Zebra mare reproductive tract
Keywords: Equus, chapmani, Chapmans, Zebra, Equus quagga chapmani, uterus, ovaries
Note: Both zebra and zebras are widely accepted words as plurals for the singular form "zebra". The author has chose to use the term zebras for the plural form in this entry.
Although some disagreement exists on the taxonomy of Zebras there appear to be seven extant species of Zebras and a few subspecies within some of those species. Equus grevyi is the largest zebra and has a diploid chromosome number of 46. Plains zebras such as the Equus quagga chapmani featured in this LORI entry, have 44 chromosomes while Mountain zebras (Equus zebra zebra) have 32. Some zebras have finer striping than others, some differ in dentition, conformation and so on.
The reproductive biology and anatomy of horses and zebras are remarkably similar.
Below, see an image of the reproductive tract of a mature (exact age unknown) Equus quagga chapmani. The parity of the mare was also unknown but examination of the tract suggested that she had been pregnant at least once. In this image, the cervix appears to be shorter than that of a domestic mare. However, detailed examinations of the reproductive tracts of 310 Equus quagga showed an average length of 4.9 cm; similar to than of domestic mares.
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The image below accentuates the remarkable similarity between this animal and domestic mares with regard to the vestibulovaginal seal (arrows) i.e. the remnants of a hymen.
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Again, below, the ovaries and adnexa of a Chapman's zebra are remarkably similar to those structures in domestic mares in both size and architecture. The author's fingertips have been inserted into the ovarian bursa.
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This mare was examined in early January (in the northern hemisphere) when most domestic mares would have been in winter anestrus. This probably explains the lack of activity in her ovaries. Multiple ovulations are rare in zebras (twins being virtually unknown) but secondary ovulations do occur during pregnancy and accessory corpora lutea form as is the case in domestic mares.
The author was struck by the similarity of the clitoris in this zebra and those in domestic mares. There was also at least one well developed clitoral sinus within dorsal surface of the clitoris. This led the authhor to contemplate the potential for Taylorella equigenitals or or other Taylorella species to cause reproductive failure in zebras.
| General commentary: Like Equus caballus, Zebras are seasonal breeders, responding to increased day length. However, there appears to a remarkable effect of increased energy availability in modulating that response to day length. From numerous observation by many authors, zebra mares appear to have standing estrous periods that are similar in duration to those in domestic mares. Their gestation is however, substantially longer. Although gestation as short as 336 days has been recorded, most zebras foal after gestations of approximately 370 days. Normal gestation can be as long as 425 days. Progesterone, eCG and estrogen profiles are similar to those in mares and certainly, urine estrogens shed during urination in sand bedding can be used to diagnose pregnancy. Similarly, assays for eCG in urine may hold promise for pregnancy diagnosis. Indeed, the latter has shown to be the case in domestic mares. Mammary development, foaling, "foal heat" and neonatal physiology in zebras are similar to the same features in domestic horses but weaning of zebra foals only occurs after 10 to 12 months. Interestingly, zebra of all stripes (!) can interbreed with domestic horse and donkeys and produce normal, live offspring. In many cases, those hybrid offspring are fertile as well. |
Selected references:
Allen, W.R. and Short, R.V. 1997. Interspecific and extraspecific pregnancies in
equids: Anything Goes. J.Heredity 8:384-392
Asa, C.S. et al. 2001. Patterns of excretion of fecal estradiol and progesterone and urinary chorionic gonadotropin in Grevy’s zebras (Equus grevyi): Ovulatory cycles and pregnancy
Zoo Biology. 20: 185–195
Smuts, G.L. 1976. Reproduction in the zebra mare Equus burchelli antiquorum from the Kruger National Park Koedoe.19: 89-132
