ASSATEAGUE — Following the birth of five foals on the Maryland side this year, National Park Service officials announced this week the trend could continue with none of the island’s mares slated for the contraception program in the coming year.
The popular horses on Assateague are wild animals and generally left to the whims of nature. However, in the interest of maintaining a healthy population size, the National Park Service many years ago began a contraceptive program for the mares. In almost each year since, selected mares have been darted with a non-invasive contraceptive called PZP in an effort to maintain the size of the herd on the Maryland side in its manageable threshold.
As recently as just a few years ago, the size of the wild horse population on Assateague had swelled to 140, or well north of the target range of 80 to 100. However, with some losses due to old age, illness or other man-made and natural causes, the number of horses in the herd now stands at 88, or right in the middle of the target range. Currently, there are 66 mares and 22 stallions.
Because the herd size is now in the ideal range, the National Park Service has recently moved from a reduction phase to an adaptive management phase, meaning rangers will not give the contraceptive PZP to any of the mares this year.
“There will be no contraception next March for several reasons,” said Assateague Island National Seashore Chief of Interpretation and Education Liz Davis this week. “The goal is now to maintain the population within the 80 to 100 range, and to do this, foaling needs to increase to match natural mortality.”
Davis said the non-invasive contraceptive does not render the targeted mares infertile for their entire lives, but there is a considerable recovery time before they can successfully birth foals. Because the herd size is now in the ideal range, the contraceptive program will be put on hold this year, meaning 2017 could see another baby boom on the barrier island.
“Most of the previously contracepted mares will need one to seven years to recover fertility,” said Davis. “At this point, mares are contracepted only once at age two so the mare can mature before first foaling and there will be no two-year-old mares in 2017.”
Davis said because many of the mares in the current population are aging and going beyond natural fertility, encouraging the younger mares to birth foals by suspending the contraceptive program is a logical part of the adaptive management strategy.
“Also, the herd currently contains a large proportion of mares aged 20 to 33,” she said. “These older mares are unlikely to foal again, so in order to refresh the reproductive population in the near future, an increase in foaling among the younger mares needs to occur. So, 2017 could be a busy year for horses and the Assateague staff.”
Meanwhile, the annual pregnancy test period for the mares on Assateague is currently underway, and while it is not entirely infallible, especially with two surprise foals birthed this year, it is a solid indicator of which mares might be expecting next year. While it certainly isn’t the most glamorous aspect of managing the famed wild horses on Assateague, a biological technician is spending much of November conducting pregnancy tests on the many mares on the barrier island in an attempt to determine how many new foals might join the herd next year.
The biological technician follows the mares in the herd essentially waiting for them to defecate. The samples are collected, frozen and sent to a lab to be analyzed to determine if any of the mares will be expecting next spring and summer.
Meanwhile, the first of five foals born on the island this year officially received its adoptive proper name last week. Christine Lloyd of Wilmington won the Name that Foal raffle conducted by the Assateague Island Alliance and named the foal “Margaret’s Thunder Heart” in honor of her father Robert Lloyd.
“I am excited and honored to provide a name for this handsome fellow,” she said. “I wanted to select a name in memory of my father. My dad was always my hero who gave me the skills I needed to make my own way. This is a great way to honor him.”