Journal of Heredity 30(12): 548-551 (1939)

Utah State Agricultural College

From time to time fertile female mules have been reported, and this is merely another instance of this rare phenomenon. Mr. Lorenzo Hubbell, a trader of Oraibi, Arizona, purchased the mule from a Navajo Indian on July 25, 1939. Five days later she gave birth to the colt shown with her. So that an unquestionable record might be had, moving pictures were made of the birth of this colt. Mr. Hubbell states that there can be no doubt that the mother is a mule, on the basis of what he has been able to learn of her ancestry as well as her conformation. According to the Indian from whom Mr. Hubbell purchased the mule, the sire of this foal is a jack. The mule is nine years old, weighs about 700 pounds, and has had one colt previously, of which no description is available.

The ninth day following foaling she was bred to a crossbred Thoroughbred-Arabian stallion. The mule and her foal were exhibited at the county fair at Nephi, Utah, September 7, 1939.

Figure 17
Photograph of the fertile mule and her male foal, which was sired by a jack and has the appearance of a normal mule. The mule has since been bred to a stallion.

W. S. ANDERSON University of Kentucky

*A few days after this note was received similar information and another illustration of the Indiana mule and her colt were submitted by Dr. Edwin E. Jacobs, Ashland College, Ashland, Ohio.—Editor.

THERE are very few living animals whose mothers are mules. Two of these rarities are owned by the Texas A. and M. College, and another is owned by W. H. Mobley and Son of Columbus, Indiana.

One of the two in Texas is a mare mule by a jack out of "Old Bec" described in this JOURNAL, Sept., 1928. She is not a three-quarter ass as her ancestry would indicate but a mule in all her features. Unlike her mother, she proved to he sterile. The other Texas animal is the son of a saddle stallion out of "Old Bec." He is a five-gaited saddle horse and has been shown as such at Texas fairs. He has none of the ass-like qualities of his mule mother. He has been mated to several mares and the foals by him show no evidence of hybrid blood.

Another animal with a mule mother, is a male foal by a Percheron stallion and was five weeks old in June, 1938, when photographed (Figure 18). He exhibits the Percheron characteristics for a foal of that age. There is complete absence of any influence of the ass ancestry coming to him from his mother. His owners intend to keep him and test him by breeding.

These three living animals explain those rare occasions when a female mule becomes a mother. It is known that the ass has a greater number of chromosomes than the horse. This difference does not interfere with an ovum of a mare developing when fertilized by a sperm of a jack; but when the hybrid reaches physical maturity and the gonads begin to function the inequality of the chromosome numbers prevents the reproductive glands from producing sex cells. The author has examined glands of male mules after castration but has never found any spermatozoa.

The production of the three animals by mare mules indicates that their ovaries did mature viable ova. Since two of the foals by stallions are horses without any features of the mule or ass, it is certain the ova that produced them carried no chromosomes from the ass. The mare mule out of "Old Bec" and by a jack is in every way a mule; not an animal with three-fourths of her genes from the ass. In this case the ovum evidently carried no chromosomes of the ass but only the chromosomes which its mother received from her mother, a female horse. The explanation appears to be, that if the female mule produces an ovum without ass chromosomes it is viable. It could happen, if in the division of the oocyte, all the ass chromosomes clung together and went into the polar, or waste cell, the final ovum would contain only the chromosomes received from the horse ancestry. If this hypothetical explanation be true then a viable ovum of a mare mule would carry the haploid number of horse chromosomes just as the ova of all female horses. On the other hand no viable ova are produced by female mules except the rare ones in which the sister polar cell carried away all the ass chromosomes.

This would explain the bearing of a horse-like colt by a fertile mule, recorded skeptically by Lloyd Jones in this JOURNAL in 1916. It was reasonably held that an animal with one-fourth ass ancestry should have some characters of the ass, but as pointed out above, this, instead of being ground for doubt, may really be what is to be expected in cases of mule fertility. As the word of the breeder was the main evidence, the rare occurrences were frequently looked upon with more doubt than interest.

Figure 18
Photographs of the female mule from Indiana and the horse-like foal she produced when mated to a Percheron stallion. The colt was five weeks old when the picture was taken. This survey of three instances of fertile mules suggests that some female mules may occasionally produce a pure horse ovum, uncontaminated by ass ancestry, and that these rare ova are fertile.

Further evidence that the foregoing hypothesis is true is found in a recent statement from the Texas A. & M. College that "the son of 'Old Bec' is fertile and has quite a number of colts around that section of the country. His daughters also are fertile. He is rather a popular sire locally and is used for saddle purposes." None of his offspring show any reversion to their ass ancestry. If he carried any of the chromosomes of the ass, hybrid qualities would appear in his offspring. The absence of ass-like qualities seems conclusive evidence that he himself originated from an ovum with no chromosomes from his grandsire, the jack.

In 1918 the Kentucky Experiment Station purchased a mare mule that had aborted a fetus of about four or five months of age. She was mated with a stallion but failed to conceive. The fetus was preserved and appears mule-like. The high fetal mortality rate of mules that conceive may perhaps be explained by combinations of chromosomes being formed which are only partially viable.

The dream of mule breeders has been to find fertile mare mules, hoping to get a race of perpetuating hybrids. The dream it seems cannot be realized as the reversion to the horse of the hybrid ancestry takes place, when they are fertile.

If the above hypothesis is correct, the two male horses out of mare mules hereditarily have no maternal grandfathers as the hereditary material which each mother mule received from her sire was all eliminated in the normal random assortment as the ova were produced, and only the mother's material gift of chromosomes entered the viable eggs from which the two horses evolved.