Every horse in the Maryland herd is given an alphanumeric code, such as those of the horses pictures here from left N6BM, N6BMT (Sonja), N10O (Tipperary), N6BMT-F (Jojo) and N6BMT-FO (Margaret’s Thunder Heart). This image was taken in 2017 and includes four generations! Notice anything about their codes? Maternal lineage is just one thing you can discover by looking closely at the horses alphanumeric code.
When researcher Dr. Ronald Keiper began studying the Assateague horses in 1975, he developed a system that could identify, and trace the maternal ancestry of, each individual horse. The system of alpha-numeric identification numbers he came up with are now known as “Keiper numbers”
In 1975, there were only 44 horses in three harem bands living on the Maryland portion of Assateague Island. A harem band consists of a dominant stallion and his mares and immature offspring. Each of these bands was designated by the letter M, N or T. The dominant stallion of each band was given the number 1, resulting in Keiper numbers M1, N1 and T1 for the stallions. The other horses in each band were numbered consecutively beginning with 2 (M2, M3, T2, etc.). The letters at the end of each horse’s number indicate that particular individual’s maternal lineage and their birth years back to the original 1975 study horses.
At birth, each horse is assigned a Keiper number created by adding the letter for its birth year to its mother’s Keiper number. Birth years began in 1976 with A, B for foals born in 1977, C for 1978, up through Z in 2001. At that point the alphabet was started over with a dash in front of the letter, resulting in -A for foals born in 2002.
Accordingly, N6BMT-FN was born in 2015 to N6BMT-F; N6BMT-F was born in 2007 to N6BMT; N6BMT was born in 1995 to N6BM; N6BM was born in 1988 to N6B; N6B was born in 1977 to N6. N6 was present in the herd during the initial 1975 survey and so does not have a birth year letter. She was the sixth horse identified in “N” herd.
And the “X” horses? During the 1980s, birth records for the herd were not as closely maintained as they are today. By 1990, there were about 25 unidentified, mostly solid-colored horses living on the island that had been born in the mid-1980s. These horses were designated with an “X” and numbered consecutively. Following a complete genetic study of the horses in 2005, correct identification and parentage for most of these “X” horses were determined based on their genotypes. An example is the mare X13, who was confirmed as being N2BH. Five of these “X” horses could not be positively identified, and so they retained their “X” IDs. However, presumed offspring of the “X” mares living at the time of the genetic study were confirmed as descending from these “X” mares, and so were named according to the same protocol as all of the positively identified horses. The stallion X15NY, for example, was born to X15N in 2000; X15N was born in 1989 to X15, who was one of the five “X” individuals living in 2005 who could not be positively identified.