each test. Because proportions of seeds eaten in the open and in cover did not reflect activity allocations, deer mice appear to have concentrated on foraging when in the open on bright nights without engaging in other activities. On dark nights, they were more active in the open (more open areas were tracked) but consumed nearly the same number of seeds in the open as on bright nights, indicating that perhaps they engaged in other activities in the open in dim moonlight (e.g., searching for other food sources). Given increased risks to predation in the open and the fact that mice never depleted all seeds available under cover, the question arises as to why deer mice consumed such a large proportion of seeds away from cover. This activity pattern may be related to balancing predation risks under cover and in the open and it also may be related to foraging benefits. Vickery et aJ. (1994) observed that deer mice (P. m. gracilis) prefer energy-rich foods over protein-rich foods and were energy maximizers. We currently are investigating if seeds that are dispersed distances from the parent plant (into the open) contain higher energy stores than those that germinate in the immediate vicinity of the parent plant in habitats of sand sagebrush. Deer mice may have been using energy-maximizing strategies, (i.e., to exploit energy-rich seeds in the open) but only sunflower seeds were available in these experiments. Although deer mice have been observed to use open areas in full moonlight in the lab and field in Montana (Clarke, 1983), Kotler's (1984a) trapping results revealed that deer mice remained in areas of cover regardless of moonlight intensity in the field in Arizona where kangaroo rats were present. A difficulty noted in interpreting trapping data regarding rodent microhabitat use is that a species' occurrence in a local habitat may not be due preference, but rather it could be excluded from more choice habitats and resources by competitors (Hol- 563 brook, 1979a, 1979b; Miller, 1967). Results of our study confirm that when deer mice were alone, they used open areas in all moonlight intensities and completely avoided open areas in the presence of kangaroo rats, abandoning activity allocation of 20% open to 80% cover that they exhibited when alone. Similar alterations in habitat use and niche shifting have been observed between other sympatric rodent species such as deer mice and voles, Clethrionomys gapperi and Microtus pennsylvanicus (Crowell and Pimm, 1976); Ord's kangaroo rats and grasshopper mice, O. leucogaster (Rebar and Conley, 1983), and chipmunks, Eutamias umbrinus and E. dorsalis (Brown, 1971). Thus, as Blaustein and Fugle (1981) noted, biotic factors (e.g., competition) can supersede abiotic factors (e.g., moonlight) in determining habitat use by rodents. When deer mice in this study ventured into the open, they were immediately chased back into cover by kangaroo rats. Chasing and fighting has been documented between members of the genus Dipodomys (Bartholomew and Caswell, 1951; Blaustein and Risser, 1976; Bowers and Brown, 1993; Bowers et aI., 1987; Brown and Munger, 1985; Frye, 1983), but not between deer mice and kangaroo rats. Those studies and our study were conducted in enclosures, which tend to escalate cases of aggression because one species cannot entirely get away from the other, as would be possible in the wild (J. Randall, pers. comm.). However, interspecific aggression has been proposed as a mechanism maintaining ecological separation between Dipodomys, with the species that is more specialized (e.g., food habits) being the most aggressive and successful competitor (Blaustein and Risser, 1976; Miller, 1967). We observed that presence and behavior of the more specialized kangaroo rats caused deer mice to shift from broad (open and cover) to narrow (cover only) microhabitat use. Subdivision of habitat by vegetative type has been identified as an important mechanism for coexistence of sympatric rodent 564 JOURNAL OF MAMMALOGY species and is related to adaptations for arboreal and terrestrial modes of locomotion (Holbrook, I 979a, 1979b; Zegers and Ha, 1981), and interspecific aggression. Deer mice exhibit a lower extinction probability and increased population size in areas where kangaroo rats have been removed or have exhibted declines (Valone and Brown, 1995; Valone et al" 1995), Based on our findings, where kangaroo rats and deer mice co-occur, deer mice may be unable to forage on resources in open areas or evade predators that remain under cover. Thus, the absence of kangaroo rats in the study areas of Valone and Brown (1995) and Valone et al. (1995) may have allowed deer mice to use a broader range of microhabitats and antipredator and foraging strategies that could not be used in the presence of Dipodomys. Our study revealed that presence of Ord's kangaroo rats, a large and often aggressive bipedal rodent competitor, directly influences microhabitat use by deer mice and may limit foraging and antipredator strategies of deer mice. ACKNOWLEDGMENTS We thank J. Randall, A. Blaustein, and S. Mackessy for their valuable comments on the manuscript; M. 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