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Estrous Cycles in the Somali Wild Ass

Much of what is currently known about the reproductive physiology of wild equine species is based on behavioral observation or data collected from culled individuals.  It is tempting to fill in gaps in knowledge of a rare species by assuming similarity to a more common relative we know more about, however this can be misleading as even closely related species may not exhibit similar reproductive patterns.   Quantitative measurement of reproductive hormones in non-invasive samples such as feces can provide much greater detail and a significantly better understanding the estrus cycle and reproductive endocrinology, which can be of great help in preserving threatened and endangered species through successful ex-situ breeding programs.

A 2018 paper by Kozlowski et al. describes a two year study at the St. Louis Zoo during which they combined behavioral observation with endocrine data to characterize esterous cycles and pregnancy in the critically endangered Somali wild ass.   Captive females between 5 and 10 years of age were observed daily from May through October each year, and three times a week fecal samples were collected from each individual’s stall in the morning.   Fecal progestagens (Arbor Assays, DetectX® Progesterone EIA Kit, K025-H) and estrogens were measured estrous cycle patterns determined for each individual throughout the entire course of the study.   All of the test subjects conceived in the fall of 2012 and delivered between August and October of the following year so both pregnancy and post-partum data was available for all animals along with normal cycles.

Iterative analysis of the fecal hormone data determined that wild asses have an estrous cycle length of 27.2 ± 1.2 days and that they cycle throughout the year.   Luteal phase progesterone concentrations were approximately 3-fold higher than follicular phase levels.   In contrast fecal estrogen levels were not seen to vary through the estrous cycle. Progesterone concentrations during early pregnancy were low, within the range of the follicular phase, and could therefore not be used to establish the date of conception. Conception dates based on observed mating behavior provide an average gestation of 56.4 weeks.   Progesterone began increasing markedly approximately 16 weeks prior to birth, reached their maximum levels during the week of delivery, and fell back to baseline at birth.   Fecal estrogen levels during pregnancy were higher overall than those observed during non-conceptive cycles; peaking at ~ 20 weeks prior to birth and falling back to baseline 2-3 weeks before delivery.  Neither foal heat, nor lactational suppression of esterous cycles were observed following delivery.

This hormonal data demonstrates that Somali ass estrous cycles are longer then the 21-day cycle described for domestic horses, but similar to those observed in domestic donkeys , Przewalski’s horses, and Persian onagers.  The patterns of fecal progestagens and estrogens observed during pregnancy were similar to what is known for Grevy’s and plains zebras and domestic horses.

This work by Kozlowski et al. is the first to describe the reproductive endocrinology of the critically endangered Somali Ass and will hopefully improve captive breeding rates and support important conservation efforts.

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