Influence of thermal environment on food habits of female cave myotis (Myotis velifer).
The cave myotis (Myotis velifer) exhibits a high level of plasticity in selection of roosts. Avila-Flores and Medellin (2004) suggested this to be true for most common, wide-ranging species because optimal roosts often are limited. Some females of this species remain in cave roosts during summer to raise their young, whereas others establish maternity roosts in warmer, manmade structures, such as barns (Barbour and Davis, 1969; Kunz, 1973, 1974; Reid, 1997). In general, cave roosts provide a relatively cool and stable thermal environment, whereas barns provide a more variable environment with temperatures fluctuating both seasonally and daily. During summer, ambient temperature in barns usually is higher than in caves (Kunz, 1973; Kunz and Lumsden, 2003). Substantial differences between ambient temperature and body temperature in cave environments, relative to above-ground roosts, make it necessary for females to regulate their body temperature and use more of their daily energetic allowance for heat production (Kunz and Lumsden, 2003; Speakman and Thomas, 2003), leaving less energy available for embryonic development. Growth of embryos and neonates can be delayed by exposure to prolonged ambient temperatures below the thermal neutral zone because neonates and females in the late stages of pregnancy are unable to adequately regulate their body temperature (Studier and O'Farrell, 1972; Humphrey et al., 1977). Because of reduced metabolic cost, selection of roosts with high ambient temperature (i.e., a smaller difference between ambient and body temperature) is beneficial (McNab, 1969; Studier and O'Farrell, 1976; Speakman and Thomas, 2003). Warm temperatures of roosts in barns, thus, could reduce energetic efforts required to maintain high body temperature. As a result, bats in caves or barns might exhibit differences in their diet (Henreid and Schmidt-Nielson, 1966; Kunz, 1973; Burnett and August, 1981; Kurta and Fujita, 1988). Different types of roosts of M. velifer present an opportunity to determine if food habits mirror differences in roost environments. We hypothesized that bats in barns and caves would exhibit differences in types of prey they consume.
MATERIALS AND METHODS--Study Location.--A population of M. velifer inhabits the Red Hills region of southern Kansas and northern Oklahoma. In south-central Kansas, it is the most common species of bat (Hibbard, 1934; Sparks and Choate, 2000). In summer 2004 (7 June-2 July), we studied maternity roosts of M. velifer in Barber, Comanche, Kiowa, and Pratt counties, Kansas. We collected representative individuals by hand to confirm their identity and deposited the voucher specimens in the Sternberg Museum of Natural History. No other species of bat was observed at any of the maternity roosts.
We located two caves (Gates and Gentry) and two barns (Bergner and Whitney) that were occupied by adult female M. velifer. Caves were <1 km from water on ranches characterized by rolling hills and canyons where vegetation was grazed, mixed-grass prairie. Adult females used one large chamber in each cave for a maternity site. About 10,000-12,000 bats inhabited each cave. Human disturbance to caves was minimal because they were relatively isolated. Barns were <0.4 km from water on active farms surrounded primarily by agricultural land. Both barns were sound structurally and were used by the owners, although use was curtailed in summer when bats were present. About 8,000-10,000 bats roosted in each barn. Although Bergner Barn had a loft and ample space in other areas, bats used only a granary that was 3 m wide, 4 m deep, and 2.5 m high located in the central area of the lower level. This space likely afforded females greater protection from disturbance by humans or predators and from environmental extremes. Bats roosted on walls and the ceiling of the granary. Bats at Whitney Barn roosted on the ceiling of the loft as well as the ceiling of the lower level. During the first week of data collection, bats remained in the loft. However, as temperatures rose and parturition neared, some bats moved to the ceiling of the lower level. Whereas bats in Gates and Gentry caves and Bergner Barn were relatively protected, bats in Whitney Barn were more vulnerable to predation, disturbance, and environmental extremes because doors at one end of the barn were open. In fact, this was the only roost at which obvious signs of predation were found (e.g., holes dug through mounds of guano in the loft, presumably by raccoons, Procyon lotor, to reach bats roosting on the ceiling of the lower level).
Measurement of Temperature-To confirm that there were differences in temperature and, therefore, in energetic demands among roosts, we mounted a Hobo H8, 2-channel, temperature data logger (Onset Computer Corporation, Bourne, Massachusetts), in each roost to record temperature at the roosting surface and air temperature ca. 1.5-1.8 m beneath the surface on which bats roosted. We took hourly temperature readings for the duration of the study.
A malfunction occurred with the data logger in Bergner Barn in the second one-half of the study, so temperature was not recorded for that period (21 June-12 July). When bats in Whitney Barn roosted in the loft and lower level, we took samples of guano and measurements of temperature at both locations.
We used a one-tailed Mann-Whitney U-test to determine if temperatures recorded in barns were significantly different than temperatures in caves. We used a non-parametric test because the distribution of our data deviated from normal. By using a one-tailed Mann-Whitney Utest, we tested our directional hypothesis that temperatures in barns were significantly higher than temperatures in caves (Zar, 1999). We also used a one-tailed variance-ratio to test the directional hypothesis that temperatures in barns were more variable than temperatures in caves (Zar, 1999).
Collection of Guano--Analysis of fecal pellets has been used in numerous studies to determine diets of insectivorous bats (e.g., Barclay, 1985; Rydell, 1989; Kurta and Whitaker, 1998; Shiel et al., 1998). We used guano from each roost to analyze food habits. We collected samples of guano weekly 25 May-3 July, which corresponded to the time when females congregated in maternity colonies until juveniles became volant and began to forage on their own. A sample is defined as the guano taken from a single container. To collect guano, we placed three containers that were 0.3 [m.sup.2] in diameter on the floor of the cave or barn beneath clusters of bats. Containers were wire frames lined with trash bags that we removed each week at, or just after, dusk (i.e., 2100-2230 h CDT) while females were foraging. We installed new bags each week.
Gates Cave offered a number of challenges to collecting guano. Bats at this location were not located until the final 2 weeks of the study. Also, because of muddy access roads and our inability to reach the study site, guano from the final 2 weeks was collected together. Because of these complications, we took only three samples from this location (29 June-13 July).
Samples from Bergner and Whitney barns obtained in the first 2 weeks of the study (25-31 May and 31 May-7 June) and from Gentry Cave (25 May-1 June and 1-8 June) were collected with plastic sheets placed beneath roosting bats because wire collectors had not been installed. We collected samples of guano from the plastic. Such differences in collection methods do not affect analysis of food habits.
Analysis of Food Habit--We used a combination of traps (e.g., malaise, canopy malaise, and flight) to collect insects near each roost to assemble a reference collection that aided in identification of dietary components. We then followed procedures for fecal analysis outlined by Whitaker (1988) and Shiel et al. (1997). We analyzed 20 pellets from each sample and identified insects in each pellet to order and, when possible, to family. Because of an inadequate sample size for statistical analysis, we quantitatively described data from the analysis of food habits. Data from fecal analyses were used to calculate percentage volume and relative frequency of each taxon of prey in each pellet. Percentage volume and relative frequency of food items from all samples from a single week were averaged. We then averaged weekly averages to yield overall percentage volume and relative frequency for each roost and all weeks of the study. We considered samples collected in the loft and lower level of Whitney Barn to be from a single locality; therefore, we did not describe samples separately. We averaged percentage volume and relative frequency of food items in samples from the loft and lower level for each week, which resulted in a single percentage volume and relative frequency by roost, not by level. Gates Cave was represented only by three samples of guano collected over a 2-week period. Thus, data for Gates Cave described herein likely do not accurately represent diet of females in that roost for the entire study period.
RESULTS--Temperature of Roost--We recorded mean, low, and high temperatures for each week when we collected samples of guano (Table 1). Bats roosting in barns were exposed to significantly warmer thermal environments than bats in caves (roosting surface: Mann-Whitney U-test, U = 0, P < 0.01, [n.sub.1] = 8, [n.sub.2] = 8; air: Mann-Whitney U-test, U = 0, P < 0.01, [n.sub.1] = 6, [n.sub.2] = 7). Temperatures at roosting surfaces in caves also showed significantly less variation than temperatures in barns (variance-ratio test, F = 5.51, P < 0.05, [n.sub.1] = 8, [n.sub.2] = 8). For example, temperature at the roosting surface in Gentry Cave remained relatively cool and constant (mean daily temperature = 21.4 [+ or -] 0.54[degrees]C), whereas temperature at the roosting surface in Whitney Barn was warm and variable (mean daily temperature = 27.7 [+ or -] 3.50[degrees]C; Fig. 1). While a difference in variation of air temperature did occur between caves and barns, it was not statistically significant (variance-ratio test, F = 3.66, P > 0.05, [n.sub.1] = 7, [n.sub.2] = 6). Over the period of 1 week in Gentry Cave, high and low temperatures at the roosting surface were 24.0 and 20.2[degrees]C, respectively. In contrast, high and low temperatures in Whitney Barn were 35.3 and 21.0[degrees]C, respectively. We observed similar patterns of temperature fluctuation throughout the study. Mean temperatures at roosting surface for the entire study period were 20.8 [+ or -] 2.0[degrees]C in the cave and 26.0 [+ or -] 3.8[degrees]C in the barn. Temperatures recorded for the duration of the study increased gradually during the first one-half of the study and then showed more marked increases during the final weeks. We observed the same trends in temperature illustrated by Gentry Cave and Whitney Barn in Gates Cave and Bergner Barn.
[FIGURE 1 OMITTED]
Food Habits--We identified arthropod prey items in samples of guano taken from maternity roosts of M. velifer (Table 2). We collected a total of 15, 14, 3, and 15 samples of guano from Bergner Barn, Whitney Barn, Gates Cave, and Gentry Cave, respectively. Percentage volume represented how much of the samples consisted of members of an order, whereas percentage relative frequency represented the number of pellets in which members of an order were found. Coleopterans were dominant food items, comprising 37-44% of volume and occurring in 31-36% of pellets analyzed (Figs. 2a and 2b). Dipterans (10-21% volume, 12-25% frequency), lepidopterans (20-25% volume, 16-19% frequency), and hemipterans (2-13% volume, 19-27% frequency) comprised the next largest portions of diet.
Percentage volume and relative frequency of the orders Coleoptera, Diptera, Lepidoptera, and Hemiptera showed little variation among roosts (Fig. 2a), except Gates Cave. This difference might have been because data for this roost were represented by only one collection of guano as opposed to 5-6 collections in the other three roosts; so, temporal variation in numbers and kinds of insects during the study were not represented. Percentage volume and relative frequency of major prey items in samples of guano from Bergner and Whitney barns and Gentry Cave were similar.
Percentage volume and relative frequency of food items of the orders Araneae, Ephemeroptera, Hymenoptera, Neuroptera, Orthoptera, and Trichoptera also showed only slight differences among roosts (Fig. 2b). As with the previous orders, percentage volume and relative frequency of these orders in Gates Cave differed from the other roosts.
DISCUSSION--Adult female M. velifer that roost in caves and barns experience trade-offs between protection from predators, disturbance, and thermoregulatory costs. However, because there is no difference in development of young in caves and barns (Kunz, 1973), bats in the two kinds of roosts must be achieving energetic equilibrium. If bats in caves were not compensating for added energetic demands, development of young might be retarded (Pearson et al., 1952; Davis, 1969; Racey, 1982) because rate of development of neonates is related to temperature (Herreid, 1967; Zahn, 1999). Slower rates of fetal development have been observed in Townsend's big-eared bat (Carynorhinus townsendii) and the pallid bat (Antrozous pallidus) in cool temperatures (Pearson et al., 1952; Davis, 1969). Additionally, lengths of forearm of juvenile mouse-eared bats (Myotis myotis) roosting in consistently cool temperatures in attics were shorter than forearms of bats roosting in warm attics and more stable, cave-like roosts in rock walls (Zahn, 1999). Delayed parturition could affect juveniles negatively because they cannot exploit food resources efficiently at the time of year when resources are most plentiful (Kunz, 1973). Therefore, reproductive females tend to occupy warmer roost sites presumably to dampen negative effects of cool temperatures (Avila-Flores and Medellin, 2004).
Kunz (1974) studied food habits of M. velifer inhabiting maternity colonies in barns and caves in southern Kansas, but his comparisons were between sexes and between juveniles and adults, not between individuals in different types of roosts. He detected no difference in prey eaten by either group. Absence of difference in diet of males and lactating females indicates that females might not be using selection of prey to compensate for high energetic demands. In this regard, our analysis of food habits of bats in four maternity roosts revealed that there was no qualitative difference in types of arthropod prey consumed. According to optimal-foraging theory, bats in need of greater intake of energy might selectively pursue insects that are higher in nutritional value (Jones and Rydell, 2003). They also might incorporate different foraging strategies in which bats that needed to decrease flight time would pursue fewer large insects rather than greater numbers of small insects. If opportunistic foraging occurs, we assumed that it might occur on a large scale, whereas bats might be more selective on a smaller scale. For example, females might opportunistically choose areas with high density of insects, but might select specific types of insects in those areas. Results of our study, however, do not indicate that bats in either thermal environment select specific types of arthropods. In all roosts, coleopterans were the most frequently consumed prey items followed by hemipterans, lepidopterans, and dipterans. Our results nearly parallel those reported by Kunz (1974, Table 2).
[FIGURE 2 OMITTED]
Lack of difference in dietary composition suggests that other mechanisms might be used to compensate for variability in energetic demands. According to Wang (1924), animals, including bats, have three strategies by which they respond to energetic demands. One strategy is to increase assimilation of energy by increasing consumption. Thus, rather than eating different prey, females under greater energetic stress (i.e., those in cooler environments) might consume more food than females in warmer roosts. A laboratory study of little brown bats (Myotis lucifugus) showed an inverse relationship between ambient temperature and consumption of food--higher intake of food occurred at lower ambient temperatures (Stones, 1965). This increase in consumption of food evidently compensated for greater energetic demands associated with thermoregulation. Racey and Swift (1981) studied a maternity colony of the common pipistrelle (Pipistrellus pipistrellus) and noted that food-deprived individuals were less able to maintain a high body temperature than those that were well fed. Thus, it is possible that, instead of consuming different types of prey, female M. velifer inhabiting maternity colonies with cool thermal environments are simply eating more food than those inhabiting warmer roosts.
Finally, there is much support for social behavior as a mechanism of energy conservation. Clustering by bats reduces surface area available for heat loss and makes digestion and assimilation more efficient (Twente, 1955; Herreid, 1967; Trune and Slobodchikoff, 1976). The cluster actually can serve as a unit for heat production and cause an increase in temperature at roost sites in caves (Twente, 1955; Betts, 1997). Accordingly, formation of clusters might partially explain the lack of difference in diet in this study, but was observed in both types of roosts. In general, caves housed a greater number of individuals than barns. Individuals in caves tended to form predominately large clusters on the ceiling and in crevices, but also formed smaller, satellite clusters. Individuals in barns also formed clusters but roosted on joists and rafters, as well as the ceiling. We did not measure density of clusters. If bats in cool cave environments were able to form denser clusters, this behavior could compensate for lower ambient temperatures. Based on these findings, we assume that bats reduce expenditures of energy by a suite of actions that culminate in maximum conservation of energy.
We express our thanks to D. W. Sparks and J. O. Whitaker, Jr., Indiana State University, for assistance with analysis of food habits. We thank S. D. Roth, Jr., Kansas Biological Survey, for guidance through the Red Hills of Kansas and for sharing many years of knowledge about the region and the bats that live there. We also thank S. M. Dunn and S. K. Nilz for field support. Finally, we thank R. A. Van Den Bussche, M. J. Hamilton, and D. M. Leslie, Jr. for comments on our manuscript, and V. L. Jackson for statistical assistance. Funding was provided by the Department of Biological Sciences and the Sternberg Museum of Natural History at Fort Hays State University, and by the Fleharty Fellowship, which is sponsored by the family of E. D. Fleharty.
Submitted 21 May 2007. Accepted 1 September 2008. Associate Editor was Troy L. Best.
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SHAUNA R. MARQUARDT * AND JERRY R. CHOATE
Missouri Department of Conservation, 6700 West Route I, Columbia, MO 65203 (SRM) Department of Biological Sciences, Sternberg Museum of Natural History, Fort Hays State University, Hays, KS 67601 (JRC)
* Correspondent. firstname.lastname@example.org
TABLE 1--Mean ((+ or -) SD), high, and low temperatures ([degrees]C) for each week that samples of guano of Myotis velifer were collected in south-central Kansas; n = number of temperatures used in calculations. Temperature at roosting surface Site Week Mean (+ or -) SD (n) High Low Bergner 31 May 7 June 25.85 (+ or -) 3.26 (168) 34.01 20.18 Barn 7-14June 29.89 (+ or -) 1.99 (168) 34.01 25.95 14-21 June 29.79 (+ or -) 2.75 (142) 36.13 24.40 Whitney 7-14 June 26.61 (+ or -) 2.63 (147) 33.59 23.24 Barn 21-28 June 22.91 (+ or -) 3.91 (144) 32.34 14.47 (lower) 4-12 July 27.96 (+ or -) 3.52 (206) 35.27 20.95 Whitney 21-28 June 27.33 (+ or -) 5.98 (144) 39.22 17.14 Barn 4-12 July 31.31 (+ or -) 4.42 (206) 41.05 22.48 (loft) Gates 29 June-4 July 20.06 (+ or -) 1.80 (139) 22.09 15.62 Cave 4-13 July 20.66 (+ or -) 0.73 (206) 22.09 19.04 Gentry 31 May-8 June 17.87 (+ or -) 1.15 (192) 21.71 15.62 Cave 8-15 June 19.98 (+ or -) 0.86 (168) 22.48 18.66 15-22 June 20.42 (+ or -) 0.66 (94) 22.09 19.42 22-29 June 20.33 (+ or -) 3.62 (168) 25.56 18.28 29 June-4 July 21.02 (+ or -) 0.61 (144) 24.01 19.81 4-12 July 21.83 (+ or -) 0.81 (207) 24.01 20.18 Air temperature 1.5-1.8 m below roosting surface Site Week Mean (+ or -) SD (n) High Low Bergner 31 May 7 June 23.96 (+ or -) 3.46 (168) 32.76 17.14 Barn 7-14June 28.44 (+ or -) 1.97 (168) 32.76 24.40 14-21 June 28.15 (+ or -) 3.16 (142) 34.85 21.33 Whitney 7-14 June 26.18 (+ or -) 3.42 (147) 35.27 20.57 Barn 21-28 June 22.58 (+ or -) 4.12 (144) 32.76 14.08 (lower) 4-12 July 27.27 (+ or -) 3.69 (206) 34.85 20.18 Whitney 21-28 June -- -- -- Barn 4-12 July 30.17 (+ or -) 4.30 (206) 40.13 22.09 (loft) Gates 29 June-4 July 18.08 (+ or -) 1.48 (139) 21.71 15.23 Cave 4-13 July 18.43 (+ or -) 1.16 (206) 22.07 16.00 Gentry 31 May-8 June 16.88 (+ or -) 1.20 (192) 20.95 14.08 Cave 8-15 June 18.94 (+ or -) 0.85 (168) 22.09 17.90 15-22 June 19.38 (+ or -) 0.58 (94) 21.33 18.28 22-29 June -- -- -- 29 June-4 July -- -- -- 4-12 July 21.02 (+ or -) 1.14 (207) 23.24 18.28 TABLE 2--Arthropod prey of female Myotis velifer in reproductive condition summarized as percentage volume and relative frequency (%) of total food items consumed. Number of samples of guano analyzed is listed in parentheses following the name of each roost. Results of the analysis of food habits by Kunz (1974) are included for comparison. Bergner Barn (15) Whitney Barn (14) Food item Percentage Relative Percentage Relative volume frequency volume frequency Araneae <0.1 0.1 0.2 0.5 Coleoptera 38.3 34.8 44.1 36.2 Alleculidae 0.3 0.7 0.5 1.6 Carabidae 3.3 2.4 3.8 2.7 Cerambycidae Chrysomelidae 4.6 5.0 4.3 2.5 Curculionidae 2.2 2.1 1.4 2.17 Dytiscidae <0.1 <0.1 <0.1 <0.1 Hydrophilidae Meloidae <0.1 <0.1 Pentatomidae 0.1 0.5 Scarabaeidae 18.7 9.6 24.5 12.8 Unidentified 9.2 14.9 9.7 14.5 Diptera 13.4 12.0 11.5 17.4 Calliphoridae Chironomidae Culicidae 2.7 2.5 0.4 1.9 Dolichopodidae Muscidae Tipulidae <0.1 <0.1 Unidentified 10.6 9.5 11.1 15.5 Ephemeroptera 0.1 0.2 Hemiptera 13.3 27.0 12.9 19.7 Cicadellidae 2.9 6.6 0.8 2.4 Corixidae 1.9 3.7 1.6 2.4 Fulgoridae Lygaeidae 7.4 13.9 10.0 14.0 Miridae Nabidae Unidentified 1.0 2.4 0.5 0.9 Hymenoptera 4.9 4.3 3.3 2.5 Lepidoptera 25.5 18.8 24.4 19.4 Mesostigmata Neuroptera <0.1 0.1 0.3 0.1 Chrysopidae Hemerobiidae <0.1 0.1 0.3 0.1 Myrmelontidae Orthoptera 0.2 2.2 Siphonaptera Ischnopsyllidae Trichoptera 2.0 0.8 2.0 1.3 Unknown 2.5 1.9 1.2 0.7 Gates Cave (3) Gentry Cave (15) Food item Percentage Relative Percentage Relative volume frequency volume frequency Araneae <0.1 0.1 Coleoptera 37.7 31.1 39.7 31.4 Alleculidae 0.1 0.6 Carabidae 2.6 1.9 4.0 1.7 Cerambycidae Chrysomelidae 1.2 1.0 1.4 0.9 Curculionidae 1.0 2.4 0.3 0.7 Dytiscidae Hydrophilidae Meloidae Pentatomidae <0.1 0.5 <0.1 <0.1 Scarabaeidae 16.7 10.7 18.6 11.1 Unidentified 16.2 15.1 15.3 16.5 Diptera 21.5 25.7 11.0 12.2 Calliphoridae Chironomidae Culicidae 0.6 1.9 1.8 1.0 Dolichopodidae Muscidae Tipulidae Unidentified 20.9 23.8 9.2 11.2 Ephemeroptera 0.3 0.5 Hemiptera 2.0 9.7 12.3 27.3 Cicadellidae 0.2 2.9 3.8 9.1 Corixidae 1.3 3.4 2.3 6.6 Fulgoridae Lygaeidae 0.3 1.9 6.2 11.6 Miridae Nabidae Unidentified 0.1 1.00 Hymenoptera 10.5 9.7 11.7 7.0 Lepidoptera 20.5 17.0 22.6 18.4 Mesostigmata Neuroptera 1.2 1.0 0.6 0.9 Chrysopidae 0.1 0.2 Hemerobiidae 1.2 1.0 0.5 0.7 Myrmelontidae Orthoptera 0.8 2.9 0.1 0.2 Siphonaptera Ischnopsyllidae Trichoptera 0.7 1.4 Unknown 5.7 2.4 1.3 1.1 This study Kunz, 1974 Average Average Average Food item percentage relative percentage volume frequency volume Araneae 0.10 0.2 Coleoptera 40.0 33.4 37.4 Alleculidae 0.2 0.7 Carabidae 3.4 2.2 4.3 Cerambycidae 0.5 Chrysomelidae 2.8 2.3 0.5 Curculionidae 1.20 1.8 Dytiscidae <0.1 <0.1 1.0 Hydrophilidae 1.0 Meloidae 0 <0.1 Pentatomidae 0.1 0.3 Scarabaeidae 19.6 11.0 15.9 Unidentified 12.6 15.3 14.0 Diptera 14.3 16.8 14.4 Calliphoridae 0.5 Chironomidae 4.8 Culicidae 1.4 1.8 Dolichopodidae 0.5 Muscidae 1.4 Tipulidae <0.1 <0.1 4.8 Unidentified 12.9 15.0 2.4 Ephemeroptera 0.1 0.2 Hemiptera 10.1 21.0 27.1 Cicadellidae 1.9 5.2 17.4 Corixidae 1.8 4.0 6.8 Fulgoridae 0.5 Lygaeidae 6.0 10.4 Miridae 1.4 Nabidae 0.5 Unidentified 0.4 1.1 0.5 Hymenoptera 7.6 5.9 Lepidoptera 23.3 18.4 11.6 Mesostigmata 4.3 Neuroptera 0.5 0.5 3.9 Chrysopidae <0.1 <0.1 Hemerobiidae 0.5 0.5 Myrmelontidae 3.9 Orthoptera 0.3 1.3 0.5 Siphonaptera 0.5 Ischnopsyllidae 0.5 Trichoptera 1.2 0.9 1.0 Unknown 2.7 1.5
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|Author:||Marquardt, Shauna R.; Choate, Jerry R.|
|Date:||Jun 1, 2009|
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