A Field Test of the Activity Budget Hypothesis: The Case of Blue Sheep in Helan Mountains of Ningxia, China

Zhensheng Liu1,2, Hui Gao1,2, Jiapeng Dong1, Mingming Zhang3 and Liwei Teng1,2,*

1College of Wildlife Resources, Northeast Forestry University, Harbin 150040, China

2Key Laboratory of Conservation Biology, State Forestry Administration, Harbin 150040, China

3College of Forestry, Guizhou University, Guiyang 550025, China

ABSTRACT

Blue sheep (Pseudois nayaur) is a sexually dimorphic ungulate. In general, males are larger than females and the two sexes live in separate groups outside the breeding season. We studied the seasonal activity of blue sheep from May 2010 through August 2012 to test the activity budget hypothesis, which predicts that females spend more time on feeding and that the activity synchronization of same-sex groups is higher than for mixed-sex groups. Our results support the hypothesis. Both males and females spent most of their time on feeding and moving. Feeding behavior differed significantly by sex but moving behavior did not. Activity synchronization indices of same-sex groups were higher than for mixed-groups and were highest for male groups. The activity budget hypothesis of sexual segregation was supported by blue sheep behavior in the Helan Mountains.

Article Information

Received 18 December 2016

Revised 21 May 2016

Accepted 29 January 2017

Available online 30 June 2017

Authors’ Contribution

LT and ZL conceived and designed the experiments. HG performed the experiments. MZ analyzed the data. JD and HG wrote the article.

Key words

Blue sheep (Pseudois nayaur), Sexual segregation, Activity budget hypothesis, Helan Mountains.

DOI: http://dx.doi.org/10.17582/journal.pjz/2017.49.4.1267.1272

* Corresponding author: [email protected]

0030-9923/2017/0004-1267 $ 9.00/0

Copyright 2017 Zoological Society of Pakistan

INTRODUCTION

 

Blue sheep (Pseudois nayaur) inhabits alpine valleys with bare rock or grass at elevations of 2 500-5 500 m above sea level (Liu et al., 2005). Blue sheep has highly economic, medicinal (Du et al., 2001), ornamental and recreational values (Hu, 1994). Blue sheep is classified as a Category II Protected Wildlife under the Wildlife Protection Law in China (Wang et al., 1998). This species is distributed in the Tibetan Plateau and the surrounding mountain areas. Helan Mountain region is one of the central distribution areas of blue sheep (Wang and Schaller, 1996). Males and females sheep have different external attribute and body size, and they live in separate groups outside the breeding season. Studies on the home range and activity rhythms of blue sheep in Helan Mountain region found sexual segregation between male and female (Cui, 2007), and more investigations should be carried out to make further understanding of the sexual segregation mechanism in blue sheep.

Sexual segregation exists in many ungulates (Ruckstuhl and Kokko, 2002; Ruckstuhl and Neuhaus, 2002). It may occur at various ecological scales and within different ‘aggregation units’, which means the social group, the habitat type, or merely the space occupied (Pérez-Barbería and Gordon, 1998). Hence, it is difficult to define sexual segregation (Mooring et al., 2003). Sexual segregation has an important implication on population dynamics, conservation and management of ungulate populations (Singh et al., 2010). Ruckstuhl and Neuhaus (2000) proposed the activity budget hypothesis to explain sexual segregation. The activity budget hypothesis has two key assumptions (Yearsley and Pérez-Barbería, 2005): (1) females digest forage less efficiently than males, because females are smaller than males in body size, mouth, and stomach, which may lead to lower metabolic rates (Demment, 1982; Robbins, 1993; Xu et al., 2008); (2) differences in activity budgets may lead to increased costs of synchrony for males and females to maintain group cohesion, so they could hardly stay in mixed-sex groups (Ruckstuhl and Neuhaus, 2002). On the one hand, males may feed forage of lower quality with more efficient digestion, which would reduce time on searching food and increase time on chewing, ruminating, and digesting. On the other hand, females may be more dependent on higher quality food resources. They would spend more time on foraging and moving in search of higher quality forage and less time on chewing, rumination, and digesting (Ruckstuhl, 1999; Conradt and Roper, 2000; Ruckstuhl and Neuhaus, 2002; Yearsley and Pérez-Barbería, 2005). The activity budget hypothesis predicts (1) Females spent more time on feeding than males; (2) In order to maintain the behavioral synchrony of group activities, the same sex ungulate tend to coalesce into single-sex groups (Ruckstuhl, 2007). We will verify these predictions for blue sheep, which is the most abundant ungulate in this area. This study will be a significant implication for understanding the sexual segregation and the evolution of sociality in this species, which was important for effective management of blue sheep populations.

 

 

MATERIALS AND METHODS

 

Study area

This study was conducted in the Suyukou National Forest Park in the Helan Mountain National Nature Reserve of Ningxia, China 38°21′ 39°22′N, 105°44′ 106°42′E (Fig. 1). Helan Mountains supports core populations of blue sheep and is the northern range limit of blue sheep in China (Wang and Schaller, 1996), which are located at the boundary between temperate grassland and desert. Elevations used by blue sheep generally range from 2000 to 3000 m above sea level, and the terrain is rocky and rough. Average annual rainfall ranges from 200 to 400 mm, with most rainfall occurring at summer. Extreme maximum temperature is 35.2°C and extreme minimum temperature is -32.6°C. Vegetation in this areas is typical temperate arid and semi-arid upland vegetation types. The vertical distribution of vegetation from low to high altitude is mountain grassland, mountain woodland steppe, mountain conifer forest, and sub-alpine shrubland and meadow. Blue sheep in Helan Mountain region mainly inhabit mountain woodland steppe (Di, 1986; Liu et al., 2004, 2005, 2009).

Behavioral observations

We observed blue sheep from May 2010 through August 2012. Because blue sheep are inactive at night, we established three daylight observation periods during four seasons: 08:00 to 18:00 in spring and autumn, 08:00 to 17:00 in winter, and 08:00 to 19:00 in summer. Observation was conducted from the 21th to 26th days every month, a total of 180 observation days including 36 days in spring (from April 11 to June 20 ), 36 days in summer (from June 21 to August 10), 30 days in autumn (from August 11 to October 10) and 78 days in winter (from October 11 to April 10). Observation distances were generally from 20 to 200 meters to avoid disturbing blue sheep. We used binoculars (8 x 42) and a telescope (40–60 x 63) to observe randomly selected groups. The groups were classified as male groups, female groups, or mixed-sex groups. The group size of the observed groups ranged from 2 to 46 individuals. Male groups consisted of at least two adult or subadult males. Female groups consisted of at least two adult or subadult females. Mixed-sex groups consisted of at least one adult male and one adult female (Li and Jiang, 2008). Following Xu et al. (2012) and Shi et al. (2003) and the field observation of blue sheep, we classified sheep activities into five categories: Feeding (biting, chewing, browsing, grazing or swallowing food), standing (feet in contact with the ground, and supporting the body in a stationary state), moving (walking and running), resting (abdomen, back or side of the body in contact with the ground, eyes closed or opened) and others (not mentioned above, such as drinking, grooming, lactating, defecating etc.).

We recorded behaviors by focal sampling and group scan sampling methods. Focal sampling was used for identifiable small groups: observers selected focal animals, observed them for a certain amount of time, and recorded the transition time from one behavior to another (Ruckstuhl, 1998). Scan sampling was used for large groups and those not easily observed, by observing each individual in turn at regular intervals until the group ran out of sight (Altmann, 1974). We recorded gender and behavioral state of blue sheep every ten minutes by using a SJ-1 electronic event recorder (Jiang, 1999; Karadas et al., 2017) (Institute of Zoology, Chinese Academy of Sciences). All observations were recorded by the same person. The total sampling time was 264 h, and we observed 115 groups in spring, 72 groups in summer, 96 groups in spring and 260 groups in winter.

Data analysis

We calculated the percentage of each behavioral category (feeding, standing, moving, resting, others). We used Kruskal-Wallis tests to determine sexual differences in each behavioral category (Zar, 1999). We used a behavior synchronization index to analyze blue sheep behavior by gender group. A synchronization factor (Sf ) was calculated for each group for which more than 50% of individuals were engaged in the same activity, When Sf=1, group behaviors were highly synchronized. When Sf=0, group behaviors were not synchronized. We used St=∑(Sf)/n and S=∑(St)/k to calculate the behavior synchronization index. The mean synchronization St for the behavioral observation sample was St=∑(Sf)/n, where n is the number of scans. The mean group synchronization index, S, was S=∑(St)/k, where k is the number of observed groups. S varied from 0 to 1, and high S values indicated high behavioral synchronization. To avoid the effect of group size, we only calculated S when group size was >5 (Ruckstuhl, 1999). Data were analyzed with SPSS 19.0 (IBM, Armonk, New York, 2010).

 

RESULTS

 

Both male and female sheep spent most time on feeding (Figs. 2, 3). Females spent over 50% of time on feeding in all seasons, while males spent over 40%. Feeding as a proportion of total time was followed in decreasing order by moving, standing, resting and other behaviors.

Male and female blue sheep showed significant differences in feeding behavior (P<0.01). The proportions of foraging behavior for females were higher than males among four seasons. Proportions of moving behavior were similar for males and females. Proportions of standing behavior were similar for males and females in spring and summer, but males stood more than females during autumn and winter (P<0.01). Proportions of resting behavior were similar for males and females during summer and winter, but males rested more than females during spring and autumn (P<0.01). Proportions of other behaviors were greater for males than for females during spring, summer and autumn, and much greater in winter (P<0.01, Table I).

Behavior synchronization varied by seasons. Behavior synchronization of mixed groups peaked during the winter breeding season and was lowest in summer. Female and male groups showed the opposite trend (Fig. 4).

 

 

 

The behavior synchronization of mixed group was obviously different from that of male group (P<0.01), and the female group (P<0.01). And a significant difference was also tested between the behavior synchronizations of male groups and females (P<0.01). The behavior synchronization of mixed groups was mostly different from that of male groups.

 

Table I.- Seasonal daytime behaviors of blue sheep in the Helan mountains.

	Feeding		Moving		Standing			Resting			Others
	Z	P	Z	P	Z	P		Z	P		Z	P
Spring	-4.155	0.000	-0.193	0.847	-0.045	0.964		-3.64	0.00		-2.13	0.03
Summer	-3.173	0.002	-1.803	0.071	-0.744	0.457		-1.54	0.12		-2.09	0.04
Autumn	-6.704	0.000	-1.233	0.218	-3.542	0.000		-4.93	0.00		-2.52	0.01
Winter	-3.311	0.001	-0.129	0.897	-5.011	0.000		-0.03	0.97		-3.73	0.00

Z, indicates the statistic; P, indicates the significance.

 

DISCUSSION

 

A variety of hypotheses have been proposed to explain sexual segregation, such as same-sex peer preference (Bon and Campan, 1996), differences in habitat preferences between males and femals (Mysterud, 2000), sexual differences in predation risks (Jakimchuk et al., 1987; Corti and Shackleton, 2002), or different activity budgets (Ruckstuhl, 1998). The activity budget hypothesis has received special attention (Conradt, 1998; Ruckstuhl, 1999; Ruckstuhl and Neuhaus, 2000, 2001, 2002; Neuhaus and Ruckstuhl, 2002; Michelena et al., 2004; Li and Jiang, 2008). A prediction of the activity budget hypothesis is that males and females separate and form same-sex groups because of their behavioral asynchrony in mixed-sex groups (Ruckstuhl and Neuhaus, 2002). Ruckstuhl and Kokko (2002) suggested body size dimorphism is a driving factor leading to the sexual differences in activity budget, and an increasing of the sexual dimorphism triggers greater differences in the activity between males and females. But the activity budget hypothesis is still debatable. Some authors found no differences between the sexes in the time spent foraging, moving, resting or ruminating (Mooring et al., 2003), and no perceptible sexual segregation and a high synchronization between the activities of males and females (Michelena et al., 2004). Females are thought to be less efficient than males at digesting fiber because females are generally smaller than males, but Yearsley and Pérez-Barbería (2005) considered this assumption was based on the species’ body masses span three orders of magnitude (Robbins, 1993), while the maximum male to female body mass ratio was 2.6 (Pe´rez-Barberı´a and Gordon, 2000), so the sexual difference in the digestion efficiency of fiber should be very small. However, Ruckstuhl (1998) proposed that energy requirements and foraging behaviour were affected by body size, and females should have a higher metabolic rate and higher nutritional need per unit body mass than males, and they found sexual dimorphism in body size would lead to different movement patterns and time budgets through studying bighorn sheep, Ovis canadensi, and adult bighorn rams are about 50% heavier than females. Whether and how the difference of body size between female and male would lead to sexual differences in activity budget still needs more evidence for further investigation.

The activity budget hypothesis predicts that females spend more time grazing than males, and that activity synchrony is higher within single-sex than mixed-sex groups (Ruckstuhl, 1998, 1999). Mooring et al. (2003) found that bighorn sheep, Ovis canadensis mexicana, had no difference in time budget between the sexes. A study by Weckerly (1993) on the black-tailed deer (Odocoileus hemionus) achieved the similar conclution. In contrast, our study found that there was a huge difference in the activity budget between female and male blue sheep, and the same-sex groups had a higher behavior synchronization index than mixed-sex groups (Fig. 4). Besides, females had a longer feeding time than males (Fig. 3). These results gave support to the activity budget hypothesis. Similar results have been reported in the sutdy of Tibetan gazelle Procapra picticaudata (Li and Jiang, 2008). Studies on Alpine ibex Capra ibex ibex showed that females and males were more synchronized in activities when they were in the same-sex groups than in mixed-sex groups (Ruckstuhl and Neuhaus, 2001). Ruckstuhl (1998) found that the female of bighorn sheep Ovis canadensis had longer foraging time, and considered it was difficult for males and females to stay in the same group for their difference in movement patterns and time budgets.

The weight of male blue sheep is 1.52 times over the female blue sheep in the Helan Mountains. The differential of size is more than 50%. Throughout our study, there is a huge difference in the activity budget between females and males. Like other temperate region ungulates, blue sheep are active during the daytime and rest at night (Sayre and Seabloom, 1994), they devote a lot of time to feeding (Liu et al., 2005). Blue sheep spent most daylight hours feeding. Ungulate females spend more time than males on feeding (Ruckstuhl, 1998; Xu et al., 2012). This phenomenon could be supported by the fact that females have low digestive efficiency, and on the other hand they have a higher nutritional need for pregnancy and lactation, females must rely on feeding high-quality food resources to satisfy their nutritional needs (van Soest, 1994). In the Helan Mountains, the dry climate and low rainfall lead to relatively resource-poor vegetation. This is compounded by complex geological and geomorphological conditions, and low quality and uneven distribution of Gramineae species. Females need higher quality nutrition to gestate and feed their offspring, therefore females need more time for feeding and moving to different types of forage. In contrast, males have a high digestive efficiency and are able to feed on low-quality food resources that are widely distributed. Thus, instead of feeding, males spend more time in resting and rumination.

We recorded significant differences in behavior synchronization between same-sex and mixed-sex groups, and between male and female groups. Behavior synchronization of mixed-sex groups varied seasonally. Mixed-sex groups had a high behavior synchronization in autumn and winter due to breeding. In the non-breeding season, mixed-group behavior synchronization was very low. Behavior synchronization of male groups was lowest during the winter breeding season, even lower than for mixed-sex groups, possibly due to the fact that male groups compete for breeding rights in winter. In the non-breeding season, behavior synchronization of male groups was the highest of the three groups.

 

CONCLUSION

 

In conclusion, our research supported the two predictions of the activity budget hypothesis. Even though there are several reasons for sexual segregation, the activity budget hypothesis could explain the sexual segregation mechanism in blue sheep, whether other hypothesis could account for this phenomenon of blue sheep in Helan Mountains or not still need further investigation.

 

ACKNOWLEDGEMENTS

 

This study was financially supported by the Fundamental Research Funds for the Central Universities (2572014CA03, DL13EA01), the National Nature Science Foundation of China (31372221, 30970371), and the Optional Project of Ningxia Hui Autonomous Region (2011-017). We are grateful for the support of Ningxia Helan Mountain National Nature Reserve.

 

Statement of conflict of interest

Authors have declared no conflict of interest.

 

REFERENCES

 

Altmann, J., 1974. Observational study of behavior: sampling methods. Behaviour, 49: 227-267. https://doi.org/10.1163/156853974X00534

Bon, R. and Campan, R., 1996. Unexplained sexual segregation in polygamous ungulates: a defence of an ontogenetic approach. Behav. Process., 38: 131-154. https://doi.org/10.1016/S0376-6357(96)00029-0

Conradt, L., 1998. Could asynchrony in activity between the sexes cause intersexual social segregation in ruminants? Proc. R. Soc. London, Ser. B, 265: 1359-1363. https://doi.org/10.1098/rspb.1998.0442

Conradt, L. and Roper, T.J., 2000. Activity synchrony and social cohesion: a fission-fusion model. Proc. R. Soc. B-Biol. Sci., 267: 2213-2218. https://doi.org/10.1098/rspb.2000.1271

Corti, P. and Shackleton, D.M., 2002. Relationship between predation-risk factors and sexual segregation in Dall’s sheep (Ovis dalli dalli). Can. J. Zool., 80: 2108-2117. https://doi.org/10.1139/z02-207

Cui, D.Y., 2007. Home range, activity rhythm and foraging strategy of Bharal (Pseudois nayaur) in Helan Mountain, China. Ph. D thesis, East China Normal University, PRC.

Demment, M.W., 1982. The scaling of ruminoreticulum size with body weight in East African ungulates. Afr. J. Ecol., 20: 43-47. https://doi.org/10.1111/j.1365-2028.1982.tb01082.x

Di, V.Z., 1986. Plantae vasculares Helan Mountain. Northwestern University Press, Xi’an.

Du, T.K., Ren, Q.F. and Ding, H., 2001. Development and application of the resource of medicine animal in Ningxia. J. Ningxia Agric. Coll., 22: 39-45.

Hu, J.C., 1994. Mammals of Sichuan Province. Sichuan Science and Technology Press, Chengdu.

Jakimchuk, R.D., Ferguson, S.H. and Sopuck, L.G., 1987. Differential habitat use and sexual segregation in the Central Arctic caribou herd. Can. J. Zool., 65: 534-541. https://doi.org/10.1139/z87-083

Jiang, Z.G., 1999. Real time recording of ethological data. In: Zoological studies in China (eds. China Zoological Society), Chinese Forestry Publishing House, Beijing, pp. 702-707.

Karadas, K., Tariq, M., Tariq, M.M. and Eyduran, E., 2017. Measuring predictive performance of data mining and artificial neural network algorithms for predicting lactation milk yield in indigenous Akkaraman sheep. Pakistan J. Zool., 49: 1-8.

Li, Z. and Jiang, Z., 2008. Sexual segregation in Tibetan gazelle: a test of the activity budget hypothesis. J. Zool., 274: 327-331. https://doi.org/10.1111/j.1469-7998.2007.00388.x

Liu, Z.S., Cao, L.R., Li, Z.G., Li, T., and Wang, X.M., 2005. Winter habitat selection of blue eared pheasant (Crossoptilon auritum) in Helan Mountain, China. Chinese J. Zool., 40: 38-43.

Liu, Z.S., Cao, L.R., Zhai, H., Hu, T.H., and Wang, X.M., 2004. Winter habitat selection by red deer (Cervus elaphus alxaicus) in Helan Mountain, China. Zool. Res., 25: 403-409.

Liu, Z.S., Wang, X.M., Li, Z.G., Cui, D.Y., and Li, X.Q., 2005. Seasonal variation of diurnal activity budgets by blue sheep (Pseudis naynur) with different age-sex classes in Helan Mountain. Zool. Res., 26: 350-357.

Liu, Z.S., Zhang, M.M., Li, Z.G., Hu T.H., and Zhai, H., 2009. Feeding and bedding habitat selection by red deer (Cervus elaphus alxaicus) during winter in the Helan Mountains, China. Acta Theriol. Sin., 29: 133-141.

Michelena, P., Bouquet, P.M., Dissac, A., Fourcassie, V., Lauga, J., Gerard, J.F., and Bon, R., 2004. An experimental test of hypotheses explaining social segregation in dimorphic ungulates. Anim. Behav., 68: 1371-1380. https://doi.org/10.1016/j.anbehav.2004.04.008

Mooring, M.S., Fitzpatrick, T.A., Benjamin, J.E., Fraser, I.C., Nishihira, T.T., Reisig, D.D., and Rominger, E.M., 2003. Sexual segregation in desert bighorn sheep (Ovis canadensis mexicana). Behaviour, 140: 183-207. https://doi.org/10.1163/156853903321671497

Mysterud, A., 2000. The relationship between ecological segregation and sexual body size dimorphism in large herbivores. Oecologia, 124: 40-54. https://doi.org/10.1007/s004420050023

Neuhaus, P. and Ruckstuhl, K.E., 2002. The link between sexual dimorphism, activity budgets, and group cohesion: the case of the plains zebra (Equus burchelli). Can. J. Zool., 80: 1437-1441. https://doi.org/10.1139/z02-126

Pérez-Barbería, F.J. and Gordon, I.J., 1998. The influence of sexual dimorphism in body size and mouth morphology on diet selection and sexual segregation in cervids. Acta Vet. Hung., 46: 357-367.

Pérez-Barbería, F.J. and Gordon, I.J., 2000. Differences in body mass and oral morphology between sexes in the Artiodactyla: evolutionary relationships with sexual segregation. Evol. Ecol. Res., 2: 667-684.

Robbins, C.T., 1993. Wildlife feeding and nutrition, Second Edition. Academic Press Inc., Pittsburgh.

Ruckstuhl, K.E., 1998. Foraging behaviour and sexual segregation in bighorn sheep. Anim. Behav., 56: 99-106. https://doi.org/10.1006/anbe.1998.0745

Ruckstuhl, K.E., 1999. To synchronise or not to synchronise: a dilemma for young bighorn males? Behaviour, 136: 805-818. https://doi.org/10.1163/156853999501577

Ruckstuhl, K.E., 2007. Sexual segregation in vertebrates: proximate and ultimate causes. Integr. Comp. Biol., 47: 245-257. https://doi.org/10.1093/icb/icm030

Ruckstuhl, K.E. and Neuhaus, P., 2000. Sexual segregation in ungulates: a new approach. Behaviour, 137: 361-377. https://doi.org/10.1163/156853900502123

Ruckstuhl, K.E. and Neuhaus, P., 2001. Behavioral synchrony in ibex groups: effects of age, sexand habitat. Behaviour, 138: 1033-1046. https://doi.org/10.1163/156853901753286551

Ruckstuhl, K.E. and Neuhaus, P., 2002. Sexual segregation in ungulates: a comparative test of three hypotheses. Biol. Rev., 77: 77-96. https://doi.org/10.1017/S1464793101005814

Ruckstuhl, K.E. and Kokko, H., 2002. Modelling sexual segregation in ungulates: effects of group size, activity budgets and synchrony. Anim. Behav., 64: 909-914. https://doi.org/10.1006/anbe.2002.2015

Sayre, R.W. and Seabloom, R.W., 1994. Summer activity patterns of bighorn ewes in the Northern Great Plains. In: Proceedings of the 9th Biennial Symposium of the Northern Wild Sheep and Goat Council, pp.104-109.

Shi, J.B., Dunbar, R.I., Buckland, D., and Miller, D., 2003. Daytime activity budgets of feral goats (Capra hircus) on the Isle of Rum: influence of season, age, and sex. Can. J. Zool., 81: 803-815. https://doi.org/10.1139/z03-055

Singh, N.J., Bonenfant, C., Yoccoz, N.G., and Côté, S.D., 2010. Sexual segregation in Eurasian wild sheep. Behav. Ecol., 21: 410-418. https://doi.org/10.1093/beheco/arp205

SPSS I.B.M., 2010. IBM SPSS 19.0 for Windows. IBM Company, Armonk, NY.

Van Soest, P.J., 1994. Nutritional ecology of the ruminant, Second Edition. Comstock Pub. Associates Cornell University Press, London.

Wang, X.M. and Schaller, G.B., 1996. Status of large mammals in western Inner Mongolia. J. East China Normal Univ., 12: 93-104.

Wang, S., Le, P.Q. and Chen, Y.Y., 1998. Chinese endangered animal redbook-Mammals. Science Press, Beijing, pp. 295-299.

Weckerly, F.W., 1993. Intersexual resource partitioning in blacktailed deer: a test of the body size hypothesis. J. Wildl. Manage., 53: 475-494. https://doi.org/10.2307/3809272

Xu, F., Ma, M., Wu, Y.Q. and Yang, W.K., 2012. Winter daytime activity budgets of Asiatic Ibex Capra sibirica in Tomur National Nature Reserve of Xinjiang, China. Pakistan J. Zool., 44: 389-392.

Xu, F., Ma, M., Yang, W.K., Blank, D. and Wu, Y.Q., 2012. Test of the activity budget hypothesis on Asiatic ibex in Tian Shan Mountains of Xinjiang, China. Eur. J. Wildl. Res., 58: 71-75. https://doi.org/10.1007/s10344-011-0542-x

Yearsley, J.M. and Pérez-Barbería, F.J., 2005. Does the activity budget hypothesis explain sexual segregation in ungulates? Anim. Behav., 69: 257-267. https://doi.org/10.1016/j.anbehav.2004.04.010

Zar, J.H., 1999. Biostatistical analysis, Fourth Edition. Upper Saddle River, Prentice Hall.