Do bees feel pain?

Even the most careful hive manipulations sometimes result in bees getting rolled between frames, or worse, crushed when reassembling the hive. Some beekeepers clip one wing of the queen to reduce the chance of losing a swarm, or uncap drone brood in the search for Varroa.

All of these activities can cause temporary or permanent damage, or may even kill, bees. A careful beekeeper should try and minimise this damage, but have you ever considered whether these damaged bees suffer pain?

Before considering the scientific evidence it’s important to understand the distinction between the detection of, for example, tissue damage and the awareness that the damage causes is painful and causes suffering.

Detection is a physiological response that is present in most animal species, the pain associated with it may not be.

What is pain?

Tissue damage, through chemical, mechanical or thermal stimuli, triggers a signal in the sensory nervous system that travels along nerve fibres to the brain. Or to whatever the animal has that serves as the equivalent of the brain 1.

This response is termed nociception (from the Latin nocēre, meaning ‘to harm’) and has been recorded in mammals, other vertebrates and in all sorts of invertebrates including leeches, worms and fruit flies. It has presumably evolved to detect damaging stimuli and to help the animal avoid it or escape.

But nociception is not pain.

Pain is a subjective experience that may result from the nociceptive response and can be defined as ‘an aversive sensation or feeling associated with actual or potential tissue damage’.

Most humans, being sentient, experience pain following the triggering of a nociceptive response and, understandably, conflate the two.

But they are separate and distinct. How do we know? Perhaps the first hint is that different people experience different levels of pain following the same harmful experience; an excruciatingly painful experience for one might be “just a scratch’ to another.

‘Tis but a scratch

With people it’s easy to demonstrate the distinction between nociception and pain – you simply ask them.

Can you feel that?

Does that hurt?

For the same stimuli you may receive a range of answers to the second question, depending upon their subjective experience of pain.

Painkillers

But you cannot ask a leech, or a worm or a fruit fly or – for the purpose of this post – a bee, whether a particular stimulus hurts.

Well, OK, you can ask but you won’t get an answer 😉

You can determine whether they ‘feel’ the stimulus. Since this is a simply physiological response you can measure all sorts of features of the electrical signal that passes from the nociceptors (the receptors in the tissue that detect damaging events) through the nerve fibres to the brain. This involves electrophysiology, a well established experimental science.

But how can we determine whether animals feel pain?

What do you do when you have a bad headache?

You take a painkiller – an aspirin or paracetamol. You self-medicate to relieve the pain.

Actually, even before you reach for the paracetamol, your body is already self-medicating by the release of endogenous opioids which help suppress the pain.

In cases of extreme pain injection of the opiate morphine may be necessary. Morphine is a very strong painkiller, or analgesic. Opioids bind to opioid receptors and this binding is blocked by a chemical called naloxone, an opiate antagonist. I’ll come back to naloxone in a minute.

But first, back to the unhelpfully unresponsive bee that may or may not feel pain …

It is self-medication with analgesics that forms the basis of the standard experiment to determine whether an animal feels pain.

The principle is straightforward. Two identical foods are prepared, one containing a suitable analgesic (e.g. morphine) and the other a placebo. If an animal is in pain it will preferentially eat the food containing the morphine.

Conversely, if they do not feel pain they will – on average – eat both types of food equally 2.

But this experiment will only work if morphine ‘works’ in bees.

Does morphine ‘work’ in bees?

An unpleasant or harmful stimulus induces a nociceptive response which might include taking defensive action like retreating or flying away. Studies have shown that the magnitude of this defensive action in honey bees is reduced or blocked altogether by prior injection with morphine.

This is a dose-response effect. The more morphine injected the smaller the nociceptive response by the bee. Importantly we know it’s the morphine that is having the effect because it can be counteracted by injection with naloxone.

So, morphine does work in bees 3.

We can therefore test whether bees choose to self-medicate with morphine to determine whether they feel pain.

And this is precisely what Julie Groening and colleagues from the University of Queensland did, and published three years ago in Scientific Reports. The full reference is Groening, J., Venini, D. & Srinivasan, M. In search of evidence for the experience of pain in honeybees: A self-administration study. Sci Rep 7, 45825 (2017); https://doi.org/10.1038/srep45825

Ouch … or not?

The experiment was very simple. Bees were subjected to one of two different injuries; a continuous pinch to the hind leg, or the amputation of part of the middle leg. They were then offered sugar syrup alone and sugar syrup containing morphine.

The hypothesis proposed was that if bees felt pain they would be expected to consume more of the sugar syrup containing morphine.

To ensure statistically relevant results they used lots of bees. Half were injured and half were uninjured and used as controls. If syrup laced with morphine tasted unpleasant you would expect the control group to demonstrate this by eating less.

Throughout the experiments the authors were therefore looking for a difference in syrup alone or syrup with morphine consumption between the injured bee and the uninjured controls.

All of the experiments produced broadly similar results so I’ll just show one data figure.

Relative consumption of morphine (M) and pure sucrose solution (S) by injured (i; amputated) or control (c) bees.

Both groups of bees preferred the pure syrup (the two box plots on the right labelled S_c or S_i) over the morphine-laced syrup (M). However, the bees with the amputation did not consume any more of the morphine-containing syrup (M_i) than the controls (M_c).

Therefore they did not self-medicate.

Very similar results were obtained with the bees carrying the hind leg clip (recapitulating an attack by a competing forager or predator, which often target the rear legs). The injured bees consumed statistically similar amounts of plain or morphine-laced syrup as the control group.

The one significant difference observed was that bees with amputations consumed about 20% more syrup overall than those with the rear leg ‘pinch’ injury. The authors justified this as indicating that the amputation likely induced the innate immune system, necessitating the production of additional proteins (like the antimicrobial peptides that fight infection), so leading to elevated energy needs. Speculation, but it seems reasonable to me.

Feeling no pain

This study, using a pretty standard and well-accepted experimental strategy, strongly suggests that bees do not feel pain.

It does not prove that bees feel pain. It strongly supports the theory that they do not. You cannot prove things with science, you can just disprove them. Evidence either supports or refutes a hypothesis; in this case the evidence (no self-medication) supports the hypothesis that bees do not feel pain because, as has been demonstrated with several other animals, they would self-medicate if they did feel pain.

In the discussion of the paper the authors suggest that further work is necessary. Scientists often make that kind of sweeping statement to:

  • encourage funders to provide money in the future 😉
  • allow them to incorporate additional, perhaps contradictory, evidence that could be interpreted in a different way to their own results.

Skinning a cat

That is painful … but the proverb There’s more than one way to skin a cat 4 means that there is more than one way to do something.

And there are other ways of interpreting behavioural responses as an indication that animals feel pain.

For example, rather than measuring self-medication with an analgesic, you could look at avoidance learning or protective motor reactions as indicators of pain.

Protective motor reactions include things like preferential and prolonged grooming of regions of the body which have been injured 5. There is no evidence that bees do this.

Avoidance learning

However, there is evidence that bees exhibit avoidance learning. This is a behavioural trait in which they learn to avoid a harmful stimulus that might cause injury.

If a forager is attacked by a predator at a food source (and survives) it stops other bees dancing to advertise that food source when it returns to the hive 6.

Whilst avoidance learning does not indicate that bees feel pain, it does imply central processing rather than a simple nociceptive response. It shows that bees are able to weigh up the risk vs. reward of something good (a rich source of nectar) with something bad (the chance of being eaten when collecting the nectar). This type of decision making demonstrates a cognitive capacity that might make pain experience more likely.

We’re now getting into abstruse areas of neuropsychology … dangerous territory.

Let’s assume, as I do based upon the science presented here and in earlier work, that bees do not feel pain. What, if anything, does this mean for practical beekeeping.

Practical beekeeping

It certainly does not mean we should not attempt to conduct hive manipulations in a slow, gentle and controlled manner. Just because rolled bees are not hurting, or crushed bees are not feeling pain, doesn’t give us carte blanche to be heavy handed.

One of the nociceptive responses is the production of alarm pheromones (sting and mandibular) which are part of the defensive response. Alarm pheromones agitate the hive and make the colony aggressive, much more likely to sting and much more difficult to inspect carefully.

So we should conduct inspections carefully, not because we are hurting the bees, but because they might hurt us.

But there are other reasons that care is needed as well. Crushed bees are a potential source of disease in the hive. One reason undertaker bees remove the corpses is to remove the likelihood of disease spreading in the hive. If bees are crushed the heady mix of viruses, bacteria and Nosema they contain are smeared around all over the place, putting other hive members at risk.

And, as we’re all learning at the moment, good hygiene can be a life-saver.


Colophon

This is the first post written under ‘lockdown’. It’s a little bit later than usual as it has had to travel a   v  e  r  y    l  o  n  g   way along the fibre to ‘the internet’. It’s going to be a very different beekeeping season to anything that has gone before.

At least spring is on the way …

Primroses, 27-3-20, Ardnamurchan

 

Footnotes

  1. Some of these studies have been done with molluscs which have no ‘brain’ in the way we would normally define that organ.
  2. Assuming the foods are equally palatable, which is why you need controls.
  3. Note that this does not demonstrate that bees feel pain. It just demonstrates that they feel. The nociceptive response can be measured using electrophysiological methods and shown to be reduced in the presence of morphine – there may be no behavioural response associated with the nociceptive response.
  4. Which interestingly originated in the 17th Century as More than one way to kill a dog than hanging. Both dogs and cats have an acute awareness of pain.
  5. In the same way that we would repeatedly rub a sore knee.
  6. Tan, K. et al. (2013) Fearful foragers: honey bees tune colony and individual foraging to multi-predator presence and food quality. PLoS One 8, e75841 … this probably deserves a post of its own, there are other examples in different bee species as well.

12 thoughts on “Do bees feel pain?

    1. David Post author

      Thanks Calum
      Hope you’re keeping well. It’s going to be a very strange beekeeping year here … and probably everywhere else.
      Cheers
      David

      Reply
  1. Simon

    David,
    Great article as usual. Thank you. I’ve a question or three.
    In the experiment described with morphine-laced sugar syrup. Wouldn’t the bees have “to learn” that the morphine might reduce the pain? We take pain relief because we have learnt through others experience and passed on knowledge that it helps reduce the pain.
    Is there evidence of opiates being used in the colony? Anything in propolis?

    Keep well and keep blogging

    Reply
    1. David Post author

      Hi Simon

      Just realised this is a similar Q to your previous one. Corona is getting in the way of everything – including me keeping up with the comments here.

      I think it’s worth adding that there is lots of evidence that humans and animals use medicinal plants in a wide variety of ways. For example, some birds use particular plants when nest building as it reduces the parasite burden. Of course, many of these will be learned – though probably through natural selection, not through the passing on of knowledge. However, bees quickly learn to exploit new sugar sources, such as the weirdly coloured honey’s produced when feeding on sweet factory refuse. I think it would be logical to think that a bee that was in pain would be able to quickly realise that the pain was eased if there were fast-acting opioids in the syrup, and would be ‘smart’ enough to act on that realisation and eat more of it.

      Cheers
      David

      Reply
  2. Bill

    Very interesting. One of the references cited in this morphine syrup study is: Adamo, S. A. Do insects feel pain? A question at the intersection of animal behaviour, philosophy and robotics. Anim Behav 118, 75–79, doi:10.1016/j.anbehav.2016.05.005 (2016).

    It makes interesting companion reading:

    Adamo: “Animals have both physiological and behavioural responses to nociception that correlate roughly with the experience of pain in humans (Allen et al., 2005). Although not definitive, these similarities can suggest that an organism experiences pain (e.g. rodents, Allen et al., 2005). However, the argument is valid only in so far as the two phenomena being compared are essentially the same. Unfortunately, invertebrates such as insects are used as examples of animals for which the argument-by-analogy is invalid (e.g. Allen, 2011), primarily because their nervous systems are different from ours (Bullock, Orkand, & Grinnell, 1977).”

    Besides us humans, both the animals cited in the morphine syrup study as examples of preferential self-medicating with analgesics – rats and chickens – are vertebrates: no examples of invertebrates exhibiting this behavior are cited.

    Adamo: “Insects have small nervous systems (typically less than a million neurons), consisting of several distributed brains (ganglia) (Bullock et al., 1977). This distributed organization is thought to limit the capacity for advanced information processing (Bullock et al., 1977). Nevertheless, their principal brains (e.g. supraesophageal ganglia) contain complex neuroanatomical features (e.g. mushroom bodies, Strausfeld, 2002) that have an intricate neural architecture (Giurfa, 2013). The functions of these complex neural arrangements are still under investigation, but insects do have areas that are functionally equivalent to reward circuits in vertebrates (Giurfa, 2013). These complex structures allow insects to vary the activity of different neural circuits, providing insects with the capacity to learn and to have ‘motivated’ behaviour (Giurfa, 2013).”

    And is the assumption that bees’ stomachs are like ours also safe? The morphine syrup study shows that morphine injected into bees is effective and suppressed by naloxone. While we humans, can effectively swallow morphine for pain relief like Thomas De Quincey – “Confesssions of an English Opium Eater”, I couldn’t see where the study confirmed that the morphine taken orally by the bees was absorbed – without that confirmation maybe the morphine syrup just tasted funny, had no analgesic effect when swallowed, and was excreted “unused”?

    The Adamo study also investigates the fun question: ARE INSECTS MORE LIKE LITTLE PEOPLE OR COMPLICATED ROBOTS? So far I haven’t pinched or amputated any of my children’s limbs, but they do find almost all of my interventions intensely irritating – apparently I’m a real pain…

    Reply
    1. David Post author

      Hi Bill

      Apologies for the delay in responding. You’ll appreciate we’re at the boundaries of physiology and philosophy … a largely uncharted territory for this molecular biologist/virologist.

      I agree that there are very significant differences in the nervous systems of insects and ‘higher’ animals such as primates. However, there are significant similarities as well. The nociception responses are the same for example. Their brains may be smaller and have very different organisation, but the underlying functions – detecting harm, learning etc. – are likely to share a number of similarities. I’d argue that the morphine/naloxone injection studies further emphasise these similarities.

      However, you are correct about the oral availability aspect of the study. I don’t know whether this has been determined, though the experiment would be straightforward. You’d have two groups of bees, one fed syrup with morphine, one without. You’d dose ’em up and then measure the nociception responses using standard electrophysiology approaches. Individual bees might vary, but averaged across the study group you should be able to determine whether the morphine was orally available.

      Here’s hoping the corona-induced home confinement leaves your children’s limbs attached … though there’s bound to be an increase in the irritation (both ways 😉 ).

      Thanks for detailed comment.
      Cheers
      David

      Reply
  3. Caz

    Find it disgusting to mutilate bees even in the name of science. Whether it hurts them in a way we understand is immaterial. It’s WRONG. For years experiments have been done on dogs people rodents fish invertebrates and plants. ITS WRONG.

    Reply
    1. David Post author

      Whilst I respect your views Caz, I don’t agree with them. Far better we understand whether pain or suffering are caused, and therefore know what to avoid, than just make emotive or anthropomorphic decisions. More generally, many of the experiments on animals are to prevent diseases – both human and animal. Without animal experimentation, for vaccines for example, we would have very high infant and youth mortality rates. Before the 20th Century about 25% of infants died in their first year, and ~50% died before sexual maturity. The global average of ~4.5% (<15 years) today is not solely due to vaccines of course, it's also due to improvements in diet through more intensive farming. All of which involves animals and plants.

      And, if we're going to test vaccines we need to know how similar or different the responses of animals are to humans. Not just in their immune responses, but in a much broader range of physiological characteristics.

      Reply
  4. Janet Wilson

    The best reason to inspect respectfully (quietly, no sudden movement, with smoke, no vibrations, no rolling of bees) is that it keeps the bees in good temper, and that extends the length of time you can stay in there, looking at frames and bees. Once you open the hive the clock starts ticking, but with good practice, it ticks more slowly, a big help to newbees and a bigger help to beekeepers who have to get something specific done.

    Greetings everyone in this most unusual and unsettling time. Nice to have the bees to focus on.

    Reply
    1. David Post author

      I agree Janet … and it’s interesting how different colonies respond. Some are very passive and you can be really leisurely about things, others are perfectly OK as long as you’re not too slow even if you are extremely careful. For normal manipulations neither of these is an issue, for the beginner the really ‘laid back’ bees are a big help.

      Enjoy your bees, it’s going to be a very strange season.
      David

      Reply
  5. Simon Rice

    Great article David. Thank you.
    Where does learning come into play in the experiment described? We have learnt that morphine reduces pain and have passed that knowledge down through generations, but the bees in the experiment have not had that opportunity have they?
    Are there pain relieving compounds contained in propolis?

    Reply
    1. David Post author

      Hello Simon

      Sorry … social distancing got in the way of my reply 😉

      Opioids work pretty quickly as anyone who has had fentanyl during routine hospital treatment will testify. There’s not a lot of learning needed if a drug is rapidly effective. I think the assumption here is that opioids in bees would work relatively rapidly. Perhaps they don’t? Perhaps – for whatever reason – they work over hours or days? The injection studies showing that opioids function in bees would argue against a significantly different temporal mode of action.

      I don’t know the answer as to whether there are analgesics in propolis. Interesting.

      Cheers
      David

      Reply

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