Technical Abstract: In a related paper, we showed that mares that reproduced earlier in life tended to have a higher fecundity rate due to a decrease in the duration of inter-birth intervals relative to mares that reproduced later. However we know that younger mares are subordinate to older mares. Hence, costs associated with dominance rank might offset the benefits of earlier reproduction. We compared harassment of foals for female Jeju ponies that first reproduced at 3-years of age with that for females that first reproduced at 5-years. As a consequence of the linear dominance hierarchy, foals born and guarded by younger mares were harassed more frequently than foals whose mothers were older when they first reproduced. A mare that reproduced earlier in life guarded more closely and intervened between her foal and neighboring mares more frequently than those mares which first reproduced when older. This need to guard their foals and the harm that might ensue from frequent harassment might counter-balance selection towards earlier reproduction in mares.