Grouping Patterns of Argali in Ikh Nart Nature Reserve, Mongolia
Navinder J. Singh1, Sukh Amgalanbaatar2 and Richard P. Reading3
1Department of Life Sciences, Imperial College London, Silwood Park Campus, Buckhurst Road, Ascot, Berkshire SL57PY U.K
2Institute of Biology, Mongolian Academy of Sciences, Ulaanbaatar 51, Mongolia.
3Department of Conservation Biology, Denver Zoological Foundation, 2300 Steele Street, Denver, CO 80205, USA
Gregariousness is a common behavioural trait observed in many large mammalian herbivores. Habitat characteristics, life history, spatio-temporal resource dynamics, population density, predation risk, competition with kin and social learning often determine gregariousness in a species. These factors may infl uence grouping patterns between species as well as between sexes within a species and several of these factors may be interrelated. In this study we examined the temporal dynamics of grouping behaviour and sexual segregation in argali (Ovis ammon) using eight years of observations in Ikh-Nart reserve, Mongolia. We measured monthly and yearly variations in typical group sizes and used a sexual segregation and aggregation statistic to assess sexual segregation. The typical group size observed was 14.97±2.74. The largest groups within the year occurred during lambing (May-June) and mating periods (November-December). On an average, females formed larger groups than males. The sexes were segregated all year round except for the mating period and this pattern was consistent for all years. Argali grouping behaviour in Ikh Nart resembles the patterns observed in other sexually dimorphic mountain ungulates and argali subspecies across Asia.
Түлхүүр үг: aggregation,segregation,typical group size,argali,open habitats,Mongolia
Argali (Ovis ammon) is a sexually dimorphic, polygynous and gregarious mountain ungulate inhabiting highly seasonal, semi-arid rangelands of Central Asia. Argali generally inhabit open, rolling mountainous terrain, plateaus, and areas with rocky outcrops (Fedosenko & Blank, 2005; Reading et al., 2006, 2009). Rut occurs in late autumn and lambing occurs in spring (Schaller, 1998, Fedosenko & Blank, 2005). As is typical for Caprinae (Schaller, 1977), ewes separate from other animals as parturition approaches and deliver lambs in isolation. Females hide lambs for the fi rst few days of life. Sex-ratio is skewed towards females (Schaller, 1977; Reading et al., 1997; Fedosenko & Blank, 2005). Wolves (Canis lupus) and snow leopards (Uncia uncia) are the main predators of argali, but lynx (Lynx lynx) and domestic dogs (Canis familiaris) also kill some animals occasionally (Reading et al., 2005, 2007). We monitored the grouping behaviour and sexual segregation patterns in argali in Ikh Nart Nature Reserve, Mongolia at monthly and yearly temporal scales. To identify the factors determining grouping behaviour in argali, we tested the following predictions. Since the predation risk to lambs during and immediately after lambing may be greater than that to only males or females in open habitats (Bleich et al., 1997; Festa-Bianchet & Cфtй, 2008), females with lambs will form larger groups compared to males in argali. As predation risk varies over time, reaching a maximum during and immediately following the lambing period the largest groups will occur during and immediately after lambing. Based on the life history and polygynous mating system, argali will form larger groups during the mating season compared to the rest of the year. Considering the high sexual size dimorphism we predict that sexes will display strong yearround segregation, except during the mating period, as commonly occurs in other sexually dimorphic ungulates (Ruckstuhl & Neuhaus, 2005).
Материал арга зүй
Study Area. The Ikh Nart Nature Reserve (Ikh Nart) lies within Dornogobi Aimag (East Gobi Province) of Mongolia (N 45.723ө, E 108.645ө). Established in 1996, Ikh Nart covers an area of about 66,760 hectares of grassland and semi-desert steppe environment and harbors one of the last remaining, large populations of argali sheep and a population of Siberian ibex (Myagmarsuren 2000; Reading et al., 2006). Ikh-Nart was established in 1996 to protect the region’s unique rocky outcrops and its wildlife on the northern edge of the Gobi (Myagmarsuren, 2000; Reading et al., 2006). The region is a high upland (~1,200 m) defi ned by semi-arid steppe vegetation. Permanent cold-water springs are available in some of the several, shallow valleys draining the reserve. Climate is strongly continental and arid, characterized by cold winters (January to March: minimum temperature -43°C), dry, windy springs (April to June: wind speed of 25 mps), and relatively wet, hot summers (July-September: Maximum temperature to 40°C). Precipitation is low and seasonal, with most precipitation falling in the summer (Reading et al., 2006). The fl ora and fauna are representative of the semi-arid regions of Central Asia, with a mix of desert and steppe species (Reading et al., 2006). The fauna comprises 33 mammal species, and several birds, reptiles and invertebrates. The vegetation is sparse and is dominated by xerophytic and hyperxerophytic semi-shrubs, shrubs, scrub vegetation, and turfy grasses.
Methods. We collected monthly data on group sizes and composition continuously during the years 2000-2008 through direct observations of groups. The data was collected mostly while tracking radio collared animals (see Reading et al., 2003, 2007, 2009; Kenny et al., 2008 for more detail). We defi ned groups as a single individual or a cluster of animals within 30 m of each other and showing coordinated movements. We classifi ed animals in each group as adult males, adult females, yearlings (subadults 1–2 years in age), and lambs (newborn to 1 year in age). We usually could not determine the sex of yearlings and lambs defi nitively. We excluded animals that we could not classify from further analysis. Group sizes and composition. We performed data analysis at monthly and yearly temporal scales. We determined typical group sizes (TGS) for argali for each month per year of observation. While mean group size represents the average number of individuals encountered, TGS is more animal-centred and represents the number of other members of a group in which any individual finds itself (Jarman, 1974). TGS is often higher than the mean group size and collates several environmental constraints acting on group formation and therefore, we believe it represents a better descriptor of social organisation than mean group size. We followed Jarman (1974) in calculating TGS as:
The average typical group size of argali was 14.97±2.74 (mean±S.E.; n=8163, Fig. 1). Argali aggregate into largest groups in May (mean ± SE TGS: 18.82 ± 9.66, n=578) and November (20.06 ± 4.71, n = 820; Fig.1). Argali females on an average form larger groups than males (F1, 22 =19.67, P < 0.001, n = 4456 females and 748 males), and in the months of May (10.76 ± 5.64) and November (10.79 ± 2.71, Fig. 2). The smallest groups are observed in August, when group sizes are similar for both sexes (males: 3.88 ± 0.37, females: 3.69 ± 0.47, Fig. 2). TGS varied by year and months and maximum variability was observed during the months from May to July (Table 1, Fig. 3).
There is temporal variation in sexual segregation with significant segregation occurring during spring (April, May and June) and significant aggregation occurring during late autumn and early winter (November to February, Fig. 4). The observed SSAS statistic always fell outside the significant confidence limits of SSAS (2.5% and 97.5%, Table 2, Fig. 4) under the null hypothesis of random association. Argali segregated in all years and during all months within years, except during rut.
Argali inhabit open, hilly to mountainous terrain and sport long legs built to run to escape from coursing predators, such as wolves and dogs. Species in open habitats tend to form large groups (Kie, 1999). Nevertheless, other factors may also be important, as animals in open habitats may also increase individual vigilance rather than increasing group size. We did not measure vigilance patterns of observed groups and so cannot address this aspect. However, being in larger groups decreases individual vigilance and may, therefore, maximise individual fi tness (Hunter & Skinner, 1998). Our observations of largest groups of argali occurring in May-June (Spring) and November-December (Autumn) support the temporal variation in predation risk and life history hypotheses for this species (Linnell et al., 1995; Reading et al., 2003, 2005). Risk of predation is highest after lambing in May and June, when new-born lambs accompany females (Lima & Bednekoff, 1999), hence, forming larger groups following parturition appears to be a behavioural response of argalis to avoiding lamb predation. The larger group size of argali females compared to males provides additional evidence for this argument. Argali females also may trade-off security for resources during the peak growth period of August to fulfi l their high lactation demands. Alternatively, lambs also may have grown enough to run well by August and hence staying in larger groups may increase the chances of being sighted by predators (Creel & Christianson, 2008). Larger group sizes in November likely result from herding behaviour of males during the rutting period and the species’ polygynous mating strategy, in which single males try to form harems and mate with many females. Males form all male groups to fi ght for dominance or form harems during the rut (Main et al., 1996). The larger number of mixed groups during these months provides further evidence for this hypothesis. Larger groups during the rut may increase mating opportunities, but also may increase competition in highly seasonal environments. Sexual segregation. Argali segregated sexually during most months and years, as predicted by the sexual dimorphism of body size and polygynous mating systems hypothesis (Main et al., 1996). Segregation peaked during spring (April-June), conforming to that found for other mountain ungulate species of similar body sizes (Bonenfant et al., 2007; Singh et al., 2010). Aggregation was observed during winter probably due to formation of mixed sex groups during the rut. At present we could only provide evidence of the occurrence of sexual segregation based on social grouping. In future studies we hope to better examine the nature of sexual segregation by testing for spatial distribution of groups and assessing habitat characteristics for each group. Our results on the temporal dynamics of grouping behavior and sexual segregation of argali in Ikh Nart resemble the patterns observed in other argali subspecies (Singh et al., 2010) and other sexually dimorphic mountain ungulates such as Ibex (Festa-Bianchet & Cote, 2008; Ruckstuhl & Neuhaus, 2002).
Several people helped with this research, including Dr. L. Amgalan, S. Batdorj, S. Buyandelger, S. Dandar, Dr. A. DeNicola, Sh. Enkhtaivan, N. Enkhtuvshiin, Dr. T. Galbaatar, Ch. Ganzorig, Dr. Ts. Janchiv, Dr. D. Kenny, Dr. B. Lhagvasuren, S. Munkhdalai, Ts. Munkhtzul, Dr. J. Murdoch, G. Onolragchaa, B. Otgonbayar, T. Purevsuren, E. Surmaakhorol, E. Tuguldur, R. Ulziiduuren, G. Tsogtjargal, G. Wingard, and all our Earthwatch volunteers. Funding was provided by Denver Zoological Foundation, Earthwatch Institute, Trust for Mutual Understanding, Mongolian Academy of Sciences, Mongolian Conservation Coalition, Argali Wildlife Research Center, and numerous private donors.
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