Bill Weir/ABC
  • Wild Mustangs

    This stunning vista is the Big Horn Canyon in Montana's Pryor Mountains, home to about 175 wild mustangs. After National Geographic captured a rare glimpse of how these untamed animals exert their dominance in the wild West, "Nightline" anchor Bill Weir traveled to the Montana-Wyoming border to see the horses up-close.
    Bill Weir/ABC
  • Wild Mustangs

    Another view of the red-rock canyons and hills of the Pryor Mountains. This is the setting for a kind of equine drama that is playing across the American West, as young wild mustangs bite, kick and attack older stallions to steal their mares.
    Bill Weir/ABC
  • Wild Mustangs

    Matt Dillon, an environmental scientist who studies the wild mustangs, was on the trip as a guide. Dillion said that each horse has a distinct personality and he knows over 140 of these wild animals by face and name. This black mustang with the wild mane is Galaxy, who recently took over a harem of mares from another stallion, including a 2-week-old foal.
    Bill Weir/ABC
  • Wild Mustangs

    Photo of the wild mustang Galaxy with the 2-week-old foal he took over from another stallion.
    Bill Weir/ABC
  • Wild Mustangs

    "Nightline" senior producer Pete Austin out on location, shooting the foal.
    Bill Weir/ABC
  • Wild Mustangs

    Matt Dillon said the largest harem he had seen in the wild was one stallion with a dozen mares. An average harem, he said, is a stallion with two or three mares and some offspring. But as head of a harem, Galaxy faces constant challenges from other stallions for control of his mares.
    Bill Weir/ABC
  • Wild Mustangs

    This group of young stallions are the bachelors. They stayed on the outskirts of the harem, waiting for their day when one of them will challenge Galaxy for his harem.
    Bill Weir/ABC
  • Wild Mustangs

    Before long a so-called "satellite bachelor" named Gringo wanders close to Galaxy's harem and begins eyeing the mares. Environmental scientist Matt Dillon said Galaxy and Gringo used to run together as bachelor buddies, but now the friendship is forgotten. "That's posturing they're doing now," Dillon said. "That's a show of strength...they're just trying to scare each other."
    Bill Weir/ABC
  • Wild Mustangs

    Galaxy and Gringo went head-to-head for the harem, but Galaxy is the reigning champion in the end. "To be a successful harem stallion, you either have to be strong or smart or both, so the strong ones do it from just scaring the young ones," Matt Dillon said.
    Bill Weir/ABC
  • Wild Mustangs

    Shown here are two mares in Galaxy's harem. Matt Dillon said there have been a few cases where the mares have rejected the male horses. "A young mare got stolen from her family for the first time and she went with a young bachelor. He wasn't very experienced and didn't really know what to do and he just kept going and playing with his little buddies... after a couple of days she just left him."
    Bill Weir/ABC
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