Journal of Insect Physiology 50 (2004) 149–156 www.elsevier.com/locate/jinsphys Metabolic rate and jump performance in seven species of desert fleas Boris R. Krasnov a,
, Irina S. Khokhlova b, Sergey A. Burdelov a, Laura J. Fielden c a b Ramon Science Center and Mitrani Department of Desert Ecology, Jacob Blaustein Institute for Desert Research, Ben-Gurion University of the Negev, P.O. Box 194, 80600 Mizpe Ramon, Israel Desert Animal Adaptations and Husbandry, Wyler Department of Dryland Agriculture, Jacob Blaustein Institute for Desert Research, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel c Science Division, Truman State University, Kirksville, MO 63501, USA Received 16 August 2003; received in revised form 3 November 2003; accepted 3 November 2003 Abstract We hypothesized that sexual and interspecific differences in jumping performance of fleas found in our previous study are correlated with differences in resting metabolic rate (RMR) between sexes and among species. To test this hypothesis, we measured RMR of seven flea species (Xenopsylla conformis mycerini, Xenopsylla ramesis, Xenopsylla dipodilli, Parapulex chephrenis, Synosternus cleopatrae pyramidis, Nosopsyllus iranus theodori and Stenoponia tripectinata medialis). We compared RMR between sexes and among species and examined whether there is intra- and interspecific correlation between RMR and jumping ability. Both mass-specific and mass-independent RMR were the highest in female S. t. medialis, whereas mass-specific RMR was the lowest in male X. dipodilli and mass-independent RMR was the lowest in three Xenopsylla species and P. chephrenis. Mass-specific and mass-independent RMR were significantly higher in females than in males in all fleas except S. t. medialis. Differences in jumping ability between males and females were found to be correlated with sexual differences in mass-specific or mass-independent RMR. Interspecific comparison showed that the length of jump in both male and female fleas was strongly affected by their mass-specific and mass-independent RMR. # 2003 Elsevier Ltd. All rights reserved. Keywords: Fleas; Jumping performance; Interspecific comparison; Resting metabolic rate; Sexual dimorphism 1. Introduction Correlation between metabolic rate and life history and feeding ecology traits has been reported for different animal taxa (McNab, 1988; Koteija and Wiener, 1993; Reinhold, 1999), suggesting that adaptations involving high levels of activity lead to increased maximum sustainable metabolic rate. Aerobic capacity hypothesis states that when selection favours enhanced aerobic metabolism, required to support high activity, metabolism at rest will also be increased (Bennet and Ruben, 1979; Benton, 1979; Pough, 1980). Consequently, species demonstrating high level of activity are expected to show higher resting metabolic rate (RMR) than less active species, all else being equal (Reinhold, 1999).
Corresponding author. Tel.: +972-8-6588764; fax: +972-86586369. E-mail address: krasnov@bgumail.bgu.ac.il (B.R. Krasnov). 0022-1910/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jinsphys.2003.11.001 The jumping of fleas is their most conspicuous locomotory character that allows these wingless parasites to attack their hosts successfully. Classical studies of flea jumping demonstrated that the size of certain parts of the locomotory apparatus can be indicative of jumping capacity (Rothschild et al., 1973, 1975; Rothschild and Schlein, 1975) and, thus, can be used for comparisons among individuals, between sexes and among species. Rothschild et al. (1973, 1975) and Rothschild and Schlein (1975) reported that one of the main sources of flea jumping power is a rubber-like protein (resilin) located in the pleural arch and that pleural arch was well developed in fleas with high jumping capacity, but was absent or greatly reduced in fleas that are poor jumpers or do not jump. Inspired with the idea that flea locomotory performance is correlated with morphometric parameters, Tripet et al. (2002) measured pleural height in preserved flea specimens (from the Rothschild collection of fleas at The Natural History 150 B.R. Krasnov et al. / Journal of Insect Physiology 50 (2004) 149–156 Museum, London) and used these measurements as indicators of flea mobility for broad comparisons among species of bird fleas from different geographic and host ranges. We studied jump performance in seven flea species and compared jump length between sexes and among species to examine whether there are morphological correlates of jumping ability (Krasnov et al., 2003). We found both between-sex and among-species differences in jumping ability. However, contrary to our expectation, we did not find a correlation between jumping ability and body size or morphometrics of the locomotory system for both intra- and interspecific comparisons. This suggested that factors other than linear body metrics affected locomotory performance. These factors could be physiological. For example, the amount of energy generated by the extensor tibiae muscles has been considered to affect jump performance in orthopterans (Burrows and Wolf, 2002). In pyrgomorphid grasshopper Phymateus morbillosus, adult males are able to fly, whereas females, although fully winged, cannot fly. Physiological and biochemical correlates of this distinct dimorphism were shown to be the lower mass-specific activity of several enzymes and the mitochondria content in flight muscles of females compared with males (Gäde, 2002). Another physiological factor that presumably affects locomotory performance is metabolic rate. Size-specific differences in metabolic rate between males and females as well as among different species have been reported for various arthropods. For example, male ticks Dermacentor variabilis demonstrated higher size-specific metabolic rates than female ticks (Fielden et al., 1999). Metabolic rate during running-wheel locomotion was higher in male beetles Phoracantha recurva and Phoracantha semipunctata than in conspecific females (Rogowitz and Chappell, 2000). Analyzing published data on RMR in different insect taxa, Reinhold (1999) found that flying insects had higher RMR than species that use energetically less demanding types of locomotion. However, little is known so far about RMR in fleas except for metabolic requirements of different developmental stage in two species, Ctenocephalides felis (Silverman and Rust, 1985) and Xenopsylla conformis (Fielden et al., 2001). Given that the rate of resting metabolism was shown to be positively correlated with locomotory activity in arthropods (Lighton and Duncan, 1995) and that RMR is expected to increase with increase in energetically demanding activity (Prestwich, 1994; Reinhold, 1999), we hypothesized that sexual and interspecific differences in jumping performance in fleas can be explained, at least in part, by differences in RMR between sexes and among species. To test this hypothesis, we measured RMR of seven species of desert fleas, namely Xenopsylla conformis mycerini Rothsch., Xenopsylla ramesis Rothsch., Xenopsylla dipodilli Smit, Parapulex chephrenis Rothsch., Synosternus cleopatrae pyramidis Rothsch., Nosopsyllus iranus theodori Smit and Stenoponia tripectinata medialis Jordan. All these fleas are parasitic on rodents in the Negev desert, Israel, but they have different hosts, occurring in different habitats and have different annual cycles (Krasnov et al., 1997, 1998, 1999, 2002a). We compared RMR between sexes and among species and examined whether there is intra- and inter-specific correlation between RMR and jumping ability. 2. Materials and methods 2.1. Fleas Fleas were obtained from our laboratory colonies started in 1997–2001 from field-collected specimens on Meriones crassus (X. c. mycerini, X. ramesis and S. t. medialis), Gerbillus dasyurus (X. dipodilli and N. i. theodori), Acomys cahirinus (P. chephrenis) and Gerbillus andersoni allenbyi (S. c. pyramidis). Fleas were reared on their specific rodent hosts. The details of rearing procedure are described elsewhere (Krasnov et al., 2002b, 2003). In brief, an individual rodent host was placed in a glass cage (60  50  40 cm3 ) that contained a steel nest box with a screen floor and a pan containing a mixture of sand and dried bovine blood (nutrient medium for larvae) on the bottom. Every two weeks, all substrate and bedding material was collected from the nest box and transferred into an incubator (FOC225E, Velp Scientifica srl, Milano, Italy), where v flea development and emergence took place at 25 C and 75% relative humidity. Only newly emerged fleas were used in measurements of CO2 emission and jumping trials. Upon emergence, fleas were placed individually in 20 ml glass vials. Vials were covered with 5  5 cm nylon screen, transferred v into incubators and maintained at 25 C air temperature and 75% RH during 24 h prior to a trial. In total, we used 65 X. c. mycerini, 78 X. ramesis, 80 X. dipodilli, 60 P. chephrenis, 78 S. c. pyramidis, 30 N. i. theodori and 28 S. t. medialis. Experimental design was found to be suitable and to meet requirements of the 1994 Law for the Prevention of Cruelty to Animals (Experiments on Animals) of State of Israel by Ben-Gurion University Committee for the Ethical Care and Use Animals in Experiments (License IL-19-04-2001). 2.2. Respirometry RMR was measured via CO2 emission, which has been extensively validated for insects (Lighton, 1991). Measurements were carried out using a flow-through B.R. Krasnov et al. / Journal of Insect Physiology 50 (2004) 149–156 respirometry system. This system consisted of a respirometer chamber made of tygon tubing (6.5 mm internal diameter, 3 ml volume). Incurrent air was scrubbed of CO2 and H2O vapour by a drierite (700 ml volume) and ascarite (25 ml volume) columns and was pumped through the system at flow rate of 50 ml min1. Flow rate was controlled by a mass flow controller (model FC-260, Tylan, Rancho Dominquez, CA, USA). This dry, CO2 free air constituted the base line measurements for all flow-through measurements. Carbon dioxide concentration (ppm) of air exiting the respirometer chamber was sampled by a calibrated CO2 analyzer (model 6262, LI-COR, Lincoln, NE, USA). Readings of the amount of CO2 produced were taken every 2 s and recorded using computerized data-acquisition software (Datacan V, Sable Systems, Henderson, NV, USA). Plumbing for the entire system excluding the respirometer chamber consisted of Tygon tubing (3.3 mm internal diameter). A stable temperature for the air inside the respirometer tubing was regulated by placing the chamber and preceding 6 m of incurrent tygon tubing into a water bath (model 1013S, Fisher Scientific, Pittsburgh, PA, USA). Production of CO2 v was measured at 25 C. Measurements of different flea species were randomized between- and within days of measurement. Carbon dioxide emission for fleas was recorded for 1 h. Baseline measurements (20 min) were made before and after each recording to determine zero CO2 and to correct for instrument drift. Fleas were weighed prior to being placed in the respirometer chamber up to nearest 0.01 mg (model 290 SCS, Precisa Instruments AG, Dietikon, Switzerland). Fleas were measured both individually and in groups (n ¼ 2 5). Gas emission recordings of individual fleas did not differ significantly from group recordings on a per flea basis (t ¼ 0:8 1:3, p> 0:001). Measurements were replicated 10 times for each sex and species with different flea individuals used in each replicate. Fleas placed into respiration chamber usually moved during 2–5 min and then stayed motionless. To ensure that fleas remained stationary during sampling, they were occasionally observed. 2.3. Data analysis All computerized CO2 emission recordings were processed using the analysis package of Datacan. Each recording was converted from ppm to ml CO2 h1. To convert rate of CO2 emission to metabolic rate, the respiratory quotient was assumed to be 0.8 as determined for flat females of tick Amblyomma marmoreum by Lighton et al. (1993), which seems a reasonable assumption for hematophagous arthropods. A RQ of 0.8 gives an energy equivalent of 24.5 J ml1 CO2 (Schmidt-Nielsen, 1980). This value was used to express RMR in terms of energy. 151 All variables did not deviate significantly from normality (Kolmogorov-Smirnov tests, NS) and, consequently, we used parametric statistics. Regression analyses were applied to test the relationship between body mass and RMR. Linear and exponential non-linear (Levenberg-Marquardt algorithm) regressions of these residuals on body mass were applied. We analyzed both mass-specific and mass-independent RMR. Values of mass-independent RMR were derived from ANCOVAs of RMR per flea with body mass as covariate (to remove the effect of body mass— Packard and Boardman, 1999). Differences between males and females in mass-specific and mass-independent RMR were tested using 1-way ANOVA with sex as fixed factor separately for each species. Data on jump length (corrected for body size) of males and females of seven flea species were taken from our previous study (Krasnov et al., 2003). The effect of RMR dimorphism on jump length dimorphism was investigated using phylogenetically independent contrasts (Felsenstein, 1985). We set initial branch length as suggested by Grafen (1989), as no information on branch length was available. To compute standardized contrast values, we used the program CACTUS 1.12 (Schwilk and Ackerly, 2001). Following Cullum (1998), we regressed standardized contrasts of the female:male ratios in size-corrected jump length on standardized contrasts of female:male ratio in mass-specific or massindependent RMR. We applied major axis (model II) regression forced through origin (Garland et al., 1993). A slope greater than zero would indicate that sexual differences in jump length were explained, at least partly, by sexual dimorphism in RMR. Analysis of the relationships between RMR and jumping ability within sexes among species also involved comparisons among taxa. We regressed standardized contrasts of jump length on standardized contrasts of metabolic rate using major axis (model II) regression through the origin (Pagel, 1992; Garland et al., 1993). We used phylogenetically independent contrasts calculated as described above. Because of the error factor associated with the use of potentially incorrect branch length (Purvis et al., 1994), we also carried out the analyses on non-standardized contrasts. However, these results did not differ from those obtained with standardized contrasts and, therefore, are not presented. The phylogenetic tree for fleas was based on the taxonomy used in Hopkins and Rothschild (1953, 1962) and Traub et al. (1983) and is represented in Krasnov et al. (2003). We avoided an inflated Type I error by Bonferroni adjustments of a. Significance is recorded at the adjusted level for all intraspecific comparisons. B.R. Krasnov et al. / Journal of Insect Physiology 50 (2004) 149–156 152 Table 1 v Rate of CO2 emission and mass-specific (MSMR) and mass-independent RMR (MIMR) in seven flea species at 25 C Species X. c. mycerini X. ramesis X. dipodilli S. c. pyramidis P. chephrenis S. t. medialis N. i. theodori a b Sex Male Female Male Female Male Female Male Female Male Female Male Female Male Female Average jump length (mm)a 78:34  3:38 110:34  3:10 95:61  3:98 114:21  3:30 80:10  4:15 90:77  3:04 115:75  3:46 127:98  3:30 90:26  3:11 104:77  3:04 113:78  7:42 108:06  6:91 110:04  2:81 127:22  3:85 Body mass (mg) 0:14  0:01 0:21  0:01 0:11  0:03 0:17  0:04 0:19  0:003 0:25  0:01 0:17  0:01 0:25  0:01 0:11  0:01 0:18  0:01 0:90  0:08 1:75  0:09 0:23  0:02 0:32  0:02 Rate of CO2 emission (ll h1) (ll mg1 h1) 0:086  0:003 0:20  0:018 0:08  0:003 0:19  0:006 0:08  0:006 0:14  0:02 0:18  0:01 0:30  0:01 0:07  0:004 0:22  0: 02 1:27  0:09 2:43  0:09 0:18  0:01 0:40  0:02 0:64  0:04 0:99  0:05 0:67  0:05 1:18  0:01 0:38  0:03 0:72  0:03 0:91  0:07 1:24  0:02 0:62  0:04 1:22  0:08 1:48  0:08 1:37  0:08 0:85  0:08 1:25  0:08 MSMR (mJ h1 mg1) MIMR (mJ h1)b 15:67  2:21 24:15  2:07 16:50  2:21 28:87  1:25 9:32  1:85 17:75  2:21 22:37  2:10 30:44  2:20 15:31  2:17 29:87  2:07 36:23  1:85 33:48  1:85 20:82  2:21 30:60  2:07 2:59  0:92 4:55  0:28 2:02  0:22 4:65  0:21 1:85  0:20 3:45  0:23 4:76  0:32 7:09  0:32 2:25  0:48 4:23  0:47 43:22  6:63 47:32  6:63 4:68  0:61 9:55  0:60 Corrected for body size (data from Krasnov et al., 2003). Calculated from the linear model. 3. Results RMR for each flea species was calculated from the mean volume of CO2 emitted (ll h1) during stable continuous gas exchange. Results of measurements of RMR in seven flea species are presented in Table 1. Both mass-specific and mass-independent RMR were the highest in female S. t. medialis, whereas mass-specific RMR was the lowest in male X. dipodilli and mass-independent RMR was the lowest in three Xenopsylla species and P. chephrenis. Grouping all the species data, we found that RMR of the whole insect (MRwi, mJ h1) scaled allometrically with body mass, MRwi ¼ 0:03BM1:14 , where BM is body mass in mg (r2 ¼ 0:89, p< 0:001). However, body mass also appeared to be a good predictor of RMR under assumption of linear relationship between these parameters (MRwi ¼ 0:03BM  0:0002, r2 ¼ 0:9, p< 0:001). Mass-specific and mass-independent RMR were significantly higher in females than in males in all fleas except S. t. medialis (Tables 2 and 3). In this species, no sexual differences were found either in mass-specific or mass-independent RMR. Table 2 Summary of ANOVAs of comparisons of mass-specific metabolic rate between males and females in seven flea species Species F ratio df X. c. mycerini X. ramesis X. dipodilli S. c. pyramidis P. chephrenis S. t. medialis N. i. theodori 28.91 72.6 22.35 18.06 45.12 0.34 16.35 1, 1, 1, 1, 1, 1, 1, p 18 18 18 18 18 18 18 <0.001 <0.001 <0.001 0.001 <0.001 0.56 0.001 Differences in jumping ability between males and females were found to be well explained by sexual differences in mass-specific or mass-independent RMR. The slopes of the major axis regressions of independent contrasts in female:male jumping performance ratios on independent contrasts in RMR ratios differed significantly from zero (slope ¼ 0:41  0:1, r2 ¼ 0:63, F ¼ 8:5 for mass-specific and slope ¼ 0:24  0:07, r2 ¼ 0:70, F ¼ 11:9 for mass-independent RMR, p< 0:01 for both, Fig. 1), suggesting a strong connection between metabolic rate dimorphism and jumping ability dimorphism. The size-corrected length of jump in male fleas was strongly affected by their mass-specific and mass-independent RMR. Regressions of independent contrasts in jump length on independent contrasts in RMR were highly significant (r2 ¼ 0:69, F ¼ 11:3 for mass-specific and r2 ¼ 0:65, F ¼ 9:13 for mass-independent RMR, Table 3 Summary of ANCOVAs of sex effect on the whole insect metabolic rate in seven flea species with body mass as covariate Species Factor F ratio df p X. c. mycerini Body mass Sex Body mass Sex Body mass Sex Body mass Sex Body mass Sex Body mass Sex Body mass Sex 5.45 15.46 9.21 57.76 4.03 22.86 6.18 20.65 0.005 10.37 5.94 0.12 1.13 24.70 1, 17 0.04 <0.001 0.01 <0.001 0.06 <0.001 0.03 <0.001 <0.001 <0.001 0.03 0.73 0.3 <0.001 X. ramesis X. dipodilli S. c. pyramidis P. chephrenis S. t. medialis N. i. theodori 1, 17 1, 17 1, 17 1, 17 1, 17 1, 17 B.R. Krasnov et al. / Journal of Insect Physiology 50 (2004) 149–156 153 Fig. 1. Plots of contrasts in female/male size-corrected jump length ratios against corresponding contrasts in female/male ratios in massspecific and mass-independent metabolic rates. Fig. 2. Plots of contrasts in size-corrected jump length against corresponding contrasts in mass-specific and mass-independent metabolic rates. p< 0:01 for both, Fig. 2). The same was true for female fleas (r2 ¼ 0:91, F ¼ 52:3 for mass-specific and r2 ¼ 0:90, F ¼ 48:6 for mass-independent RMR, p< 0:001 for both, Fig. 2). It is commonly accepted that fleas originated from the winged ancestors similar to mecopteran families Boreidae or Nannochoristidae (Smit, 1982; Whiting et al., 1997; Lewis, 1998). Rothschild et al. (1973) suggested that although fleas have lost their wings during their evolution and have returned to a more primitive form of locomotion, they have retained a great part of flight adaptations and use them during jumping. In particular, the resilin pad in fleas is homologous with the wing hinge ligaments in flying insects. In other words, the flight mechanism of ancestors is incorporated into the new jumping mechanism. Therefore, energetic cost of flea jump is presumably high compared with costs of resting metabolism or that of other modes of their locomotion. This is analogously with much higher metabolic cost of flight compared with metabolism at rest in flying insects (Bartholomew and Casey, 1978). Results of this study confirm the hypothesis about a trade-off between low resting metabolism and efficient metabolism during activity (Reinhold, 1999). Reinhold (1999) suggested that in species that spend less than half of their daily metabolic energy on resting metab- 4. Discussion The results of the study supported our hypothesis that jumping ability in fleas is correlated with RMR. Indeed, males of most studied fleas are poorer jumpers than females (Krasnov et al., 2003), and in the present study RMR in males was proved to be lower than that in females. The only exception was S. t. medialis. This flea did not show sexual dimorphism either in jumping ability (Krasnov et al., 2003) or in RMR (this study). Furthermore, jumping ability and RMR in interspecific comparisons appeared to be correlated. Higher RMRs were found in species with higher jumping abilities. In addition, values of flea RMR received in this study are similar to those found in our previous study (Fielden et al., 2001). 154 B.R. Krasnov et al. / Journal of Insect Physiology 50 (2004) 149–156 olism, selection will favour mutations that increase RMR but simultaneously decrease metabolic cost of activity; whereas this will not be the case for species where resting metabolism comprises the large amount of the daily energy requirements. Similarly, mutations that decrease RMR but increase the metabolic cost of daily activity will be favoured in species that allocate more than half of their time budget to rest. It was predicted that animals that spend more energy on activity would demonstrate higher RMR. Indeed, flying insects demonstrated higher RMRs than insects with energetically less costly types of locomotion (see Reinhold, 1999; Harrison and Roberts, 2000 for review). For example, when studying RMR in six Scarabaeus species, Davis et al. (2000) found that the lowest mass-specific RMR were found in the wingless species, while the volant species showed significantly higher RMRs. Moreover, differences in RMR between individuals with different locomotory ability have been reported from intraspecific comparisons (Zera and Mole, 1994; Gäde, 2002). It is difficult to evaluate proportion of time that a flea allocates for jumping, although this time definitely constitutes less than half of the daily time budget. Nevertheless, fleas demonstrate vigorous jumping activity. For example, Rothschild et al. (1975) reported that newly emerged female Xenopsylla cheopis were able to perform up to 250 jumps per 0.5 h. Therefore, the metabolic cost of flea daily jumping can be considered to be relatively high, being in-between that of a flight and that of energetically less demanding locomotory modes, such as walking or running. Fleas with different jumping abilities represented, thus, a convenient model to test the trade-off hypothesis of Reinhold (1999). Moreover, the present study supported the prediction of this hypothesis in both intraspecific and interspecific comparisons. Differences in RMR and as a consequence in jumping performance between sexes and among species of fleas require some ecological interpretations. These interpretations can provide insight into probable causes of selection for higher or lower levels of RMR that, in turn, were determined by selection for a higher or lower levels of jumping ability. We discussed possible explanations of within- and between species differences in jump performance elsewhere (Krasnov et al., 2003). Briefly, these differences between sexes can be related to association of flea reproduction to blood feeding. Furthermore, male fleas are able to mate after a single blood meal (Iqbal and Humphries, 1970), but females of most flea species have to feed repeatedly for their eggs to mature (Vatschenok, 1988). Consequently, the urgency of a blood meal is more critical for females than for males, As a result females would benefit from being more mobile and selection would favour females with higher metabolism and, consequently, with increased jumping abilities. S. t. medialis represented an exception from a general trend of sexual dimorphism in jumping abilities and RMR. Imagoes of this flea have extremely short period of activity (3–4 winter months only), surviving summer in the cocooned stage and as mature adults (Krasnov et al., 2002a). The short period of activity likely requires both males and females to mate as early in the season as possible. Consequently, both natural and sexual selection would favour more mobile males and, thus, between-sex difference in mobility and in RMR disappears. Interspecific differences in RMR and jumping ability seemed to be associated with their occupied habitats and seasonality. The highest RMR in our study was in either parasites of psammophylic rodents (S. c. pyramidis) or fleas that are active during short winter season only (S. t. medialis) or both (N. i. theodori). These species proved to be the best jumpers too (Krasnov et al., 2003). Other studied species never occur in sand habitats (Krasnov et al., 1999). We suggest that takeoff from a mobile sand substrate presumably requires a higher energy investment than that from a hard substrate. This can explain higher RMR and jumping ability in sand-dwelling fleas. The above explanation of the absence of sexual locomotory and RMR dimorphism in S. t. medialis (to begin feeding and, consequently, reproduction as early as possible) can be invoked also for the explanation of relatively high mass-independent RMR and, as a result, locomotory ability in both winter-active species (S. t. medialis and N. i. theodori). Other studied species are active all year round, although their winter activity is low (Krasnov et al., 1997, 2002a). Causal relationships between RMR and those properties of occupied habitats that determine metabolic costs of activity were reported for different insects. Chironomids from streams had higher metabolism than related species from ponds (Walshe, 1948) because the energetic cost of staying in place or swimming against current is higher in streams (Reinhold, 1999). Another evidence is represented by Stenus beetles with different habitat preferences (Betz and Fuhrmann, 2001). S. comma that inhabits bare grounds had higher RMR than plant-mounting S. pubescens potentially because of higher energetic cost of foraging in open sites. The results of our study confirm this trend, showing that fleas differing in habitat preferences and locomotory capacity differed also in RMR. Acknowledgements A. Allan Degen (Ben-Gurion University of the Negev) read an earlier version of the manuscript and made helpful comments. This study was partly supported by the Israel Science Foundation (Grant no. 663/01-17.3 to B.R.K. & I.S.K.). This is publication no. 150 of the Ramon Science Center and no. 403 of the Mitrani Department of Desert Ecology. B.R. 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