Dr. Bruce Nock is a tenured faculty member at Washington University School of Medicine, and founder of Liberated Horsemanship.
I previously described what happens to a wild horse’s physiology when he/she suffers the trauma of being chased by a helicopter and sequestered into captivity. I suggested, if a wild horse doesn’t die straight off from the immediate devastation and commotion, it puts him on a path of accelerated deterioration.
Common sense should be sufficient to warn the BLM against compounding the problem by doing “gathers,” i.e., stampedes, in the heat of the summer. But then, there are also plenty of public warnings about hot weather activities. So, does our government or the individuals managing our wild horses really need the testimony of a scientist to convince them chasing horses with a helicopter during the summer is a bad
idea? If so, it’s a sad state of affaires.
So, let me tell you a little about horses and heat. It might surprise you but it may take less than 20 minutes of moderate exercise to raise a horse’s body temperature to dangerous levels when it is hot and humid. Add the sympathetic nervous system activation triggered by being chased by a helicopter in summer heat and you have the potential for real problems … deadly problems. Dehydration which can cause hypotension, colic, convulsions, and heart and renal failure may be the most obvious.
Horses can sweat profusely ... up to 30 liters per hour. It is their primary eans of thermoregulation. Sweat is mainly water. But other important stuff is also lost along with the water … minerals and trace elements, for example. Salts, compounds like NaCl, MgCl and KCl, are four times more concentrated in horse sweat than in our sweat. This “other stuff” has to be replenished too. Water alone is not sufficient. In fact, plain water, without electrolytes, may even exacerbate dehydration. 
But the high ambient temperatures of summer aren’t the only problem. The year’s crop of foals are only months old during their first summer. It may seem like the young should be more resilient and more able to survive stampedes than older horses. That may be so in some ways. But early development is a fragile time when stress/trauma can have a devastating impact that can last a life time. Let me tell you about epigenetics.
Epigenetics is about the microenvironment surrounding a gene. The microenvironment determines how active a gene is. I’ll give you an example. The genes in the cells that produce the white spots and black spots of a calico cat are exactly the same. But the expression of certain genes differs within the black and white areas due to
differences in the epigenome.
But color is just one example. The same sort of differences in gene expression
determine whether an organ will develop into a heart, liver or brain, for example. All three organs have exactly the same genes but the expression of the genes is different in each organ due to epigenetic differences … differences in the genome microenvironment.
Here’s the kicker. Genes are fixed. A horse is stuck with those he/she inherits. But the epigenome, isn’t. It can change throughout life, increasing or decreasing the activity of certain genes in the process. Epigenetic changes can even turn a gene completely off.
So what? What does this have to do with stampedes and foals? Well, adversity early in life is now thought to cause epigenetic patterns/marks that can adversely affect mental and physical health and increase reactivity to stress throughout life. The damage might not be immediately evident as colic or some other malady but it is real nevertheless.
If this isn’t enough to infuriate you about summer stampedes conducted by the BLM, think about this. Some epigenetic marks appear to be inheritable despite extensive reprogramming of the epigenome in the gametes and during early embryo development. In plain words, that means what the government does to our wild horses may not only affect the health and welfare of the current generation, but future generations as well.
 B. Nock, PhD, Neurobiologist, Washington University School of Medicine: Wild Horses—The Stress of Captivity. Liberated Horsemanship Press. Commissioned by The American Wild Horse Preservation Campaign. wildhorsepreservation.com, March 16, 2010.
 M.I. Lindinger, MSc, PhD, animal and exercise physiologist at the University of Guelph cited by T. Pitman, University of Guelph: Summer Riding: When the Rider is Hot, the Horse is Hotter, TheHorse.com, Article # 16625, 2010.