Miniature Ponies: Similarities and Differences from Larger Breeds in Follicles and Hormones during the Estrous Cycle

Recent direct and indirect comparisons between Miniature ponies and larger breeds during the estrous cycle are reviewed. There were many similarities but profound differences in the Miniature ponies, including (1) greatly reduced follicle population within each of several follicle diameter classifications, (2) smaller number of growing follicles 10 mm or larger per ovulatory wave (1.5 ± 0.3 vs. 5.8 ± 0.9 in Miniature ponies vs. horses), (3) lower incidence of detectable diameter deviation, (4) ovulatory luteinizing hormone (LH) surge that involved three distinct phases, and (5) LH increase and decrease during the early luteal phase that followed the ovulatory surge. Diameter of the preovulatory follicle was smaller in Miniature ponies than in horses, but the difference was slight (38 vs. 44 mm; 14%) when compared with an extreme difference in median body weight (123 vs. 465 kg; 74%). The great reduction in follicle population with only a slight reduction in diameter of the preovulatory follicle in Miniature ponies compared with larger breeds is an important consideration during both experimental and clinical examinations.