Michaela Giles explains how pigs regulate their body temperature, and the dangers that can arise when they’re unable to do so
Pigs are endotherms (warm-blooded), like all other mammals, meaning that they maintain their body temperature through internal biological processes that produce heat.
In mammals, two separate homeostatic mechanisms are required to maintain a ‘thermoregulation equilibrium’ – one increases body temperature, while the other decreases it, providing a high degree of internal temperature control. This is important because the core temperature of mammals can be controlled to be as close as possible to the optimum internal temperature for the essential enzyme activities involved in all aspects of body function; from digestion through to movement. This makes the animal relatively independent of the temperature of the environment.
If heat loss exceeds heat generation, metabolism increases to make up for the loss and/or the animal shivers to raise its body temperature. If heat generation exceeds the heat loss, then mechanisms such as panting or perspiring are triggered to increase heat loss. This means that endotherms can be active and survive at quite low external temperatures but, because they must produce heat continuously, they require adequate quantities of food to keep the whole process running.
A pig’s natural habitat would typically have been moist woodlands, swamps and shrublands, especially oak forests and regions where reeds are abundant. They originated in cooler climates, and so evolved with few sweat glands; these are only to be found in limited numbers on the snout, and on the lower limbs. In practice, these are inadequate for use in cooling the body’s core temperature. So, to help counteract this practical shortfall and assist the internal homeostatic processes, pigs employ specific ‘behavioural temperature regulation’.
Indoor, commercially-kept pigs have their conditions regulated for them to keep them at a comfortable temperature for the most part, with climate-controlled barns maintained with fans and even water sprinklers. Outdoor pigs, on the other hand, must be given a choice of surroundings that’ll enable them to maintain their own ideal temperature – which changes depending upon the season. Throughout a year, pigs will need somewhere warm to sleep without draughts, companions to sleep with, natural or artificial shade, a wallow or shaded water and plenty of palatable water to drink.
Pigs lose heat in the form of ‘sensible’ and latent (evaporative) heat. Convection, conduction and radiation are primary mechanisms by which sensible heat loss occurs, and each requires a temperature difference between the warmer animal and its cooler environment. Therefore, as the environmental temperature increases, animals redistribute blood towards the skin in an attempt to increase radiant heat loss.
With a further increase in temperature, reducing the difference in temperature between the pig and the environment, the transfer of heat by conduction, convection and radiation modes decreases, eventually leaving evaporation as the only route of heat loss.
Apart from having few functional sweat glands, a pig’s ability to regulate its temperature is further complicated by a thick subcutaneous adipose tissue (fat) layer, which can impede sensible heat loss; thus, pigs depend more on the respiratory route (ie, panting) for heat dissipation. But if their attempts at increasing heat loss prove inadequate, then the animal will initiate a variety of behaviours to minimise heat production and maximise heat loss, which include the following.
- Pigs eat less when they’re hot, to minimise internal metabolic processes generating heat. To prevent loss of condition or reduced growth during periods of hot weather, it’s important to feed early and/or late, when ambient temperatures are cooler. Research has shown that voluntary feed intake is influenced by temperature, air speed and humidity. However, the reduction is dependant upon genotype, diet composition, body weight and ambient temperature. Lower protein feeds require less water intake etc.
- Pigs will also move less when hot. All movement generates heat, which is what they’re trying to avoid.
- Hot pigs drink more to compensate for the additional water lost in cooling the body, so an adequate supply of palatable water must be continuously provided if dangerous dehydration is to be avoided.
- They lose heat to the environment by convection, conduction and radiation, and by evaporation through their respiratory tracts.
This is the transfer of heat from an object to the air around it. In warmer conditions, pigs increase blood flow by dilating their blood vessels (vasodilation) to body surfaces, which maximises heat transfer across to the cooler conditions. Up to 35% of total heat loss is through convection, primarily through the skin and ears, although heat transfer through the ears is not thought to be efficient, especially on some of the hairier breeds.
To keep the core temperature warmer in colder conditions, the blood vessels constrict (vasoconstriction), to minimise the amount of blood flowing close to the surface of the skin, thus preserving body heat. Heat loss through convection can be a potential detriment during cold and mild weather, if wind speeds are too high.
This refers to the direct transfer of heat from one surface to another and, in pigs, accounts for 10-15% of total heat loss. This heat transfer mechanism is why pigs can be seen scraping the topsoil of an area; they’re attempting to expose the cooler soil beneath, before lying down on it. It’s also why the provision of a wallow is so important, and explains why pigs urinate more in hot weather. This is an easy way of dispelling large volumes of heat straight out of the body.
Pigs use a form of conduction to keep warm in colder outside temperatures, known as kleptothermy. This process involves sharing or even stealing body warmth from ark mates.
This is the transfer of heat from the pig to its surroundings without direct contact. For example, a hot pig standing in a shady but breezy spot in the paddock can lose up to 30% of total heat loss in this way.
Evaporation is the process whereby a substance in a liquid state changes to a gaseous state, as the result of an increase in temperature and/or pressure and, with pigs, it can account for 20-25% of total heat loss. Because pigs don’t sweat to any great extent, most of this loss is through the respiratory tract, via an increased respiration rate. A certain amount of evaporation also occurs when a wet pig leaves a wallow or stream it had previously been using.
Heat stress occurs when the environmental temperature rises above the point where the pig is producing more heat from digestion and/or receiving more heat from its surroundings than it is releasing to the surrounding environment. The animal’s first reaction to this situation is that its blood vessels that run close to the skin’s surface become enlarged, which increases blood flow and the skin’s surface temperature. In turn, this increases the heat transfer rate to the environment.
Increased respiration serves to increase the water vapour output and, consequently, heat output and this provides a clinical sign of heat stress. Other physiological responses such as skin temperature, rectal temperature, tympanic (ear drum) temperature and vaginal temperature, can be measured.
The upper critical temperature is the point at which a pig’s internal core temperature becomes dangerously high. The animal will be in ‘Heat stress danger’, and will typically display increased respiration (panting) and a loss of appetite. At best, this causes reduced growth in feeder pigs, or reduced milk production in lactating sows and, at worst, the pig is at severe risk of dying if appropriate measures are not taken.
Upper critical temperatures can easily be reached inside ventless vans without fans so, when transporting young pigs home, or to shows, don’t travel for any distance during the hottest times of a summer’s day – the pigs may well be dead by the time you reach your destination because, under these conditions, evaporation will be their only method of heat loss.
Cold water should NEVER be applied to a hot pig to cool it down, as this will cause the blood vessels to vasoconstrict, triggering an increase in core temperature. The correct approach is to apply tepid water frequently, preferably in a spray or mist. A good tip for those that exhibit pigs is to have sprayable water readily available to help the pigs keep cool, and don’t wait until the animals have started to pant before you start applying it.
When exhibiting pigs and returning from a show – especially on hot days – it’s a good idea to remove any pig oil from the skin with a detergent and water. Any remaining oil on the skin will interfere with the cooling process, until it wears off.
There’s emerging evidence suggesting that some of the leaner, modern pig breeds that have been selected for their ideal production traits – lean meat, milk yield, fecundity etc – can exhibit reduced heat stress tolerance, because those breeding traits are associated with increased metabolic heat production. So this perhaps ought to be a consideration when selecting pigs for outdoor production systems.
The provision of shade in the pig pen is an essential requirement. Trees or artificial barriers (such as galvanised sheeting, shade cloths etc) can minimise exposure to the direct heat of the sun, and will also reduce the surrounding surface temperatures. Practical design considerations for shade structures include orientation, pitch, height and the material used in their construction.
Not providing pigs with the ability to regulate their own body temperatures can result in economic losses associated with heat stress, shown by reduced and inconsistent growth, decreased feed efficiency, decreased carcass quality (increased fat deposition and decreased muscle production), poor sow performance, increased mortality (especially in sows and market hogs) and morbidity.
In addition, reduced reproductive performance can be characterised by anoestrus (failure to come into season), increased wean-to-oestrus interval, decreased farrowing rate and reduced litter size. Similarly, poor semen production and quality occur in boars exposed to heat stress, so it really is vital to ensure that your pigs are able to control their body temperatures effectively.
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