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Oecologia

, Volume 154, Issue 2, pp 435–444 | Cite as

Intraspecific variation in group size in the blackbuck antelope: the roles of habitat structure and forage at different spatial scales

  • Kavita Isvaran
Behavioral Ecology

Abstract

The main ecological factors that are hypothesized to explain the striking variation in the size of social groups among large herbivores are habitat structure, predation, and forage abundance and distribution; however, their relative roles in wild populations are not well understood. I combined analyses of ecological correlates of spatial variation in group size with analyses of individual behaviour in groups of different sizes to investigate factors maintaining variation in group size in an Indian antelope, the blackbuck Antilope cervicapra. I measured group size, habitat structure, forage, and the occurrence of predators in ten blackbuck populations, and, at a smaller spatial scale, within an intensively studied population. To examine the processes by which these ecological factors influence group size, I used behavioural observations and an experiment to estimate the shape of the relationship between group size and potential costs and benefits to individuals. Group size varied extensively both among and within populations. Analyses of spatial variation in group size suggested that both forage and habitat structure influence group size: large-scale, among-population variation in group size was primarily related to habitat structure, while small-scale, within-population variation was most closely related to forage abundance. Analyses of individual behaviour suggested that larger groups incur greater travel costs while foraging. However, individuals in larger groups appeared to experience greater benefits, namely the earlier detection of a “predator”, a reduction in vigilance, and an increase in the time spent feeding. Overall, these findings suggest that individuals in groups experience a trade-off between predation-related benefits and costs arising from feeding competition. Habitat structure and forage likely influence the nature of this trade-off; thus, variation in these ecological factors may maintain variation in group size. The role of predation pressure and other factors in explaining the remaining variation needs further exploration.

Keywords

Antilope cervicapra Intraspecific variation Group living Habitat structure Forage 

Notes

Acknowledgments

I am very grateful to J. Brockmann, C. St Mary, Y.V. Jhala, C. Chapman, M. Sunquist, J. Eisenberg, B. Bolker, and S. Quader for helpful discussions and valuable comments on the manuscript; the University of Florida, the Animal Behaviour Society, the American Society for Mammalogists, and Sigma Xi for research funds; the Forest Departments of Andhra Pradesh, Gujarat, Maharashtra, Rajasthan, and Tamil Nadu in India for logistical support and kind permission to work in various protected areas; Y.V. Jhala, the USFWS-funded project entitled Conservation of the Indian Wolf, and the Wildlife Institute of India for considerable logistical help; K.V.R. Priyadarshini for help in the field; E. Mungall, W. Kyle and J. Kyle for extensive support with the Texas population; and J. Ganzhorn and three anonymous referees for valuable comments on the manuscript. This work complies with the current laws of the countries in which the work was carried out.

Supplementary material

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Copyright information

© Springer-Verlag 2007

Authors and Affiliations

  1. 1.Department of ZoologyUniversity of FloridaGainesvilleUSA
  2. 2.Centre for Ecological SciencesIndian Institute of ScienceBangaloreIndia

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