Stallions are often shy, she said, because "they're in a barn with people and strangers and strange animals."
So mare and stallion are often strangers, not that different from nervous teenagers on a first date.
"We have, in many ways, an analogous situation," she said, between horses and humans.
What, pray tell, can be done?
"Most of what we do," she said, "is just give them positive experiences."
What McDonnell and her colleagues at New Bolton offer, among other remedies, is a mare that, instead of being sexy every 21 days, has been given enough hormones to be ever ready to be near and dear.
And, lo and behold, soon a stallion becomes a worthy stallion, as only a stallion can truly be a stallion.
"It's no accident," she said, "that stallions are referred to so much in literature as a symbol of potency."
Since 1991, McDonnell and two New Bolton veterinarians - Robert Kenney and Charles Love - have been studying the sexual function of horses with a five- year, $338,000 grant from the National Institutes of Health.
One reason for the NIH interest, McDonnell said, is that what helps a stallion can help a man whose body has developed a similar disinclination.
"It's very fortunate for us in our research that the horse is more like the human than any other animal - in its copulatory behavior and its anatomy and, we think, the physiology of . . . erection and ejaculation."
Consider what she now says as though she were speaking of a young man who expected to become a father, but is paralyzed in an accident.
"Occasionally we have horses that, for some reason are unable to
breed . . .
"We have a method for inducing ejaculation with a drug so that we can obtain a semen sample for freezing," and later insemination of a female.
The drug, she said, "simply causes the ejaculation reflex without any stimulation or copulation."
She has happened upon such drugs in reading medical studies in which the drugs were used to treat problems other than sexual dysfunction.
Men with spinal cord injuries might benefit from such drugs, she said, as might "diabetic men who, because of their diabetes, have had degeneration of the nerves and blood vessels serving those functions . . .
"I've talked to urologists," she said, "who are beginning to use some of these compounds to enhance erection or ejaculation."
New Bolton is "contributing a fair amount" to such work with humans, she said, "but I wouldn't say it was leading the way in the human field.
"We're really working in the veterinary field."
Inside a New Bolton barn one sweltering morning last week, McDonnell, technician Karen Soule and a visiting vet were engaged in an everyday effort: collecting semen.
In one low stall was Red Brick Road, a 20-year-old mare. Outside the stall was Smartest Remark, a 9-year-old stallion, a former racehorse.
The stallion was led to the mare and, with a sniff of her aroma that drew his teeth back into a semblance of a grin, seemed to be ready to ask whether she was a Libra or Scorpio.
The mare was led across the room, to stand near a leather dummy resembling the gymnastic equipment known as, well, as a horse.
The stallion was led to the dummy and, with the mare clearly in view and apparently in mind, the stallion mounted the dummy and, while one of the women held a closed-end sleeve, did his duty.
"We also have horses," McDonnell said, "who get their feelings hurt.
"If a mare is difficult with them, they get sexually depressed. If she kicks him, refuses him, . . . it can damage the stallion's confidence."
This stallion was certainly not one of the troubled patients.
Stallion and mare both are New Bolton property, like 20 ponies that had been gathered from the pastures earlier that morning, for research.
Last month, McDonnell and her colleagues published a paper in a scholarly journal. This month, she said, they intend to submit four more.
They publish. They sell semen to owners of mares across the nation hoping to breed with good blood lines.
And, most important to owners of high-priced horse flesh, they return stallions to productive lives.
The work is special enough that vets come from afar to study it.
The vet working with McDonnell and Soule that morning at the center northwest of Longwood Gardens was Margosia Pozor, beginning a four-week visit
from the University of Agriculture in Cracow, Poland.
In the afternoon, she was joined by Itzak Zilberdriver a large-animal vet
from the Golan Heights, the territory occupied by Israel.
"I got my degree in Vienna," he said, "and as far as I know, there is nothing like this in Vienna."