Sweating Like a Pig

With summer fast approaching in the northern hemisphere and global temperatures rising, dealing with heat stress and preventing overheating is an issue not only for humans, but for pets and other animals. Animals have developed through evolution many ways of coping with heat stress depending on their environment, energy expenditure, and access to water. Some of their biological mechanisms are more effective than others, and animals with less efficient approaches are at an increased risk of heat related illnesses. Understanding the differences can help humans protect themselves and their pets from the heat.

Humans cope with heat in a unique way—we sweat, and as a species, we are very good at it. On a hot day in the desert most humans can produce 12 liters of sweat at an average rate of one liter per hour with a maximal capacity of around 3.5 liters per hour. Sweating works by the process of evaporative cooling. When the sweat on our skin evaporates, transforming from a liquid into a gas, heat from the blood in vessels just under our skin is carried with it, helping to reduce excess body heat. When environmental temperatures are higher than skin temperatures, we depend on sweating to prevent overheating.

Sweat is produced in humans by two different types of specialized glands in the skin, apocrine and eccrine sweat glands. The latter are by far the most abundant and are responsible for most of our sweat production. Eccrine sweat glands are most numerous on our palms and soles, then in decreasing order on our head, trunk, and extremities.  By contrast, apocrine glands are highly concentrated in our axilla or underarm regions and contribute little to thermal cooling because of their location.

The sweat produced by eccrine glands is primarily water with a small amount of sodium chloride, or salt, and even smaller amounts of potassium. As we become acclimated to a hot environment, the amount of salt in our sweat decreases, increasing the efficiency of evaporation. Sweat produced by apocrine glands, in contrast, is rich in lipids, making it viscous, and contains proteins, sugars, and ammonia. While the primary function of the eccrine glands is to help dissipate heat, the function of apocrine glands in humans is less clear. In non-human primates they are believed to function as scent glands and are responsible for pheromone production.

While hoofed animals, such as donkeys, cows, horses, and camels all have sweat glands, the role sweating plays in their thermoregulation varies. For example, horses, like humans, are prodigious sweaters. Also, like humans, their skin is well vascularized, which, combined with their summer short hair coats, facilitates evaporative cooling when they sweat. Donkeys and camels, however, have thick, dense coats year-round that can help insulate them from heat gain in direct sunlight. Their skin is not highly vascularized, and they do not sweat to the same degree as horses.

Although both are equines, donkeys (Equus asinus) and horses (Equus ferus caballus) evolved different mechanisms for thermal cooling which likely reflect different evolutionary pressures. Horses evolved on the plains of North America, then crossed a land bridge and spread across Eurasia, where they were subsequently domesticated. In these regions, where water was relatively plentiful and the climate temperate, sweating allowed the horse to maintain a high activity level during hot weather, a capacity valued by the tribes that used them for transportation and conquest. In contrast, donkeys evolved and were domesticated in North Africa, where water was scarce and the climate hot and dry. Sweating under these conditions could predispose the animals to dehydration, and therefore thick, insulating coats and a reduced activity level were more advantageous approaches.

Thermal cooling by sweating, however, is not an option for most pets. Although dogs do have sweat glands, located primarily on their paws, panting—rapid, shallow, open-mouth breathing—is their most effective mechanism to eliminate excess body heat. Like sweating, it is a form of evaporative cooling, utilizing well vascularized tissues of the canine respiratory tract and excessive production of saliva.

The impact of the length and density of a dog’s coat on heat tolerance is controversial among some dog owners. Most dog breeds originating in colder climates, such as Alaskan Malamutes and Huskies, have long coats, often with dense undercoats, which serve primarily to keep them warm. Whether or not those thick coats help or hurt the dog stay cool in warm weather is debated. Owners who believe the heavy coats impair the dogs’ cooling capacity shave them off.  However, according to the American Kennel Club, shaving these dogs increases their risk of sunburn and insect bites, while potentially causing long term damage to the coat. In addition, in strong sunlight, these coats may help decrease the amount of heat gained through environmental exposure, much like the coats of donkeys and camels.

A study of dogs presenting with heatstroke in the United Kingdom doesn’t clear up the debate. Researchers reported that Chows, known for their long, thick coats, were the breed most seen in heat stress. However, Bulldogs and French Bulldogs, both short-coated breeds, were also at a significantly elevated risk of developing heatstroke compared to other dog breeds. Both are brachycephalic, having extremely shortened muzzles, which may decrease the effectiveness of panting as a means of thermal cooling due to the decreased surface area of the oral cavity. These study results suggest that there are likely many factors that contribute to a dog’s risk of developing heat related injuries, which include coat length, muzzle confirmation, activity level, and body mass.

Little is known about the heat tolerance of domesticated cats. They have sweat glands, like dogs, primarily on their paws, so the contribution of sweat to thermal cooling is likely minimal. They pant, also like a dog, and will produce excess saliva when overheated. Cats also lick their fur when hot, another form of evaporative cooling, and they demonstrate behavioral responses to heat, such as seeking out cool floors or shady areas, behaviors also observed in dogs. Because they are generally less active than dogs, they may be at a lower risk of developing heat-related injuries, but cat owners, too, should be aware of the risk higher temperatures pose to their pets.

Finally, pigs also have sweat glands, but relative to their size, they do not have enough to make sweating an effective mechanism for thermal cooling. Their respiratory tract is also small compared to their size, so panting is not an efficient mechanism either. Wild boar, the ancestor of the domesticated pig, keep cool by staying in the shade and, yes, wallowing in mud and watering holes during the hottest part of the day. Modern production facilities for swine don’t allow for wallowing, but use a number of methods to try to keep the pigs cool, including ventilating fans, sprinklers, and even cool rooms for pregnant sows. Like pigs, cattle are also not effective sweaters. Pastured animals must be provided with adequate shade and plenty of water, while those in feedlots or milking parlors are provided with misters and cooling fans like pigs.

So why do we say we are “sweating like a pig” when we are sweating profusely? The ‘pig’ in ‘sweating like a pig’ is short for ‘pig iron’. Pig iron is an intermediate product of the iron industry, also known as crude iron. To mold the iron, it is heated to a very high temperature to liquify it, and then the liquid is poured into molds. The traditional shape of the mold used for pig iron consists of a single primary channel with multiple smaller molds branching from it resembling a litter of piglets nursing.  As the metal cools, moisture from the surrounding air condenses on the surface of the iron, which looks like sweat, thus sweating like a pig.
‍

‍

Pet owners are learning that they need to adapt to a warming globe. In 2016, there were 395 confirmed cases of heat related illnesses in dogs in the United Kingdom, not a country known for its particularly warm weather. Avoiding extreme heat is not always possible and therefore a number of devices, including cooling vests, pads, and collars, have been developed for dogs. However, one study found that teaching military working dogs to voluntarily dunk their heads in water to retrieve a toy or food treat was the most effective cooling method. It may also be the most amusing!

– Cindy Cole DVM, PhD, DACVCP

‍

First Five
First Five is our curated list of articles, studies, and publications for the month.‍

1/ The Secret of Orange Cats
The vast majority of fully orange cats are male, which suggests that the genetic code for orange color is carried on the X chromosome. As with other mammals, female cats have two X chromosomes, while males have one X and one Y. Any male cat carrying the orange trait on its one X chromosome will be entirely orange. A female would need to inherit the trait on both X chromosomes (one from each of her parents) to be completely orange, making it less likely. Instead, most female cats with orange fur have patchy patterns, calico or tortoiseshell, that include black and white. Scientists recently discovered the unique genetic mutation responsible for orange color in cats, which is a deletion of a very small section of the X chromosome. The deletion results in the activation of a gene in pigment producing cells that blocks production of black pigment, so the only pigment produced is orange. The same mutation was found in all orange cats studied over a wide geographic area, suggesting a single mutation event is responsible. The mutation must be quite old because there are depictions of calico cats in Chinese art dating back to the 12th century.

2/ Natural Selection at Work
According to a recent study, the use of human-made hummingbird feeders has changed the beak sizes and shapes of Anna’s hummingbirds, and spread their range from a narrow pocket of California all the way up the coast to British Columbia. It appears that urbanization, particularly after the Second World War, brought hummingbird feeders up the California coast, and Anna’s hummingbirds followed. The hummingbirds with longer and more tapered beaks were able to best adapt to the new food source. Scientists indicate that this evolutionary event was remarkably fast compared to morphological changes observed in other species.

3/ Bird Brains?
Researchers recently reported on a Cooper’s hawk who was taking advantage of stopped traffic to hide its approach to attack a flock of birds in a neighborhood yard. It appears that the hawk was keying in on the sound of the triggered pedestrian signal that would result in a long line of stopped traffic. The bird then flew behind the cars to hide its approach from birds feeding in the yard. Pretty smart for a bird brain.

4/ The Power of a Dog’s Nose
The ability of dogs to detect the smallest of scents never ceases to amaze us. In a recent study, dogs were trained to sniff out the eggs of the spotted lanternfly, a leaf-hopping invasive pest first detected in the U.S. a decade ago. Although harmless to humans, the insects damage trees and fruit crops. In the study, four dogs searched out egg masses in foliage. Each egg mass can produce 30 to 50 spotted lanternflies, and the dogs are credited with finding more than 4,000 of them, which could then be destroyed. Researchers hope to expand the program to help slow the progress of this destructive, invasive insect.

5/ Horsepower
Horses were domesticated some 5,000 years ago, and we have been molding them as a species ever since. We have developed very fast breeds, such as the Quarter Horse Thoroughbred, as well as immensely powerful ones, including Percherons and Clydesdales. What these two ends of the spectrum share is their ability to perform intense exercise, whether to run or to pull, for extended periods of time. The mass-adjusted oxygen consumption of elite equine athletes is more than double that of elite human athletes. The results of a recent study found that a single point mutation in their genome results in the production of an altered protein that enhances aerobic energy production, while mitigating oxidative stress. The mutation appears to be very old, because it is also present in the modern horse’s only living relatives, zebras and donkeys, indicating it occurred in an ancient, shared ancestor. While the athleticism of zebras is readily on display as they roam the plains of Africa, the donkey is also capable of tremendous aerobic outputs similar to the horse, but is rarely asked to perform them.

‍