This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Pigs are one of the top animals consumed across the world. According to the US Census Bureau, in 2010, around one hundred million metric tons of pork were consumed that year, with 10% of this being in the US (although it does seem that overall meat consumption is declining).
With so many of us eating pork, you might think we’d know a bit more about these animals. A lot of people are surprised to hear about some of the cognitive abilities of the average pig. While it’s problematic to call an animal 'intelligent' or not, as this is a term is ill-defined and too often related to human cognition, pigs have shown us that they have a number of cognitive abilities tested across many different types of test. They have good learning and memory in many contexts (both short- and long-term), including episodic memory (memory for past events in their life), the ability to differentiate between familiar and unfamiliar pigs, and an inclination to explore novel objects.
In addition to these behavioural feats, the pig brain is well-developed. For example, the volume of the prefrontal cortex is around 24% of the total neocortex and 10% of the total brain volume, comparable to primates including humans.
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I’m not sure why, despite this research, pigs have a reputation for being ‘stupid’. Similar to the ‘three-second memory’ myth with fish, I wonder if it’s perpetuated to make people not feel bad about eating these animals, or the conditions under which they are often reared.
These conditions include not having enough space, being in barren environments bereft of stimulation and being denied the opportunity to perform their natural behaviours. In addition to this, many piglet’s tales are docked (80% of piglets in the UK), they have their teeth clipped, and males are castrated without anaesthetic (now banned in the UK but still legal throughout much of Europe).
One recent study looked specifically at one aspect of pig rearing: how enriched the pigs’ environment was, to see its effects on the animals' cognition. In particular, they looked at the effect of enrichment on the pigs' spatial memory: the ability to remember where an object is.
Two groups of pigs were tested: one from ‘barren’ housing and one from ‘enriched’, which contained wood shavings, peat, straw and branches.
The task consisted of sixteen buckets, four of which had chocolate raisins in them (extremely yummy to pigs). The pigs were given access to these buckets over 30 trials, to see if they remembered where the chocolate was between trials, and how long it took them to learn this. To make sure that the pigs were actually remembering where the chocolate was and not just smelling it, all buckets had chocolate raisins placed under them under a mesh (to allow the odour through but not allowing the pigs to access them).
The pigs housed in the barren pens were actually faster to reach the first baited bucket than the pigs from the enriched housing. This agrees with previous findings, and is thought to be because barren-housed animals are basically bored, and will work harder for food due to the lack of stimulation in their own pens. Additionally, the barren-housed pigs were just as good at remembering where the food was between the trials (using their long-term memory) as the enriched pigs. However, the barren-housed pigs had a worse short-term memory within each trial for the chocolate-rewarded buckets they had already visited and emptied, revisiting these buckets more often than the enriched pigs. The fact that the pigs’ housing condition affected short- but not long-term memory is interesting and supports the fact that these are different memory systems are underpinned by different neural substrates.
Rather than thinking of the enriched environment as enhancing the pigs’ cognitive abilities, the scientists instead present the poorer performance of the barren-housed pigs as a symptom of stress at their housing situation. This kind of ‘barren’ housing is common for pigs in many farms, and although consumers may not care if the pig they are eating is cognitively inferior to an enriched pig, they should care if the reason for this is due to stress or suffering caused to the animal.
References
Bolhuis, J. E., Oostindjer, M., Hoeks, C. W., de Haas, E. N., Bartels, A. C., Ooms, M., & Kemp, B. Working and reference memory of pigs (Sus scrofa domesticus) in a holeboard spatial discrimination task: the influence of environmental enrichment. Animal cognition DOI 10.1007/s10071-013-0646-7
Kornum, B. R., & Knudsen, G. M. (2011). Cognitive testing of pigs (Sus scrofa) in translational biobehavioral research. Neuroscience & Biobehavioral Reviews, 35(3), 437-451.
Photo credits
Meat graph by Angela Wong at NPR, data taken from here
Pig brain taken from Kornum & Knudsen (2011)
Pig with piglets: woodlywonderworks
Pig in house (photo not from experiment): Sean
German saddleback, a breed of domestic pig: Silke