Canadian Journal of Zoology
Ecomorphological correlates of microhabitat selection in two sympatric Asian box turtle species (Geoemydidae: Cuora)
Journal: Canadian Journal of Zoology
Manuscript ID cjz-2016-0218.R2
Manuscript Type: Article
Date Submitted by the Author: 23-May-2017
Complete List of Authors: Xiao, Fanrong; Hainan Normal University, College of Life Sciences Wang, Jichao;Draft Hainan Normal University, College of Life Sciences Shi, Haitao; Hainan Normal University, College of Life Sciences Long, Zaizhong; Hainan Normal University, College of Life Sciences Lin, Liu; Hainan Normal University, College of Life Sciences Wang, Wei; Hainan Normal University, College of Life Sciences
keeled box turtle, Cuora mouhotii, Indochinese box turtle, Cuora Keyword: galbinifrons, leaf litter, microhabitat, rock crevice
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Ecomorphological correlates of microhabitat selection in two sympatric Asian box turtle species (Geoemydidae: Cuora )
Fanrong Xiao, Jichao Wang, Haitao Shi, Zaizhong Long, Liu Lin, and Wei Wang
F. Xiao, J. Wang, H. Shi, Z. Long, L. Lin, W. Wang:
College of Life Sciences, Hainan Normal University, Haikou 571158, China
H. Shi:
Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu 610041, China (e mail: haitao shi@ 263. net) Draft
Corresponding authors: H. Shi, Hainan Normal University, Longkun south road no.
99, Haikou, Hainan province, China. Tel: 0898 65803298 (e mail: haitao shi@ 263.
net)
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Ecomorphological correlates of microhabitat selection in two sympatric Asian box turtle species (Geoemydidae: Cuora )
F. Xiao, J. Wang, H. Shi, Z. Long, L. Lin, and W. Wang
Abstract: Closely related species that co occur in homogeneous environments often possess differing morphologies, which can result in niche divergence that minimizes interspecific competition. In the present study, we examined the relationship between the ecomorphological characteristics and microhabitat selection of two Asian box turtle species, the keeled box turtle Cuora mouhotii (Gray, 1862) and Indochinese box turtle C. galbinifrons (Bourret, Draft 1939), which have sympatric distributions in the rainforest of Hainan, China. We found that C. mouhotii had a relatively flat shell and preferred microhabitats with rock crevices and steep slopes in the field, whereas C. galbinifrons had a domed shell and was restricted to microhabitats of deciduous leaves under bamboo growing on gentle slopes. We conclude that morphological divergence allows the two Cuora spp. to use different microhabitats and, thereby, to successfully co occur.
Key words: keeled box turtle, Cuora mouhotii , Indochinese box turtle, Cuora galbinifrons , leaf litter, microhabitat, rock crevice.
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Introduction
One of the most important objectives of ecomorphology is to determine the
underlying mechanisms of species co occurrence in relation to their morphological
and ecological characteristics, both in the present and over evolutionary time (Motta
and Kotrschal 1992; Vanhooydonck et al. 2000). Generally, co occurring species
either possess similar morphologies, owing to convergent evolution from adaptation
to the same habitat (Grant 1972), or divergent morphologies that result in niche
divergence and minimize interspecific competition (Hutchinson 1959; Brown and
Bowers 1985). In particular, closely related species that co occur in homogeneous environments should be morphologicallyDraft distinct to partition limited resources, such as microhabitats, and, thus, to reduce competitive interactions and, ultimately, achieve
co occurrence. Numerous studies on vertebrate taxa have shown that ecomorphology
correlates with habitat selection among sympatric species (e.g., mammals: Davies et
al. 2007; birds: Guillemain 2002; reptiles: Pounds 1988; Lindeman 2000;
Vanhooydonck et al. 2000; Higham and Russell 2010; and fish: Robinson and Wilson
1994). However, such studies have typically correlated morphological features with
habitat classes, and only a few have quantified ecological characteristics, such as
microhabitat (Lindeman 2000; Vanhooydonck et al. 2000).
Both the keeled box turtle ( C. mouhotii Gray, 1862; formerly Pyxidea mouhotii )
and Indochinese box turtle (C. galbinifrons Bourret, 1939) are distributed in south
China, including Hainan Province (Zhao and Adler 1993). C. mouhotii is typically
found in moist evergreen forests at low altitude (Wang et al. 2011 a; Wangyal et al.
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2012; Struijk et al. 2016), whereas C. galbinifrons is found in similar forests but at high altitude (Wang et al. 2011 b). Previous studies have reported the occurrence of natural hybrids in Hainan Province (Shi et al. 2005), which implies that, in some areas, the distributions of the two species overlap, and the species have since been reported to co occur at 700–914 m in the Hainan Diaoluoshan Natural Reserve by Wang et al.
(2011 a,b).
Interestingly, the morphology of C. mouhotii is extremely divergent from its sympatric species, C. galbinifrons , as well as from all other species in the genus. For example, C. mouhotii possesses a relatively flat carapace for a member of Cuora , with a flattened top, whereas C. galbinifronsDraft has a highly domed carapace. In general, carapace shape is correlated with the habitat preference of turtles: aquatic turtles have flattened carapaces, whereas terrestrial turtles have highly domed carapaces, and semiaquatic turtles have carapaces with intermediate morphology (Romer 1967;
Claude et al. 2003; Bonnet et al. 2010; Benson et al. 2011). However, previous studies about the microhabitat preference of flat topped turtle species remain scarce, such as
C. mouhotii . Additionally, although C. galbinifrons has been considered to exhibit a semiaquatic habit (Ernst and Barbour 1989), Wang (2007) and Wang (2008) tracked nine individuals in Diaoluoshan Natural Reserve for two years, using radio telemetry, and found that the species never entered the water. Therefore, the ecological function of the two species’ divergent morphologies has been unclear.
Recently, one individual of C. mouhotii was found in a rock crevice at the Deo
Ca Protected Forest, Phu Yen Province, Vietnam (Ly et al. 2013). In contrast, C.
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galbinifrons is more frequently observed in microhabitats of deciduous leaves under
dense bamboo (Wang et al. 2011 b). Therefore, in the present study, we quantified the
microhabitat use of co occurring C. mouhotii and C. galbinifrons at Diaoluoshan
Natural Reserve. We hypothesized that the two species would occupy different
microhabitats, owing to their divergent shell morphology.
Materials and methods
Ethics statement
This study was approved by the Animal Research Ethics Committee of Hainan Provincial Education Centre forDraft Ecology and Environment, Hainan Normal University (HNECEE 2014 002), and was carried out in strict accordance with the
institutional guidelines. Fieldwork was carried out with permission from the
Diaoluoshan Forest Bureau. No turtles were sacrificed for this study or incurred injury
or death while in the traps.
Microhabitat selection
We studied the microhabitat selection of C. mouhotii and C. galbinifrons at the
Diaoluoshan Natural Reserve in Hainan Province, China (18°43′53″N, 109°52′10″E)
from April 2015 to February 2016. The field site was located in a rainforest between
860 and 914 m in altitude, where the average highest temperature is 28 °C in July, the
average lowest temperature is 15.4 °C in January, and the annual rainfall ranges from
1870 to 2760 mm.
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The turtles were captured in traps baited with salty fish or rancid pork skin; thereafter, a radio tracking transmitter (RI 2B, 216.000–216.999 MHz; Holohil
Systems, Ltd., Caro Ontario, Canada) was fitted to the last costal scute of each turtle.
The size (diameter × height = 21 × 8 mm), mass (6 g), and location of the transmitters would not have prevented turtles from using any of the microhabitats. Then the turtles were released at the site of capture. The turtles were tracked daily from 09:00 to 16:00, using a handheld receiver (TRX 1000S, 216.000–216.999 MHz; Wildlife Materials
International, Inc., Murphysboro, Illinois, USA) with a three element folding antenna
(Wildlife Materials, Inc.), and the locations were recorded using a handheld Magellan Triton 400 Global Positioning SystemDraft (Magellan Navigation, Inc., Santa Clara, USA). Radio tracked turtles were tracked for an average of 164 days. All radio tracking transmitters were carefully removed from the turtles’ shells when the tracking study was complete.
We hypothesized that the locations at which the turtles remained for >3 d indicated preferred microhabitats and quantified 14 microhabitat characteristics within two different sized quadrats (10 × 10 m and 1 × 1 m) at each preferred site. In the large quadrats, we quantified (1) slope gradient (°), (2) slope position (1 = upper slope,
2 = middle slope, 3 = lower slope; slope position refers to the location of slope occupied by turtle in the mountain, which was classified as upper slope, middle slope or lower slope when the turtle occur near peak, middle or low position of the mountain, respectively), (3) canopy cover (%), (4) number of fallen logs, (5) number of bamboo clumps, (6) number of tree holes, and (7) number of rock crevices; and in
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the small quadrats, we quantified (8) bamboo density (plants/m 2), (9) herbage height
(cm), (10) herbage density (plants/m 2), (11) deciduous leaf cover (%), (12) deciduous
leaf thickness (cm), (13) stone cover (%), and (14) type of concealment (1 = rock
crevice, 2 = tree hole, 3 = fallen log, 4 = grass, 5 = bamboo clump, 6 = deciduous
leaves, 7 = bare ground). In total, we collected data from 44 preferred sites from seven
C. mouhotii individuals and from 27 preferred sites from five C. galbinifrons
individuals .
Chi squared tests (SPSS 16.0; SPSS, Inc., Chicago, IL, USA; same below) were
used to assess whether the two species differed in their preference of slope position and type of concealment. The Kolmogorov SmirnovDraft test was performed to test the normality of the 12 numeric microhabitat variables. One way analysis of variance
(ANOVA) was subsequently used to test whether the two species differed in slope
gradient, herbage height, and deciduous leaf cover, and the Mann Whitney U test was
used to determine whether the two species differed in any the nine remaining
microhabitat variables. Furthermore, discriminant function analysis (DFA) was
performed to assess the differences in the 12 numeric microhabitat variables between
the two species. The stepwise method of DFA was performed to determine the best
variables significantly separating the two species.
In addition, we measured the slope gradient at stopover sites at which the turtles
remained for <1 d, using a clinometer. Whether the two species differed in slope
gradient was examined using one way ANOVA.
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Morphological measurements
Morphological measurements were taken from all the experimental individuals
(n = 9 for C. mouhotii , n = 8 for C. galbinifrons ), as well as from other individuals that were more recently encountered in the field (nine individuals for C. mouhotii and five individuals for C. galbinifrons ), using a Vernier caliper (accuracy to 0.02 mm).
For each individual, we measured carapace length, carapace width (measured at the sixth marginal scute), and carapace height (measured at the sixth marginal scute). In addition, we also calculated the ratio of carapace height to width (R) as a measure of shell contour (Domokos and Várkonyi 2008). After the normality and Draft homogeneity of variance were tested for all morphological measurements, one way ANOVA was used to test whether the two species differed in carapace length, and to remove the effect of body size, one way analysis of covariance (ANCOVA; carapace length as the covariate) was performed to test whether the two species differed in any of the other morphological variables.
Results
Microhabitat selection
No individuals of either C. mouhotii or C. galbinifrons were found in the water during the field study. Among the 14 microhabitat variables, slope gradient, number of rock crevices, deciduous leaf cover, deciduous leaf thickness, stone cover (Table 1), and type of concealment (χ2 = 69.69, df = 6, P < 0.0001) significantly differed
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between the two species . This finding indicates that C. mouhotii was more commonly
observed on rocky substrates and steep slopes (Table 1) and typically selected rock
crevices for concealment (56.82%; Fig. 1B), whereas C. galbinifrons was usually
observed in deciduous leaves on gentle slopes (Table 1) and selected deciduous leaves
for concealment (93.93%; Fig. 1C).
The stepwise DFA indicated that three variables were selected which
significantly discriminate C. mouhotii and C. galbinifrons (stone cover, slope gradient,
and bamboo density; Table 2), and the efficiency of the analysis was high (62 of 71
quadrats 87.3% were correctly classified). The distribution of the quadrats on the DFA axis showed that the microhabitatsDraft selected by C. mouhotii were mostly distinct from those selected by C. galbinifrons , and only a small overlap between the two
species (Fig. 2). For instance, both C. mouhotii and C. galbinifrons selected the same
terrestrial habitat with high canopy cover (Table 1; Fig. 1A).
In addition, the slope gradients of the stopover sites of the two species differed
significantly (one way ANOVA; F1, 324 = 128.214; P < 0.0001), with C. mouhotii
(mean ± SD: 30.3 ± 8.7°, range: 5 – 60°) occurring on steeper slopes than C.
galbinifrons (22.3 ± 7.5°, 5 – 40). This result was in accordance with the slope
gradients from the preferred sites.
Morphological measurements
The carapace length of the two species did not differ significantly (one way
ANOVA; Table 3). However, one way ANCOVA, with carapace length as the
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covariate, indicated that the carapace height and ratio of carapace height to width of the two species all differed significantly, whereas the carapace width of the two species was similar (Table 3). These findings indicated that C. mouhotii had a relatively flat shell, whereas C. galbinifrons had a highly domed one. In addition, the plastron hinge of C. mouhotii only allowed the shell to close partially, whereas that of
C. galbinifrons allowed the shell to close completely; and C. mouhotii also possessed a carapace with serrated posterior edges and a flattened top, whereas C. galbinifrons possessed a carapace with smooth edges.
Discussion Draft In agreement with previous studies, we found that both adult C. mouhotii and C. galbinifrons were terrestrial species and selected the same high canopy cover habitat
(Wang 2007; Lian 2009); however, we also observed that the microhabitats of the two species were significantly different. Notably, C. mouhotii individuals were mostly observed in areas with greater stone cover and often selected rock crevices for concealment, where they hid for up to 20 d, whereas C. galbinifrons individuals were mostly observed in deciduous leaf litter under bamboos, where they often hid for long periods, as well. In addition, C. mouhotii also occupied microhabitats with slopes that were much steeper than those of the microhabitats occupied by C. galbinifrons . This suggests that the two species co occur by occupying different niches.
Understanding the relationship between microhabitat selection and morphology is critical for understanding ecomorphological adaption (Vanhooydonck et al. 2000;
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Rivera 2008; Rivera et al. 2014). In the present study, we found that body size
(carapace length) did not differ between C. mouhotii and C. galbinifrons , but the shell
of C. mouhotii was significantly more flat than that of C. galbinifrons . Our
microhabitat experiment confirmed that C. mouhotii prefers and selects microhabitats
with rock crevices, and since flattened bodies often allow animals to fit into narrow
crevices (Miles 1994) and because previous studies have reported that terrestrial
turtles with flat carapaces, such as the African pancake tortoise (Malacochersus
tornieri Siebenrock, 1903), are adapted to lifestyles in rocky crevices (Ireland and
Gans 1972; Malonza 2003), we believe that the relatively flat shell of C. mouhotii is an adaptation for using microhabitatsDraft with rock crevices. In addition to possessing a flatter shell than C. galbinifrons , the flat topped carapace of C. mouhotii was
important in its adaptation to the rock crevice microhabitats, as in several species of
the genus Homopus Duméril and Bibron, 1835 (Ernst and Barbour 1989; Bonin et al.
2006).
We also observed that C. mouhotii has a posteriorly serrated carapace, which
might help prevent the turtles from being dragged out of rocky crevice by predators.
In addition, C. mouhotii occupied microhabitats with slopes that were much steeper
than those of the microhabitats occupied by C. galbinifrons . This difference may be
correlated with the shell shapes of the two species. Since flat shells impart a lower
center of gravity (Domokos and Várkonyi 2008) and thus increase stability on steeper
surfaces, the relatively flat shell of C. mouhotii could also be an adaptation for
climbing on smooth rocks and inclined substrates; however, this morphofunctional
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hypothesis should be confirmed by further investigation.
In contrast to the relatively flat shell of C. mouhotii , the highly domed shell of C. galbinifrons is typical of terrestrial turtles, and both the present and previous studies
(Wang 2007) have reported that C. galbinifrons inhabits terrestrial habitats. Indeed, the species is well adapted to terrestrial environments, since its shell can tolerate much stronger mechanical forces, such as those from terrestrial predators (Greene
1988; Stayton 2011); and although the highly domed shell may also restrict the species to hiding in soft leaves and to microhabitats with relatively gentle slopes, the species possesses several alternative defensive features. For example, C. galbinifrons also has a single transverse hingeDraft across the middle of its plastron that allows complete retraction and protection of its extremities from predators (Pritchard 2008), and the color and pattern of the species’ carapace is similar to that of deciduous leaves, which may function to reduce predator detection via camouflage (Fig. 1C; Stevens and Merilaita 2009; Xiao et al. 2016). Moreover, the smooth marginal scutes of C. galbinifrons allow it to burrow deeper than C. mouhotii under deciduous leaves.
Notably, we found that C. galbinifrons was a leaf dwelling species. However, many terrestrial species that hide under leaf litter, such as the black breasted leaf turtle ( Geoemyda spengleri Gmelin, 1789; R = 0.54; Benson et al. 2011), forest cane turtle ( Vijayachelys silvatica Henderson, 1912; R = 0.51; Whitaker and Vijaya 2009), and spiny turtle ( Heosemys spinosa Gray, 1831; R = 0.53; Spinks et al. 2012), have flat shells. Therefore, it is actually quite surprising that the highly domed C. galbinifrons also uses this microhabitat. One possible way in which C. galbinifrons
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can mitigate the disadvantage of its tall shell is that the coloration of C. galbinifrons is
somewhat similar to that of other leaf dwelling species, as well as to that of leaves
(i.e., camouflage). It is also possible that the leaf litter in the habitat of C. galbinifrons
affords high cover and is thick enough to hide its highly domed shell.
Intriguingly, other Cuora spp. possess different shell morphologies and occupy
diverse habitats. For example, both C. galbinifrons (R = 0.70) and the
yellow margined box turtle ( C. flavomarginata Gray, 1863; R = 0.65; Benson et al.
2011) have highly domed shells and occupy terrestrial habitats, i.e., evergreen forests
(Lue and Chen 1999), whereas the Malayan box turtle ( C. amboinensis Daudin, 1802), which also has a relatively domedDraft shell (R = 0.61; Benson et al. 2011), is aquatic (Ernst 1989; Joyce and Gauthier 2004). Similarly, C. mouhotii , which has a relatively
flat shell (R = 0.58), occupies terrestrial environments with rocky crevices; whereas,
the Chinese three striped box turtle ( C. trifasciata Bell, 1825), golden headed box
turtle (C. aurocapitata Luo and Zong, 1988), and Pan’s box turtle ( C. pani Song,
1984), which are morphologically similar and possess very flat and streamlined shells
(R = 0.48, 0.45, and 0.47, respectively; Zhang et al. 1998) that are typical of aquatic
species (R under approx. 0.6; Domokos and Várkonyi 2008), are completely aquatic
(Zhang et al. 1998; Cheung 2007). Phylogenetic analysis will be needed to elucidate
the patterns of shell evolution and habitat diversification of Cuora spp. in the future.
In summary, we found that the two species occupy different niches. C. mouhotii
prefers microhabitats with rock crevices and was more commonly observed on slopes
with steep inclines in the field, for which its flattened shell is well suited, whereas the
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dome shelled of C. galbinifrons restricts the species to hiding in deciduous leaves with bamboos and to microhabitats with relatively gentle slopes. Therefore, we conclude that morphological divergence allows the two Cuora spp. to partition the available microhabitats, and, thereby, to successfully co occur.
Acknowledgements
We are grateful to F. Kong, Z. Zhao, and Q. Wang for assistance with field work.
We are also grateful to the editors, R.M. Brigham and H. Guderley, and an anonymous reviewer for comments that improved this article. This study was supported by the National Natural Science FoundationDraft of China (nos. 31260518 to J.W. and 31372228 to H.S.).
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Table 1 Characteristics of microhabitat used by C. mouhotii and C. galbinifrons. Values indicate the mean ± SD measurements for each species, as well as the statistical significance of their difference.
one way ANOVA Quadrat size C. mouhotii C. galbinifrons or Mann Whitney U test
Slope gradient (°) 10 × 10 m 33.55 ± 8.5 26.81 ± 6.25 F = 12.679, P = 0.001 Canopy (%) 10 × 10 m 75.45 ± 10.39 80.19 ± 6.12 Z = 1.74, P = 0.082 No. fallen logs 10 × 10 m 2.84 ± 1.78 2.85 ± 1.23 Z = 0.491, P = 0.623
No. bamboo clumps 10 × 10 m 4.95 ± 2.47 4.00 ± 2.62 Z = 1.791, P = 0.073
No. tree holes 10 × 10 m 1.25 ± 0.87 0.89 ± 0.75 Z = 1.759, P = 0.079
No. rock crevices 10 × 10 m 5.45 ± 4.81 0.19 ± 0.48 Z = 5.846, P < 0.0001
Bamboo density (plants/m 2) 1 × 1 m 4.43 ± 4.37 3.85 ± 5.19 Z = 1.007, P = 0.314
Herbage height (cm) 1 × 1 m Draft41.01 ± 23.59 38.70 ± 16.68 F = 0.203, P = 0.654
Herbage density (plants/m 2) 1 × 1 m 4.41 ± 2.49 4.33 ± 2.97 Z = 0.552, P = 0.581
Decid. leaf cover (%) 1 × 1 m 48.07 ± 20.78 80.93 ± 14.08 F = 52.566, P < 0.0001
Decid. leaf thickness (cm) 1 × 1 m 6.18 ± 3.38 17.56 ± 24.88 Z = 5.779, P < 0.0001
Stone cover (%) 1 × 1 m 58.48 ± 35.21 2.78 ± 2.25 Z = 6.185, P < 0.0001
Table 2 Stepwise discriminant analysis of microhabitat variables of two Cuora spp.
Variable No. Variables Discriminant coefficients Wilk’s λ F P
1 Stone cover 0.962 0.51 66.29 < 0.0001
2 Slope gradient 0.424 0.463 39.429 < 0.0001
3 Bamboo density 0.36 0.432 29.382 < 0.0001
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Table 3 Difference in the morphology of two Cuora spp. Values indicate the mean ± SD
measurements for each species, as well as the statistical significance of their difference (one way
ANOVA for carapace length and one way ANCOVA for the remaining parameters, with carapace
length as the covariate).
C. mouhotii C. galbinifrons F P
Sample size 18 13
Carapace length (mm) 162.91 ± 18.47 168.77 ± 8.61 1.12 0.298
Carapace width (mm) 114.24 ± 7.82 113.98 ± 4.73 1.77 0.194
Carapace height (mm) 65.81 ± 4.19 79.26 ± 3.67 101.53 < 0.0001
Carapace height / width 0.58 ± 0.03 Draft 0.70 ± 0.03 113.65 < 0.0001
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Figure Legends
Fig. 1 The shared macrohabitat and divergent preferred microhabitats of two sympatric Cuora spp . in Hainan, China. A, the high canopy cover habitat shared by both C. mouhotii and C. galbinifrons . B, the preferred microhabitat of C. mouhotii (rock crevices) , with a turtle visible in the crevice (arrow). C, the preferred microhabitat of C. galbinifrons (deciduous leave litter under bamboo clumps), with a turtle visible under the leaf litter (arrow). All photos were taken by F.
Xiao.
Fig. 2 Distribution of the quadrats on the DFA axis. Draft
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Draft
Fig. 1 The shared macrohabitat and divergent preferred microhabitats of two sympatric Cuora spp. in Hainan, China.
382x821mm (300 x 300 DPI)
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Draft
Fig. 2 Distribution of the quadrats on the DFA axis
105x75mm (300 x 300 DPI)
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