August 02, 2007
My Favorite Mule is a Hinny! - Part 1
By Amy K. McLean, Dr. Mel Yokoyama, and Dr. Sue Hengemuehle
Sowhatchet Mule Farm, Inc.
Part I. Why a hinny?
Have you ever seen or owned a mule that just seemed a little more like a horse or maybe a little more like a donkey or only associated with donkeys in the pasture? Maybe the hybrid was not a mule instead it was a hinny. I have always been intrigued with hinnys. I almost bought a beautiful chestnut hinny with a star on his forehead in Houston, TX one year from Dianne Mangrum. The hinny was very horse like and quite fancy. Needless, to say my father would not allow me to buy a hinny. It is my theory that there are more hinnys around than we realize.
When trying to raise hinnys versus mules claims have been made that it is more challenging to get a jenny to conceive when being bred to a stallion. According to Dr. David Pugh, a well renowned mule and donkey veterinarian at Auburn University, “when breeding a stallion to a jenny, which produces a hinny, the conception rate is quite low, only about 20%.” I have had other conversations with industry experts such as, Dr. Tex Taylor, a retired veterinarianary surgeon from Texas A&M University, who has maintained his own private mammoth donkey research herd for decades. He has suggested that the decreased conception rate is actually due to problems associated with an outer protein covering on the stallion’s sperm cells commonly referred to as the acrosome. It is possible that the acrosome can not penetrate the outer covering of the jenny’s ovum called the zona pellucida. The outer membrane layer of the ovum (egg) or zona pellucida is made up of carbohydrate glycoprotein receptors and if the proteins are not compatible among the two species, fertilization may not be able to take place. Therefore, it is possible that the receptors can not or do not likely bind to the stallion spermatozoa.
Typically, the acrosome acts as an enzyme and breaks down the zona pellucida by drilling into the outer shell. In order to better understand why or why not the acrosome of the stallion maybe different from that of the jack I spoke with a reproduction specialist. Ms. Angela Maschari-Busta, a reproduction specialist at Michigan State University, provided me with more information about proteins found on the head of the sperm cells as well as a better understanding of what happens to sperm cells in the process of fertilization. She has worked exclusively for years with bovine sperm cells, specifically sexed semen (mostly all female cells) and has suggested to me that it’s possible that the head of the sperm which is covered in proteins are species specific. Furthermore, the intrauterine environment according to Angela could also have a negative impact on the stallion’s spermatazoa viability. Typically the sperm cells attach to the oviductal epithelium cells found in the lining of the uterine tubal(s). This process is mediated by glycoproteins found on the spermatozoa’s head especially galactose-binding proteins (Sabeur, 2006). Researchers have noted that the carbohydrates vary among animal species but the adhesion of equine spermatozoa binding to oviductal epithelium is carbohydrate dependent (DeMott, 1995). At this point the spermatozoa restore their energy by absorbing nutrients such as carbohydrates (needed for binding to the epithelium) or calcium (Dobrinski, 1996).
If the jenny’s intrauterine environment for example is higher or lower in carbohydrates or calcium this could create a hostile environment for the stallion’s spermatozoa and could cause the cells to either (1) die or (2) be motile but not fertile. In mares galactosyl residues are responsible for binding to the stallions spermatozoa in the epithelium but what about in the jenny is it the same residue (Ball, 1997). If the ligand binding residue is different then it is likely that the stallion’s sperm cell could die. To make matters even more interesting, some research has been dedicated to comparing a protein known as zonadhesin, glycoprotein found on equine spermatozoa. Among all three equine species, zebra, donkey and horse, this particular protein has been reported to be the same (Breazeale, 2002). Other reasons on why the conception rate is so much lower could be related to sperm-zona interactions which are also believed to be mediated by carbohydrate recognition (Yanagimachi, 1994).
According to Ms. Maschari, each mammalian species varies in the type of protein found on the head of the sperm or outer shell of the female’s ovum. So, the theory of why conception rates are lower when producing hinnys may simply be due to specifies specific related issues. Until more research is done one can only assume why the conception rate is typically thought to be lower. Although there are hinnys found throughout the world but just how many? Of course I’m sure you are wondering how we can produce mules if the proteins or receptors are not compatible. Great question and this is my assumption that the reserve cross, the jack sperm cells are more adaptable and have less problems binding to the receptor and penetrating the zona as well as possibly be able to adapt and live in the oviduct epithelium of the mare. Before we go any further I would like to know how many people breed for hinnys each year. I would love to get some feedback and have a better idea!
There are many different ideas and beliefs related to producing hinnys. Some fallacies and myths about hinnys include ideas such as, “the hinny is not physically sound when compared to mules, and hinnys often have internal problems such as, organs that are not developed.” Unfortunately, our industry is lacking in scientific research in the area of hinnys and mules and many of these questions or beliefs can not be answered or disputed. I did have a rare chance a few months ago to help with a hinny foal. Back in February, I had visited the Turning Point Donkey Rescue in Dansville, Michigan and met Ms. Sharon Windsor in sub-degree temperature! It was so cold that the diesel had frozen in the truck that day! Anyway, it did not stop me from visiting the farm and seeing the donkeys. A few weeks later Ms. Windsor called to ask me “how do you imprint a hinny foal?” I told her I assumed it would be like imprinting a donkey or mule foal but I had never seen a baby hinny in my twenty plus years of mules and donkey babies. About an hour later, I received another call from her and the foal was not nursing so I made another visit to the farm. The foal like Ms. Windsor said, “She was the funniest looking donkey foal she had ever seen.” We both concluded the foal was a hinny based on its physical appearance.
The foal was a bright red sorrel and cute as a button to say the least but the jenny was not thrilled about the foal nursing. It made me wonder if the jenny was not accepting the foal because it was a hybrid and not a donkey. After an army of volunteers and Dr. Colby (the vet on call) trying to get the foal to nurse on its own and that didn’t work the jenny was milked. The foal was fed colostrum from a frozen source as well as colostrum that were milked straight from the jenny but throughout the day the condition of the foal deteriorated. Later that evening we (Sharon and I) took the foal and jenny to the Michigan State University Veterinary Teaching Hospital. By that point, the donkey mom, Curly, had become quite attached to her baby. The foal rode the whole way in my lap in the cab of the truck so we could keep her warm.
Unfortunately, once arriving at the vet school hospital, there were some difficulties in passing a feeding tube and it took 2 and half hours before the foal received plasma and she passed away later that night. There are some anatomical differences in donkeys, mules, and hinnys, when compared to horses that makes it more difficult to some times pass a tube or even collect blood from the jugular. In 2002 at the American Association Equine Practitioners Annual Meeting, several seminars were dedicated to mules and donkeys and how to treat them. If you email me I can provide you with copies of the articles if you are interested in receiving them and sharing with your vet(s). The proceedings include great diagrams and descriptions on many of the anatomical differences such as laryngeal anatomy which is obviously somewhat different than that of a horse because donkeys don’t whinny, they bray!
After the foal died I felt that it was very important to find out why this foal had died. I couldn’t help but wonder if some of the myths about hinnys were true such as the tales I had heard about their internal organs not being developed properly. Considering that the foal was a hinny to our belief, I thought this was a somewhat rare and unique opportunity to learn from the first hinny foal I had ever seen in 20 plus years! The foal was submitted and taken to necropsy by Dr. Carla Carleton, an endocrinologist at MSU’s vet school, who has an interest in mules and donkey and is serving on my graduate committee. Once the foal was submitted to necropsy, Dr. Dalen Agnew, was the pathologist assigned to the case and he did an outstanding job and really cared about why this mysterious animal passed away.