Do Pigs Sweat? Why Pigs Really Roll in Mud to Keep Cool

Why can’t pigs sweat?

Mammals have two main kinds of sweat glands. Eccrine glands release watery sweat straight onto the skin, and humans have millions of them spread over the body. When this sweat evaporates, it carries heat away. Apocrine glands are linked to hair follicles and release a thicker, oily fluid.

Pigs have apocrine glands in their skin, but these produce very little fluid and do not help them lose heat. Their eccrine glands are found only on the snout. As a result, a hot pig cannot rely on sweating, and the old phrase “sweat like a pig” gets the animal exactly wrong.

Panting helps only a little. Pigs breathe faster when they are hot, losing some heat from the moist lining of the mouth and airways, but they are less efficient at this than dogs. Their thick layer of fat under the skin also holds in heat, which suits cold winters but makes hot days harder.

How hot is too hot for a pig?

Adult pigs are most comfortable in a fairly narrow band of temperatures, roughly 16–22 °C (61–72 °F). This is called their thermoneutral zone: within it, a pig does not need to work to warm up or cool down. Newborn piglets are the opposite. They have little fat and thin coats, so they need much warmer conditions and huddle together and against their mother.

Above that comfortable range, adult pigs start to change their behavior. Studies of wallowing show that adult pigs begin to wallow once the air reaches about 17–21 °C (63–70 °F). In hotter weather, they become less active, eat less, lie stretched out on cool ground and spend more time in water or mud. On farms, heat stress is a serious problem in summer, and many pig houses use shade, sprinklers, misting or cooled air to keep pigs comfortable.

Why do pigs roll in mud?

A pig wallowing in mud is doing much the same job that sweat does for us. Wet mud on the skin slowly dries, and as the water evaporates it takes heat from the body. Because mud holds water and dries more slowly than plain water, a good coat keeps cooling the pig for longer, often well after it has left the wallow.

Mud has other benefits too:

• Sun protection. Pigs have sparse bristles, and pale breeds such as the Large White can burn in strong sun. A layer of dried earth works like sunscreen.
• Protection from pests. Mud helps keep away flies and biting insects and may help remove lice and other skin parasites when it dries and is rubbed off.
• Scent and social signals. In wild boar, wallowing increases in males during the breeding season, and researchers think the mud and the trees boar rub against afterward may spread their scent.

A pig settles into the wallow, rolls from side to side and covers itself from head to tail. It does not usually sink completely under the mud, but it adjusts how deep and how long it wallows to the weather. On hot days, a pig may return to the wallow many times.

Are pigs really dirty?

Pigs have a messy reputation, but given enough room they are tidy animals. Free-ranging pigs choose separate places to sleep, feed and dung, and keep their dunging area well away from the nest. Even young piglets soon learn to leave the nest to relieve themselves.

The dirty image comes mostly from wallowing, and partly from how pigs have often been kept. A pig shut in a crowded, hot pen with no clean water or mud may end up lying in wet dung, simply because it is the only damp, cool surface available. Researchers who study pig welfare see this as a sign of heat stress and poor housing, not of the animal’s nature.

Pigs are also intelligent and curious animals, as described in How smart are pigs?. Their rooting and wallowing are normal behaviors that they are strongly motivated to perform, even when they are otherwise well cared for.

Do wild boar and other animals wallow?

The domestic pig inherited the habit from its ancestor, the wild boar. Wild boar wallow in muddy pools, stream banks and wet hollows, especially in summer, and the same wallows are often used again and again. After wallowing, boar typically rub their bodies against tree trunks, which can leave the bark polished and caked with mud at boar height.

Many other large mammals with little hair use the same trick. Rhinoceroses, African buffalo, elephants and water buffalo all wallow or throw mud and dust over their skin. Hippos spend the hot day in water and also produce a reddish skin secretion that acts as a sunscreen, explained in Do hippos sweat blood?.

Where does “sweat like a pig” come from?

The origin of the phrase is uncertain. A widely repeated explanation is that it refers to pig iron, the bars of cast iron produced by smelting. According to this story, the bars “sweated” droplets as they cooled, showing workers they were safe to move. It is a neat tale, but etymologists have not found firm evidence for it, so it is best treated as a possibility, not a fact.

Whatever its roots, the saying is now used simply to mean sweating heavily. Biologically it is a myth: the real pig does almost the opposite, which is why mud, shade and water matter so much to it on a summer day.