Sensory Capacity of Pigs

A pig’s eyes are bigger and more functional than people often think¹. The eye of a pig is about 24mm in diameter, similar to a human’s eye. The total optical power of the pig eye is estimated to be 78 diopters, which is greater than the optical power of humans at 60 diopters. So, pigs are likely nearsighted compared to humans. Pigs have approximately 310-degree vision, and the ability to detect colours. The retina of a pig contains two types of cones which are sensitive to blue and green-yellow light. Thus, the pig is believed to have dichromatic colour vision (bluish and yellowish) and can identify blue better than any other colours.

In terms of lighting preferences, pigs prefer well-lit areas to dark areas. So, it is easier to move pigs to a lighter area than a darker area². Pigs also have a very sensitive sense of smell¹. Sows identify their piglets through smell, and in turn, piglets find their dam and locate teats through olfactory clues. Piglets can recognise their dam’s faeces by seven days of age. In a group, pigs can recognise pen-mates and dominant individuals mainly using olfactory cues³. The strong sense of smell in pigs has been used in swine production, such as using an odour-masking agent to suppress aggression among pigs⁴.

Pigs have a sense of taste that is similar to humans, and can discriminate between sweet, sour, salty, and bitter tastes¹. Pigs prefer sweet to sour taste and will reject bitter food⁵. Sweet, meaty, and cheesy are the preferred flavors to pigs. Sweetener additives can be used to encourage newly weaned piglets to consume solid feed^5,6.

The hearing range of pigs is also similar to humans but with greater sensitivity in the ultrasonic range¹. Pigs frequently use audio cues to communicate with each other. Sows emit a series of grunts, varying in frequency, tone and magnitude to call their piglets and indicate imminent let-down of milk^7,8. Lactating sows also respond to alarm calls of piglets⁹. Loud, sudden noises can stress pigs and interfere with the farrowing process. Playing recorded nursing grunts to sows can stimulate and synchronise nursing in a room⁷.

Normal Behavioural Patterns of Swine

Normal behaviour in pigs refers to behaviours that are observed in pigs under natural conditions. These behaviours promote biological functions in pigs, such as survival and reproduction¹⁰. Since rearing environments for commercial production are different from natural conditions, indoor raised pigs do not display the entire repertoire of behaviours they perform under natural conditions.

For example, free-ranged pigs spent 35 to 55 per cent of their time foraging for food¹¹, while indoor housed pigs do not forage because food is readily available. The benefit and impact of performing the full spectrum of natural behaviours is not the focus of this article; in this article, the author discusses behaviours in pigs for indoor commercial production in order to help pork producers improve animal care by applying behavioural knowledge.

Farrowing and nursing behaviour

On average, sows spend two to four hours giving birth to a litter, with an interval of 15 to 20 minutes between each piglet. However, there is large variation in farrowing duration (45 minutes to 12 hours) and inter-birth intervals (five to 85 minutes) among individual sows^12,13. During farrowing, sows spend 85 per cent of their time lying on their side¹⁴.

At birth, piglets can present normally in either a ‘nose first’ or ‘hind legs first’ position. ‘Broadside on’ is considered a malpresentation. Piglets can get on their feet within a few minutes and successfully suckle milk within 45 minutes after birth.

Birth order may affect piglet survival and growth¹⁵. Later-born piglets usually have less vigour, suckle later, obtain less colostrum and have poor teat order compared to piglets born earlier. In addition, later-born piglets have a high risk of being stillborn or being born enveloped in afterbirth¹⁵.

Nursing tends to be continuous at farrowing, and later becomes rhythmic^7,8. A sow nurses her piglets approximately once per hour¹⁶. Nursing is usually synchronized among sows in the same room, meaning that sows within a room usually nurse at approximately the same time. Each nursing takes about three minutes but the actual time of milk letdown only lasts 20 to 30 seconds. On average, each piglet obtains about 0.7oz (20ml) of milk at each nursing^7,8.

Resting behaviour

Pigs spend the majority of their time resting or lying. On average, grow-finish pigs spend 75 to 85 per cent of their time lying, and five to 10 per cent eating, with the remainder of their time involving in other activities such as walking, sitting, rooting/nosing and drinking³. Within the thermal comfort zone (i.e. the temperature range when pigs spent the least energy to maintain their body temperatures), lying on their side (lateral lying) is the predominant posture, with about 40 to 50 per cent of pigs touching each other¹⁷.

When temperatures are below the comfort zone, pigs adopt a sternal lying posture (on their chest and stomach) and huddle together to reduce the exposed skin surface area for heat loss. Under very cold conditions, pigs may lie on top of each other to keep warm.

Above the upper limit of the thermal comfort zone, pigs change their lying position from sternal to lateral with legs, stretching out and avoid contact with other pigs. Under extremely hot conditions, pigs increase their respiratory rate (panting) to dissipate heat through evaporation from the respiratory tract.

Eating and drinking behaviour

On average, pigs eat 10 to 25 meals per day, with younger pigs having more meals than older pigs¹⁸. Nursery pigs can eat 20 to 25 meals per day,while finisher pigs may eat only 10 to 15 meals per day. As pigs grow, they eat faster, so older pigs spend less time eating compared to younger pigs^18,19.

Eating events peak in the morning and evening. Even though pigs will eat during the night, the number and duration of meals is less than during the day^18,19. When feeder space is limited, pigs will increase the amount of time spent eating during the night, which changes their diurnal (daily) pattern of eating behaviour²⁰. For example, diurnal variation in eating behaviour is reduced when there are 20 pigs per feeder space, and is completely diminished with 30 pigs per feeder space, compared to 10 pigs per feeder space²⁰.

Increasing the length of the light period can also increase the number of meals and diminish the diurnal variation in eating behaviour in pigs [Gonyou, unpublished data]. Eating behaviour can be socially facilitated. When a hungry pig is eating adjacent to a satiated pig, the satiated pig typically starts eating²¹. Furthermore, pigs in adjacent pens tend to eat at the same time. However, when feeders are close together without protective dividers, pigs will avoid eating together and are aggressive towards other pigs that are eating next to them²¹.

Pigs learn what to eat and where to eat from other pigs. Pigs that have observed other pigs eating a novel diet are more likely to eat the novel diet than pigs that have not. In addition, pigs that have observed other pigs eating from one of three feeder troughs will eat from the same feeder if given a choice²². Within the thermal comfort zone, drinking behaviour usually occurs within 10 minutes of eating, hence the number of times per day that a pig drinks is similar to the number of eating events³.

Grow-finish pigs usually spend 20 to 30 minutes drinking per day, with 15 to 30 seconds for each drink. Under conditions of heat stress, however, pigs spend more time drinking as well as playing with the drinker. Pigs with diarrhea will also consume more water. Thus, excessive water consumption can be used as an animal based welfare measure to monitor the thermal comfort and health status of pigs.

Excretory behaviour

Pigs like to excrete in cool and wet areas, and lie in warm and dry areas. Pigs often drink, urinate, and defecate in close proximity of each other²³.

Because pigs take an unstable stance when they are excreting, they tend to excrete in a place that is away from commotion, and most commonly in a corner or against a wall. In most pens, the majority of the commotion is near the feeding areas so pigs tend to excrete away from feeders. If water is spilled, the pigs lie away from that area and then use it as an excretory area. So, pigs usually create the dunging area near drinkers.

Pigs also like to excrete along the opening partitions between pens due to either draughts or the natural marking behaviour when they see other pigs in the next pen²⁴. Both higher room temperatures and a higher stocking density can cause pigs to foul the pen^24,25.

Abnormal Behaviours and Possible Causes under Production Conditions

In contrast to normal behaviours, abnormal behaviours refer to behaviours in pigs that have not been seen under natural conditions. Abnormal behaviours are considered an indicator of poor welfare in pigs under production conditions. In many cases, abnormal behaviours compromise the production performance or health of pigs.

Piglet savaging

‘Piglet savaging’ is the behaviour that gilts or sows kill their piglets after birth, and has been categorized as cannibalism. The incidence of piglet savaging was reported as 0.3 per cent in farrowing sows and gilts,with a higher frequency observed in gilts than in sows[26]. Gilts that savaged at their first farrowing were at higher risk of savaging in their next farrowing compared to gilts that did not savage in their first farrowing. Continuous lighting in the farrowing room may reduce incidence of piglet savaging in gilts²⁶.

Belly-nosing

Belly-nosing has been categorized as a stereotypy. Stereotypies are defined as repetitive, non-functional behaviours. Belly-nosing is most often seen in early weaned piglets, and peaks at three to seven days after weaning^27,28. It is considered to be caused by stress associated with early weaning. Piglets that are weaned at two weeks of age display two to five times more belly-nosing than piglets that are weaned at four weeks of age.

Early weaned piglets can spend 15 to 25 minutes per day engaging in belly-nosing. Prolonged belly-nosing can result in lesions on the recipient pig. Pigs performing belly-nosing usually spend less time eating and grow slower than their counterparts²⁷. Liquid feeding has been reported to reduce belly-nosing and improve growth rate in newly weaned pigs²⁹. Compared to nipple drinkers, bowl drinkers have been reported to reduce belly nosing as well³⁰. Belly-nosing can also be observed in grow-finish pigs but the incidence is much lower than in nursery pigs³¹.

Oral, nasal and facial behaviours

Oral, nasal, and facial behaviours (ONF) are also considered stereotypic behaviours. Oral, nasal and facial behaviours are most often seen in pregnant sows and can be associated with limit feeding and barren environments³². Gestating sows in stalls may spend 30 per cent of their time performing ONF behaviour. Compared to sows in gestation stalls, group-housed sows have been reported to spend less time engaging in ONF³³.

However, Daily and McGlong³⁴ did not find differences in ONF performed by sows kept on pastures or indoors in stalls. They argued that ONF maybe natural pre- and post-feeding appetitive and consummatory chewing and rooting activities, and not necessarily indicate poor welfare in sows. Although the motivation and consequence of ONF is still under debate, ONF has been perceived as a behavioural indicator of poor welfare. Feeding sows ad libitum with diets containing high fibre decreases the incidence of ONF in sows housed both in stalls and in pens^35,36.

Tail-biting

Tail-biting, which is more often observed in grow-finish pigs than in any other production stages, is the most damaging abnormal behaviour in pigs. The incidence of tail-biting in grow-finish pigs with normal tails has been reported to be nine per cent, which is higher than the three per cent reported for tail-docked pigs³⁷. Tail-biting is categorised as cannibalism in pigs. Causes of tailing biting are not well understood,but are known to be related to several factors including malnutrition, discomfort,and lack of environmental enrichment^38,39. Low-salt diets, draught or high air speed, high ammonia or carbon dioxide concentrations, limited feeder and/or drinker space, and over-crowding have all been reported to be associated with tail biting³⁸.

Since tail-biting can be caused by multiple factors, there are no specific solutions to the problem. In practice, victim pigs are usually removed once their tails become injured. Timely removal of both tail biters and victim pigs is important in preventing an outbreak of tail biting in a pen. Once tail biters have tasted blood from the tails of victim pigs, it is hard to stop them from biting more tails because pigs prefer the taste of blood. Continued tail biting becomes rewarding and the frequency of tail biting behaviour will increase³⁹.

Behaviours in sick and compromised pigs

Sick and compromised pigs usually display behaviours that are different from healthy animals⁴⁰.Typical sickness behaviours in swine include reduced feeding, drinking, and interactions with pen mates, and increased resting, huddling,and shivering. Sick pigs are less successful in competing in a group,so these pigs should be moved to a hospital pen and receive individual care⁴⁰.

Summary

Pigs have well-developed senses of vision, smell, hearing and taste. In general, pigs prefer well-lit areas and sweet taste.

They rely on olfactory and auditory cues to recognise and communicate with other pigs.

Under good management conditions (provision of adequate feed and space), pigs display diurnal patterns of feeding and activities.

Improper management and environments could cause changes in behavioural patterns, and may induce abnormal behaviours.

Understanding the sensory capabilities and normal behavioural patterns of pigs will help us prevent the occurrence of abnormal behaviours in swine production.

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November 2014