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Eur J Wildl Res DOI 10.1007/s10344-012-0641-3 ORIGINAL PAPER Influence of population density on group sizes in goitered gazelle (Gazella subgutturosa Guld., 1780) David Blank & Kathreen Ruckstuhl & Weikang Yang Received: 7 January 2012 / Revised: 20 March 2012 / Accepted: 22 May 2012 # Springer-Verlag 2012 Abstract We conducted our study in Ili depression, south-eastern Kazakhstan during 1981–1989 to investigate how group sizes and group class frequencies change with increasing population densities in goitered gazelles. In addition, we compared our study to data on group size and group class frequency of various goitered gazelle populations in Kazakhstan with very variable population densities. We found that mean group size was a more variable index than group class frequency. Population density had some effect on mean group sizes, but the strength of the influence was quite weak, and only in cases where densities of two populations varied more than sevenfold did group sizes start to change. Group class frequency was not correlated with population density at all. The impact of the yearly breeding cycle on group size was bigger than population density. The density-dependent response of goitered gazelle population was curvilinear in fashion, and it may be classified as Communicated by P. Acevedo D. Blank (*) : W. Yang Key Laboratory of Biogeography and Bioresource in Arid Land, Xinjiang Institute of Ecology and Geography, The Chinese Academy of Sciences, Urumqi 830011, China e-mail: [email protected] D. Blank Institute of Zoology, Kazakh Academy of Sciences, Alma-Ata, Kazakhstan K. Ruckstuhl Department of Biological Sciences, University Calgary, Calgary, Canada K. Ruckstuhl Zoology Department, University of Cambridge, Cambridge, UK intermediate between social-dwelling ungulate species, living in large groups and demonstrating continuous (linear) increases of group size with population density and those that are solitary or territorial ungulate species with no relationship between population size and group size, though the goitered gazelle population’s weak response was distinctively closer to the one of solitary ungulate species. Keywords Goitered gazelle . Group size class . Mean group size . Population density Introduction Density dependence is a key concept in population dynamics because it determines resource availability and the partitioning of food among individuals. Most studies on density dependence deal with physical conditions, growth, births and mortality rates (Caughley 1970; Kie et al. 1980; Skogland 1983, 1985). Less often, researchers considered the influence of population density on ungulate social behaviour (Berger 1978; Fowler 1987). Obviously, the available forage biomass declines with increasing ungulate density, and per capita food intake declines with decreasing availability (Wickstrom et al. 1984). Increasing density should force ungulates to change their behaviour and, in the first place, group size and sometimes the whole social structure, due to scramble competition over limited food supplies. Various ungulate species live in different habitats, have different body sizes, variable diets and feeding styles and, as a consequence, have different group sizes or social structures (Brashares et al. 2000; Jarman 1974). Environmental factors have been considered as a key factor in explaining ungulate social organisation, although other factors, such as predation risk, reproductive strategies and social affinities, have Eur J Wildl Res been identified as being equally important in shaping group types and sizes (Bon et al. 2001; Hamilton 1971; Roberts 1996; Underwood 1982). According to the optimal group size hypothesis, every individual prefers to be in a group whose size is a close as possible to the value that maximises its vital physiological and social requirements (Pepin and Gerard 2008). Groups of some animals (cetaceans, proboscideans and many primates) are quite stable, and their mean group size is independent of population density (Dittus 1987; Henzi et al. 1997; Lehmann and Boesch 2004; Karczmarski et al. 2005; Wittemyer et al. 2005), while others (some ruminants and kangaroos) form fission–fusion groups, whose size is very sensitive to population density (Gerard et al. 2002) and their group sizes increase with population density (Barrette 1991; Lawes and Nanni 1993; Taylor 1983; Toigo et al. 1996; Wirtz and Lorscher 1983). Fowler (1987) considered the density dependence of group sizes in 21 large herbivores species and reported that 17 of them had such dependence. He supported the idea of Eberhardt (1977) that group sizes had different sensitivities to changes in population density. The group size and its size frequency in some social-dwelling deer species (Axis axis, Capreolus capreolus, Cervus nippon, Cervus elaphus and Dama dama), which prefer to stay in large groups (3–6 and even larger size), were positively affected by population density (Barrette 1991; Borkowski 2000; Hebblewhite and Pletscher 2002; Stuwe and Hendrichs 1984; Thirgood 1996; Vincent et al. 1995). An increase in group size with population density has also been reported for some African antelopes (Kobus and Redunca—Spinage 1969; Wirtz and Lorscher 1983). Alpine ibex (Capra ibex) and chamois (Rupicapra pyrenaica), which usually live in large groups of 3–10 individuals (Lovari and Consentino 1986; Toigo et al. 1996), also have a distinct tendency of group sizes to increase with population density (Pepin and Gerard 2008; Toigo et al. 1996). Peccaries (Tayassu pecari), which are usually very social ungulates living in large groups of 20–300 individuals, decreased their group size with decreasing population density (Reyana-Hurtado et al. 2009). In contrast, moose (Alces alces) did not show such density dependence on group sizes, and in Sweden, home range sizes were not affected by differences in moose density (Sweanor and Sandegren 1989). Comparison of various populations of different subspecies of moose in Alaska, Minnesota and Montana demonstrated that group size varied seasonally according to the yearly breeding cycle, and though the largest group sizes did occur with the densest populations, density did not appear to influence trends in aggregation sizes through the year (Peek et al. 1974). The oribi (Oerebia ourebia) is a territorial antelope with small group sizes, which may increase with population density (Arcese et al. 1995), but within very limited changes (Rowe-Rowe et al. 1992). Gorals (Nemorhaedus goral), duiker (Sylvicapra grimmia) and steenbok (Raphicerus camelus) are not particularly social and predominantly solitary species, and their group size is very stable and does not change over the seasons (Bergstrom and Skarpe 1999; Pendharkar and Goyal 1995). From this, it becomes clear that population density is more likely affecting group size in social species, which form mostly large groups and even huge aggregation under some conditions, whereas such impacts would be weak for ungulates that prefer a solitary lifestyle or are staying in small groups. Goitered gazelles (Gazella subgutturosa Guld., 1780) are able to gather in groups of several tens of individuals, though singletons and small groups (<4 individuals) are more typical than large herds (Zhevnerov et al 1983; Blank 1990, 1992). Males of goitered gazelles have individual territories, but only during the rutting period (November– December), whereas the rest of the year they roam freely all over their home range and gather in groups of adult and yearling males, which were expelled by territorial males from the family groups during the rutting season (Blank 1998). Females leave their herds and stay alone for giving birth (May), but they gather into groups several weeks after the appearance of offspring and later they unite into larger groups including several mothers and their young (Blank 1986). Mixed-sex groups are uncommon for goitered gazelles during most of the year and only during migration in March–April and October do they gather in large mixed-sex groups of several hundred individuals (Blank 1990, 1992). Goitered gazelles may thus change their group size according to their biological life cycle, with most females staying solitary during birthing in spring and males being solitary during the rut in autumn–winter but gathering in especially large herds for migrations twice a year (Blank 1992). Therefore, goitered gazelles have a wide span of group sizes from singletons to herds of several tens, occupying an intermediate position between social-dwelling species, living in large groups and exclusively solitary or territorial species, for which large groups are atypical. That is why the consideration of relationship between group size and population density in goitered gazelles would be especially interesting. We thus hypothesise that goitered gazelle groups sizes will grow with increasing population density and that the frequency of such large groups will increase as well. The Kapchagaj population of goitered gazelle is protected from poaching and therefore demonstrated the highest indices of population density (Tables 1 and 2). Other populations in Kazakhstan have virtually no protection from poaching and have thus suffered substantial losses due to poaching (Blank 1990), and as a result, they occur at uncharacteristically low densities (Tables 4 and 5). In fact, we reported before (Blank 1990) that poaching was the single-most factor explaining the striking differences in population densities between the Eur J Wildl Res protected Kapchagaj population and the other desert populations. Because of the severe effect of poaching on gazelle densities, we did not consider the impact of rainfalls and consequently plant densities, composition and biomass on goitered gazelle group size and its frequencies. We thus were primarily interested in testing if mean group size and frequency of gazelle groups would correlate with their local population densities. Materials and methods We conducted the study on goitered gazelles living in the Kapchagaj Nature Reserve (Ili depression, south-eastern Kazakhstan) from 1981 to 1989. This area is now within the Altyn-Emel National Park, with a size of 4,600 km2. Periodically, additional population censuses were carried out in various deserts of Kazakhstan (Aktau, 300 km2; Panfilov Karakum, 250 km2; Boguty, 1,200 km2; Taukum, 8,000 km2; Saryishikotrau, 24,000 km 2; Muunkum, 37,500 km2; Betpakdala, 75,000 km 2—Skotselias 1995). We used two kinds of indices: mean group size (number observed individuals per encountered group) and group size class frequency (10 classes from 1 to 10 individuals and the 11th class for groups that are larger in size than 10 individuals). For checking our hypothesis, we compared the number of two types of groups with each other: singletons and groups from 2 to 4 individuals. We proceeded from two assumptions. (1) If the group size of goitered gazelle increases with density, then firstly, the number of singletons are expected to decrease and the number of small groups (2–4 individuals) to increase, as was observed for sika deer, for example (Borkowski 2000); and (2) singletons and small groups (2–4 individuals) are the most numerous types of groups in gazelle population in Kapchagaj reserve (X±SE086.9 %±1.2, n027) and in other deserts (X±SE093.5 % ±2.5, n013). We collected two data sets to verify our hypothesis that population density affects average group sizes. Firstly, we used the data from our long-term study of goitered gazelle ecology and behaviour in the Kapchagaj Reserve, when this population increased their density (from 1.63 to 2.75 gazelles per 1 km2) between 1981 and 1989 (Tables 1 and 2). Secondly, we used the census data from the various desert populations, which turned out to have very different population densities (from 0.05 to 2.75 gazelles per 1 km2— Tables 4 and 5). Values of the goitered gazelle density in various deserts were taken from published materials (Blank 1990; Blank and Kovshar 1988; Kovshar and Blank 1986). In the Kapchagai Nature Reserve, we counted gazelles along pedestrian transects (total of 2,000 km) and car routes (10,000 km). Gazelle counts were done once every month. To avoid re-sampling, the same individual during a census we used the following method. We did a south–north parallel transects every 5 km, which covered the whole study area (24 transects, between 8 and 20 km in length each). We moved not more than 20 km/h (vehicle) from west to east along transects, stopped every 3 km and counted gazelles along transects from each side forward using binoculars (magnification ×8) and telescopes (magnification ×30, ×60), but did not count any on the way back when crossing an already sampled area. During focal observations, we always moved the telescope clockwise and registered antelopes within distances of 0.5 km. In other deserts, we used the same method of counting along parallel transects every 15–20 km (along existing roads in the sandy deserts) covering different areas within every desert and stopping every 3 km for sampling gazelles from elevated watch points and registered all visible ungulates within distances of 0.5 km. According to our estimations, we sampled more than 80 % of the entire gazelle population in the Kapchagaj Nature Reserve, Aktau, Panfilov Karakum and Boguty, while not more than 25 % of the population was sampled in Taukum, Saryishikotrau, Muunkum and Betpakdala because of the huge sizes of these deserts. It is possible that the gazelle populations of the Aktau have some kind of limited connections with the Kapchagaj Nature Reserve population, while gazelle population from other deserts do not have any connections with each other at all. During scans, we recorded the number, size and location of groups. Gazelles were noted as member of a group if they were <50 m from each other, moved in the same direction and stayed together longer than half an hour. These are measures commonly used in defining groups of ungulates (Ruckstuhl 1998). The differences in mean group size over years were tested with one-way ANOVA, as tests for normality of these data were satisfied (Kolmogorov–Smirnov test). Fisher’s least significant differences post hoc comparison was used to compare means between separate pairs of values (subgroups). In addition, we used a general linear model (GLM, type 4 sum of squares) to test the impact of month and density on group size of the gazelle population in Kapchagaj Nature Reserve and other deserts. In addition, we performed independentsample t tests for comparing group size for pairs of different goitered gazelle populations, living in different deserts. We used chi-square-goodness-of-fit tests to analyse changes in the frequency of various group sizes under different population densities. Results Kapchagaj Reserve population We found significant monthly variability for mean group size and group class frequency between 1982 and 1985 (one-way ANOVA, Eur J Wildl Res Table 1 Mean group size of goitered gazelle population in the Kapchagaj Nature Reserve enlarging its population density (individuals per square kilometer) over years Years–density ind/km2 Months April May June July September November 1981–1.53 – – – – – N0230 3.23±0.20 1982–1.65 N0256 2.53±0.15 N0211 2.26±0.14 – N0186 2.89±0.17 N053 2.30±0.16 N091 2.55±0.17 1983–1.74 – N0211 2.23±0.11 N0286 N085 1.73±0.14 N0433 – – – N0162 – – 2.14±0.12 2.29±0.10 2.09±0.10 N0516 N0412 N0752 N0760 N0254 N0313 3.96±0.23 2.45±0.12 2.28±0.08 2.71±0.09 3.71±0.25 2.88±0.16 – N0328 N0439 2.26±0.09 N0853 2.39±0.14 2.35±0.08 – – – ANOVA F00.998 df04 P>0.05 1984–1.69 1985–1.83 1986–2.15 1987–2.29 t test F013.537 N0285 1.76±0.08 N01037 1.83±0.07 ANOVA F06.872 df0771 P<0.0001 df05 P<0.0001 1989–3.67 Significance of difference F021.902, df05, P<0.000), though such differences were much less significant for 1982 (one-way ANOVA, F02.289, df05, P00.044). We thus decided to only compare yearly indices of the same calendar month for our analyses. The Kapchagaj population with the highest density of goitered gazelle in Kazakhstan had the following distribution December N0127 2.80±0.21 – N0429 N0306 2.85±0.13 – 3.89±0.25 – – – ANOVA F06.633 N01517 3.29±0.12 ANOVA F05.252 ANOVA F05.422 ANOVA F08.269 df03 P<0.0001 df02 P00.005 df03 P00.001 df02 P<0.0001 N0217 4.29±0.39 of group class frequency (Fig. 1). Singletons were the most often noted kind of group, followed by herds of two and three individuals (chi-square test, χ2 04.587, df01, P00.032 and χ2 010.286, df01, P00.001). The proportion of bigger size groups was considerably smaller, decreasing continuously from 7.5 % (groups from 4 individuals or class 4) to 0.1 % Table 2 Proportion (%) of singletons to groups (2–4 individuals) of the goitered gazelle in the Kapchagaj Nature Reserve population enlarging its population density (individuals per square kilometer) over years Years–density ind/km2 Months April May June July September 24/61 30/64 1981–1.53 November December 33/46 1982–1.65 46/39 50/40 1983–1.74 43/48 59/38 1984–1.69 54/38 43/50 46/49 1985–1.83 28/48 42/45 49/42 32/57 30/46 40/44 40/52 36/57 38/48 36/39 1.254-0.001 0.592-0.045 1986–2.15 1987–2.29 60/36 1989–3.67 Chi-square (χ2) df01 P value 58/33 35/48 37/46 27/50 70/23 41/43 7.332 208.957–0.580 96.481-0.662 1 P00.000–0.416 P00.000–0.104 P00.000–0.650 P00.263–0.980 P00.442–0.832 Totally 6 of 10 cases were P<0.05 Totally 4 of 6 cases were P<0.05 Totally 2 of 3 cases were P<0.05 Totally 6 cases were P>0.05 Totally 3 cases were P>0.05 P00.000-0.446 0.007 Totally 12 of 15 cases were P<0.05 25.154-2.636 12.145-0.206 Eur J Wildl Res 45 Groups 40 Portion in percentage, % Fig. 1 Portion of different group-size classes in goitered gazelle among all observed groups (groups) and number of individuals observed inside of every class (individuals) Individuals 35 30 25 20 15 10 5 0 1 2 3 4 5 6 7 8 9 10 >10 Group size (from 10 individuals or class 10) and, after that, an abrupt increase in the portion of groups of more than 10 individuals (from 0.1 to 2.4 %—Fig. 1). In regards to the proportion of individuals staying in the different group size classes, most gazelles were found to form groups of 3 individuals (17.7 % of all gazelles), 2 (16.6 %), groups of more than 10 individuals (15 %) or remain as singletons (14.3 %). These portions were not significantly different from each other (chi-square-goodnessof fit test, χ2 00.625, df03, P00.891). The rest of the gazelles stayed in groups of 4–10 individuals (10.8 % of gazelles stay in groups of 4 and 2.3 % in groups of 10 individuals). Changing characteristics of the Kapchagaj Reserve population over years The mean group size of the Kapchagaj Reserve population varied significantly over the years for all checked months, except for June (Table 1). In April and December, mean group sizes increased over the years, whereas in May, they generally had a decreasing trend. During July, September and November, mean groups size increased and decreased without any clear trend (Table 1, r0−0.19, N08, P00.964). Neither population density (GLM, F00.465, df07, P00.843) nor month significantly affected mean group size (GLM, F00.339, df06, P00.155). Unlike mean group size, which significantly changed over the years for all months, the group size class frequency in the Kapchagaj Reserve population varied significant only in some cases (25 of 44 cases, Table 2). However, the correlation between population density and proportion of singletons and groups was significant only for May and partially for December (Table 3). There was no effect of density and month on group size frequency (GLM, F00.707, df07, P00.668 and F00.339, df06, P00.903, respectively). Comparing characteristics of the gazelle populations of different deserts The mean group size of the goitered gazelle populations of various deserts with different population densities only significantly differed between some cases but not others (Table 4); there were no differences for desert comparisons (GLM, F02.232, df016, P00.353). The high-density population of Kapchagaj Reserve had the same mean group sizes as the considerably lowerdensity populations of Aktau and Boguty and other comparisons yielded similar results despite having different population densities (Taukum, Saryishikotrau, Muunkum and Betpak-Dala). Only Panfilov Karakum, Taukum and sometimes the Boguty populations had significantly smaller group sizes compared to the Kapchagaj population, which had the highest density among all of them (Table 4). Group size only changed when population densities were more than seven times higher, whereas there were no significant changes in mean group sizes if population densities did not get above this value. In regards to the frequency of occurrence of different group size classes occurring at different population density, the only effect was found for populations which had difference in density of more than seven times larger that typically observed (Table 5). The only one case of comparison of Aktau and Panfilov Karakum was an exception to this. GLM analyses demonstrated insignificant impact of density on singletons (GLM, F03.047, df017, P00.375) and a low effect of density on groups of two to four individuals (GLM, F0403.645, df017, P00.032). Discussion Our results demonstrated that group sizes in goitered gazelles are highly variable across seasons and years. This variability is mostly driven by their breeding cycle, when females prefer to stay alone during the birthing period in May–June and males protect their individual territories during the rutting period in November–December. As a result of these two events, mean group sizes decrease considerably during these seasons, especially distinctively during the birthing period (Blank 1986; 1998). The group size frequency in the Kapchagaj population was the following: Eur J Wildl Res Table 3 Pearson Correlation index for proportion (singletons/groups of 2–4 gazelles) changing in the Kapchagaj population over years Months Group size Pearson index, N and P May Singletons 0.855, N06, P00.030 Groups −0.852, N06, P00.031 June Singletons Groups 0.083, N05, P00.894 −0.265, N05, P00.666 July Singletons −0.084, N04, P00.916 September Groups Singletons 0.117, N04, P00.883 0.680, N03, P00.524 Groups −0.995, N03, P00.061 November Singletons 0.514, N04, P00.486 December Groups Singletons 0.471, N04, P00.529 −0.448, N03, P00.704 Groups −1.000, N03, P00.013 The portion of singletons as a kind of group was more than others and groups of two to four individuals followed after singletons. Most gazelles stayed in groups of one to three individuals. The bigger groups were noted much less often decreasing their frequency with enlarging group sizes. We thus conclude that goitered gazelles seem to prefer smaller groups, and those singletons are the most typical kind of group found for this species. Such group sizes are likely due to the Table 4 The mean group size of goitered gazelle among populations in various deserts with different density arid environment goitered gazelles live in and the sparse distribution of their forage. This is likely why Jarman (1974) classified all gazelles as animals with small to mid-size groups. Other authors also reported that goitered gazelles prefer to stay in small groups in Saudi Arabian hot deserts (Cunningham and Wronski 2011b) and in Central Asian’s cold arid areas (Qiao et al. 2011). Our results showed that mean group sizes changed significantly over the years for all checked months; however, these changes were not correlated with increasing population density in the Kapchagaj population. The group size class frequency also did not change with the population density except for in May. Moreover, in contrast to our expectation, the portion of singletons increased and the number of groups of two to four individuals each decreased with increasing of the population density during May (Table 3). This pattern was completely contrary to our hypothesis of group size increasing with population density. However, as mentioned above goitered gazelles preferred to stay in small groups because of sparse distribution of their forage which limits group size. Our results thus confirm Krause and Ruxton’s (2002) prediction that median group sizes initially increase with population density, until the preferred group size is reached, and that a further increase in the population density will subsequently lead to higher numbers of Populations Density ind/km2 Mean group size ind per group N t test P value Kapchagaj Reserve Aktau (12.1986) Kapchagaj Reserve Aktau (06.1987) 2.15 0.30 2.29 0.54 3.55±0.266 4.00±0.712 2.39±0.144 2.56±0.287 221 34 853 54 F00.943 df0243 F00.185 df0905 0.333 Kapchagaj Reserve PanfilovKarakum Aktau PanfilovKarakum Kapchagaj Reserve Boguty1 (05.1987) Kapchagaj Reserve Boguty1 (06.1987) Kapchagaj Reserve Boguty2 (08.1989) Kapchagaj Reserve Taukum (12.1982) Taukum (09.1988) Chu Muunkum Taukum (07.1983) Saryishikotrau Chu Muunkum Taukum Saryishikotrau 2.29 0.35 0.54 0.35 2.29 0.23 2.29 0.23 3.67 0.27 1.65 0.22 0.31 0.17 0.22 0.07 0.10 0.18 0.08 2.39±0.144 1.33±0.106 2.56±0.287 1.33±0.106 2.39±+0.144 1.73±+0.080 2.39±0.144 1.41±0.113 3.17±0.119 2.81±0.322 4.05±0.266 2.62±0.299 1.88±0.147 1.90±0.204 1.50±0.107 1.70±0.300 1.90±0.246 1.73±0.159 1.95±0.223 853 39 54 39 853 234 853 56 1506 48 266 31 69 24 46 19 20 30 22 F04.259 df0890 F019.323 df091 F011.839 df01085 F05.261 df0907 F01.872 df01552 F06.763 df0295 F00.372 df087 ANOVA F02.770 df02 ANOVA F00.532 0.039 BetpakDala (09.1986) 0.05 2.00±0.246 12 df02 0.667 0.000 0.001 0.022 0.171 0.010 0.543 0.155 0.591 Eur J Wildl Res Table 5 Group size classes of the goitered gazelle in various populations with different density Populations Density ind/ km2 Singles/ Chi-square (χ2) groups Kapchagaj Reserve Aktau (12.1986) 2.15 0.30 32/44 44/29 18.021, df01, P00.000 Kapchagaj Reserve Aktau (06.1987) 2.29 0.54 58/33 41/46 38.574, df01, P00.000 Kapchagaj Reserve PanfilovKarakum 2.29 0.35 58/33 74/26 584.616, df01, P00.000 Aktau PanfilovKarakum 0.54 0.35 41/46 74/26 18.062, df01, P00.000 Kapchagaj Reserve Boguty1 (05.1987) 2.29 0.23 58/33 62/35 263.108, df01, P00.000 Kapchagaj Reserve Boguty1 (06.1987) 2.29 0.23 58/33 71/27 518.423, df01, P00.000 Kapchagaj Reserve Boguty2 (08.1989) 2.75 0.27 41/43 38/46 7.040, df01, P00.008 Kapchagaj Reserve Taukum (12.1982) 1.65 0.22 29/40 39/42 11.481, df01, P00.001 Taukum - (09.1988) Muunkum 0.31 0.17 48/49 40/60 2.391, df01, P00.122 Taukum 0.22 Saryishikotrau (07.1983) 0.07 63/37 50/50 3.130, df01, P00.077 Taukum Muunkum (07.1983) 0.22 0.10 63/37 45/50 2.000, df01, P00.157 Saryishikotrau Muunkum 0.07 0.10 50/50 45/50 0.053, df01, P00.819 Taukum 0.18 Saryishikotrau (09.1986) 0.08 50/50 42/58 0.519, df01, P00.471 Taukum BetpakDala (09.1986) 0.18 0.05 50/50 33/67 3.320, df01, P00.068 Saryishikotrau BetpakDala (09.1986) 0.08 0.05 42/58 33/67 0.712, df01, P00.399 groups, but not to further increases in group size. Mean group size and group size class differed significantly between May and other months, with a considerable decrease with population density. During birthing in May, most females leave their herds and stay alone for several weeks. In addition, the high level of synchronisation of birthing, with most females giving birth within a few days of each other is very typical for this species (Blank 1986). Since goitered gazelles have female-skewed populations where the portion of females in the population may exceed 60 % of the entire population (Zhevnerov et al. 1983), and in May, when most females leave their groups, mean group size and group size frequency of the whole population decreases considerably. Comparisons of various populations with different densities revealed that mean group sizes did not change within a wide range of densities, and started to change only when such differences were considerable. It was clear from these results that group sizes changed with population density in a non-linear or abrupt fashion. Such a non-linear density-dependent response of group sizes was found in other animals, where group size varied with the square root of population density (for fishes, Bonabeau and Dagorn 1995; Gueron and Levin 1995) or in a logarithmic fashion [for red kangaroo (Macropus rufus), Johnson 1983, and for Alpine ibex (Capra ibex), Toigo et al. 1996), and a threshold (abrupt) or curvilinear response was found for population growth rates in some African antelopes (Tragelaphus, Connochaetes, Owen-Smith 2006). The group size class frequencies showed the same pattern as for mean group size with the same sevenfold difference threshold for population density, with the exception of the AktauPanfilov Karakum population. We did not find any correlation pattern for group class frequencies entirely. Our research indicates that mean group sizes were more variable than group size class frequencies, likely because goitered gazelles preferred to be alone or stay in small groups within very wide ranges of population densities. The group size frequencies had no correlation with population density or if they had it was completely the opposite of what we had hypothesised, when the singleton portion increased and group (from 2–4 individuals) frequency decreased with rising population density during birthing period in May. This means that the impact of the breeding cycle and especially the birthing period is more distinctive in goitered gazelle than the impact of population density, as most females continued to stay alone during birthing and most males led a solitary lifestyle during the rut in the condition independent of the population density. Thus, population of the goitered gazelle did not show a density-dependent response and behaved as an ungulate species with a solitary lifestyle would be expected to behave. A similar social structure was found for Arabian sand gazelle (Gazella subgutturosa marica), which mainly formed small groups in Saudi Arabia (Cunningham and Wronski 2011a), and Gazella gazella farasani (Cunningham and Wronski 2011b) with their mainly solitary lifestyle and for forest antelope species from the genus Tragelaphus, which also have an “almost solitary” lifestyle (Wronski et al. 2009). Various factors, other than population density, will also affect group sizes: Human hunting pressure, for example, can lead to an increase in group sizes (probably because animals feel safer in larger numbers) regardless of density (Jedrzejewski et al. 2006). Predominantly solitary gorals formed groups of more than 10 individuals under habitat disturbance (Pendharkar and Goyal 1995). However, in mountain gazelles (Gazella gazella), human disturbance had opposite effects and lead to a decrease in mean group sizes of this species (Manor and Saltz 2003). The openness of the habitat (Estes 1974; Korte 2008; Walther 1972), food abundance (Borkowski 2000; Elgar 1989; Raman 1996; Rowe-Rowe et al., 1992) or snow depth have also been found to positively correlate with group size (Maruyama, 1981; Peek et al., 1974), and the reproductive cycle and Eur J Wildl Res even daily events can also significantly affect group sizes (Jedrzejewski et al. 2006). Conclusions To conclude, we found that mean group size in goitered gazelles increased with population density in a non-linear and abrupt fashion, and significant responses of group sizes was found only for populations with more than sevenfold difference in population density. Group class frequency was not correlated with population density at all. Such a densitydependent response of goitered gazelle population may be classified as intermediate between social-dwelling ungulate species, demonstrating continuous and even linear increases of group size with population density and solitary and territorial ungulate species that have no such response at all. However, the goitered gazelle population response to increasing density is more akin to that found in solitary ungulate species than that found in social ungulates. Acknowledgment We are grateful to the International Science & Technology Cooperation Program of China (2010DFA92720), the Chinese Academy of Sciences (Visiting Professorships for Senior International Scientists—2009Z2-5), the Chinese Academy of Sciences Xi Bu Zhi Guang (LHXZ200701), and SINO-UAE Cooperation Project (0866031) for granting our work and creating all conditions for writing this paper. 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