Reining in the Wild Horses

1
Reining in the Wild Horses

• Data story from DASL
• Reference Eagle, Asa, Garrot et al. (1993).
  Wildlife Society Bulletin, 21(2), 116-121.
• Research Question Will sterilization of the
  dominant male in a wild horse herd reduce foaling
  (birth) rates for 2 or more years?

2
Background

• December 1985 Dominant male in wild horse bands
  from two Herd Management Areas vasectomized.
  Control group male left untreated
• June 1986 July 1988 aerial survey of bands 3-4
  times a year to record number of adults and foals
  in the treated and untreated bands

3
Data Analysis

• Data entered into SPSS
• Explore command used to produce descriptive
  statistics for treated and untreated groups
• Clustered Bar graphs created to display findings.

4
Program Effectiveness

• Mean number of foals in the sterilized bands was
  5 (range of 1 to 17) compared to a mean of 26
  (range of 16 to 39) in the untreated bands
• Treatment effect was observed in both herds

5
Adjustment for Herd Size

• Mean number of adults differed in the treated and
  untreated groups (M(trt) 46 adults vs. M(untrt)
  174)
• Calculate number of foals/number of adults to
  adjust for this

6
Conclusions

• Sterilization of dominant male in herd reduced
  average number of births by 21 foals per season
  (26 down to 5)
• Adjusted for herd size, this represents a
  decrease from 1 foal per 15 adults (0.15) down to
  1 foal per 10 adults (0.10)
• Over time, this decrease in birth-rate will
  improve herd health and grazing land condition