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Why does my son's mare seem
interested in another mare in a bordering pasture? --via e-mail
Without further information, I can offer
explanations for a couple of likely scenarios. If the mare is just
buddying up with another mare along a common fence line, it could simply
be social behavior. Some pairs just have an affinity for one another;
they would buddy up if they were allowed direct contact, and still do
despite the fence. But I am guessing that there is more to it than just
standing near one another.
A common complaint is a mare that shows
stallion-like interest in other mares, and might also behave with males
as if she were another male. This would include prancing and vocalizing
with an arched neck like a stallion, sniffing expelled urine and feces
of other horses, posturing to urinate or defecating over the excrement
of other horses, sniffing a mare in the characteristic teasing pattern
of a stallion, performing the flehmen response, and even herding or
mounting other mares.
In the case of mares, any stallion-like
behavior is abnormal and reflects exposure to male hormones or very high
levels of estrogens (which can induce male-type behavior in the mare).
There are two common culprits. One is steroids given to enhance
performance. Race fillies are commonly given anabolic steroids or
testosterone to boost their athletic performance and increase muscle
mass. These drugs can make a mare's behavior almost indistinguishable
from that of a stallion in some cases, and the stallion-like behavior
can linger for as much as six months to a year after the medications are
no longer given. If you have owned this mare for a year or more and she
has had no steroids administered, then you can probably rule that out as
a cause.
Another source of hormones that can
produce male behavior in a mare is a tumor of the ovary, called a
granulosa cell tumor (GCT). If the problem is due to an ovarian tumor,
the behavior can be pretty variable over time. The tumor can produce too
much of several different male and female hormones, and depending upon
which combination of hormones the tumor is releasing, the behavior might
look fairly confused. She might be in estrus one minute, and fighting a
stallion the next. The possibility of a GCT can be easily investigated
by a veterinary examination and blood tests.
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