Drs. Christina Nagel, Maria Melchert, Christine Aurich and Jörg Aurich created a study to determine if the size of the mare affected the birthing process. The team used 23 pregnant mares and studied hormone and heart-related changes that occur in the mare and the fetus around birth. The groups consisted of six Shetland Pony (small); eight Haflinger (medium) and nine warmblood (large) mares.
They discovered that foal weight was approximately 10 percent of mare weight, no matter the size of the mare. Placenta weight was highest in Warmbloods; it did not differ between the small- and medium-sized mares. Progestins and cortisol were also highest in the larger mares. Progestin concentrations decreased as foaling got closer and cortisol increased as parturition was imminent in all sizes of horse studied.
Heart rates in all mares increased before foaling, but the most pronounced increase was in the small mares. Overall, the small mares foaled earlier than the larger mares.
Missed heartbeats during foaling occurred regularly in the larger mares, but only occasionally in the small- and medium-sized mares. The scientists report that this is most likely because of differences in heart efficiency. Small- and medium-sized mares occasionally had missed beats during the expulsive phase of foaling and the first two hours after delivery. The researchers suggest that this is an effect of breed, not size.
The team concludes that fit, athletic horses have highly efficient respiratory and cardiovascular systems; while at rest, these horses have reduced heart rates. Fetal heart rate also went down toward birth, with the most prominent reduction in full-sized horses.
The authors determined that mare of different sizes show both similarities and differences in cardiac and endocrine changes as the time for them to give birth nears. They note that late pregnancy puts substantial strain on the mare's cardiovascular system, but that mares are well adapted to these demands, no matter their size.
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