pig in pen
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Few people would deny that animals are able, at least to some degree, to experience pain. However, in many cases farm animals routinely undergo procedures which are likely to cause pain without pain relief (often called “analgesia”).

For example, the American Veterinary Medical Association reports that surgical castration of male piglets “is typically performed without anaesthesia or analgesia” 1 and a recent RSPCA report stated that “It is estimated that 80 per cent of male piglets in the EU are castrated each year (approximately 100 million piglets), many without anaesthetic/analgesic” 2. A survey of U.S. animal science faculty members found that only 34% of respondents agreed with the following statement; “acute interventions that cause pain (e.g., castration) should be performed under local anaesthesia (or general, if animal’s age suggests that)”.

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Respondents frequently defended their answers with statements such as “How do we know the animal is experiencing pain?”. Perhaps this question lies at the crux of why animal pain is overlooked in some industries; animals can’t communicate their feelings in the same way as we can, so it is easier to dismiss their feelings altogether.

Pain is a complex phenomenon defined by the International Association for the Study of Pain (IASP) as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage” 3. This definition and the associated notes emphasize the importance of verbal self-report, i.e. describing how much pain you are in, in the assessment of pain. Unfortunately, non-human animals are unable to verbally tell us whether they are in pain, or describe where the pain is located and how much it hurts, which makes pain assessment in animals difficult.

The IASP definition differentiates the subjective, emotional experience of pain from the sensory experience of pain, which is also known as “nociception” (nociception is defined by the IASP as the “neural process of encoding noxious stimuli” 3). The subjective experience of pain is arguably the hardest component of pain to measure, even in humans; for example, the phrase “severe pain” is likely to mean different things to different people. It is harder still to measure this component in non-humans, since they can’t tell us how they are feeling… or can they? Recent advances in animal behaviour science have revealed techniques which can be used to effectively “ask” non-human animals about their feelings; we will describe two examples below.

Judgement Bias Testing

Judgement bias testing (JBT) measures an individual’s emotional state (or “affective state”) by observing how they interpret an ambiguous situation. The individual may interpret the situation in an “optimistic” way or a “pessimistic” way. In human psychology, it is known that people who are in a generally negative affective state are more likely to interpret situations in a more pessimistic manner 5. Put simply: if you’re in a bad mood you’re more likely to think bad things are going to happen to you in the future.  

So how can this be demonstrated in non-human animals if they can’t directly tell us their predictions about the future? A simple paradigm which measures expectation of reward/punishment is the most widely used form of JBT in non-human species 6. These tasks normally involve training an animal to associate one cue with a reward and another cue with a punishment/lesser reward, then presenting ambiguous cues and observing whether the animal behaves like they are expecting a reward or not. For example, a group of researchers (Neave et al., 20147) wanted to find out if dairy calves were in a more negative affective state following hot-iron disbudding (a method of dehorning routinely used in young cattle, see here for more information).


baby cow calf


Firstly, the calves were trained that if they touched a computer screen with their nose they would get a reward (milk!), but they would only get a reward if the screen was red when they touched it, if the screen was white they would get nothing. The calves were soon running up to touch the screen when it turned red, and ignoring it when it turned white (see the video embedded at the end of the online study article here, it’s really quite cute!). Once the calves had learnt that red= reward and white= no reward, they were presented with ambiguous screens (various shades of pink).

The researchers found that the calves were more likely to go up and touch the darker pink screens (i.e. closer to red) than the lighter pink screens (closer to white). The calves then underwent routine hot-iron disbudding before being exposed to the screens again.

When shown the ambiguous pink screens before disbudding, the calves treated the darker pink and lighter pink screen colours as being similar to the red and white screens, approaching these screens in 92% and 23% of trials, respectively. The calves were more ambivalent about the medium pink screen (halfway between red and white), approaching it in 69% of the trials. After disbudding calves were significantly less likely to approach all three of the ambiguous pink screens (they approached the dark pink in 88% of trials, the medium pink in 55%, and the light pink in 12%).

These findings indicate a pessimistic judgement bias in dairy calves following a painful experience (hot-iron disbudding), which suggests that the procedure had a negative impact on their affective state.

It is interesting to note that calves in this study were sedated and given local anaesthetic before the procedure took place, which is standard practice in many countries to protect calf welfare. However, there are some countries which do not mandate the use of local anaesthesia before dehorning cattle under a certain age, these include Australia, New Zealand, and Denmark 8. It can reasonably be assumed that withholding local anaesthesia would lead to the procedure having an even greater negative impact on calf welfare.

Self-selection of analgesics

Another technique which could be used to “ask” animals if they are in pain, is to allow them the opportunity to self-medicate with analgesics (pain killers). For example, a group of researchers (Danbury et al., 2000 9) trained broiler chickens (chickens raised for meat) to discriminate between two different coloured chicken feeds, one of which contained pain-relieving medication (carprofen).

The researchers found that chickens who were lame consumed a greater proportion of feed treated with carprofen than untreated feed, compared to non-lame chickens. These researchers also found that the more severe the bird’s lameness, the greater the proportion of treated feed they consumed.

These findings suggest that lame chickens will actively choose to relieve their pain, which suggests that pain is an unpleasant experience which they will seek to avoid. These findings are worrying in the light of a large-scale study which assessed the walking ability of 51,000 broiler chickens in the UK and found that 27.6% showed poor locomotion, and 3.3% were “almost unable to walk” 10. This high prevalence of lameness in broiler flocks is thought to be associated with selective breeding for high growth rates (most broiler chickens reach slaughter weight in just 40 days).


Many people reading this will probably find the studies mentioned above pointless… surely anyone who has spent time with non-human animals will know that they can experience pain and suffering?

If you accidentally tread on your dog’s paw, or close a door on your cat’s tail, you’ll know from the noises they make that they are hurt, and you will instinctively feel empathy towards them, and want to comfort them. However, in industries where animals are reared for profit, it is often the case that welfare and profit are inversely correlated: analgesics cost money, castrating pigs improves the quality of the meat (by eliminating boar taint), breeding birds to grow fast increases the rate of meat production.

This means that improved profits, which can be easily proven, will trump animal welfare concerns if all there is to back up these concerns is sentimental assumptions about their feelings. This is why, in my opinion, there needs to be more empirical evidence to support our animal advocacy campaigns; so that those who do not instinctively feel compassion for non-human animals will be forced to question their opinions and beliefs.



  1. AVMA. Welfare Implications of Swine Castration. AVMA Literature Reviews. 2013. (link)
  2. RSPCA. Welfare Standards for Pigs. RSPCA Farm Animal Welfare Standards. 2016 (link)
  3. IASP. Report of International Association for the Study of Pain Subcommittee on Taxonomy. Pain. 1979;6(3):249-52. (link)
  4. Heleski CR, Mertig AG, Zanella AJ. Assessing attitudes toward farm animal welfare: a national survey of animal science faculty members. Journal of Animal Science. 2004; 82(9):2806-14. (link)
  5. Bower, G.H., 1983. Affect and cognition. Philosophical Transactions of the Royal Society of London Series B-Biological Sciences 302, 387-402. (link).
  6. Mendl M, Burman OHP, Parker RMA, Paul ES. Cognitive bias as an indicator of animal emotion and welfare: Emerging evidence and underlying mechanisms. Applied Animal Behaviour Science. 2009;118(3-4):161-81. (link).
  7. Neave HW, Daros RR, Costa JHC, von Keyserlingk MAG, Weary DM. Pain and Pessimism: Dairy Calves Exhibit Negative Judgement Bias following Hot-Iron Disbudding. Plos One. 2014;9(4):1. (link).
  8. AVMA. Welfare Implications of Dehorning and Disbudding Cattle. AVMA Literature Reviews. 2014. (link).
  9. Danbury TC, Weeks CA, Chambers JP, Waterman-Pearson AE, Kestin SC. Self-selection of the analgesic drug carprofen by lame broiler chickens. Veterinary Record. 2000;146(11):307-+. (link).
  10. Knowles TG, Kestin SC, Haslam SM, Brown SN, Green LE, Butterworth A, Pope SJ, Pfeiffer D, Nicol CJ. Leg Disorders in Broiler Chickens: Prevalence, Risk Factors and Prevention. Plos One. 2008; 3(2): e1545 (link).

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