What salt water means for wild horses

Shackleford Banks is an 8-mile-long barrier island off the coast of North Carolina made up of sandy beaches, salt marshes, and maritime forests. There are no vacation rentals, boardwalks, or seafood restaurants serving the residents of Shackleford Banks. That’s because the residents are more than 100 wild horses who call the sand dunes of this island home.

The delicate ecosystem of Shackleford Banks faces the effects of a changing climate, including increasingly volatile storms and floods, droughts, erosion and saltwater intrusion. The island’s low elevation means that its freshwater sources can be infiltrated with salt water during king tide events and storm surges. During periods of drought, sources of fresh water for the island’s equine residents can be depleted, leaving horses to compete for these vital resources.

“These horses have been here long enough to adapt and survive, but the availability of fresh water is a critical resource,” he said. Mateo Syriannia geoscientist at East Carolina University who present your research on December 16 at the AGU Annual Meeting. Sirianni and his colleagues monitored freshwater sources on Shackleford Banks and found that horse behavior changed when freshwater sources were scarce. As saltwater intrusion and coastal hazards increase along the islands off the coast of North Carolina known as Outer benchesThese findings can improve understanding of how to manage wildlife during a changing climate.

Searching for fresh water

Wild horses have lived on Shackleford Banks for ages. One theory suggests they arrived in the 16th century, swimming to shore after Spanish explorers were shipwrecked on the east coast. Genetic testing suggests that today’s Shackleford Banks horses are related to Spanish horse breeds, but early English settlers also brought horses that may also have escaped or been abandoned along the Outer Banks.

Today, the Shackleford Banks wild horse population must be kept to a manageable number so that the island and its resources are not overwhelmed. National Park Service workers give mares hormonal contraceptives each year to keep herd size down. The small and resistant equines They have adapted to a life of eating marsh grass, sea ​​oatsand wax myrtle. They drink from freshwater ponds and seeps, and also dig holes in the sand to reach groundwater freshwater when other sources have dried up.

In the new study, researchers monitored surface and groundwater levels and conductivity (an indirect measure of salinity because higher conductivity values ​​mean saltier water) at six water sources (two ponds, a groundwater seepage, and three excavation sites) located across the island.

“Barrier islands often develop a freshwater lens “In the underground it floats on denser, saltier water,” said Sirianni. “By monitoring the level and conductivity of the water, we can, over time, see if the freshwater lens is shrinking, growing or becoming saltier, which tells us how the island’s water resources are responding to things like tides, storms or droughts.”

The researchers installed motion-activated trail cameras near water sources to capture photos of animals drinking. They then grouped time-stamped photographs to connect the horses’ drinking activities to water level and conductivity data. From there, they looked for patterns of water use, as well as information on how long holes dug by horses (some up to 3 feet deep) remained filled with fresh water.

“Preliminary results from July 2024 to April 2025 indicate that horses spend more time drinking at excavation sites, where conductivity is lower and more stable, compared to ponds and groundwater seeps, where conductivity is higher and more variable. However, when rainfall is low, excavation sites often dry out, leading horses to drink from these higher-conductivity sources,” said Sirianni.

A saltier future?

“With the research that [Sirianni] is doing, we have learned that these ponds can be very brackish,” said Linda Kuhn, volunteer veterinarian for the National Park Service who did not participate in the investigation. “In the past, we said horses wouldn’t drink brackish water, but we were wrong. They drink brackish water when it’s all they have available.”

If freshwater sources become saltier, history has already shown that wild horses could be in trouble. On Chincoteague Island, Virginia, equine deaths and illnesses were bound to a toxic increase in salinity in freshwater supplies, possibly caused by storm surge from Hurricane Erin.

It took volunteers and observers two weeks to figure out what was wrong in Chincoteague. “Here we have [Sirianni] providing us with data in real time. It also has cameras so you can see who’s drinking.” In light of what happened to the Chincoteague ponies after Hurricane Erin, “it’s a very important study right now,” Kuhn said.

And with the possibility of stronger, more damaging storms in the future, wild horses on this precarious island habitat may face further water and food shortages, as well as dangers from the land itself. Pre-modeling studies They suggest that rising sea levels will cause the already shallow water table to rise to the surface, as well as the coastline to recede as land subsidence and erosion worsen.

Sirianni plans to continue monitoring Shackleford Banks wild horses and water sources until at least July 2026 while working on a final study manuscript on his findings. But he hopes to fund this research in the future. “These horses have been here for many years and have weathered many storms, so I like that they are a symbol of wildness and freedom even in the face of adversity,” he said.

—rebecca owen (@beccapox.bsky.social), scientific writer

Citation: Owen, R. (2025), What salt water means for wild horses, Eos, 105, https://doi.org/10.1029/2025EO250433. Published on November 21, 2025.

Text © 2025. The authors. CC BY-NC-ND 3.0
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