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.
Madison, GA
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.
September 09, 2007
My Favorite Mule is a Hinny! - Part 2
By Amy K. McLean, Dr. Mel Yokoyama, and Dr. Sue Hengemuehle
Sowhatchet Mule Farm, Inc.
Madison, GA
Part II. Are hinnys different than mules?
The preliminary report indicated the foal was stressed and the
lungs were bright red, there was a possibility the foal had passed
away from equine herpes virus infection or septicemia. There was
some evidence that the foal was stressed and possibly the jenny had
an intrauterine infection. Curly, the jenny was later cultured and
was cultured clean (no infection was found). One finding in the
preliminary report which was interesting was a tubular structure
attached to the broad ligament. This structure created a lot of
conversation among many professionals in regards to what it was and
was the foal possibly a hermaphrodite. The final necropsy report
showed the additional tubular structure to be an enlarged blood
vessel that is larger in size when compared to a horse. It’s
possible that the larger blood vessel is normal in desert creatures
like donkeys, mules, and hinnys. The report indicated that the foal
most likely died from lack of nutrition and not due to an abnormal
organ. Another very interesting event associated with the foal was
the results of the karotyping.
Only 62 chromosomes were counted from this obscure sample
indicating the foal was not a hinny but actually a donkey. I’m still
not convinced the foal was a donkey due to its physical appearance
and more blood samples from known hinnys, mules, and donkeys are in
the process of being taken and submitted for karotyping. Also, when
reviewing the photos and slides of the foal I had the chance to look
at the ovarian tissue of this animal. There was no follicular
activity meaning no follicles were present and follicular growth was
occurring to produce eggs (ovums) for reproduction. However,
typically sterile animals do not reproduce so is this unusual in a
hinny or mule to lack follicles? Again, some mules and hinnys in
China and Africa (Morocco, Ethiopia, and Kenya) have reported mules
and hinnys that have produced foals but would more mules and hinnys
have foals if they were bred? When compared to horse foals (fillies)
multiple follicular growth could be identified in the tissue. This
leads me to believe the animal along with its phenotypical
characteristics was a hinny. A sterile animal such as a hinny or
mule can produce ovums and sperm cells but not all of them due to
complications with genes pairing with one another. Although, maybe
this is typical in a jenny foal to lack follicular growth in her
ovarian tissue, or maybe this is typical of mule and hinnys, we
don’t know because there were no other samples to compare the tissue
to. Also, this brings up the question about how many hinnys have
been karotyped to compare to and some of the journal articles I have
read and reviewed just suggested that typically a mule or hinny has
63 but who really knows because how many mules and hinnys have been
tested? So, you can be the judge look at the photos and make up your
own mind. Hinny or donkey foal?
Donkeys have 31 pairs of chromosomes or 62 in total. Typically,
mules and hinnys have 63 chromosomes, they inherit 31 chromosomes
from the donkey (E. asinus) and 32 from the horse (E. caballus)
(Trujillo). After reading several scientific articles I am not 100%
convinced that all hybrids contain 63 chromosomes and maybe more
research needs to be done. Most of the reports I have read in
regards to hinnys and mules are the rare cases where both have
produced offspring. Even more interesting one article found that an
intact male hinny contained both horse and donkey sperm cells in
regards to chromosome counts (i.e. some contained 62 chromosomes and
others contained 64 but none contained 63 chromosomes) (Trujillo).
The mystery continues!
