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FARM ANIMAL COALITION
PIGS

Learn about pigs

In Chinese culture, pigs symbolize luck and prosperity (1).  As for the west, commendatory images of pigs have appeared in various culturally significant works, the most popular of which is Wilbur from the ever delightful “Charlotte’s Web”. Yet, pigs are now often associated with derogatory terms like ‘sloth’, ‘dirty’, ‘lazy’ and ‘greedy’ among others due to their images taken from factory farms (2). It is high time these amazing creatures be understood as sentient beings, independent of the negative affiliations that many people are attributing them to.  It is our hope that with information summarized from the insightful “Thinking pigs: A comparative review of Cognitive, Emotion and Personality in Sus domesticus” by Lori Marino and Christina M. Colvin, the amazing facts about pigs could help do them justice (3).

Let's get scientific about pigs!

a) Senses

 

According to Kruska (1988), the pig’s snout is the part which has the highest density of sensory receptors, which enable the pig to exhibit a wide range of maneuvers, like rooting, carrying, pushing and social engagements (Stolba & Wood-Gush, 1989). 

 

For pigs, their sense of smell is the most sensitive. This is mostly utilized when they go foraging. The acute sense of smell helps pigs find food items they could eat (Croney, Adams, Washington & Stricklin, 2003).  According to Heffner & Heffner in 1992, pigs can hear sounds between 42-40500 Hz, which means that they could discern ultrasounds. 

The senses of sight, smell and touch are used by pigs to discern objects in a natural setting (Arave, 1996; Lomas, Piggins & Phillips, 1998), but they can do this by vision alone (Koba & Tanida, 2001). To distinguish between humans, according to Tanida & Nagano (1998), sight and hearing are used more than the sense of smell. 

 

 

 

 

 

b) Non-social cognition 

 

  • Object differentiation

 

In various studies, pigs have displayed their keen capabilities to differentiate objects. To illustrate, based on the work of Gifford, Sylvie Cloutier & Newberry in 2007, after getting acquainted with an object for the period of 2 days, pigs could remember said object for 5 days and indicated a preference for novel objects over previously introduced ones.

 

In a food-searching task in a study by Held et al in 2005, given a choice between 2 food sources, they showed regular preference for the source with more food which they remembered. According to Croney et al (2003), pigs can find food sources by following clues of certain colors and food odors. 

Similar to other mammals, pigs have been shown to understand symbolic language. In studies, pigs demonstrated the ability to distinguish between three choices of objects and learn action-object tasks. These findings lend credence to the fact that pigs are capable of complex cognitive processes.

  • Time sensing

 

Despite the fact that more concrete data is needed to evaluate the perception of time among pigs, many studies have provided evidence that may suggest that the prospects of future research finding pigs to be able to perceive time, anticipate different scenarios and even plan could be positive.

 

 

  • Spatial learning and memory

 

In a research by van de Staay et al (2012), the holeboard procedure was used to evaluate pigs’ spatial learning abilities and memory. This procedure involves an open field that has various wells that can be baited, which constitutes a natural foraging setting for pigs to use their snouts to search for food.  Pigs could perform well in this setting and was not affected by external stress (Arts, van der Staay and Ekkel, 2009). 

 

Flexibility was also detected in pigs’ use of spatial memory as well. The research of Laughlin & Mendl in 2000 suggested that pigs can be taught to come back to an area where they found food in the past or alternatively, direct their foraging activities elsewhere away from the locations they previously found food.

 

In a study by Held et al in 2005, it was suggested that pigs could both distinguish and retain the memory of different food locations with different quantities, which could also point towards the possibility of a quantity assessment ability (numerosity).

 

  • Play behaviors and sense of exploration

 

A research led by Horback in 2014 studying play behaviors in pigs yielded findings which pointed towards pigs having complex play behaviors, which include both social and object play. Pigs generally engage in object play by shaking, carrying and tossing objects (Bolhuis et al, 2005; Dudink et al, 2006; Newberry et al, 1988). Pigs’ play behaviors also include locomotor play that involves various uses of limbs and head in movements like tossing head, jumping, pawing and gamboling, hopping around and flopping on the ground (Martin, Ison & Baxter, 2015). Pigs also play-fight, push and run after other pigs in social play situations (Horback, 2014). 

Playing is a behavior that fosters the need to discover and emotional development. A research by Martin et al in 2014 suggested that pigs reared in challenging settings develop more in terms of social cognition than their counterparts reared in farrowing crates. The former tend to engage more with objects and other pigs. This is further supported by the works of Olsen et al (2002) and Douglas et al (2012), with the former indicating that pigs are more inclined to play and other positive behaviors if given materials and the latter suggesting that in an environment characterised by stimulation, pigs are more likely to have a positive bias. 

 

 

 

 

 

 

c) Social cognition 

  • Distinguishing between conspecifics and others

The works of de Souza et al (2006), Kristensen et al (2001) and McLeman et al (2005) suggested that pigs possess the ability to recognize conspecifics and they prefer those that are familiar to them over strangers. In the study carried out by McLeman et al in 2005, young pigs were found to use learning as the basis to form discrimination. They can use all normal social cues to tell the difference between two familiar conspecifics and furthermore, can even distinguish between closely-related ones.

Findings from other studies were consistent with these suggestions.  According to Mendl et al (2002), with only urinary samples, juvenile pigs can distinguish between familiar and unfamiliar individuals. Illman et al (2002) found that pigs could recognize the voice of their offspring from others and respond accordingly. The findings of these studies showcase flexibility in pigs’ ability to differentiate between conspecifics under different conditions.

As regards discrimination between humans, various studies indicated that pigs possess the abilities to some extent. Tanida and Nagano in 1998 suggested that pigs can distinguish between two humans based on previous experiences in which one of them treated them gently and brought food. Each pig employed different senses like smelling, seeing and hearing as cues to different levels. 

On the other hand, in situations in which pigs were treated roughly by handlers in commercial contexts, pigs do not discriminate between their handlers. This could be explained by the fact that all handlers subjected them to tough treatments. This suggests that pigs have different responses to different previous experiences. Moreover, the findings from Koba and Tanida’s study in 1999 indicated that pigs could utilize body size and possibly certain facial features to differentiate between humans when they were put in recurrent contact with those wearing the same attire.

  • Perspective-taking

Perspective-taking refers to a complex mental process in which one must put oneself in the point of view of others to discern their thoughts, feelings and motivation. This information can be used to one’s advantage such as for exploitation. In some studies, pigs have been shown to possess this ability.

In a study by Held et al in 2000, pigs were put into pairs to forage for food with only one knowing the food source’s place. Interestingly, the pig without the information followed the one with the information to the food source and claimed the food first. In the later trials, the informed pigs adjusted their behavior to lower the chance of being exploited and elevated their speed to outrun their followers (Held et al, 2010). These strategies can be considered a form of tactical deception. Perspective-taking is most evident in the competitive context for pigs.

 

Pigs can also perceive the state of attention in humans. In a study conducted by Nawroth et al in 2013, pigs successfully detected the attentive person using head cues. In another study by the same team in the same year, pigs could also locate food using the pointing cues given by humans under some settings. This points towards the direction that pigs have a some degree of sense when it comes to others’ intentions.

d) Sense of self

Self-awareness refers to the ability to have a sense of self in animals, both physically and emotionally. Traditionally, mirror self-recognition (MSR) is used to measure self-awareness in animals, for example, to see if the pigs recognize the reflection in the mirror is of themselves. These tests’ results were inconclusive in pigs although in some, many of the pigs with previous experience with mirrors could promptly find food after viewing. Many pigs also appeared to watch themselves in the mirror. However, this is not conclusive. 

In terms of self-agency or the ability to understand the effects of actions caused by themselves, in a study by Croney (1999), pigs could maneuver joysticks to move the cursors on computer screens. They seemed to understand that the joystick moves the cursor. Surprisingly, their performances were better than dogs in moving the cursor to hit a target on screen. 

e) Feelings and emotions

Reimert et al (2013) found that emotional contagion occurs among pigs, as evidenced by the findings of their study in which pigs detected the feelings in other pigs and responded to those anticipating the outcomes of future events. This was further supported by another study of the same team in 2014.

Murphy et al (2014) suggested that play behaviors could also be indicative of emotions in pigs. There is a high likelihood that positive emotions arising from anticipation of positive events could be an indirect indicator of happy feelings, exhibited by an increase in play behaviors, both with objects and with others. 

f) Personality

Personality deals with consistency displayed in emotions, cognitive processes and behaviors of an individual, of both humans and animals. A sense of individuality can be acknowledged in animals when the existence of personality is recognized among them when they are seen as not just any indistinguishable members of a species. In the case of pigs, studies by Forkman, Furuhaug & Jensen (1995) and Gosling & John (1999) have shown that pigs possess personality dimensions and structures similar to other species. 

Scientists derived their evaluation of personality in pigs from observations of their temperament,  behaviors, responses and coping mechanisms when placed in different situations. In a study conducted by Ruis et al (2000), the most significant personality trait of female pigs in a group-feeding competition was individual aggression. In another study by Forkman et al in 1995, researchers studied the responses of piglets to various situations as well as their behaviors. At least three key elements of pigs' personality emerged, namely aggression, sociability and exploration, which could be extrapolated to human traits like agreeableness, extraversion and openness. These findings pointed to complex personalities in pigs.

 
 

Fun facts about pigs!

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