ASSATEAGUE =- While it certainly wasn’t the most glamorous aspect of managing the wild horses on Assateague, National Park Service staffers have completed the annual November pregnancy tests on most of the mares on the barrier island.
Each November, Assateague Island National Seashore biologists conduct pregnancy tests on the mares among the population of wild horses in an attempt to predict how many, if any, new foals are expected to join the herd in the coming year. The size of the wild horse population on the Maryland side of Assateague has dipped to 75, or the low side of the ideal range of 80-100, so the pregnancy tests completed this month will be watched perhaps more closely than in other years.
The pregnancy testing targeted 36 mares between the ages of two and 24, according to Assateague Island National Seashore Chief of Interpretation and Education Liz Davis.
“Of the 53 mares in the current herd, 36 were pregnancy tested,” she said. “We expect the results to come back by mid-December.”
While the annual pregnancy testing each November is an essential component of the management of the wild horse population on the barrier island, it certainly isn’t the most glamorous. Essentially, staffers follow the mares and wait for them to defecate. The pregnancy testing program began 25 years ago in 1994.
Samples are collected, frozen and sent to a lab to be analyzed to determine if any of the mares will be expecting next year. There could one or two new foals next year, or as many as five or six, or possibly even zero.
For over two decades, Assateague staffers have been monitoring the birthing habits of the island’s most famed residents and the information collected this month, or more specifically the fecal matter, will tell the story for the coming year. It’s important to note the horses on Assateague are indeed wild and are free to roam all over the roughly 27 miles of barrier island.
Assateague staffers know each the mares in the herd by their alpha-numeric name, their markings, the bands they belong to and the areas of the island they tend to frequent. Although it is not entirely scientific, the pregnancy tests generally serve as a good barometer for the expected foals in the next year.
However, tracking the horse’s whereabouts is not always easy. Some bands tend to congregate around the semi-developed areas of the islands, while others roam freely in the back country. Again, the ideal population range established by the National Park Service is 80-100 and the current population has dipped below that range at 75 due to a variety of factors including old age, illness, injury and even some man-made causes such as horses being struck by vehicles.
There have been years when the population soared past the ideal range, pushing 140 as recently as just a few years ago. In the interest of maintaining a healthy population size, the National Park Service in 1994 began a contraceptive program for the mares in the herd. In almost every year since, selected mares have been injected, or darted more appropriately, with a non-invasive contraceptive called PZP in order to curb sudden baby booms and effectively manage the size of the herd.
However, with the population now at 75, including 53 mares and 22 stallions, the NPS has changed its contraception program from a reduction phase to a adaptive management phase, according to Davis.
“The Assateague Island National Seashore wild horse contraceptive program is an adaptive management phase,” she said. “Mares will have the opportunity to foal until the population approaches the upper end of the established range of 80-100 horses. At that time, contraception will resume in order to stabilize the population.”
It remains uncertain just how many mares on the barrier island are expecting and the results of the recently completed pregnancy tests won’t be known until mid-December. With no mares contracepted last year, there could be a foal boom at Assateague next year and the pregnancy tests will help predict that.
In the meantime, National Park Service staffers will continue to monitor the size of the herd with a full census collected six times per year in February, March, May, July, September and November.
“During each census, horses are identified by their distinguishing characteristics, mapped and counted,” said Davis. “Individual horses that are not observed during multiple census periods are presumed dead. The purpose of the census is to monitor the overall population dynamics of the horse herd in support of the long-term fertility control program that was initiated in 1994.”