Influence of population density on group sizes in goitered gazelle (Gazella subgutturosa Guld., 1780)

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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,                    intermediate between social-dwelling ungulate species, living
south-eastern Kazakhstan during 1981–1989 to investigate              in large groups and demonstrating continuous (linear)
how group sizes and group class frequencies change with               increases of group size with population density and those that
increasing population densities in goitered gazelles. In              are solitary or territorial ungulate species with no relationship
addition, we compared our study to data on group size                 between population size and group size, though the goitered
and group class frequency of various goitered gazelle                 gazelle population’s weak response was distinctively closer to
populations in Kazakhstan with very variable population               the one of solitary ungulate species.
densities. We found that mean group size was a more
variable index than group class frequency. Population                 Keywords Goitered gazelle . Group size class . Mean group
density had some effect on mean group sizes, but the                  size . Population density
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                 Introduction
class frequency was not correlated with population den-
sity at all. The impact of the yearly breeding cycle on               Density dependence is a key concept in population dynamics
group size was bigger than population density. The                    because it determines resource availability and the partitioning
density-dependent response of goitered gazelle population             of food among individuals. Most studies on density
was curvilinear in fashion, and it may be classified as               dependence deal with physical conditions, growth, births
                                                                      and mortality rates (Caughley 1970; Kie et al. 1980; Skogland
Communicated by P. Acevedo                                            1983, 1985). Less often, researchers considered the influence
D. Blank (*) : W. Yang
                                                                      of population density on ungulate social behaviour (Berger
Key Laboratory of Biogeography and Bioresource in Arid Land,          1978; Fowler 1987). Obviously, the available forage biomass
Xinjiang Institute of Ecology and Geography,                          declines with increasing ungulate density, and per capita food
The Chinese Academy of Sciences,                                      intake declines with decreasing availability (Wickstrom et al.
Urumqi 830011, China
e-mail: blankdavidalex@yahoo.com
                                                                      1984). Increasing density should force ungulates to change
                                                                      their behaviour and, in the first place, group size and
D. Blank                                                              sometimes the whole social structure, due to scramble
Institute of Zoology, Kazakh Academy of Sciences,                     competition over limited food supplies. Various ungulate
Alma-Ata, Kazakhstan
                                                                      species live in different habitats, have different body sizes,
K. Ruckstuhl                                                          variable diets and feeding styles and, as a consequence, have
Department of Biological Sciences, University Calgary,                different group sizes or social structures (Brashares et al.
Calgary, Canada                                                       2000; Jarman 1974). Environmental factors have been
                                                                      considered as a key factor in explaining ungulate social
K. Ruckstuhl
Zoology Department, University of Cambridge,                          organisation, although other factors, such as predation
Cambridge, UK                                                         risk, reproductive strategies and social affinities, have
Eur J Wildl Res

been identified as being equally important in shaping             1992). Gorals (Nemorhaedus goral), duiker (Sylvicapra
group types and sizes (Bon et al. 2001; Hamilton 1971;            grimmia) and steenbok (Raphicerus camelus) are not
Roberts 1996; Underwood 1982).                                    particularly social and predominantly solitary species,
   According to the optimal group size hypothesis, every          and their group size is very stable and does not change
individual prefers to be in a group whose size is a close as      over the seasons (Bergstrom and Skarpe 1999; Pendharkar
possible to the value that maximises its vital physiological      and Goyal 1995). From this, it becomes clear that population
and social requirements (Pepin and Gerard 2008). Groups of        density is more likely affecting group size in social species,
some animals (cetaceans, proboscideans and many primates)         which form mostly large groups and even huge aggregation
are quite stable, and their mean group size is independent of     under some conditions, whereas such impacts would be weak
population density (Dittus 1987; Henzi et al. 1997; Lehmann       for ungulates that prefer a solitary lifestyle or are staying in
and Boesch 2004; Karczmarski et al. 2005; Wittemyer et al.        small groups.
2005), while others (some ruminants and kangaroos) form              Goitered gazelles (Gazella subgutturosa Guld., 1780) are
fission–fusion groups, whose size is very sensitive to            able to gather in groups of several tens of individuals,
population density (Gerard et al. 2002) and their group sizes     though singletons and small groups (
Eur J Wildl Res

protected Kapchagaj population and the other desert              we used the following method. We did a south–north parallel
populations. Because of the severe effect of poaching            transects every 5 km, which covered the whole study area (24
on gazelle densities, we did not consider the impact of          transects, between 8 and 20 km in length each). We moved not
rainfalls and consequently plant densities, composition          more than 20 km/h (vehicle) from west to east along transects,
and biomass on goitered gazelle group size and its               stopped every 3 km and counted gazelles along transects from
frequencies. We thus were primarily interested in testing        each side forward using binoculars (magnification ×8) and
if mean group size and frequency of gazelle groups               telescopes (magnification ×30, ×60), but did not count any on
would correlate with their local population densities.           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
Materials and methods                                            deserts, we used the same method of counting along parallel
                                                                 transects every 15–20 km (along existing roads in the sandy
We conducted the study on goitered gazelles living in the        deserts) covering different areas within every desert and
Kapchagaj Nature Reserve (Ili depression, south-eastern          stopping every 3 km for sampling gazelles from elevated
Kazakhstan) from 1981 to 1989. This area is now within           watch points and registered all visible ungulates within
the Altyn-Emel National Park, with a size of 4,600 km2.          distances of 0.5 km. According to our estimations, we
Periodically, additional population censuses were carried out    sampled more than 80 % of the entire gazelle population in the
in various deserts of Kazakhstan (Aktau, 300 km2; Panfilov       Kapchagaj Nature Reserve, Aktau, Panfilov Karakum and
Karakum, 250 km2; Boguty, 1,200 km2; Taukum, 8,000 km2;          Boguty, while not more than 25 % of the population
Saryishikotrau, 24,000 km 2; Muunkum, 37,500 km2;                was sampled in Taukum, Saryishikotrau, Muunkum and
Betpakdala, 75,000 km 2—Skotselias 1995). We used                Betpakdala because of the huge sizes of these deserts. It
two kinds of indices: mean group size (number observed           is possible that the gazelle populations of the Aktau
individuals per encountered group) and group size class          have some kind of limited connections with the Kapchagaj
frequency (10 classes from 1 to 10 individuals and the           Nature Reserve population, while gazelle population from
11th class for groups that are larger in size than 10            other deserts do not have any connections with each other at
individuals). For checking our hypothesis, we compared           all.
the number of two types of groups with each other:                   During scans, we recorded the number, size and location
singletons and groups from 2 to 4 individuals. We                of groups. Gazelles were noted as member of a group if they
proceeded from two assumptions. (1) If the group size            were
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            December

1981–1.53                          –                  –                –                  –                     –                 N0230
                                                                                                                                  3.23±0.20
1982–1.65                          N0256              N0211            –                  N0186                 N053              N091
                                   2.53±0.15          2.26±0.14                           2.89±0.17             2.30±0.16         2.55±0.17
1983–1.74                          –                  N0211            N085               –                     –                 –
                                                      2.23±0.11        1.73±0.14
1984–1.69                                             N0286            N0433              N0162                 –                 –                   N0127
                                                      2.14±0.12        2.29±0.10          2.09±0.10                                                   2.80±0.21
1985–1.83                          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
1986–2.15                                             –                N0328              N0439                 –                 N0429               N0306
                                                                       2.26±0.09          2.35±0.08                               2.85±0.13           3.89±0.25
1987–2.29                                             N0285            N0853              –                     N0217             –                   –
                                                      1.76±0.08        2.39±0.14                                4.29±0.39
1989–3.67                                             N01037           –                  –                     N01517            –                   –
                                                      1.83±0.07                                                 3.29±0.12
Significance of difference         t test             ANOVA            ANOVA              ANOVA                 ANOVA             ANOVA               ANOVA
                                   F013.537           F06.872          F00.998            F06.633               F05.252           F05.422             F08.269
                                   df0771             df05             df04               df03                  df02              df03                df02
                                   P
Eur J Wildl Res

Fig. 1 Portion of different                                                 45
group-size classes in goitered
                                                                            40                                                     Groups   Individuals
gazelle among all observed
groups (groups) and number of                                               35

                                                 Portion in percentage, %
individuals observed inside of
every class (individuals)                                                   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                             but not others (Table 4); there were no differences for
increase in the portion of groups of more than 10 individuals                            desert comparisons (GLM, F02.232, df016, P00.353).
(from 0.1 to 2.4 %—Fig. 1). In regards to the proportion of                              The high-density population of Kapchagaj Reserve had
individuals staying in the different group size classes, most                            the same mean group sizes as the considerably lower-
gazelles were found to form groups of 3 individuals (17.7 % of                           density populations of Aktau and Boguty and other
all gazelles), 2 (16.6 %), groups of more than 10 individuals                            comparisons yielded similar results despite having different
(15 %) or remain as singletons (14.3 %). These portions were                             population densities (Taukum, Saryishikotrau, Muunkum
not significantly different from each other (chi-square-goodness-                        and Betpak-Dala). Only Panfilov Karakum, Taukum and
of fit test, χ2 00.625, df03, P00.891). The rest of the gazelles                         sometimes the Boguty populations had significantly
stayed in groups of 4–10 individuals (10.8 % of gazelles stay in                         smaller group sizes compared to the Kapchagaj population,
groups of 4 and 2.3 % in groups of 10 individuals).                                      which had the highest density among all of them (Table 4).
                                                                                         Group size only changed when population densities were
Changing characteristics of the Kapchagaj Reserve population                             more than seven times higher, whereas there were no
over years The mean group size of the Kapchagaj Reserve                                  significant changes in mean group sizes if population
population varied significantly over the years for all checked                           densities did not get above this value. In regards to the
months, except for June (Table 1). In April and December,                                frequency of occurrence of different group size classes
mean group sizes increased over the years, whereas in May,                               occurring at different population density, the only effect was
they generally had a decreasing trend. During July, September                            found for populations which had difference in density of more
and November, mean groups size increased and decreased                                   than seven times larger that typically observed (Table 5). The
without any clear trend (Table 1, r0−0.19, N08, P00.964).                                only one case of comparison of Aktau and Panfilov Karakum
Neither population density (GLM, F00.465, df07, P00.843)                                 was an exception to this. GLM analyses demonstrated
nor month significantly affected mean group size (GLM,                                   insignificant impact of density on singletons (GLM, F03.047,
F00.339, df06, P00.155).                                                                 df017, P00.375) and a low effect of density on groups of two
   Unlike mean group size, which significantly changed                                   to four individuals (GLM, F0403.645, df017, P00.032).
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                                    Discussion
correlation between population density and proportion of
singletons and groups was significant only for May and                                   Our results demonstrated that group sizes in goitered
partially for December (Table 3). There was no effect                                    gazelles are highly variable across seasons and years. This
of density and month on group size frequency (GLM,                                       variability is mostly driven by their breeding cycle, when
F00.707, df07, P00.668 and F00.339, df06, P00.903,                                       females prefer to stay alone during the birthing period in
respectively).                                                                           May–June and males protect their individual territories during
                                                                                         the rutting period in November–December. As a result of
Comparing characteristics of the gazelle populations of                                  these two events, mean group sizes decrease considerably
different deserts The mean group size of the goitered gazelle                            during these seasons, especially distinctively during the
populations of various deserts with different population                                 birthing period (Blank 1986; 1998). The group size
densities only significantly differed between some cases                                 frequency in the Kapchagaj population was the following:
Eur J Wildl Res

Table 3 Pearson Correlation index for proportion (singletons/groups    arid environment goitered gazelles live in and the sparse
of 2–4 gazelles) changing in the Kapchagaj population over years
                                                                       distribution of their forage. This is likely why Jarman (1974)
Months                  Group size           Pearson index, N and P    classified all gazelles as animals with small to mid-size
                                                                       groups. Other authors also reported that goitered gazelles
May                     Singletons           0.855, N06, P00.030       prefer to stay in small groups in Saudi Arabian hot deserts
                        Groups               −0.852, N06, P00.031      (Cunningham and Wronski 2011b) and in Central Asian’s
June                    Singletons           0.083, N05, P00.894       cold arid areas (Qiao et al. 2011).
                        Groups               −0.265, N05, P00.666         Our results showed that mean group sizes changed
July                    Singletons           −0.084, N04, P00.916      significantly over the years for all checked months;
                        Groups               0.117, N04, P00.883       however, these changes were not correlated with increasing
September               Singletons           0.680, N03, P00.524       population density in the Kapchagaj population. The group
                        Groups               −0.995, N03, P00.061      size class frequency also did not change with the population
November                Singletons           0.514, N04, P00.486       density except for in May. Moreover, in contrast to our
                        Groups               0.471, N04, P00.529       expectation, the portion of singletons increased and the
December                Singletons           −0.448, N03, P00.704      number of groups of two to four individuals each decreased
                        Groups               −1.000, N03, P00.013      with increasing of the population density during May
                                                                       (Table 3). This pattern was completely contrary to our
                                                                       hypothesis of group size increasing with population
The portion of singletons as a kind of group was more than             density. However, as mentioned above goitered gazelles
others and groups of two to four individuals followed after            preferred to stay in small groups because of sparse
singletons. Most gazelles stayed in groups of one to three             distribution of their forage which limits group size.
individuals. The bigger groups were noted much less often              Our results thus confirm Krause and Ruxton’s (2002)
decreasing their frequency with enlarging group sizes. We              prediction that median group sizes initially increase with
thus conclude that goitered gazelles seem to prefer smaller            population density, until the preferred group size is
groups, and those singletons are the most typical kind of group        reached, and that a further increase in the population
found for this species. Such group sizes are likely due to the         density will subsequently lead to higher numbers of

Table 4 The mean group size of
goitered gazelle among popula-       Populations              Density ind/km2   Mean group size ind    N        t test          P value
tions in various deserts with dif-                                              per group
ferent density
                                     Kapchagaj Reserve        2.15              3.55±0.266             221      F00.943         0.333
                                     Aktau (12.1986)          0.30              4.00±0.712             34       df0243
                                     Kapchagaj Reserve        2.29              2.39±0.144             853      F00.185         0.667
                                     Aktau (06.1987)          0.54              2.56±0.287             54       df0905
                                     Kapchagaj Reserve        2.29              2.39±0.144             853      F04.259         0.039
                                     PanfilovKarakum          0.35              1.33±0.106             39       df0890
                                     Aktau                    0.54              2.56±0.287             54       F019.323        0.000
                                     PanfilovKarakum          0.35              1.33±0.106             39       df091
                                     Kapchagaj Reserve        2.29              2.39±+0.144            853      F011.839        0.001
                                     Boguty1 (05.1987)        0.23              1.73±+0.080            234      df01085
                                     Kapchagaj Reserve        2.29              2.39±0.144             853      F05.261         0.022
                                     Boguty1 (06.1987)        0.23              1.41±0.113             56       df0907
                                     Kapchagaj Reserve        3.67              3.17±0.119             1506     F01.872         0.171
                                     Boguty2 (08.1989)        0.27              2.81±0.322             48       df01552
                                     Kapchagaj Reserve        1.65              4.05±0.266             266      F06.763         0.010
                                     Taukum (12.1982)         0.22              2.62±0.299             31       df0295
                                     Taukum (09.1988)         0.31              1.88±0.147             69       F00.372         0.543
                                     Chu Muunkum              0.17              1.90±0.204             24       df087
                                     Taukum (07.1983)         0.22              1.50±0.107             46       ANOVA           0.155
                                     Saryishikotrau           0.07              1.70±0.300             19       F02.770
                                     Chu Muunkum              0.10              1.90±0.246             20       df02
                                     Taukum                   0.18              1.73±0.159             30       ANOVA           0.591
                                     Saryishikotrau           0.08              1.95±0.223             22       F00.532
                                     BetpakDala (09.1986)     0.05              2.00±0.246             12       df02
Eur J Wildl Res

Table 5 Group size classes of the goitered gazelle in various popula-     sizes was found in other animals, where group size
tions with different density
                                                                          varied with the square root of population density (for
Populations              Density ind/   Singles/ Chi-square (χ2)          fishes, Bonabeau and Dagorn 1995; Gueron and Levin
                         km2            groups                            1995) or in a logarithmic fashion [for red kangaroo
Kapchagaj Reserve        2.15           32/44    18.021, df01, P00.000
                                                                          (Macropus rufus), Johnson 1983, and for Alpine ibex
Aktau (12.1986)          0.30           44/29                             (Capra ibex), Toigo et al. 1996), and a threshold
Kapchagaj Reserve        2.29           58/33    38.574, df01, P00.000    (abrupt) or curvilinear response was found for population
Aktau (06.1987)          0.54           41/46                             growth rates in some African antelopes (Tragelaphus,
Kapchagaj Reserve       2.29            58/33    584.616, df01, P00.000
PanfilovKarakum         0.35            74/26
                                                                          Connochaetes, Owen-Smith 2006). The group size class
Aktau -                 0.54            41/46    18.062, df01, P00.000
                                                                          frequencies showed the same pattern as for mean group
PanfilovKarakum         0.35            74/26                             size with the same sevenfold difference threshold for
Kapchagaj Reserve       2.29            58/33    263.108, df01, P00.000   population density, with the exception of the Aktau-
Boguty1 (05.1987)       0.23            62/35                             Panfilov Karakum population. We did not find any
Kapchagaj Reserve       2.29            58/33    518.423, df01, P00.000
Boguty1 (06.1987)       0.23            71/27
                                                                          correlation pattern for group class frequencies entirely.
Kapchagaj Reserve       2.75            41/43    7.040, df01, P00.008
                                                                              Our research indicates that mean group sizes were more
Boguty2 (08.1989)       0.27            38/46                             variable than group size class frequencies, likely because
Kapchagaj Reserve       1.65            29/40    11.481, df01, P00.001    goitered gazelles preferred to be alone or stay in small
Taukum (12.1982)        0.22            39/42
                                                                          groups within very wide ranges of population densities.
Taukum - (09.1988)      0.31            48/49    2.391, df01, P00.122
Muunkum                 0.17            40/60
                                                                          The group size frequencies had no correlation with population
Taukum                   0.22           63/37    3.130, df01, P00.077     density or if they had it was completely the opposite of what
Saryishikotrau (07.1983) 0.07           50/50                             we had hypothesised, when the singleton portion increased
Taukum                  0.22            63/37    2.000, df01, P00.157     and group (from 2–4 individuals) frequency decreased with
Muunkum (07.1983)       0.10            45/50
                                                                          rising population density during birthing period in May. This
Saryishikotrau          0.07            50/50    0.053, df01, P00.819
Muunkum                 0.10            45/50
                                                                          means that the impact of the breeding cycle and especially the
Taukum                   0.18           50/50    0.519, df01, P00.471     birthing period is more distinctive in goitered gazelle than the
Saryishikotrau (09.1986) 0.08           42/58                             impact of population density, as most females continued to
Taukum                  0.18            50/50    3.320, df01, P00.068     stay alone during birthing and most males led a solitary
BetpakDala (09.1986)    0.05            33/67
                                                                          lifestyle during the rut in the condition independent of the
Saryishikotrau          0.08            42/58    0.712, df01, P00.399
BetpakDala (09.1986)    0.05            33/67
                                                                          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
groups, but not to further increases in group size. Mean                  gazelle (Gazella subgutturosa marica), which mainly formed
group size and group size class differed significantly                    small groups in Saudi Arabia (Cunningham and Wronski
between May and other months, with a considerable                         2011a), and Gazella gazella farasani (Cunningham and
decrease with population density. During birthing in                      Wronski 2011b) with their mainly solitary lifestyle and for
May, most females leave their herds and stay alone for several            forest antelope species from the genus Tragelaphus, which
weeks. In addition, the high level of synchronisation of                  also have an “almost solitary” lifestyle (Wronski et al. 2009).
birthing, with most females giving birth within a few                         Various factors, other than population density, will also
days of each other is very typical for this species (Blank 1986).         affect group sizes: Human hunting pressure, for example,
Since goitered gazelles have female-skewed populations                    can lead to an increase in group sizes (probably because
where the portion of females in the population may exceed                 animals feel safer in larger numbers) regardless of density
60 % of the entire population (Zhevnerov et al. 1983), and in             (Jedrzejewski et al. 2006). Predominantly solitary gorals
May, when most females leave their groups, mean group size                formed groups of more than 10 individuals under habitat
and group size frequency of the whole population decreases                disturbance (Pendharkar and Goyal 1995). However, in
considerably.                                                             mountain gazelles (Gazella gazella), human disturbance
   Comparisons of various populations with different                      had opposite effects and lead to a decrease in mean group
densities revealed that mean group sizes did not change                   sizes of this species (Manor and Saltz 2003). The openness
within a wide range of densities, and started to change                   of the habitat (Estes 1974; Korte 2008; Walther 1972), food
only when such differences were considerable. It was                      abundance (Borkowski 2000; Elgar 1989; Raman 1996;
clear from these results that group sizes changed with                    Rowe-Rowe et al., 1992) or snow depth have also been
population density in a non-linear or abrupt fashion.                     found to positively correlate with group size (Maruyama,
Such a non-linear density-dependent response of group                     1981; Peek et al., 1974), and the reproductive cycle and
Eur J Wildl Res

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Chinese Academy of Sciences (Visiting Professorships for Senior                  Conservation of Nature and Natural Resources, Morges, Switzerland,
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Project (0866031) for granting our work and creating all conditions for          large mammals. In: Genoways H (ed) Current mammalogy. Plenum,
writing this paper. We thank also the Institute of Zoology, former               New York, pp 401–441
Academy of Sciences of Kazakhstan, which has given us possibility           Gerard JF, Bideau E, Maublanc ML, Loisel P, Marchal C (2002) Herd
for investigations of goitered gazelles in natural environment over a 10         size in large herbivores: encoded in the individual or emergent?
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