RELATIONSHIP BETWEEN PLASMA TESTOSTERONE CONCENTRATIONS AND AGE, BREEDING-SEASON AND HAREM SIZE IN MISAKI FERAL HORSES

Jugular vein blood samples were collected from 23 young and sexual mat ure feral stallions to examine the relationship between plasma testost erone concentration and age, breeding season or harem size. Testostero ne concentration increased with the age of the stallions until they fo rmed their own harems, at about 4 to 6 years old. Seasonal variations in testosterone concentrations were observed, and found to be signific antly higher (P<0.001) throughout the breeding season than non-breedin g season, from 3 years of age. Testosterone levels were correlated wit h harem size for individual stallions. It can be inferred from these r esults that there is a relationship between plasma testosterone concen tration and age, breeding season and harem size.