Parasites and social behavior of island feral horses

Equus caballus at Shackelford Banks, North Carolina, are heavily parasitized by intestinal worms and harassed by dipterans. Although both types influence behavior only internal parasites affect bodily condition and the structuring of horse society. Thirteen species of internal parasites were identified, but only 4 of 13 groups contain them all and even within groups differences among individuals are large. The most important environmental factors influencing egg production are season and a group's location on the island, presumably because of salinity and soil differences and their effects on ova survival. Of the social and life history factors, age and group size, but neither reproductive state nor dominance status, are important. The fitness consequences of internal parasitism may be large since the number emitted is negatively correlated with next year's bodily condition. Biting fly burdens are also affected by a variety of environmental factors. In general, horses are covered with more flies on sunny days, when winds are moderately brisk, when occupying dunes, and around mid-day. Fly burden is affected by reproductive condition and dominance status and tends to decrease as groups increase in size. -from Authors