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Title: Age of the Jeju pony (Equus caballus) at first reproduction: potential costs for foals and mothers that first reproduce early in life

item RHO, JEONG - Seoul Zoo
item Srygley, Robert
item CHOE, JAE

Submitted to: Ethology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/22/2008
Publication Date: 1/17/2009
Citation: Rho, J.R., Srygley, R.B., Choe, J.C. 2009. Age of the Jeju pony (Equus caballus) at first reproduction: potential costs for foals and mothers that first reproduce early in life. Journal of Ethology. Journal of Ethology. 27(3):483-488.

Interpretive Summary: We studied the reproductive behavior of Jeju ponies on Jeju Island, Korea. Older mares are dominant to younger ones, and so we compared the reproductive behavior of mares that first foaled earlier in life to those that first foaled later. Mares harassed the offspring of young mothers, and as a consequence, young mothers guarded their foals more closely and intervened to defend their foal more frequently than older mothers. Relative to mares that first breed later in life, mares that begin to breed at a younger age might have more offspring in their lifetime, but they also need to defend their foals more vigorously. This research is important to the conservation of the Jeju pony, a population of wild horses established by the Mongols during the 13th-14th centuries.

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.