Different competitive potential in two coexisting mouse lemur species in northwestern Madagascar.код для вставкиСкачать
AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 145:156–162 (2011) Different Competitive Potential in Two Coexisting Mouse Lemur Species in Northwestern Madagascar Sandra Thorén,1* Miriam Linnenbrink,2 and Ute Radespiel1 1 2 Institute of Zoology, University of Veterinary Medicine Hanover, 30559 Hanover, Germany Evolutionary and Functional Genomics, Department of Biologie II, University of Munich, 82152 Plonegg-Martinsried KEY WORDS admixture; human population genetics; anthropometry ABSTRACT Interspeciﬁc competition has been suggested to inﬂuence the biogeographic distribution patterns of species. A high competitive potential could entail species-speciﬁc advantages during resource acquisition that could translate into a higher potential for range expansion. We investigated whether differences in the competitive potential of the morphologically similar and partially sympatric gray mouse lemur (Microcebus murinus) and golden-brown mouse lemur (Microcebus ravelobensis) may help to explain differences in their geographic range sizes. We carried out encounter experiments with 14 pairs of captured female mouse lemurs of both species. The experimental dyads were tested in a two-cage arrangement, with individuals being separated from each other outside the experiments. Two days of habituation and four subsequent days of 1-h encounter experiments were conducted, before releasing the animals again in the wild. In general, the M. murinus individuals won signiﬁcantly more conﬂicts than their partners. In eight of 14 tested pairs, there was a signiﬁcant species bias in winning conﬂicts, and in 87.5% of these dyads, M. murinus was the ‘‘dyad winner’’. A high competitive potential did not depend on body mass. Furthermore, ‘‘dyad winners’’ spent more time feeding (P < 0.05) and were less spatially restricted than ‘‘dyad losers’’. To conclude, our results suggest that the widely distributed M. murinus may indeed have a higher competitive potential than the regional endemic M. ravelobensis, which may, among other possible factors, have enabled this species to expand geographically, despite the presence of other competing congeners. Am J Phys Anthropol 145:156– 162, 2011. V 2011 Wiley-Liss, Inc. Interspeciﬁc competition occurs frequently within species communities (review in Schoener, 1983). Species with high competitive abilities may gain higher access to essential resources and consequently better ﬁtness compared to species with lower competitive ability (Kaufmann, 1983; Bernstein, 1981; Magnuson et al., 1979). Consequently, interspeciﬁc competition may have implications for species survival and stability of species communities (Gause, 1934; Brown and Wilson, 1956; Hardin, 1960; Chase and Leibold, 2003). Interspeciﬁc competition has also been hypothesized to inﬂuence large-scale biogeographic patterns (Cox and Moore, 2000; Case et al., 2005). Competitive advantage during resource acquisition may translate into a higher potential for range expansion, and may explain why widely distributed species have been able to expand geographically despite the presence of competing species (Hanski, 1982; Darwin, 1959; Brown, 1984; Wilson and Keddy, 1986; Holway, 1999; Walck et al., 1999). On the other hand, competitive superiority has also been associated with a narrow distribution (Glazier and Eckert, 2002). A high competitive potential in geographically restricted species may have allowed them to maintain their often specialized ecological niches in the presence of widely distributed generalists (Miller, 1967). For instance, small mammals found in geographically restricted areas were shown to be dominant over related widespread species (Hallett, 1982; Glazier and Eckert, 2002). In this study we use two mouse lemur species, endemic primates of Madagascar, as models to investigate whether interspeciﬁc differences in competitive potential can help to explain striking differences in their geographic range size. Microcebus murinus, the gray mouse lemur, has the widest distribution of all mouse lemur species and ranges from the south to the northwest of the island. In contrast, M. ravelobensis, the golden-brown mouse lemur, occurs only in the area between two large rivers, the Betsiboka and the Mahajamba (Olivieri et al., 2007). The two mouse lemur species have a high potential for resource competition due to their phylogenetic proximity, reﬂected in ecological and morphological similarities (Zimmermann et al., 1998; Radespiel et al., 2006; Thorén et al., 2011). They (a) have partially overlapping feeding niches (Radespiel et al., 2006; Thorén et al., 2011), (b) do not differ in body length (both species are about 117 mm from snout tip to anus) or body mass (M. murinus: 53.9 6 0.9 g, M. ravelobensis: 56.2 6 1.8 g, Zimmermann et al. 1998), and (c) show similar nocturnal activity and seasonal reproduction (Schmelting et al., 2000). We tested the hypothesis that the biogeographic patterns of sympatric species reﬂect differences in competitive potential. We used an experimental design to C 2011 V WILEY-LISS, INC. C Grant sponsor: DFG; Grant numbers: Ra 502/9-1. *Correspondence to: Sandra Thorén, Institute of Zoology, University of Veterinary Medicine Hanover, Bünteweg 17, 30559 Hanover, Germany. E-mail: Sandra.Thoren@tiho-hannover.de Received 7 July 2010; accepted 30 January 2011 DOI 10.1002/ajpa.21516 Published online 16 March 2011 in Wiley Online Library (wileyonlinelibrary.com). COMPETITIVE POTENTIAL IN TWO MOUSE LEMUR SPECIES test for agonistic asymmetries between two partially sympatric Microcebus species. First, we investigated whether one species had a higher competitive potential, i.e. won more conﬂicts during encounter experiments, than the other species. Second, we investigated whether individuals with high competitive potential had priority of access to food and were less spatially restricted than individuals with low competitive potential. Third, we investigated whether one species consistently won more conﬂicts, had priority of access to food and was less spatially restricted than the other species. METHODS Study site and capturing We conducted this study during 2007 (May to July) and 2008 (June to August) in the dry deciduous forest in the Ankarafantsika National Park in northwestern Madagascar (135.000 ha), located about 120 km southeast of Mahajanga. For our experiments, we used mouse lemurs captured from two sites located near the forestry station of Ampijoroa (168190 S, 468480 E): Jardin Botanique A (JBA), a 30.6 ha area where M. ravelobensis lives sympatrically with M. murinus, and Jardin Botanique B (JBB), a 5.1 ha area where M. ravelobensis is found exclusively (Rendigs et al., 2003). We trapped mouse lemurs weekly to obtain the animals for our cage experiment. We placed 100 Sherman live traps, baited with banana, at the intersections of the trails (about every 25– 50m) in the evening, following standard procedures (Rendigs et al., 2003). The traps were controlled the next day in the early morning. Since food is believed to essentially limit the reproductive success of females, but not as much of males (Trivers, 1972), we tested only female pairs in this study. During the study period, we captured 61 different females in 2007 (16 M. murinus and 6 M. ravelobensis in JBA, 39 M. ravelobensis in JBB) and 46 different females in 2008 (9 M. murinus and 11 M. ravelobensis in JBA, 26 M. ravelobensis in JBB). Fourteen female pairs were chosen from these captures for the experiments. We paired preferably unfamiliar individuals of similar body mass and age, and tested them in only one dyad. Home range sizes of female mouse lemurs of both species range from 0.3 ha to a maximum of 2.7 ha (idealized maximum diameter: 195 m; Ehresmann 2000; Radespiel 2000; Lutermann 2001; Weidt et al. 2004; Quietzsch 2009). Females were therefore only paired if they were captured at a distance [300 m to each other (two pairs), or preferably originated from different study sites (12 pairs). We determined the sex and weighed the trapped animals. We marked new individuals with unique ear patterns (small triangular cuts of about 2 mm2 in size in the outer part of the pinna) and each animal obtained a subcutaneous transponder (Trovan Small Animal Marking System, Telinject1). At dusk of the same day, the chosen test animals were placed in the test cages, while all other individuals were released at their individual capture location in the forest. 157 during the actual experiments. To avoid a potential bias in the behavior of the animals due to side effects in the twocage arrangement, we placed each species equally often in the left and right cage. We used three of these two-cage arrangements, which were placed in the forest, about 20 m away from each other. Each cage (volume: 0.72 m3, height: 1.2 m, length: 1.0 m, width: 0.6 m) contained a sleeping box (bamboo cane), branches, leave litter on the ground, a feeding place and a water bottle (see Fig. 1). All cages were furthermore provided with a sun roof. All observations and experiments were carried out during the 6-day period of temporary captivity. We tested each dyad for one hour per evening, starting with two evenings of closed tunnels, in order to habituate the animals to the presence of the observer and the technicalities of the observations. During the following four evenings, one-hour encounter experiments (tunnels open) were conducted per night and pair between 6 pm and 10 pm. At the beginning of each experiment, we provided banana (food that was successfully used to capture both mouse lemurs species) at one feeding place (Days 1 and 2: in the upper connecting tunnel, Days 3 and 4: alternatively in the M. murinus cage and the M. ravelobensis cage). We observed both animals simultaneously. We recorded all observational data on a Dictaphone (Olympus digital voice recorder, WS-320M) using a ﬂash light (Maglite, USA) and a head lamp (Petzl MYO 5 or Petzl tikka plus, France). The animals seemed undisturbed by the talking into a Dictaphone. The tunnels were closed at the end of the 1-h encounter experiment and both animals received further banana food on their feeding places. None of the interspeciﬁc interactions resulted in any visible injuries. The animals were held continuously in the cages throughout the six-day period and were released at dusk on the day after the experiments were completed at their exact capture location. Data collection and statistical analyses Both animals were observed simultaneously, and proximity as well as all social behaviors were recorded with a continuous focal animal sampling (Altmann, 1974). To obtain information on space use, we noted the position of both animals (own or other cage, tunnels, sleeping box) by means of an instantaneous sampling with an interval duration of 15 s. We also noted the beginning and end of the time spent feeding and the time spent inside the sleeping box. Cage experiments We conducted encounter experiments with 14 dyads, each composed of one female M. murinus and one female M. ravelobensis. We placed each experimental pair for a period of six days in a cage arrangement that consisted of two separate cages. The two cages were connected by two tunnels (length of ca 0.3 m), which were only opened Fig. 1. The experimental two-cage arrangement, connected by two tunnels, which were only open during the experiments. In black: sleeping place. In grey: feeding place. In hatched pattern: connecting tunnels. American Journal of Physical Anthropology 158 S. THORÉN ET AL. Any series of interactions that contained agonistic behaviors (aggressive or submissive), was termed a conﬂict. A conﬂict was recorded as a new one, if four seconds or more had passed since the previous conﬂict. We classiﬁed all interactions during conﬂicts as either ‘‘neutral’’ (proximity 5 to be within 0.5 m from another without any further interaction), ‘‘aggressive’’ (chase 5 rapidly follow and displace the other individual, attack 5 animal A rapidly approaches and bites and/or beats animal B, ﬁght 5 both individuals engage in an agonistic body contact that consists of reciprocal biting and/or beating), or ‘‘submissive’’ (avoid 5 rapidly increase distance to the partner without being chased, ﬂee 5 rapidly increase distance to the partner as a response to being chased). We distinguished between decided and undecided conﬂicts. A conﬂict was decided if one female showed only submissive behaviors, and undecided when none of the females or both showed submissive behaviors. Any conﬂict with both individuals showing aggressive behavior was deﬁned as not decided. We noted the context of each conﬂict as (a) feeding context (one or both animals were ingesting food at the time of the interaction), (b) sleeping box context (one or both animals were sitting inside or on the sleeping box at the time of the interaction), (c) social context (when the two animals were in body contact prior to the conﬂict), or (d) unspeciﬁc (the encounter was not associated with any above described contexts). We used a Binomial test (P \ 0.05) to test whether one animal in a dyad won signiﬁcantly more conﬂicts than the other animal. In those dyads where one species won signiﬁcantly more conﬂicts than the other one, the winner is termed ‘‘dyad winner’’ whereas the loser is termed ‘‘dyad loser’’. We used the Wilcoxon-test to test for differences in dependent datasets (STATISTICA 6, StatSoft, Inc. 2004). RESULTS Interspeciﬁc difference in competitive potential We observed a total of 359 interspeciﬁc conﬂicts in the 14 dyads (range, 0–65 per dyad). Of these, 25.1% (n 5 90) occurred in the feeding context, 27.6% (n 5 99) in the sleeping place context, 0.8% (n 5 3) in the social context and 46.5% (n 5 167) in the unspeciﬁc context. A total of 281 (78.3%, 0–58 per dyad) conﬂicts had a decided outcome, 228 of which were decided for M. murinus (81.1%) and 53 were decided in favor of M. ravelobensis (18.9%). In general, M. murinus won signiﬁcantly more conﬂicts than M. ravelobensis (M. murinus: median, 6.0 per dyad; range, 0–50 per dyad; M. ravelobensis: median, 2.5 per dyad, range, 0–10 per dyad; Wilcoxon test: T 5 12.0, Z 5 2.34, n 5 14, P 5 0.02). The M. murinus females were not statistically heavier than the M. ravelobensis females (M. murinus: median 5 52.5 g, range, 36.0–64.0 g, n 5 14 M. ravelobensis: median, 53.5 g; range, 34.0–80.0 g; n 5 14; Wilcoxon test: T 5 35.0, Z 5 0.73, n 5 14, P 5 0.46). In 11 of 14 dyads, M. murinus won more conﬂicts than M. ravelobensis whereas M. ravelobensis won more conﬂicts than M. murinus in only two dyads (see Fig. 2). In one dyad, there were no conﬂicts at all. In eight of the 14 dyads (Dyads A, B, C, E, F, G, J, L), the number of decided conﬂicts was sufﬁciently high to statistically determine whether one individual won signiﬁcantly more conﬂicts than the other (Binomial test: P \ 0.05; Fig. 2). In seven of these eights dyads (87.5%), M. muriAmerican Journal of Physical Anthropology nus was the ‘‘dyad winner’’, while M. ravelobensis was the ‘‘dyad winner’’ in only one dyad (12.5%; Fig. 2). The ‘‘dyad winners’’ were not statistically heavier than the ‘‘dyad losers’’ (winners: median mass 5 49.0 g, range, 36.0–64.0 g, n 5 8; losers: median mass 5 49.5 g, range, 34.0–68.0 g, n 5 8; Wilcoxon test: T 5 15.0, Z 5 0.4, n 5 8, P 5 0.67). Competitive potential and access to food In the dyads with a statistically biased competitive potential (n 5 8), the ‘‘dyad winners’’ spent signiﬁcantly more time feeding than the ‘‘dyad losers’’ during the four hours of observations (winners: median 5 26:44 min, losers: median 5 19:21 min; Wilcoxon test: T 5 4.0, Z 5 1.96, P 5 0.049; Fig. 3). However, the ‘‘dyad winners’’ did not spend signiﬁcantly more time at the feeding place than the ‘‘dyad losers’’ (winners: median 5 10.7%, range, 4.2–19.0% of scans; losers: median 5 8.5%, Fig. 2. Number of conﬂicts won by M. murinus minus number of conﬂicts won by M. ravelobensis in the 14 test dyads (Letters A-N). In eight of these dyads, one species won signiﬁcantly more conﬂicts than the other species (dark gray bars), and in 87.5% of those (n 5 7), M. murinus won signiﬁcantly more conﬂicts than M. ravelobensis (dark gray bars above x-axis). In six dyads, the number of conﬂicts was too low to statistically determine an ‘‘overall winner’’ (light grey bars). Numbers above bars: number of decided conﬂicts in each dyad. Fig. 3. Comparison of the time spent feeding by the ‘‘dyad winners’’ and the ‘‘dyad losers’’ in the eight decided dyads during the total observation time; *P < 0.05. COMPETITIVE POTENTIAL IN TWO MOUSE LEMUR SPECIES range, 6.0–12.6%; Wilcoxon test: T 5 15.0, Z 5 0.42, n 5 8, P 5 0.67). Interspeciﬁc comparisons showed that there were no signiﬁcant difference in how long time M. murinus and M. ravelobensis spent feeding during the four hours of observations (M. murinus: median, 22:17 min; range, 1:00–55:44 min; M. ravelobensis: median, 20:03 min; range, 3:55–41:50 min; Wilcoxon test: T 5 39.0, Z 5 0.85, n 5 14, P 5 0.40), or how much time they spent at the feeding place (M. murinus: median, 10.0%, range, 0.3–26.8% of scans; M. ravelobensis: median, 12.5%; range, 7.6–30.0%; Wilcoxon test: T 5 27.0, Z 5 1.60, n 5 14, P 5 0.11). Competitive potential and space use In the dyads with a statistically biased competitive potential (n 5 8), there was no difference in how much time the ‘‘dyad winners’’ and the ‘‘dyad losers’’ spent in their sleeping boxes (winners: median, 41.9%; range, 9.1–76.0% of scans; losers: median, 11.8%, range, 3.5– 64.3%; Wilcoxon test: T 5 10.0, Z 5 1.1, n 5 8, P 5 0.26). Calculated on the basis of the number of scans spent outside their sleeping boxes, the ‘‘dyad winners’’ spent signiﬁcantly more time in the cage of their partner than the ‘‘dyad losers’’ (winners: median, 41.4%, range, 19.3–70.4.8% of scans; losers: median, 13.8%; range, 7.2– 32.6%; Wilcoxon test: T 5 3.0, Z 5 2.1, n 5 8, P 5 0.04). Conversely, the ‘‘dyad losers’’ spent signiﬁcantly more time in their own cage (losers: median, 68.8%; range, 42.1–81.4%; winners: median, 39.1%; range, 16.6–60.3% of scans; Wilcoxon test: T 5 2.0, Z 5 2.2, n 5 8, P 5 0.03). There was no difference between the ‘‘dyad winners’’ and the ‘‘dyad losers’’ regarding the time they spent in the connecting tunnels (winners: median, 20.1%, range, 8.7–36.8% of scans; losers: median, 17.1%, range, 6.7–26.1%; Wilcoxon test: T 5 12.0, Z 5 0.8, n 5 8, P 5 0.40). Interspeciﬁc comparisons showed that individuals of M. murinus spent signiﬁcantly more time in their sleeping boxes than individuals of M. ravelobensis (M. murinus: median, 56.7%; range, 9.1–95.6% of scans; M. ravelobensis: median, 22.3%; range, 1.6%–72.2%; Wilcoxon test: T 5 11.0, Z 5 2.6, n 5 14, P 5 0.01). Calculated on the basis of the number of scans spent outside their sleeping boxes, there was no difference in how much time M. murinus and M. ravelobensis spent in their own cage (M. murinus: median, 46.1%; range, 6.9–100.0%; M. ravelobensis: median, 58.6%; range, 18.3–81.4% of scans; Wilcoxon test: T 5 37.0, Z 5 1.0, n 5 14, P 5 0.33), in the cage of their partner (M. murinus: median, 36.6%; range, 0.0–70.4.8% of scans; M. ravelobensis: median, 21.5%; range, 7.2–44.8%; Wilcoxon test, T 5 29.0, Z 5 1.5, n 5 14, P 5 0.14), or in the connecting tunnels (M. murinus: median, 17.2%; range, 0.0–23.9% of scans; M. ravelobensis: median, 16.8%; range, 6.7–36.8%; Wilcoxon test: T 5 33.0, Z 5 1.2, n 5 14, P 5 0.22). DISCUSSION Our study revealed evidence for a modest difference in competitive potential in two partly sympatric congeners in an experimental setting consisting of a series of encounter experiments. Because of few conﬂicts between some M. murinus and M. ravelobensis, our competition experiments yielded a clear winner in solely 57% of the dyads. In general, however, M. murinus won signiﬁ- 159 cantly more conﬂicts than M. ravelobensis. Moreover, in 87.5% of the dyads with a clear winner, M. murinus was the ‘‘dyad winner’’. Despite this overall higher competitive potential of M. murinus compared to that of M. ravelobensis, access to food and space was not generally better for individuals of M. murinus, but rather for individuals with high competitive potential (mainly M. murinus). The modest difference in competitive potential between the two species will be subsequently discussed with respect to the possible role of body mass but also to interspeciﬁc niche differentiation and interspeciﬁc variations in biogeographic patterns as possible consequences of these interspeciﬁc differences in competitive potential. Body mass Competitive ability is typically inﬂuenced by body size, with larger species tending to have a competitive advantage over smaller species. A positive correlation between body size and the ability to win conﬂicts has been observed in microtine rodents (Randall, 1978), birds (Travaini et al., 1998; Shelley et al., 2004), ﬁsh (Blann and Healey, 2006), and primates (Peres, 1996). Despite the overall morphological similarities of our model species (Zimmermann et al., 1998), we found evidence for an interspeciﬁc difference in competitive potential between M. murinus and M. ravelobensis. This disparity, however, was not based on differences in body mass between the tested M. murinus and M. ravelobensis, which exclude the possibility that the overall bias in competitive potential was based on an overall bias in strength. It is possible that the two species differ in aggressiveness. However, since we did not perform control experiments with intraspeciﬁc dyads, this hypothesis could not be tested in our study. Our primary aim was to determine the relative competitive potential of both species in interspeciﬁc conﬂicts, and not to estimate the general frequency of conﬂicts in a captive setting or to compare the ﬁndings to rates of intraspeciﬁc aggression. Proximate parameters such as behavioral differences, differences in personality and differences in ‘‘dominance style’’ may have inﬂuenced the biased outcome of the interspeciﬁc conﬂicts, and need further comparative investigation on the intra- as well as on the interspeciﬁc level. Niche differentiation As already shown in various studies (Magnuson et al., 1979; Bernstein, 1981; Kaufmann, 1983), our results suggest that a relatively high competitive potential increases the potential to monopolize resources. The ‘‘dyad winners’’ in our study, mainly individuals of M. murinus (87.5%), spent more time feeding than the ‘‘dyad losers’’. In addition, the ‘‘dyad winners’’ spent more time in the cage of their partner and less time in their own cage compared with the ‘‘dyad losers’’, which might further indicate that the individuals with relatively high competitive potential were less spatially restricted by the presence of their partner than vice versa. Under natural conditions, differences in interspeciﬁc competitive potential may lead to a niche shift in the subordinate species (Schoener, 1983). Several studies show how competitively inferior species change their habitat usage pattern (microtine rodents: Randall, 1978) or their feeding strategies in the presence of American Journal of Physical Anthropology 160 S. THORÉN ET AL. competitively superior species (birds: Sandlin, 2000; primates: Houle et al., 2006). Previous studies of M. murinus and M. ravelobensis revealed ecological differences between the congeners such as species-speciﬁc bias in habitat use (Rakotondravony and Radespiel, 2009), feeding niche differentiation, as well as differences in locomotor activity (Radespiel et al., 2006; Thorén et al., 2011) and sleeping site ecology (Radespiel et al., 2003, Thorén et al., 2009). The difference in sleeping site preferences, with M. murinus more often sleeping in tree holes, whereas M. ravelobensis is found more often in open vegetation (Radespiel et al., 2003, Thorén et al., 2009), may explain why M. murinus in our study spent more time in their sleeping boxes (about half of the time) compared M. ravelobensis (about one fourth of the time). If these ecological differences are the results of past and/or ongoing interspeciﬁc competition or independent evolutionary niche divergence is not yet known. However, the spatial expansion of M. murinus into northwestern Madagascar seems to be rather recent, which suggests that M. murinus and M. ravelobensis may not have undergone a long-term sympatry and coevolution in this area (Schneider et al., 2010). The evolutionary implications of these ﬁndings, however, need to be further explored in future studies. Biogeographic patterns Interspeciﬁc competition is expected to have implications for species survival and stability of species communities (Gause, 1934; Hardin, 1960; Brown and Wilson, 1956; Chase and Leibold, 2003), but has also been suggested to inﬂuence large-scale biogeographic patterns (Cox and Moore, 2000; Case et al., 2005). Our study suggests that M. murinus, the species with the widest distribution of all known mouse lemurs, possesses a moderately higher competitive potential than the geographically restricted M. ravelobensis. In general, the impact of interspeciﬁc competition on range size should be especially apparent in ecologically and morphologically similar congeners. Across its range, M. murinus occurs in sympatry with at least two other geographically restricted congeners. These congeners are M. griseorufus in southwestern Madagascar (Yoder et al., 2002), and M. berthae in western Madagascar (Schwab, 2000; Schwab and Ganzhorn, 2004; Dammhahn and Kappeler, 2005, 2008). Although M. murinus and M. griseorufus occur in contact zones of restricted hybridization (Gligor et al., 2009), they are typically found in different habitat types separated by a transition zone (Rasoazanabary, 2004). The colonization history of these two species is not known, and it is not clear whether the observed niche divergence is the result of historical or ongoing interspeciﬁc competition or independent evolutionary histories of the two species. M. murinus and M. berthae are also spatially separated, but on a much smaller geographic scale (Dammhahn and Kappeler, 2008). Direct evidence of the relative competitive potential of this species pair is lacking so far, although qualitative accounts exist about the competitive superiority of M. murinus over the much smaller M. berthae (Schwab and Ganzhorn, 2004). The spatial separation of these species has been explained by M. berthae avoiding direct competition with its larger congener (Schwab and Ganzhorn, 2004) and proﬁting from a patchy distribution pattern of M. murinus that American Journal of Physical Anthropology originates from communal breeding of closely related females (Dammhahn and Kappeler, 2008). Our study showed partly very few conﬂicts between M. murinus and M. ravelobensis. However, when there was a clear dyad winner (57% of dyads), the winner was almost always M. murinus, which suggests that M. murinus has a higher competitive potential compared with M. ravelobensis. In evolutionary terms, even a modest bias in agonistic outcome may be sufﬁcient to result in ﬁtness differences and inﬂuence evolutionary processes such as colonization success. Recent genetic data (Schneider et al., 2010) suggest that M. murinus most likely colonized northwestern Madagascar relatively recently (late Pleistocene). The high competitive potential that could be shown in this study may be one factor that enabled this species to expand into areas that were already inhabited by other congeners. CONCLUSIONS We showed that M. murinus has a competitive advantage over M. ravelobensis that can lead to advantages during food acquisition and space use. The bias in competitive potential might be best described as moderate, since conﬂicts could be won by both species, but with different relative frequencies. Regardless of the observation that 43% of the dyads had little conﬂicts, the fact that when there was a clear winner, the winner was almost always M. murinus, should have important biological signiﬁcance. Our results suggest that the widely distributed M. murinus might have been able to expand geographically despite the presence of other competing species due to this elevated competitive potential. Whether the interspeciﬁc difference in relative competitive potential in our experimental setup corresponds to reproductive and survival advantages under natural conditions, needs to be further explored. Due to difﬁculties conducting focal observations of small nocturnal solitary foragers such as mouse lemurs, we still lack quantitative data on interspeciﬁc competition in the wild. Anecdotal observations, however, are available on occasional interspeciﬁc aggressive interactions (S.T., unpublished data; Franziska Quietzsch & Marine Joly, pers. com). These accounts suggest that interspeciﬁc encounters like the ones that we provoked in our experiments, may be paralleled by occasional interspeciﬁc encounters in the wild which probably just occur at lower frequencies. To conclude, our study provides support for the hypothesis that a high competitive potential may positively inﬂuence the range size of species (Hanski, 1982; Brown, 1984; Holway, 1999). Contrasting ﬁndings in other model species (small mammals: Hallett, 1982; Glazier and Eckert, 2002), suggest that interspeciﬁc competition alone does not explain all cases of divergent distribution patterns of species. Further potentially important parameters are the degree of ecological specialization of species (Harcourt et al., 2002; Eeley and Lawes, 1999), their body size (Eeley and Lawes, 1999) and their phylogeographic history (Grubb, 1982; Diamond and Hamilton, 1980; Schneider et al., 2010). Future studies should aim to model the relative importance of each of these factors in an integrated approach. ACKNOWLEDGMENTS The authors thank the Department des Eaux et Forêts (DEF) and the members of CAFF/CORE for approving COMPETITIVE POTENTIAL IN TWO MOUSE LEMUR SPECIES our research proposal and issuing the research authorization. They also thank the University of Antananarivo (D. Rakotondravony, and the late O. Ramilijaona) for its continuous support and the Association pour la Gestion des Aires Protégées (ANGAP) for the permission to work in the Ankarafantsika National Park. They particularly thank Solofo Rasoloharijaona and Blanchard Randrianambinina for practical help during the beginning of the study. They also highly appreciate the continuous support of the local staff of the National Park. They are also thankful to the editor and two anonymous reviewers for helpful comments on the manuscript. ETHICAL STANDARDS The study was approved by CAFF/CORE, the Department des Eaux et Forêts (DEF) and the Association pour la Gestion des Aires Protégées (ANGAP). Capturing and cage observation procedures adhered to the legal requirements of Madagascar and were approved by the Malagasy Ministry of Environment, Water and Forests. We have complied with all ethical standards concerning the treatment of primates and with the national laws and research rules formulated by the Malagasy authorities. LITERATURE CITED Altmann J. 1974. Observational study of behavior: sampling methods. Behaviour 49:227–267. Bernstein IS. 1981. Dominance: the baby and the bathwater. 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