The Chincoteague Pony is thought to be a descendant of domestic horses turned wild, that were brought to Assateague Island in the 17th century by mainland owners to avoid taxation and fencing laws. This island is 37-miles long and is a barrier island off the coast of Maryland and Virginia. A fence separates the borders of Virginia and Maryland thus separating the herds. Probably due to their environment and poor diet these horses are actually the size of ponies. Those that are taken from the island and have received a higher protein diet grow to the size of a horse. The Chincoteague Ponies have two main herds, one in Virginia and one in Maryland. Each herd is maintained separately to control and maintain the damage to the islands ecology and are kept at around 150 ponies. They roam freely in groups of two to twelve ponies each occupying a certain area. The Virginia Herd is owned and managed by the Chincoteague Volunteer Fire Company and this is the herd referred to as the 'Chincoteague' Ponies. These ponies are allowed to graze on Chincoteague National Wildlife Refuge through a special use permit. This permit to protect the island restricts the population size of the herd to about 150 adult animals. To manage this they have the "Pony Penning" event which began in 1924, which was used as an effort to raise money for the Chincoteague Volunteer Fire Company. This event is still held the last Wednesday and Thursday of July. The Virginia herd are rounded up and made to swim across the channel to Chincoteague in front of a crowd of spectators. The foals are auctioned off and the remaining ponies are swum back to Assateague Island the next day. These foals adapt well to domestic life. The Virginia herd is checked by a veterinarian twice a year and is vaccinated against encephalitis, rabies and tetanus and tested for EIA. They are dewormed and have their hooves trimmed regularly. On the Virginia side of the Island the ponies are fenced off from the roads to discourage people from feeding them. The Maryland herd is owned and managed by the National Park Service. To keep the size of the herd between 120 to 150 animals the Maryland herd is given contraceptives by dart guns, the non-hormonal contraceptive vaccine stimulates the horse's immune system to produce antibodies and at sufficiently high levels these antibodies block fertilization and prevent pregnancy. This contraceptive last about one year, but can be extended with a yearly booster shot. This contraceptive program of maintaining the herds growth began in 1994 and is better than 95% effective.