Broom, D.M., Sena, H. and Moynihan, K.L. 2009. Pigs learn what a mirror image 1"
represents and use it to obtain information. Anim. Behav., 78, 1037-1041. 2"
DOI: 10.1016/j.anbehav.2009.07.027 3"
Pre-publication copy 6"
Pigs learn what a mirror image represents and use it to obtain information 7"
Donald M. Broom, Hilana Sena and Kiera L. Moynihan 9"
Centre for Animal Welfare and Anthrozoology 10"
Department of Veterinary Medicine 11"
University of Cambridge 12"
Madingley Road, Cambridge CB3 0ES, U.K. 13"
Correspondence: D. M. Broom, Centre for Animal Welfare and Anthrozoology, 15"
Department of Veterinary Medicine, University of Cambridge, Madingley Road, 16"
Cambridge CB3 0ES, U.K. firstname.lastname@example.org. 17"
Mirror usage has been taken to indicate some degree of awareness in animals. Can pigs 20"
obtain information from a mirror? When put in a pen with a mirror in it, young pigs 21"
made movements while apparently looking at their image. After 5 hours spent with a 22"
mirror, the pigs were shown a familiar food bowl, visible in the mirror but hidden 23"
behind a solid barrier. Seven out of eight pigs found the food bowl in a mean of 23s by 24"
going away from the mirror and around the barrier. Naïve pigs shown the same, looked 25"
behind the mirror. The pigs were not locating the food bowl by odour, did not have a 26"
preference for the area where the food bowl was and did not go to that area when the 27"
food bowl was visible elsewhere. To use information from a mirror and find a food 28"
bowl, each pig must have observed features of its surroundings, remembered these and 29"
its own actions, deduced relationships among observed and remembered features and 30"
acted accordingly. This ability indicates assessment awareness in pigs. The results may 31"
have some effects on the design of housing conditions for pigs and may lead to better 32"
pig welfare. 33"
Keywords: awareness, cognition, learning, mirror, pigs, 35"
The E.U. Treaty of Amsterdam refers to domestic animals as sentient and a sentient 39"
being has been defined as “one that has some ability: to evaluate the actions of others in 40"
relation to itself and third parties, to remember some of its own actions and their 41"
consequences, to assess risk, to have some feelings and to have some degree of 42"
awareness” (Broom 2007). Hence the extent to which an animal can learn about 43"
complex aspects of its world and the level of awareness which it has can influence 44"
human attitudes to the moral status of such animals and hence the ways in which they 45"
are treated (Mendl et al 2001, Broom 2003). 46"
Griffin (1981) said that awareness involves the experiencing of inter-related mental 47"
images and awareness has been defined as “a state in which complex brain analysis is 48"
used to process sensory stimuli or constructs based on memory” (Broom 1998). The 49"
term “complex brain analysis” implies that there is some degree of interpretive thought 50"
over and above perceptual processing and a gradation has been proposed with four 51"
categories of awareness: unaware but responsive, perceptual awareness, cognitive 52"
awareness, assessment awareness and executive awareness (Sommerville and Broom 53"
1998). For example, in assessment awareness the individual is able to assess and deduce 54"
the significance of a situation in relation to itself over a short time span. The individual 55"
would not only be sensible to stimuli but would have memory of events and mental 56"
images of non-current events that could be used when taking appropriate action, both to 57"
avoid the negative and to increase positive consequences. This use of the concept of 58"
awareness is similar to that of Snyder et al (2004) who refer to awareness of concepts 59"
and equate consciousness with executive awareness. Mendl and Paul (2004, 2008) 60"
discuss “basic awareness” of sensations, feelings, emotions and memories. One level of 61"
self-cognition is to be self-referent and to discriminate labels of self from labels of non-62"
self (Hauber and Sherman 2001) and this has been described as different from being 63"
self-aware “the cognitive process that enables an individual to discriminate between its 64"
own body or possessions from those of others” Bekoff and Sherman 2004). However, 65"
this is a description of a consequence rather than a definition of self-aware as an 66"
individual could be self-aware in the absence of any cue from others. Most discussions 67"
of awareness refer to the social context and to whether animals are able to infer the 68"
mental states of others (Gallup 1998). 69"
A prediction if an individual can have assessment awareness is that if it has a novel 70"
visual experience, like viewing images in a mirror, this could be followed by learning 71"
about what it sees in the mirror in relation to itself and then using such information at a 72"
later time. Human infants can use mirrors in the course of shape discrimination (Itakura 73"
and Imamizu 1994) and, if given sufficient exposure to mirrors at appropriate age, will 74"
discover the contingency between visual and proprioceptive feedback from their own 75"
body movements (Lewis and Brooks-Guy 1979). At five months of age they look more 76"
at their own image in a mirror than at the image of another infant or a puppet (Bahnick 77"
et al 1996) and at nine months they are able to discriminate self from other in a mirror 78"
(Rochat and Striano 2002). At 14-18 months, when looking in a mirror infants are 79"
described as showing self-referencing activities, self-labeling and embarrassment at 80"
rouge on the face (Bertenthal and Fisher 1987). These children would have been told 81"
that the image in the mirror is of themselves. Povinelli et al (1996) allowed children to 82"
see a television image of themselves, very similar to a mirror image, and found that 83"
when a sticker was put on their head, no 2-year-olds reached for the sticker, 25% of 84"
three-year-olds reached for it and 75% of 4-year-olds reached for it. Also using live 85"
television images, Menzel et al (1985) reported that chimpanzees could use these 86"
images to find targets visible only on the television screen. Iriki et al (2001) observed 87"
that Japanese monkeys could use televised images of their hands to pick up food while 88"
Anderson et al (2009), showed live television images of themselves to capuchin 89"
monkeys and concluded that their behaviour strongly suggested recognition of the 90"
correspondence between kinaesthetic information and external visual effects. Dolphins 91"
have been reported to use a television image, apparently to explore themselves visually 92"
(Marten and Psakoros 1995). Tests with chimpanzees, an elephant, dolphins and 93"
magpies that had had previous experience of mirrors, using marks on the body visible 94"
in a mirror, led to the individuals touching or apparently looking at the marks (Gallup 95"
1982, Reiss and Marino 2001, Plotnik et al 2006, Prior et al 2008). 96"
The abilities indicated by these mirror and television image studies range from 97"
discrimination of images, through learning that what is seen in a mirror is on the same 98"
side as the observer, learning that own movements can be monitored by looking at the 99"
mirror, to appreciating that the image is the self. As Rochat (2002) puts it, in this last 100"
case the specular image is standing for the identified or conceptual self, not somebody 101"
else, and the self is as seen by others. 102"
Pigs have complex social behaviour (Jensen 1982, Broom and Fraser 2007) and a 103"
series of experimental studies have also provided evidence of their substantial cognitive 104"
ability (Mendl et al 1997, 2001, Croney et al 2003, Laughlin and Mendl 2004, Held et al 105"
2005). For example, pigs can recall where food was encountered, integrate this 106"
information with information about type of food and replenishment rate and avoid 107"
unproductive visits to potential food sites (Mendl and Paul 2008). The present study 108"
was designed to find out whether or not pigs could use information from a mirror to 109"
locate an object that could only be seen in the mirror. Pigs which had not had 110"
experience of a mirror were compared with pigs which had. 111"
The vision of pigs is adequate for mirror images to be perceived and eyeball size, 112"
retina, pupil and lens are similar to those of humans (Piggins 1992, Zonderland et al 113"
2007). Pigs have fewer cone cells than humans so their spatial discrimination is poorer 114"
(Zonderland et al 2007). However they show preferences for food bowls of certain 115"
colours (Deligeorgis et al 2006). Olfactory signals are used for social recognition and 116"
regulation of sexual behaviour but pigs can successfully find food sources using visual 117"
or olfactory cues (Kristenson et al 2001, Croney et al 2003, Zonderland et al 2007). 118"
In a preliminary study the behaviour of pigs was recorded when they first encountered 119"
a mirror and after 24 hours with it. The response of pigs naïve to a mirror was then 120"
compared with pigs with five hours experience of a mirror when they saw a food bowl 121"
reflected in the mirror. The distribution of time in different parts of the test pen prior to 122"
seeing the food bowl in the pen was also observed in separate trials. In a subsequent 123"
test, the mirror was replaced by wire-mesh with the food in the position visually the 124"
same as when the mirror was present. 125"
The subjects were 4-8 week Large White x Landrace pigs housed in strawed pens 127"
with natural light and food and water ad libitum. All were familiarised with a red food 128"
bowl as a food container. None had seen a mirror, or other reflecting surface, before the 129"
studies described here. 130"
The trials took place in a 4.6 x 2.8 m. strawed pen located approximately 30 m away 131"
from the home pen. All behaviour was video-recorded. The 0.6 x 0.7 m. mirror was in a 132"
1.2 x 1.4 m. frame. A 1.7 m. long 1.4 m. high barrier could be attached to the mirror 133"
frame, 0.09 m. from the mirror, so the pig couldn’t pull it and pass between it and the 134"
frame (Fig. 1). In the preliminary study, seven pigs were put individually into the pen 135"
for 24 hours with the mirror and food present. Their behaviour was recorded for the first 136"
two hours and from 23 to 24 hours. 137"
During the trials with the mirror and food bowl (Mirror Test), the pig was put in a small 138"
pen (area 7) with solid wooden walls. A curtain covered the exit from the small pen so 139"
that the pig could not see outside it. The curtain was opened and the pig left inside this 140"
small pen for 1 minute before the front gate of the small pen was opened with a pulley 141"
to allow the pig to leave. During the minute before the gate was opened in the Mirror 142"
Test the pig could see the barrier and the right hand side of the mirror with the image of 143"
the food bowl through the front section. When in Area 4 or Area 7 the pig could see the 144"
food bowl but could not see whether or not there was food in it. 145"
Fig. 1 Plan of the pen where the experiments were carried out, showing the small pen with solid walls 149"
(area 7), mirror (or wire mesh) in a frame, solid wood barrier, fan position (above pig head level in the 150"
pen) and the numbers of floor sections used to describe the position of the pig. Area 3 is where the red 151"
food bowl, whose reflection was visible in the mirror when the pig was in areas 4 or 7, was placed during 152"
the mirror test. The food bowl would appear to be in area 1 to a naïve pig that had not had experience 153"
with a mirror. 154"
The mirror tests were carried out during nine non-consecutive weeks between 09.00 157"
and 18.00 hours. Firstly, pigs with no previous experience of a mirror were tested, then 158"
pigs that had experience with a mirror. 159"
Eleven “mirror naïve” pigs, six males and five females, which had never seen a mirror 161"
were released, singly, into the pen (Fig. 1) with the red food bowl, containing food, 162"
present on the left side of the barrier and visible only in the mirror. A fan was 163"
positioned slightly behind and above the food. This was intended to ensure that the 164"
smell of the food could not be localised by the pig. Observation of the movement of 165"
particles in the air indicated that air flowed initially from the front towards the back of 166"
the pen but then became mixed throughout the pen. The behaviour of the pigs was 167"
recorded during the Mirror Test with the intention of continuing for one minute or, if it 168"
occurred earlier, until the pig moved behind the barrier or mirror. Each of the 11 pigs 169"
was observed in the Mirror Test once and not used in any subsequent test. 170"
In order that the “mirror experienced” would have the opportunity to learn about a 172"
mirror, eight pigs, four females and four males, were put into the pen with a mirror in it 173"
for 5h. They were in pairs, so that they would not associate the visits to the pen with 174"
social isolation. This provided company but also allowed them to observe the other 175"
animal as a moving reference point in the mirror. The subsequent tests, described 176"
below, were conducted on the same day. 177"
In order to find out where the “mirror experienced” pigs would go by chance in the test 179"
pen after leaving the small pen, two males and two females were observed. Only four of 180"
the eight pigs used in the Mirror Test were used because the desirability of this control 181"
study was only appreciated after the first four pigs had been tested. They were left in the 182"
small pen for 15 s and then allowed to go out of it for 25 s. The barrier and mirror were 183"
in place but no food or food bowl was present. The amount of time spent in areas 1 to 7, 184"
including area 1 behind the mirror and area 3 where the food was located in the mirror 185"
test, was recorded. 186"
The Mirror Tests were done once with each of eight “mirror experienced” animals using 188"
the pen shown in Fig. 1. The bowl with food was placed on the left side of the barrier in 189"
such a way that it could be seen from the small pen via the mirror. Each pig was in the 190"
Area 7 pen and then released, as explained above. After release it was left in the test pen 191"
for a maximum of one minute and its behaviour video-recorded. 192"
After the “mirror experienced” pigs had completed the Mirror Test, in order to check 194"
whether the pigs had just changed their behaviour to show a preference for Area 3 195"
(behind the barrier) wire mesh of mesh diameter approximately 3 cm was put in the 196"
frame in place of the mirror. The food in the food bowl was put behind the frame so the 197"
pig could see it through the wire mesh in the same position that a mirror image would 198"
appear to have. The same methodology was used in the Wire-Mesh Test as in the 199"
Mirror Test for each of the eight pigs. In an extra, subsequent test, with only the last of 200"
the pigs previously tested, wire-mesh was in place of the mirror, the familiar bowl 201"
behind the wire-mesh was clean and empty and there was food in a bowl on the other 202"
side of the barrier, i.e. in the place where the food was put in the Mirror Test. 203"
Initial observations: qualitative descriptions of first contact with the mirror 209"
When first encountering the mirror, all seven pigs whose behaviour was recorded in 210"
detail walked towards it, sometimes vocalising, stopped with nose pointing towards the 211"
mirror, moved forward again and made contact with the mirror surface with their nose. 212"
Some pigs looked behind the mirror after looking at their reflection in it. One female 213"
pig, observed during preliminary studies, moved rapidly towards the mirror and broke 214"
it, perhaps attacking her mirror image. After initially encountering the mirror the pigs 215"
moved back from the mirror surface, oriented nose and eyes towards it apparently 216"
looking at it and made movements looking again from different angles. Three pigs 217"
showed some weaving movements. In the preliminary studies, the mean time before 218"
there was a break of more than 30s in attending to the mirror was 20 minutes. Some 219"
habituation to the mirror was apparent and from 23 to 24 hours after the mirror was put 220"
in the pen, much less time was spent looking at it than in the first hour. Similar 221"
behaviour was shown during the 5h exposure to the mirror by the pigs that would 222"
experience the Mirror Test. Sometimes pigs lay down in front of the mirror, looking at 223"
it or in parallel with it as if lying beside another pig. 224"
“Mirror naïve” pigs in the Mirror Test. 226"
Of the eleven pigs that had never seen a mirror, in the Mirror Test where they could 227"
see a familiar food bowl reflected in a mirror, but not directly visible because it was 228"
behind the barrier, nine approached the mirror then walked behind it to area 1 (Table 1). 229"
One pig knocked over the barrier and one walked around the whole pen including going 230"
behind the barrier. In each case, the trial was then terminated. The nine pigs that went 231"
behind the mirror did so in 15-50 seconds (mean 25.7, s.d. 11.6). 232"
“Mirror experienced” pigs: activity in Mirror Test pen prior to Mirror Test. 234"
The animals observed were able to go anywhere in the test pen for 25s with no food 235"
present, so the total time, during four repeats for 4 animals, was 400 seconds. In the 16 236"
periods, Area 1 was visited by three pigs on one occasion each whilst Area 3 was 237"
visited by four pigs on one occasion each as the pig walked around the pen. The total 238"
time spent in Area 3 was 66s (mean per individual 16.5s, S.D. 9.8). The 66s spent in 239"
Area 3 out of a total time observed of 400 s gives a probability of one in six of a pig in 240"
this pen being in Area 3 at any one time and a probability of one in four of visiting Area 241"
3. A statistical comparison with Mirror Test data is not accurate because some cell sizes 242"
are too small. 243"
“Mirror experienced” pigs in the Mirror Test. 246"
When the eight pigs with previous experience of the mirror were released from the 247"
small pen during the Mirror Test, they walked out, looked around the test pen and 248"
looked at the mirror where the food dish was visible. Seven of the eight pigs went to 249"
Area 3 on the left side of the barrier and reached the food (Table 1). They all moved 250"
away from the mirror, around the end of the barrier, and then directly to the food. The 251"
times taken to reach it were 11s, 29s, 23s, 10s, 46s, 13s, 32s, mean 23.4 s, S.D. 13.3s. 252"
One pig took 41s to decide and then went to Area 1 behind the mirror. For comparisons 253"
of the numbers of naïve and experienced pigs reaching Area 1: p<0.01 and Area 3: 254"
p<0.01 (Fisher Exact Test). 255"
Table 1. Comparison of “Mirror naïve” and “Mirror experienced” pigs in the Mirror 257"
Test and Wire-Mesh Test. 258"
“Mirror naïve” “Mirror experienced” “Mirror experienced” 259"
Mirror present Wire-Mesh present 260"
n 11 8 8 261"
Number going to: 262"
Area 1 (behind mirror) 9 1 6 263"
Area 3 (with food bowl) 1 7 2 264"
Other action 1 265"
Mean latency if reached Area 1 41s (n=1) 14s SD 3.9s 267"
Mean latency if reached Area 3 23s SD 13.3s 43s (n=2) 268"
“Mirror experienced” pigs in the Wire-Mesh Test. 271"
In this test, conducted after the Mirror Test, six of eight pigs went to Area 1, behind 272"
the wire-mesh frame, and reached the food (Table 1). Of the two pigs that went to Area 273"
3, one took 44s to decide and was the individual that did not reach the food in the 274"
Mirror Test, whilst the other took 42s to decide before going to the wrong place. Both 275"
showed frequent hesitation when moving. Comparing 6 out of 8 pigs going to Area 1 276"
with 1 out of 8 in the Mirror Test, p < 0.01 (Fisher Exact Test). In the test on a single 277"
pig with an empty bowl behind the wire mesh and a full food bowl behind the barrier, 278"
the pig went behind the mirror to the empty bowl in Area 1. 279"
The aim of this study was to find out whether or not pigs can obtain information from 285"
a mirror, as has been demonstrated for humans and other primates, dolphins, elephants, 286"
magpies and an African grey parrot (Pepperberg et al 1995). The 4-6-week-old pigs 287"
studied responded to a mirror initially as if to another pig but later by looking at it as 288"
they moved. They moved and then stopped still, apparently looking at their image and 289"
its surroundings, oriented either with nose towards the mirror or with the head parallel 290"
to it. As a consequence of the lateral position of the pig’s eye, it is not possible to record 291"
duration of looks towards the mirror and pigs show little change in facial expression. 292"
They do vocalise and some of these pigs did so when exposed to the mirror. As with the 293"
movements in front of a novel mirror described for chimpanzees, humans, capuchin 294"
monkeys, dolphins and elephants (Gallup 1982, Reiss and Marino 2001, Keenan et al 295"
2003, Plotnik et al 2006, Anderson et al 2009) some of the movements of these young 296"
pigs suggest that they could have been monitoring the movements in the mirror image 297"
when they moved their own head or body. As Anderson et al (2009) put it, the animals 298"
could be comparing the kinaesthetic information and the external visual effects. 299"
Although the naïve pigs exposed to the Mirror Test went behind the mirror to the 301"
apparent position of the food bowl, five hours experience with the mirror in a pen 302"
changed the behaviour of the pigs. When they were subjected to the Mirror Test, all but 303"
one of them went away from the mirror to the actual position of the food bowl within 304"
23s. This movement is first with the air-stream, then against it. The results in total, in 305"
particular the difference between the naïve and mirror-experienced pigs, makes it clear 306"
that the pigs were not locating the food bowl by odour. Pigs often use smell to reach 307"
food (Zonderland et al 2007), but the fan blew air away from the food bowl and 308"
circulated it in the pen. The single pig in the Wire Mesh Test that could see a bowl 309"
through the wire mesh but could not see that the bowl was empty went to the empty 310"
bowl rather than to a bowl containing food behind the barrier. It would seem that 311"
localisation of the food bowl when the fan was on was impossible, or at least more 312"
difficult than using the visual information. The association between visual cues and 313"
food reward is sometimes not an easy task for pigs (Zonderland et al 2007) but it seems 314"
that they learned how to do so in this study. They also learned in five hours to use the 315"
mirror in a way that later allowed them to locate the food. In the Mirror Test, “mirror 316"
experienced” pigs went to the position of the food behind the barrier in Area 3 much 317"
more often than had four of their number, after mirror experience but prior to the Mirror 318"
Test, when their activity was monitored in the Mirror Test pen with no food in it. 319"
The possibility that all pigs had developed a preference for Area 3 at the time of the 321"
Mirror Test was shown not to be the case when the same animals were tested soon 322"
afterwards with the wire-mesh in place of the mirror (Wire-Mesh Test) and six out of 323"
eight went to the food bowl behind the wire mesh in Area 1. One pig went to the wrong 324"
side in both trials, behind the mirror (Area 1) in the Mirror Test and to the left side of 325"
the barrier (Area 3) in the Wire-Mesh Test. This animal either could not learn, or did 326"
not have enough time to learn, about a mirror as it was confused in both trials, taking 41 327"
s and 44 s respectively. Another pig, which reached the food in the Mirror Test but not 328"
in the Wire-Mesh Test, also seemed to be confused in the latter and took 42 s to decide 329"
to go to the left side of the barrier (Area 3) instead of the back of the frame where the 330"
food bowl was located. 331"
A reflecting surface, such as the mirror, was novel to the pigs studied and changes in 333"
their behaviour were apparent when they were exposed to the mirror. Each of the seven 334"
pigs that used information from the mirror and rapidly found the food bowl must have: 335"
observed features of its surroundings, remembered these and its own actions, deduced 336"
relationships among observed and remembered features, and acted accordingly. When a 337"
mirror-experienced pig saw the food in the mirror, it could not smell the food directly, 338"
although it was likely to be able to detect the presence of food throughout the test 339"
The pig has looked at the mirror and appreciated that what it sees is related to its own 342"
movements and that the image reveals objects that are not directly visible and that have 343"
an actual position that has a certain relationship with where they appear to be. When it 344"
looked at the red bowl and then turned away from the mirror to go around the barrier, it 345"
must have remembered that the mirror image gives information about what is positioned 346"
somewhere to the left of perpendicular to the mirror surface. The action of turning away 347"
from the mirror and going behind the barrier to reach the food bowl necessitates 348"
remembering the position of the food while it is navigating around the barrier. The 349"
concept of the food and its position must be remembered while it is carrying out the 350"
actions to get to the food. Some kind of map of its environment and awareness of its 351"
movement ability is needed to do this. The behaviours and ability shown fulfil the 352"
criteria described above for assessment awareness (Sommerville and Broom 1998). 353"
In studies of human infants, and in most studies of other Primates, with mirrors or 354"
television self images, the subjects had prolonged experiences of the images. Human 355"
subjects are generally given much information about mirror images and television 356"
images by their parents and others. The pigs in this study had only five hours of 357"
experience of a mirror before they demonstrated that they could use information from it. 358"
However, no test for self-recognition has been conducted on pigs. Just as in other 359"
studies, e.g. that of Paukner et al (2004) with capuchin monkeys, information from a 360"
mirror or television self-image does not necessarily imply awareness by the subject that 361"
the image is that of itself. 362"
Work with various species of animals indicates that the presence of a mirror or 363"
television image may add complexity to the environment of an individual and improve 364"
its welfare (Plattner and Novak 1997, McAfee et al 2002). These abilities of pigs, and 365"
the awareness indicated by them, may result in some people housing and treating pigs 366"
better than previously, so that poor welfare is minimised. The relationship between the 367"
cognitive ability of animals, sentience and how they should be treated is discussed by 368"
Mendl et al (2001), Broom (2003, 2007), Panksepp (2005), Webster (2006). 369"
We thank Sophie Prowse for help in caring for pigs, supplying materials and practical 371"
guidance, Francisco Bernal for loan of a camera, Gregorio Pesinato for help during 372"
experimental trials and the editor and reviewers for helpful suggestions.. 373"
Anderson, J.R., Kuroshima, H., Paukner, A. and Fujita, K. 2009. Capuchin monkeys (Cebus 376"
apella) respond to video images of themselves. Animal Cognition, 12, 55-62. 377"
Bahnick,L., Moss, L. and Fadil, C. 1996. Development of visual self-recognition in infancy. 378"
Ecological Psychology, 8, 189-208. 379"
Bekoff, M. and Sherman, P.W. 2004. Reflections on animal selves. Trends in Ecology and 380"
Evolution. 19, 176-180. 381"
Bertenthal, B. and Fisher, K. 1987. Development of self-recognition in the infant. 382"
Developmental Psychology, 14, 44-50. 383"
Broom, D.M. 1998. Welfare, stress and the evolution of feelings. Advances in the Study of Behavior, 27, 384"
Broom, D.M. 2003. The Evolution of Morality and Religion (pp.259). Cambridge: Cambridge 387"
University Press. 388"
Broom, D.M. 2007. The evolution of morality. Applied Animal Behaviour Science, 100, 20-28. 389"
Broom D.M. and Fraser, A.F. 2007. Domestic Animal Behaviour and Welfare (pp.437). 390"
Wallingford: CABI. 391"
Croney, C.C., Adams, K.M., Washington, C.G., Stricklin, W.R. 2003. A note on visual, 392"
olfactory and spatial cue use in foraging behavior of pigs: indirectly assessing cognitive 393"
abilities. Applied Animal Behaviour Science, 83, 303-308. 394"
Deligeorgis, S.G., Karalis, K., Kanzouros G. 2006. The influence of drinker location and colour 395"
on drinking behavior and water intake of newborn pigs under hot environments. Applied Animal 396"
Behaviour Science, 96, 233-244. 397"
Gallup, G. G. 1982. Self-awareness and the emergence of mind in primates. American Journal 398"
of Primatology, 2, 237-248. 399"
Gallup, G.G. 1998. Can animals empathize? Yes. Scientific American, 1998, 9, 66-71. 400"
Griffin, D.R. 1981. The Question of Animal Awareness. New York : Rockefeller University 401"
Hauber, M.E. and Sherman, P.W. 2001. Self-referant phenotype matching: theoretical 403"
considerations and phenotype matching. Trends in Neuroscience, 24, 609-616. 404"
Held, S., Baumgartner, J., Kilbride, A., Byrne, R.W. and Mendl, M. 2005. Foraging behaviour in 405"
domestic pigs (Sus scrofa): remembering and prioritizing food sites of different value. Animal 406"
Cognition, 8, 114-121. 407"
Iriki,A., Tanaka, M., Obayashi, S. and Iwamura, Y. 2001. Self-images in the video monitor 408"
coded by monkey intraparietal neurons. Neuroscience Research, 40, 163-173. 409"
Itakura, S. and Imamizu, H. 1994. An explanatory study of mirror-image shape-discrimination 410"
in young children – vision and touch. Perceptual and Motor Skills, 78,83-88. 411"
Jensen, P. 1982. An analysis of agonistic interaction patterns in group-housed dry sows—412"
aggression regulation through an 'avoidance order'. Applied Animal Ethology, 9, 47-61. 413"
Keenan, J. P., Gallup, G. G. and Falk, D. 2003. The Face in the Mirror: the Search for the 414"
Origins of Consciousness. New York: Harper Collins. 415"
Kristensen, H.H., Jones, R.B., Schofield, C.P., White, R.P., Wathes, C.M. 2001. The use of 416"
olfactory and other cues for social recognition. Applied Animal Behaviour Science, 72, 321-333. 417"
Laughlin, K. and Mendl, M. 2004. Costs of acquiring and forgetting information affect spatial 418"
memory and its susceptibility to interference. Animal Behaviour, 68, 97-193. 419"
Lewis, M. and Brooks-Guy, J. 1979. Social Cognition and the Acquisition of Self. New York: 420"
Marten, K. and Psakoros, S. 1995. Using self-view television to distinguish between self-422"
examination and social behavior in the bottlenose dolphin (Tursiops truncatus). Consciousness 423"
and Cognition, 4, 205-224. 424"
McAfee, L.M., Mills, D.S. and Cooper, J.S. 2002. The use of mirrors for the control of 425"
stereotypic weaving behaviour in the stabled horse. Applied Animal Behaviour Science, 78,159-426"
Mendl, M., Burman, D., Laughlin, K. and Paul, E. 2001. Animal memory and animal welfare. 428"
Animal Welfare, 13, S17-S25. 429"
Mendl, M., Laughlin, K., Hitchcock, D. 1997. Pigs in space: spatial memory and its 430"
susceptibility to interference. Animal Behaviour, 54, 1491-1508. 431"
Mendl, M. and Paul, E. S. 2004. Consciousness, emotion and animal welfare: insights from 432"
cognitive science. Animal Welfare, 13, S17-S25. 433"
Mendl, M. and Paul, E.S. 2008. Do animals live in the present? Current evidence and 434"
implications for welfare. Applied Animal Behaviour Science, 113, 357-382. 435"
Menzel, E.W., Savage-Rumbaugh, E.S. and Lawson, J. 1985. Chimpanzee (Pan troglodytes) 436"
spatial problem solving with the use of mirrors and televised equivalents of mirrors. Journal of 437"
Comparative Psychology, 99, 211-217. 438"
Panksepp, J. 2005. Affective conciousness: core emotional feelings in humans and animals. 439"
Consciousness and Cognition,14, 30-80. 440"
Paukner, A., Anderson, J.R. and Fujita, K. 2004. Reactions of capuchin monkeys (Cebus 441"
apellus) to multiple mirrors. Behavioural Processes, 66, 1-6. 442"
Plattner, D.M. and Novak, M.A. 1997. Video stimulation as enrichment for captive rhesus 443"
monkeys (Macaca mulatta). Applied Animal Behaviour Science, 52, 139-155. 444"
Pepperberg, I.M., Garcia, S.E., Jackson, E.C. and Marconi, S. 1995, Mirror use by African grey 445"
parrots (Psittacus erithacus), Journal of Comparative Psychology, 109, 182-95. 446"
Piggins, D. 1992. Visual perception. In Phillips, C. and Piggins, D. (eds) Farm Animals and the 447"
Environment, 159-184. Wallingford; C.A.B. International. 448"
Plotnik, J. M., de Waal, F. B. M. and Reiss, D. 2006. Self-recognition in an Asian elephant. 449"
Proceedings of the National Academy of Sciences, 103, 17053-17057. 450"
Povinelli, D.J., Landau,K.R. and Perilloux, H.K. 1996. Self-recognition in young children using 451"
delayed versus live feedback: evidence of a developmental asynchrony. Child Development, 67, 452"
Prior, H., Schwarz, A. and Güntürken, O. 2008. Mirror-induced behavior in the magpie (Pica 454"
pica): evidence of self recognition. PLoS Biology, 6 (8),: e202.doi:10.137 journal.pbio.0060202. 455"
Reiss, D. and Marino, L. 2001. Mirror self-recognition in the bottle nose dolphin: a cased 456"
cognitive consequence. Proceedings of the National Academy of Sciences, 98, 5937-5942. 457"
Rochat, P. 2002. Origins of self concept. In J.G. Bremner and A. Fogel (eds), Blackwell 458"
Handbook of Infant Development. Oxford: Blackwell. 459"
Rochat, P. and Striano, T. 2002. Who’s in the mirror? Self-other discrimination in specular 460"
images by four- and nine-month-old infants. Child Development, 73, 35-46. 461"
Sommerville, B.A. and Broom, D.M. 1998. Olfactory awareness. Applied Animal Behaviour 462"
Science, 57, 269-286. 463"
Snyder, A., Bossomaier, T. and Mitchell, D.J. 2004. Concept formation:object attributes 464"
dynamically inhibited from conscious awareness. Journal of Integrative Neuroscience, 3, 31-46. 465"
Webster, J. 2006. Animal sentience and animal welfare. Applied Animal Behaviour Science,100, 466"
Zonderland, J.J., Cornelissen, L., Wolthuis-Fillerup, M., Spoolder, H.A.M. 2007. Visual acuity 468"
of pigs at different light intensities. Applied Animal Behaviour Science, 111, 28-37. 469"
Lay summary (150 words). 471"
How clever are pigs? We tested whether pigs can learn that what they see in a mirror is 472"
in front of it in a certain position and not behind it. Young pigs, shown a mirror for five 473"
hours, moved while apparently looking at their image. Afterwards, the pigs were shown 474"
a familiar food bowl, visible in the mirror but hidden behind a solid barrier. Seven out 475"
of eight pigs rapidly found the food bowl by going away from the mirror and around the 476"
barrier. Naïve pigs shown the same, looked behind the mirror. The pigs were not 477"
locating the food bowl by odour and did not have a preference for the area where the 478"
food bowl was. In order to be aware of the food bowl position, each pig must have 479"
learned how to use a mirror image. Views about pig management and welfare may be 480"
changed by such results. 481"
For version with Figure contact authors. 482"
Figure 1. Plan of the pen where the tests were carried out, showing the small pen with 483"
solid walls (Area 7), mirror (or wire mesh) in a frame, solid wood barrier, fan position 484"
(above pig head level in the pen) and the numbers of floor sections used to describe 485"
the position of the pig. Area 3 is where the red food bowl, whose reflection was visible 486"
in the mirror when the pig was in Areas 4 or 7, was placed during the Mirror Test. The 487"
food bowl would appear 488"