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Different competitive potential in two coexisting mouse lemur species in northwestern Madagascar.

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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.
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