Socio-cognitive abilities of free-ranging pigs (Sus scrofa domesticus): their indicators of welfare and ethical implications
Dissertation - University of Veterinary Medicine Vienna - 2020
The social intelligence hypothesis is currently the leading hypothesis, which attempts to explain the evolution of higher intelligence. It proposes that the cognitive evolution is a result of selection for socio-cognitive skills. While primates and social birds dominate the field of research in socio-cognitive abilities of animals, domesticated species are still underrepresented. Similar to dogs, the domestic pig has a long domestication history and is strongly connected to the development of the modern human during the Neolithic Revolution. Other than dogs, the wild ancestors of pigs are still available, widespread and relatively easily accessible. However, the cognitive abilities of pigs remain mainly unknown. To contribute to the social intelligence hypothesis and to learn more about the socio-cognitive abilities of pigs, a sounder of 39 free-ranging pigs living in semi-natural conditions was established on an eight hectar field at Haidlhof Research Station, Bad Vöslau, Lower Austria. To cover three main cornerstones of social intelligence, three experiments with complementary topics were conducted.
Study 1 focused on individual learning of visually presented categories and generalization of the learned features to novel instances. For this purpose, we constructed a Touchscreen box suitable for pigs where we could present two pictures of which each one represented one category. We used pictures of human heads, photographed from the front vs. the backside. After reaching a learning criterion of more than 80% correct choices in three consecutive sessions, the test phase started with a generalization test, where novel picture pairs were presented. All pigs generalized the learned features highly significantly above chance. After this first test, five feature tests followed in a randomized order for each pig. We presented the known positive stimulus and a manipulated face of the same person. In condition Inverted Face, the inner face contours were inverted, while the outer silhouette was the same. In Condition Inverted Picture, the whole picture was turned upside down. In condition No Face, all facial features were erased. For condition Scattered Face, the facial features (eyes, mouth, nose) were randomly arranged in the face area. Condition No eyes No Mouth paired a face with erased eyes to a face with erased mouth. The majority of the pigs did not show any preference for the manipulated or the original picture. Some individuals, however, seemed to use distinct features to discriminate between the categories and chose significantly the original face or the manipulated one. Contrary to previous studies, we could show the remarkable abilities of pigs to discriminate categories of pictures using visual cues only. In addition, we established a new method for pig cognition experiments using the touchscreen technology.The result provide some evidence that pigs would use a feature set to generalize the learned features to novel pictures.
In Study 2, we investigated the mechanisms of information transfer from adult pigs to their offspring in an extractive foraging situation. The piglets observed their mother or aunt while they were opening a box with a sliding door. Each piglet was assigned to either their mother or aunt as a demonstrator. One demonstrator opened it to the left, the other to the right side. After the observation phase, the piglets got access to the box and were allowed to open it themselves. One group served as non-observer baseline. Both observer groups were significantly faster in touching and starting to manipulate the box than the non-observer group. They also showed a significant preference to move the sliding door in the direction they observed. They did not learn via local or stimulus enhancement, as they did not use the same area or coloured bar as the demonstrator to manipulate the door. But they moved the door to the same side as the demonstrator, which suggests that they learned via object-movement re- enactment. More controls, especially a ghost control, would be necessary to rule out asocial factors like spacial position of the open or closed doors.
The information content of formants in piglets’ grunts was investigated in Study 3. Acoustic allometry is a tool to identify the specific acoustic features, which are correlated with body size of the emitting individual, and formants are often a useful cue for this question. We took repeated X-ray radiographs of the same 20 piglets at four occasions within the first six months of life to measure the length of the vocal tract. At the same occasions, we recorded samples of grunts of each individual separately. The formants encoded in the grunts were correlated with the size of the vocal tract and the body size of the piglets. We could find that body weight is a strong predictor for the vocal tract length. The formants in the grunts are in turn determined by the vocal tract length. We therefore found evidence that the formants in grunts potentially carry information about the body size of the emitting individual. However, if pigs are able to use this information needs further research including playback experiments.
Taken together, the three experiments round up three major topics of social cognition of free ranging pigs. We gained insight in the mechanisms how pigs gain and process information visually and socially and learned that also the constantly emitted grunts might carry some information content. It seems that pigs have even more complex social abilities than expected based on what is known so far. By now, research topics about pigs in general focus mainly on productivity in meat production and physical health. To assess the welfare status of an animal, one needs not only physical parameters but also knowledge about their social needs, their perception and abilities to be able to install cognitive enrichment, which is truly suitable to improve their living conditions.
Dissertation - Veterinärmedizinische Universität Wien - 2020
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