Demography of the Pryor Mountain Wild Horses, 1993–2007
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Demography of the Pryor Mountain Wild Horses, 1993–2007 Scientific Investigations Report 2010-5125 U.S. Department of the Interior U.S. Geological Survey Front and back cover photographs. Jason I. Ransom, U.S. Geological Survey. Demography of the Pryor Mountain Wild Horses, 1993–2007 By James E. Roelle, Francis J. Singer, Linda C. Zeigenfuss, Jason I. Ransom, Linda Coates-Markle, and Kathryn A. Schoenecker Scientific Investigations Report 2010-5125 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior KEN SALAZAR, Secretary U.S. Geological Survey Marcia K. McNutt, Director U.S. Geological Survey, Reston, Virginia: 2010 For product and ordering information: World Wide Web: http://www.usgs.gov/pubprod Telephone: 1-888-ASK-USGS For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment: World Wide Web: http://www.usgs.gov Telephone: 1-888-ASK-USGS Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted material contained within this report. Suggested citation: Roelle, J.E., Singer, F.J., Zeigenfuss, L.C., Ransom, J.I., Coates-Markle, L., and Schoenecker, K.A., 2010, Demography of the Pryor Mountain wild horses, 1993–2007: U.S. Geological Survey Scientific Investigations Report 2010-5125, 31 p. iii Contents Abstract ...........................................................................................................................................................1 Introduction.....................................................................................................................................................1 Study Area.......................................................................................................................................................2 Herd Behavior and Management .......................................................................................................4 Methods...........................................................................................................................................................5 Field Data Collection.............................................................................................................................5 Analysis ..................................................................................................................................................5 Results .............................................................................................................................................................6 Population Size and Growth ................................................................................................................6 Removals ................................................................................................................................................7 Sex and Age Structure .........................................................................................................................7 Survival ...................................................................................................................................................8 Foaling Rates .......................................................................................................................................10 Harem Stallions and Band Composition .........................................................................................13 Bachelors .............................................................................................................................................16 Population Regulation ........................................................................................................................16 Density Dependence .................................................................................................................16 Weather .......................................................................................................................................17 Discussion .....................................................................................................................................................17 Population Size and Growth ..............................................................................................................17 Sex and Age Ratios.............................................................................................................................19 Survival .................................................................................................................................................20 Foaling Rates .......................................................................................................................................22 Harem Stallions and Band Composition .........................................................................................24 Bachelors .............................................................................................................................................25 Population Regulation ........................................................................................................................25 Acknowledgments .......................................................................................................................................25 References Cited..........................................................................................................................................27 Figures 1. Map showing location of the Pryor Mountain Wild Horse Range and study area ............3 2-14. Graphs showing: 2. Trend in growth rate of the Pryor Mountain wild horse (Equus caballus) population, 1993–2007 .......................................................................................................................................7 3. Sex ratio of wild horse (Equus caballus) foals born in the Pryor Mountain herd, 1993–2007 .......................................................................................................................................8 4. Variation in the sex ratio of wild horse (Equus caballus) foals by age of dam in the Pryor Mountain herd, 1993–2007 ................................................................................................8 5. Annual sex ratio of adult (≥1 year old) wild horses (Equus caballus) in the Pryor Mountain herd, 1993–2007 ...........................................................................................................8 6. Average age of (A) male and (B) female wild horses (Equus caballus) in the Pryor Mountain herd, 1993–2007 ...........................................................................................................9 7. Trend in annual survival of Pryor Mountain wild horses (Equus caballus) for adults and foals, 1993–2007 ...................................................................................................................11 8. Age of wild horse (Equus caballus) stallions holding harems in the Pryor Mountain herd, 1996–2007 ...........................................................................................................................14 9. Age at which wild horse (Equus caballus) stallions first held a harem in the Pryor Mountain herd, 1996–2007 .........................................................................................................15 10. Frequency distribution of wild horse (Equus caballus) harem size in the Pryor Mountain herd, 1996–2007 .........................................................................................................15 11. Mean number of wild horse (Equus caballus) mares held relative to age of harem stallions in the Pryor Mountain herd, 1996–2007 ...................................................................15 12. Number of bachelor wild horses (Equus caballus) in the Pryor Mountain herd, 1996–2007 .....................................................................................................................................16 13. Age distribution of bachelor wild horses (Equus caballus) in the Pryor Mountain herd, 1996–2007 ...........................................................................................................................17 14. Relation of (A) survival and (B) population growth rate to the previous year’s population for wild horses (Equus caballus) in the Pryor Mountain herd .........................18 v Tables 1. Size and growth rate of the Pryor Mountain wild horse (Equus caballus) herd, 1993–2007 .......................................................................................................................................7 2. Removals of wild horses (Equus caballus) from the Pryor Mountain herd, 1993–2007 ....8 3. Annual survival rates of Pryor Mountain wild horses (Equus caballus), 1993–2007 .......10 4. Age-specific survival rates of Pryor Mountain wild horses (Equus caballus), 1993–2007 .....................................................................................................................................11 5. Annual foaling rates of Pryor Mountain wild horse (Equus caballus) mares, 1993–2007 .....................................................................................................................................12