For nearly three decades, Dr. Jay Kirkpatrick has spearheaded a project to keep the wild horse population on Assateague Island off the coast of Maryland at a healthy number that the environment can support. This has been done through effective use of birth control delivered by dart gun. But he and his research partner’s first attempt in the 1980s was a dismal failure—they not only didn’t prevent pregnancies, they increased the fertility rate among the horses. It wasn’t until they tried a new vaccine, PZP, developed by a third scientist, that they began to achieve success, although it’s taken years of monitoring to be certain of the project’s long-term efficacy. The trial and error, collaboration, and patience required in scientific work as seen through Dr. Kirkpatrick’s research is one fascinating dimension of this volume, but so, too, is the broader information it provides about horses. Everything from breeds and color patterns to their evolutionary and migratory history to their relationship with humans makes for intriguing reading. Brief information is also provided about ongoing efforts to address population issues among wild horses in the West in this welcome new addition to the Scientists in the Field series. (Ages 10–15)