Cycle of life gets longer for Assateague's beloved ponies

June 3, 2015 - Evidence shows fertility vaccine is helping the National Sea Shore's live healthier and longer.

Seven horses in Assateague Island's Maryland herd died this past winter.

It's nothing to be surprised by, park officials said. They were just old, in their late 20s and early 30s. So it goes for a herd of horses that have been around since the 17th century, before development tried and failed on the island and long before the National Park Service moved in.

For officials, managing the herd is a challenge they have down to a science after 40 years. The Park Service has even been recognized by the Humane Society of the United States for its careful care of the horses.

According to the latest survey, completed in 2012, the majority of horses on the island are older than 15, so more deaths than usual are expected in the coming years simply because of how the population is distributed, according to Kelly Taylor, Assateague Island National Seashore science communicator.

So why manage the pony population? To help reduce its impact on native species.

"If we were in a vacuum, then a huge population would be sustainable," Taylor said. "But we're not in a vacuum."

The horses are heavy grazers, chewing grasses down to the ground, which can contribute to erosion, Taylor said. The horses stand in the same place for a long period of time, compacting the dirt beneath them and harming other species, such as fiddler crabs and insects, that use tall grasses.

"They're a form of competition for the native species," Taylor said. "They could be a ginormous ecological disaster."

But they're not because the herd is kept between 80 and 100 horses using a birth control vaccine. The vaccine is unique because it is effective at preventing pregnancy but doesn't impact the horse's hormones, meaning their social behavior remains the same.

"We're not actually altering the way they interact with each other," Taylor said.

There is also evidence that the birth control has helped the horses live longer. In 1990, hardly any horses lived past 15; now, the horses are living to 30 and older.

The mares, or females, in the herd receive the vaccine starting when they're 2, but then go off of it when they're 4 until they deliver a live foal. It's the equivalent of preventing a scenario similar to a teen pregnancy, where the horse's body isn't fully developed and ready to give birth, Taylor said.

The birth control is also allowing the mares to live much longer — instead of focusing on raising babies throughout their lives, they give birth once, typically, then have the rest of their years to focus on themselves.

It's clear which foal belongs to which mare, but it is harder to know who he father is, Taylor said. Mares can switch from one stallion's band, or group of horses, to another easily.

"It's kind of 'As The World Turns' on the beach," she said.

While the Assateague Island Alliance gives names to the horses, the National Park Service assigns each with an alphanumeric code, indicating the horses maternal lineage back to 1975, when the government first started surveying the horses.

With the birth of a new foal this May, the herd is right at the center of it's target range at 93, which science communicator Taylor said gives them the ability to "wiggle" some.

"This past winter, we had seven animals die," she said. "We can take the liberty to maybe not provide birth control to another mare this year to let her foal again."

That means there will, hopefully, be more births in the coming year.

Originally published by DelmarvaNow on June 3, 2015

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