One of the principal threats to species is the continued loss, fragmentation, and transformation of their habitats. As a consequence, the persistence of many species hinges on their ability to adapt their diets to modified and changing habitats, and for many species, successful conservation management will depend on having at least a baseline knowledge of feeding ecology (Korschgen 1980, Bagchi et al. 2004, Mishra et al. 2004, Liu et al. 2007a). The dwarf blue sheep (Pseudois schaeferi, Haltenorth 1963), a Caprinae, is confined to a narrow distribution along the steep rocky slopes of the upper Yangtze River gorge between c. 28°15′N and 31°0′N in Sichuan, Yunnan, and Tibet, China (Wang and Hoffman 1987, Wang et al. 2000, Harris 2008a). As a consequence of unrestricted overhunting, habitat loss, and competition with livestock, the distribution and abundance of dwarf blue sheep have sharply declined since the 1950s (Wang et al. 2000, Wang and Wang 2003), raising concern for the fate of the species, and providing rationale for the species to be listed by the IUCN as ‘Endangered’ (IUCN 2011). Pastoralism in the region is widespread, invasive weeds are abundant, infrastructural development is rapid, and pressure from locals on natural resources is pervasive. Still, remarkably little is known of the basic biological requirements of P. schaeferi and how these factors relate to its survival in a changing environment.
The relatively sparse distribution in difficult terrain has made data collection on Pseudois schaeferi challenging, so that few studies have been completed on the species. The best previous dietary information on P. schaeferi relied on a few direct observations made in summer and late autumn, and information from local herders and hunters in Batang, Sichuan (Wu et al. 1990, Wang et al. 2000, Harris 2008a). Wu et al. (1990) observed P. schaeferi in spring and autumn eating a mixed diet of more than 20 plant species, including Solomon’s seal (Polygonatum sp.), flaccid grass (Pennisetum flaccidum), eragrotis (Eragrotis nigra), yellow foxtail (Setaria pumila), primula (Androsace erecta), and clubmoss (Selaginella sanguinolenta). From their observations, Wang et al. (2000) suggest P. schaeferi derives most of its nourishment from graminoids, such as P. flaccidum, S. pumila, as well as the drought-tolerant S. sanguinolenta. These observations are congruent with those made of the bharal Pseudois nayaur (Hodgson, 1833), the only close relative of P. schaeferi, but which occupies a wide range of mostly treeless mountain habitats across the high ranges of the Himalayas from Pakistan to Inner Mongolia (Harris 2008b). Pseudois nayaur preferentially select habitats near cliffs (Mishra et al. 2004), where they preferentially graze on graminoids, and to a lesser degree on shrubs, trees, forbs, and lichens where graminoid availability is low or has been reduced (Harris and Miller 1995, Shrestha et al. 2005, Liu et al. 2007a, Suryawanshi et al. 2009). For P. schaeferi, no quantitative estimate of diet for the species has ever been completed, and nothing has been reported on the population at Rini Mountain, Yunnan.
This study aimed to redress the dearth of information on the diet of Pseudois schaeferi as a preliminary step in better understanding which food resources the species utilizes in its harsh environment, as well as to identify potential resource conflicts or threats to its existence. The principle objectives of this study were to quantify and compare winter diets of male and female, and adult and juvenile P. schaeferi based on direct field observations of P. schaeferi. We chose to collect data during the winter when quantity and nutritional quality of forage species was likely to be lowest and intraspecies competition for resources presumably higher than at other times of the year. Intraspecific gender and age-class comparisons of feeding choice can provide valuable biological information (Main et al. 1996), helping to determine the presence and causes of intraspecific competition and sexual segregation (Shank 1982, Bowyer 1984, Young and Isbell 1991, Bleich et al. 1997, Kie and Bowyer 1999), differences in physiological requirements (Harrison 1983, Dardaillon 1989), and learning modalities of feeding in young individuals (Bergerud 1972, Edwards 1976). We predicted the winter diet of P. schaeferi, like Pseudois nayaur, to be mixed, rich in browse species. We also predicted animals should prefer the same food (based on quality or abundance), though believed some selection differences may occur due to body architecture (Hildebrand 1974), and allometric effects of metabolic requirements and digestive retention (McCullough 1979, Barboza and Bowyer 2000), or underlying nutritional requirements (Main et al. 1996, Bleich et al. 1997).
Materials and methods
Rini Mountain (3650 m asl, 28°15′–28°18 N, 99°16′–99°18′ E) is a low conical hill situated above the Yangtze River in Baima Snow Mountain National Nature Reserve, Deqin County, Yunnan (Figure 1). Baima Snow Mountain is part of the extensive Hengduan Mountain Range that forms the high spine between the Jinsha (Yangtze) and Lancang (Mekong) Rivers in Yunnan and Tibet. Rini Mountain is separated from the main range by a low saddle and a mixed broadleaved-conifer forest belt (above 3200 m asl) of mainly Pinus yunnanensis, Cupressus duclouxiana, and Quercus guajavifolia. The lower and midslopes are rocky steep and unstable; massive cliffs, are interspersed by small pockets of sparse grassland, shrubland, scrub, and loose scree deposits. The scrub and shrubland vegetation within the main habitat used by the study group (2050–3300 m asl) mainly comprised the shrubs Vitex negundo var. microphylla, Osyris quadripartita, Sophora davidii, and the introduced cactus Opuntia ficus-indica. Scattered Pistacia weinmanniifolia grow along the river. The site is sparsely populated by sedentary agropastoralists who raise goats (Capra hircus), yak (Bos grunniens), and grow barley. Livestock are grazed within the habitat used by the Pseudois schaeferi study group (Li pers. obs.).
The site is characterized by a warm, arid climate, with mean annual temperatures from 14°C to 17°C, and with monthly means ranging between 24°C in July to 8°C in January (Jin 2002). Total annual precipitation is <350 mm, with 90% occurring as rainfall during the wet season (May–September). Annual mean evaporation is 3000–4000 mm (Jin 2002). Snow occurs regularly in winter down to ca. 3200 m asl (near lower treeline).
After observing the herd for 10 days and speaking to local herders and hunters, seven prominently located sites (A–F) were selected for observing the Rini Mountain Pseudois schaeferi population (Figure 1). The sites were located more-or-less evenly throughout the estimated home range of the study population and offered observational visibility to approximately 87% of the home range (Li unpublished data). The quantitative observations of behavior were made from 13 October 2006 to 27 January 2007 using a scan sampling method at 10-min intervals (Altmann 1974). During each 10-min observation, a visual scan was made of the group, recording the activity of each visible individual at that instant (<5 s/animal). The behavior types recorded during scan samples included resting/lying, standing/watching, moving, feeding, drinking, fighting, grooming, and defecating. When feeding behavior was recorded, the food plant species eaten were recorded at the species level and foraging class. The nomenclature follows eFloras (2008). For each scan record, the animal was classed as either adult male, adult female, or juvenile. Observations were made daily from ca. 0800 h to 1830 h using a Nikon spotting scope (40×–60×) at distances, where possible to 300 m, though more typically observations were made between 50 and 200 m. The prohibitively steep and dangerous terrain precluded tracking the sheep on some occasions when they moved out of view. When the study group was disturbed by local herders, data collection was terminated until the following day.
To test the adequacy of the scan sample size for representing the winter feeding diet, 388 feeding records (where the foraging species was positively identified) were analyzed using the methods outlined by Foster et al. (2010). The total number of species consumed was calculated for 10 randomly selected scans for an initial sample, and then repeated with each incremental addition of five scans to the sample up to 300 scans. This procedure was repeated for 100 sequences of random selections to create an iterative species accumulation curve (Foster et al. 2010).
The habitat types were recorded for visible individuals every 30 min, classed as cliff cactus scrub, scree cactus scrub, scree scrub, cliff shrubland, and shrubland. For better visualization, the habitat was presented as percentages of the total records per age-gender class, and the differences in habitat use were tested for independence by a χ2-test.
Food niche overlap
The frequency of feeding scans was used to evaluate the differences in the diet of adult and juvenile dwarf blue sheep. A χ2 analysis was used to test for independence of age-gender class (male, female, juvenile) and most commonly selected food items (where the expected value was >1), and foraging classes. A Fishers test of two proportions was used to test the usage of each food item. The relationship between the number of records age-gender class and habitat use was tested for independence by a χ2-test. For better visualization, habitat use was presented as percentages of the total records per age-gender class. Dietary overlap between adults and juveniles was calculated using the Pianka’s index (O) (Pianka 1973). Pianka’s index formula is:
where pij (or pik) is the proportion of food species (or plant category) i recorded in the diet of age class j (or k), and n is the total number of diet items. Index values range from 0 (no overlap) to 1 (complete overlap). It is generally considered that values of Pianka’s index (Ojk) higher than 0.6 means that there is a biologically significant niche overlap for the resource(s) examined (Wallace 1981).
In total, 630 scans were made recording some 1893 feeding records of Pseudois schaeferi from October to February; 734 on adult males, 626 on adult females, and 533 on juveniles. Of these records, 388 records were positively identified to species, and an additional 27 records, totaling 415, were identified to foraging class.
Based on these records, 17 plant species were eaten, though only six species (Opuntia ficus-indica, Themeda triandra, Festuca durata, Polygonum thunbergii, Elsholtzia cypriani, and Excoecaria acerifolia) made up nearly 90% of the observed identifiable diet (Table 1). Exotic succulent cactus prickly pear (O. ficus-indica) alone accounted for almost half of the observed feeding occurrences (Table 1). By foraging class, the diet was dominated by succulents (43%), with graminoids (18%), herbs (17%), and woody plants (21%) making up about equal proportions. The lycopods accounted for <1% of the diet.
A species accumulation curve of 300 randomly selected scans showed some leveling, but did not reach an asymptote (Figure 2). However, as few as 30 scans were able to detect the six most common food items (i.e., those species making up >5% of the items in the full sample).
The dwarf blue sheep used all five habitats, though more than 60% of the observed times were in cliff cactus scrub. The habitat use between age-gender groups among the five major habitat types at the site was significantly different than the expected (Pearson’s χ2=97.388, df=8, p<0.001). The adult males used the cliff cactus scrub habitat more than expected, while the juveniles and adult females used the scree scrub habitat more often than expected (Figure 3). A significant difference was also observed when comparing the adult male, female, and juvenile use of cactus-dominated habitats in comparison with other habitats (Pearson’s χ2=37.986, df=8, p<0.001).
Food niche overlap
According to the scan survey data, the diet of adults was significantly different from that of the juveniles based on the nine species with expected values >1 (Pearson’s χ2=52.508, df=8, p<0.001) and foraging classes (Pearson’s χ2=44.844, df=3, p<0.001). The trophic niche overlap (O) was high, both by species and by foraging class (Table 1). The adult sheep fed more frequently on the introduced succulent cactus, Opuntia ficus-indica, whereas the juvenile sheep fed more frequently on the herbs, Polygonum thunbergii and Elsholtzia cypriani, and the woody shrub, Buddleja caryopteridifolia. These results were significant (Table 1).
Feeding niche differences also existed between age-gender classes. The adult male, adult female, and juvenile dwarf blue sheep fed unevenly on the most common species (Pearson’s χ2=90.996, df=16, p<0.001). A similar result was found for the foraging class (Pearson’s χ2=72.380, df=6, p<0.001) (Table 2). The trophic niche overlap (O) was high between males and females, and females and juveniles, both by species and by foraging class; however, feeding niche overlap was less than between males and juveniles (Table 3).
Our observations showed that dwarf blue sheep at Rini Mountain exhibit a narrow winter diet. The animals were observed to feed on 17 species, from which just six species formed the bulk of the diet (Table 1). This, however, is not extraordinary. Pseudois nayaur show a similarly narrow feeding breadth across its range, e.g., in Ninxia Hui Autonomous Region, Inner Mongolia (Liu et al. 2007a), Qinhai Province (Harris and Miller 1995), India (Mishra et al. 2004), and Nepal (Shrestha et al. 2005), and dietary studies among various grazing animals show that a narrow feeding breadth is a common character even in more diverse habitats (e.g., Hayward 2005, Kowalczyk et al. 2011).
According to the classification set out by Hoffman (1989), the winter diet of the dwarf blue sheep at Rini Mountain was sufficiently mixed across the foraging classes to be grouped as an intermediate feeder. Wang et al. (2000) and Wu et al. (1990) each observed the dwarf blue sheep to have mixed diets, though specific proportions of the foraging classes were not given. Many specialist herbivores show considerable temporal variation in the contribution of foraging classes to their diets in response to environmental factors (Bodmer 1990, Brown and Doucet 1991, Liu et al. 2007a). Pseudois nayaur is, in its natural habitat, predominantly a grazer, preferring graminoids, though shifts its diet along the grazer-browser continuum as a result of seasonal availability, competitive exclusion by sympatric grazers, and local plant composition (Harris and Miller 1995, Miller and Schaller 1998, Schaller 1998, Mishra et al. 2004, Shrestha et al. 2005, Liu et al. 2007a, Suryawanshi et al. 2009). This shift is particularly pronounced in winter when graminoid resources become limited or where competition with domestic livestock or other wild ungulates occurs (Mishra et al. 2004, Liu et al. 2007a, Suryawanshi et al. 2009). Ultimately, the local plant composition determines the availability; for example, a complete lack of woody species in some areas totally precludes that component from entering the diet (Harris and Miller 1995, Miller and Schaller 1998).
Season, competitive exclusion by other grazers, and local plant composition are also likely to have been critical to defining the dietary pattern of dwarf blue sheep at Rini Mountain. While the winter food niche of both adults and juveniles overlapped, significant quantitative differences in the food composition were observed (Table 1). The adults, particularly the males, fed more frequently on the succulent, Opuntia ficus-indica, whereas the juvenile sheep fed more frequently on the herbs, Polygonum thunbergii and Elsholtzia cypriani, and the woody shrub, Buddleja caryopteridifolia. The adult females had an intermediate diet. Rumen samples of three poached animals also broadly support this disparity (Li unpublished data). The fleshy cladode of Opuntia spp. is highly palatable, high in soluble carbohydrates, Ca, K, and vitamin A, and a good source of water (Chavez-Ramirez et al. 1997, Hernández-Urbiola 2011), making Opuntia spp. an important food source for domestic, feral, and native species in the arid and semiarid parts of the world where it is found (Russell and Felker 1987, Biru and Bekele 2012). Graminoid density is exceedingly sparse in the dry winter months (Li unpublished data). Assuming the dwarf blue sheep have the same preference for graminoids and dietary plasticity as Pseudois nayaur, the animals may have shifted their diet to include cactus as a result of low graminoid abundance or quality or in response to a new resource. Numerous animal species are reported to browse Opuntia spp., though not always without ill consequences. Mouth injuries and chronic infection are common (Chavez-Ramirez et al. 1997). Some livestock will only browse the cactus when starving or preferentially browse where spines have been removed (Chavez-Ramirez et al. 1997). The observations in this study also recorded the adult sheep frequently raking their broad (ca. 9 cm diameter; Liu et al. 2007b) horns along the broad cladodes (flattened photosynthetic stems of Cactaceae) effectively removing spines from the fleshy cladodes before feeding. The juveniles with less-developed horns were never observed exhibiting this behavior and only rarely ate the cactus, usually targeting new growth. While the behavior of rubbing cacti to remove the spines is known in the Caprinae, e.g., big horn sheep (Ovis canadensis) in North America (Warrick and Krausman 1989, Tarango et al. 2002), no records exist for this behavior in Pseudois, and no other native plant with a similar morphology or defense mechanism exists within the range of Pseudois schaeferi or P. nayaur. In the current study, domestic yaks and goats were observed to eat O. ficus-indica, as was the Chinese goral, Naemorhedus griseus Milne-Edwards, 1871, on one occasion, though none displayed the behavior of using their horns to remove the spines.
Intraspecific feeding niche partitioning of sexes is nearly ubiquitous in polygynous, sexually dimorphic ruminants. Intraspecific differences in the diet are particularly common where genders occupy seasonally different home ranges or habitats and have access to different resources (e.g., Shank 1982, Bowyer 1984, Young and Isbell 1991, Main et al. 1996, Bleich et al. 1997, Kie and Bowyer 1999). The males and females at Rini Mountain were not especially segregated in winter. The animals had highly overlapping home ranges during the breeding season and used habitats in similar proportions. Although all-male groups were still seen on occasion in October and early November, and the males chose cliff cactus scrub more often than the females and juveniles, most males were moving with the females and juveniles during the period of the study, and the differences in habitat use accounted for <10% of the observed time. This suggests that large-scale spatial segregation and habitat use differences are not likely to be the most important factors in determining the difference in winter diet.
Instead, differential selection of food resources between age-gender classes is likely to have arisen from allometric effects of gut capacity and metabolic rates (McCullough 1979, Barboza and Bowyer 2000), physiological requirements (Harrison 1983, Daraillon 1989, Bleich et al. 1997), or differences in accessibility resulting from body architecture (Hildebrand 1974), e.g., the dimensions of the incisor arcade, the placement of new vegetative growth, the distribution of the spines on the plant, or the smaller horn size. The allometric hypothesis proposes that the males have a larger ratio of ruminal volume to body mass, and a lower metabolic rate per unit of body weight than the females or subadults and, therefore, are able to accommodate and digest bulkier forage of lower quality (McCullough 1979, Bowyer 1984, Barboza and Bowyer 2000). This leads to an expected greater retention time in the larger males. The greater retention time is associated with an ability to utilize slower digesting matter, such as mature plant fiber and crystalline celluloses, and reduces protein intake requirements due to the greater capacity for the recycling of urea (Wales et al. 1975, Barboza and Bowyer 2000, Parker et al. 2005). The smaller individuals, on the other hand, constrained by gut size, must ingest more energy-rich, quickly digestible diets because they need more energy per kilogram. However, cactus is likely to differ from most other available winter forages in having high digestible energy, low levels of fiber, and very low protein contents (Ramírez et al. 2000, Hernández-Urbiola et al. 2011). The ability to accommodate more matter in the rumen may enable the recovery of more nitrogen from microbial activity and reduce the minimal density of protein required in the diet of the males. Thus, males would be enabled to consume proportionately more cactus. However, whether the high-energy content of the cactus provides carbohydrates that sustain microbial activity and facilitate endogenous recycling of nitrogen is unknown. Protein needs would presumably be particularly important in the developing juveniles and lactating or pregnant females, thus a rich diet of cactus could present nutritional challenges (Dardaillon 1988, Parks et al. 2005, Guevara et al. 2009). Several studies of domestic sheep have shown that the growth rates of juveniles decrease with increasing levels of Opuntia spp. in the diet, unless supplemented with protein (Ben Salem et al. 2002, Tien and Beynen 2005, Mendez-Llorente et al. 2008). Conversely, the young animals and lactating females may benefit more from the high water content of cactus. Several studies in arid environments have shown that the Opuntia spp. greatly reduces water consumption in domestic sheep (Mendez-Llorente et al. 2008). The larger-bodied mammals have lower rates of water loss than the smaller-bodied ones (Gordon et al. 1982), so that the females, particularly the lactating females, and the juveniles would probably have successively higher water requirements than the males, which should, in the absence of other variables, cause them to consume proportionately more cactus.
The winter diet of Pseudois schaeferi at Rini Mountain has undoubtedly changed with the invasion of the highly palatable exotic succulent Opuntia ficus-indica. Opuntia ficus-indica is a long-domesticated crop species estimated to have originated in central Mexico sometime in the last 9000 years (Griffith 2004). Probably due to its ease in clonal propagation, edibility, and drought tolerance, O. ficus-indica was both intentionally and unintentionally established around the world by early explorers and sailors (Griffith 2004), with sometimes catastrophic ecological and economic consequences (Richardson et al. 2000). In many of its destinations, e.g., South Africa, Australia, Pacifica, its invasive qualities enabled it to rapidly spread and, in several instances, gaining complete dominance of the ecosystem (Richardson et al. 2000). Opuntia ficus-indica was first introduced in China around 1645 as a hedge and for its edible young joints and fruit (eFloras 2008). Since then, it has become well established in the upper Yangtze River valley, becoming a conspicuous element of the arid dryland ecosystem, particularly on the steep and sparsely vegetated rocky slopes below 2800 m asl. No other records exist of P. schaeferi or its close relative Pseudois nayaur consuming O. ficus-indica, although O. ficus-indica extends as far north as Batang. The literature conspicuously lacks information on the species in Yunnan, and it is not clear whether its naturalized distribution has reached an equilibrium, to what extent native and domestic animals are contributing to its spread, or how it is interacting with the ecosystem. Our results give the first evidence that the winter diet of the adult dwarf blue sheep was highly enriched by this exotic and frequently invasive species, and given the ecosystem-level issues caused by O. ficus-indica in parts of its range, highlight the need for a better understanding of the interactions between the dwarf blue sheep, O. ficus-indica, and the environment. While wild native ungulates are commonly reported foraging on exotic species (e.g., Parks et al. 2008), to our knowledge, the diet of no other wild population of threatened or endangered ungulate depends so much on an invasive exotic species. While O. ficus-indica cactus is now an integral winter forage crop for the dwarf blue sheep at Rini Mountain, and may be directly contributing to their survival, the long-term physiological needs of the species and the balance of ecosystem-level interactions are still troublingly unclear.
This study was supported by the following: Education Department of Yunnan (08Z0072), Key Subject of Wildlife Conservation and Utilization in Yunnan (XKZ200904) and Ministry of Environmental Protection (Ecological Assessment of Priority Conservation Areas of the Trans-Himalaya). Permission for the study was granted by the Administration of Baimaxueshan Nature Reserve. Plant identification was graciously provided by Ding Li and Yang Hong-Min from Southwest Forestry University. Logistical support during the study was provided by Ti-Bu. Also, we would like to offer many thanks to A-Ba for making our fieldwork easier.
Anthony, R.G. and N.S. Smith. 1974. Comparison of rumen and fecal analysis to describe deer diets. J. Wildl. Manage. 38: 535–540.Google Scholar
Bagchi, S., M. Charudutt and Y.V. Bhatnagar. 2004. Conflicts between traditional pastoralism and conservation of Himalayan ibex (Capra sibirica) in the Trans-Himalayan Mountains. Anim. Conserv. 7: 121–128.Google Scholar
Bergerud, A.T. 1972. Food habits of Newfoundland caribou. J. Wildl. Manage. 36: 913–923.Google Scholar
Ben Salem, H., A. Nefzaoui and L. Ben Salem. 2002. Supplementing spineless cactus (Opuntia ficus-indica f. inermis) based diets with urea-treated straw or oldman saltbush (Atriplex nummularia). Effects on intake, digestion and sheep growth. J. Agric. Sci. 138: 85–92.Google Scholar
Biru, Y. and A. Bekele. 2012. Food habits of African elephant (Loxodonta africana) in Babile Elephant Sanctuary, Ethiopia. Trop. Ecol. 53: 43–52.Google Scholar
Bleich, V.C., R.T. Bowyer and J.D. Weyhausen. 1997. Sexual segregation in mountain sheep: resources or predation? Wildl. Monogr. 134: 1–50.Google Scholar
Brown, D.T. and G.T. Doucet. 1991. Temporal changes in winter diet selection by white-tailed deer in a northern deer yard. J. Wildl. Manage. 55: 361–376.Google Scholar
Chavez-Ramirez, F., X.G. Wang, K. Jones, D. Hewitt and P. Felker. 1997. Ecological characterization of Opuntia clones in south Texas: implications for wildlife herbivory and frugivory. J. Prof. Assoc. Cactus 2: 9–19.Google Scholar
Dardaillon, M. 1989. Age-class influences on feeding choices of free-ranging wild boars (Sus scrofa) Can. J. Zool. 67: 2792–2796.Google Scholar
eFloras. 2008. Flora of China. Available: http://www.efloras.org (accessed 17 June 2011). Missouri Botanical Garden, St. Louis, MO and Harvard University Herbaria, Cambridge, MA.
Foster, R.J., B.J. Harmsen and C.P. Doncaster. 2010. Sample-size effects on diet analysis from scats of jaguars and pumas. Mammalia 74: 317–321.Google Scholar
Gordon, M.S., G.A. Bartholomew, A.D. Grinnell, C.B. Jorgensen and F.N. White. 1982. Animal physiology: principles and adaptations. 4th ed. MacMillan, New York, NY. pp. 635.Google Scholar
Guevara, J.C., P. Suassuna and P. Felker. 2009. Opuntia forage production systems: status and prospects for rangeland application. Range. Ecol. Manage. 62: 428–434.Google Scholar
Harris, R.B. 2008a. Pseudois schaeferi. In: IUCN 2011. IUCN Red List of Threatened Species. Version 2011.1. Available: http://www.iucnredlist.org. (accessed 16 June 2011).
Harris, R.B. 2008b. Pseudois nayaur. In: IUCN 2011. IUCN Red List of Threatened Species, version 2011.1. Available: http://www.iucnredlist.org (accessed 16 June 2011).
Harris, R.B. and D.J. Miller. 1995. Overlap in summer habitats and diets of Tibetan Plateau ungulates. Mammalia 59: 197–212.Google Scholar
Hayward, M.W. 2005. Diet of the quokka (Setonix brachyurus) (Macropodidae: Marsupialia) in the northern jarrah forest of Western Australia. Wildl. Res. 32: 15–22.Google Scholar
Hernández-Urbiola, M.I., E. Pérez-Torrero and M.E. Rodríguez-Garcí. 2011. Chemical analysis of nutritional content of prickly pads (Opuntia ficus indica) at varied ages in an organic harvest. Int. J. Environ. Res. Public Health 8: 1287–1295.CrossrefGoogle Scholar
Hildebrand, M. 1974. Analysis of vertebrate structure. John Wiley & Sons, New York, NY. pp. 710.Google Scholar
IUCN. 2011. IUCN Red list of Threatened Species, version 2011.1. Available: http://www.iucnredlist.org (accessed 16 June 2011).
Jin, Z.Z. 2002. Hot valleys of Yunnan and Sichuan and floral characters of dry-warm valley plants. Yunnan Science and Technology Publishing House, Kunming, China. pp. 226.Google Scholar
Kie, J.K. and R.T. Bowyer. 1999. Sexual segregation in white-tailed deer: density-dependent changes in use of space, habitat selection, and dietary niche. J. Mammal. 80: 1004–1020.CrossrefGoogle Scholar
Korschgen, L.J. 1980. Procedures for food-habits analysis. In: (S.D. Schemnitz, ed.) Wildlife management techniques manual. The Wildlife Society Inc., Washington, DC. pp. 113–128.Google Scholar
Kowalczyk, R., P. Taberlet, E. Coissac, A. Valentini, C. Miquel, T. Kaminski and J.M. Wojcik. 2011. Influence of management practices on large herbivore diet – case of European bison in Bialowieza Primeval Forest (Poland). For. Ecol. Manage. 261: 821–828.Google Scholar
Liu, Z.S., X.M. Wang, L.W. Teng, L.R. Cao. 2007a. Food habits of blue sheep, Pseudois nayaur in Helan Mountains, China. Folia Zool. 56: 13–22.Google Scholar
Liu, Y.D., C.Q. Zhou, C.S. Huang and J.C. Hu. 2007b. Comparison of morphological characters of skulls between the blue sheep and the dwarf blue sheep. Acta Theriol. Sinica 27: 376–379.Google Scholar
McCullough, D.R. 1979. The George Reserve deer herd: population ecology of a K-selected species. University of Michigan Press, Ann Arbor, MI. pp. 271.Google Scholar
Mendez-Llorente, F., R.G. Ramírez-Lozano, J.I. Aguilera-Soto and C.F. Arechiga-Flores. 2008. Performance and nutrient digestion of lambs fed incremental levels of wild cactus (Opuntia leucotrichia). Conference on International Research on Food Security, Natural Resource Management and Rural Development. 7–9 October 2008. University of Hohenheim, Tropentag, Germany. pp. 7.Google Scholar
Miller, D.J. and G.B. Schaller. 1998. Rangeland dynamics in the Chang Tang Wildlife Reserve, Tibet. In: (I. Stellrecht, ed.) Karakorum-Hindukush-Himalaya: dynamics of change. Rudiger Koppe Verlag, Koln, Germany. pp. 125–147.Google Scholar
Mishra, C., S.E. Van Wieren, P. Ketner, I.M.A. Heitkönig and H.H.T. Prins. 2004. Competition between domestic livestock and wild bharal Pseudois nayaur in the Indian Trans-Himalaya. J. Appl. Ecol. 41: 344–354.CrossrefGoogle Scholar
Parker, K.L., P.S. Barboza and T.R. Stephenson. 2005. Protein conservation in female caribou (Rangifer tarandus): effects of decreasing diet quality during winter. J. Mammal. 86: 610–622.CrossrefGoogle Scholar
Ramírez, R.G., R.R. Neira-Morales, R.A. Ledezma-Torres and C.A. Garibaldi-González. 2000. Ruminal digestion characteristics and effective degradability of cell wall of browse species from northeastern Mexico. Small Rum. Res. 36: 49–55.Google Scholar
Richardson, D.M., W.J. Bond, W.R.J. Dean, S.I. Higgins, G.F. Midgley, S.J. Milton, L.W. Powrie, M.C. Rutherford, M.J. Samways and R.E. Schulze. 2000. Invasive alien species and a global change: A South African perspective. In: (H.A. Mooney and R.J. Hobbs, eds.) Invasive species in a changing world. Island Press, Washington, DC. pp. 303–349.Google Scholar
Schaller, G.B. 1998. Wildlife of the Tibetan Steppe. University of Chicago Press, Chicago, IL. pp. 383.Google Scholar
Shrestha, R., P. Wegge and R. Koirala. 2005. Summer diets of wild and domestic ungulates in Nepal Himalaya. J. Zool. 266: 111–119.Google Scholar
Tien, D.V. and A.C. Beynen. 2005. Growth performance of lambs in Phangrang, Vietnam: effects of a dietary supplement containing prickly-pear cactus. Trop. Anim. Health Pro. 37: 237–244.CrossrefGoogle Scholar
Wales, R.A., L.P. Milligan and E.H. McEwan. 1975. Urea cycling in caribou, cattle, and sheep. Biol. Pap. Univ. Alaska 1: 297–307.Google Scholar
Wallace, R.K. 1981. An assessment of diet-overlap indexes. Am. Fish. Soc. 110: 72–76.Google Scholar
Wang, X. and R.S. Hoffman. 1987. Pseudois nayaur and Pseudois schaeferi. Mammal. Spec. 278: 1–6.Google Scholar
Wang, Y. and X.M. Wang. 2003. Population ecology of dwarf blue sheep (Pseudois schaeferi). Biodivers. Sci. 11: 59–62.Google Scholar
Warrick, G.D. and P.R. Krausman. 1989. Barrel cacti consumption by desert bighorn sheep. Southwest. Nat. 34: 483–486.Google Scholar
Wu, Y., C.G. Yuan, J.C. Hu, J.T. Peng and P.L. Tao. 1990. A biological study of dwarf blue sheep. Acta Theriol. Sinica 10: 185–188.Google Scholar
Young, T.P. and L.A. Isbell. 1991. Sex differences in giraffe feed ecology: energetic and social constraints. Ethology 87: 79–89.Google Scholar