population size and demographics, and more. Finally, harem-changing by mares occurs to varied degrees in horse populations in varied ecological contexts in uncontracepted populations (see, e.g., Feist and McCullough, 1975; Berger, 1977, 1986; Nelson, 1978; Rubenstein, 1981; Stevens, 1990; Goodloe, 1991; Jensen, 2000).
Figure 4-2 shows a frequency distribution of the percentage of mares observed changing bands in population studies before or without contraception (Feist and McCullough, 1975; Nelson, 1978; Rubenstein, 1981; Berger, 1986; Rutberg, 1990; Stevens, 1990). Values range from 8 to 61 percent (mean, 27 percent; median, 25 percent). The study by Madosky et al. (2010) found that 70 percent of PZP-treated mares changed bands; that is significantly higher than the percentage of mares that change bands in uncontracepted populations (Wilcoxon signed-rank test, T = –18, p = 0.008, df = 7). The percentage of control mares changing bands (33.3 percent) did not differ from that of mares in uncontracepted herds (Wilcoxon signed-rank test, T = –6, p = 0.44, df = 7) (analysis provided by D. Rubenstein).
Whether Shackleford Banks is a unique case or not, additional study is needed to understand whether the absence of foaling as a result of contraception has an effect on band stability. Gray (2009) argued that sexual behavior and the ability to form consortships were adequate to maintain band stability in her study in Nevada. The studies on Shackleford Banks (Nuñez et al., 2009; Madosky et al., 2010) suggest that there is an interaction between pregnancy and social cohesion. The importance of harem stability to mare well-being is not clear, but considering the relatively large number of free-ranging mares that have been treated with liquid PZP in a variety of ecological settings, the likelihood of serious adverse effects seems low.
Side Effects: Demography and Population Processes. The easiest way to envision the effect of contraception on population processes is to examine its effect on demographic vital rates
FIGURE 4-2 Percentage of band changes by mares as shown in a review of published literature. DATA SOURCE: Feist and McCullough (1975), Nelson (1978), Rubenstein (1981), Berger (1986), Rut-berg (1990), Stevens (1990).