Intensive population management by means of fertility control has been shown to change the age profile of a wild horse herd. The primary change has been an increase in the number and percent of older animals, as expected, but also the appearance of new and older age classes. An examination of direct effects of fertility control on two groups of treated animals shows a significant increase in longevity over non-treated animals that is associated with contraceptive treatment. The mean age at death (MAD) was calculated for 128 wild horses for which precise birth and death dates were known, including 56 stallions, 42 untreated mares, 11 mares treated with a porcine zona pellucida contraceptive vaccine for 1-2 years, and 19 mares treated with the same vaccine for >/=3 years. The MAD for stallions (10.3+/-0.84 [SEM] years), and mares treated for 1-2 years (10.2+/-0.56), was significantly greater (P<0.05) than for untreated mares (6.4+/-0.85), and significantly <19.9+/-1.66 for mares treated >/=3 years (19.9+/-1.66). Zoo Biol 26:237-244, 2007. (c) 2006 Wiley-Liss, Inc.