By Nick Kilvert
Up until the mid-1980s, human babies didn't feel pain.
Studies indicate bees experience both stress and pessimism. Photo: 123RF
Of course that's not actually true, but due to research conducted in the 18th and early 19th centuries, it was an attitude that still lingered among a small minority of scientists and medical professionals.
So much so that some infant surgery was still conducted without, or with very little, anaesthesia in the US into the '80s.
Today, the question of physical and emotional experience has moved beyond humans to animals, including invertebrates like insects, crabs and octopus.
And it's not just the feeling of pain that is being debated, but the whole spectrum of emotional experience: stress, joy, apprehension, even pleasure.
"Given the growing number of studies finding markers of emotions in animals, we've reached a tipping point," philosopher Kristin Andrews tells the ABC.
"Science is generating more and more reasons to accept animals' emotions, without finding any new reasons to deny them."
Professor Andrews is a research chair in animal minds at the University of York in Canada.
Alongside biologist and primate behaviour expert Frans der Waal, Andrews argues in a recent issue of Science that the weight of evidence says many invertebrates experience what we might call emotions, and that this morally matters.
"If they can no longer be considered immune to pain, invertebrate experiences will need to become part of our species' moral landscape," they wrote.
So what is some of the evidence that points to invertebrate sentience? And if we accept that some invertebrates experience emotions and pain, how should this change how we interact with them?
Are shaken bees stirred?
Similar to the example of babies and pain, scientifically quantifying the emotional experience of animals is difficult because they can't tell us what they feel.
But we may be able to infer the emotional state of animals by how they respond to things.
In humans, negative emotions are correlated with "pessimistic cognitive biases" - basically, when we're in a bad frame of mind, we tend to expect things to not go well for us.
Honey bees were less likely to reach for a reward after being shaken. Photo: AFP
An experiment where honey bees were shaken for 60 seconds to simulate a predatory attack found the bees' equivalent of blood - called haemolymph - was lower in dopamine and serotonin after shaking, compared with bees that weren't shaken.
Both dopamine and serotonin are involved in mood regulation in humans.
But more than that, the shaken bees seemed to be less optimistic about their chance of getting a treat.
At the beginning of the experiment, the bees were conditioned to recognise a reward - a sugary liquid - and a punishment in the form of a bitter quinine solution.
After shaking, the bees were then offered mixtures made of different ratios of quinine and sugar liquid, including a half-half mix.
The bees that hadn't been shaken were significantly more likely to extend their proboscis to taste the half-half mix than those that had.
The inference was that "negative cognitive bias" in the shaken bees meant they were more likely to presume the borderline mixture was the punishment mixture, whereas the unshaken bees presumed the opposite.
The bees' behaviour has "more in common with that of vertebrates than previously thought", the authors of that study wrote at the time.
If a similar response was measured in humans, we'd have no doubt that emotions were involved, professors Andrews and der Waal argue.
"In any mammal, a pessimistic behavioural response associated with physiological indications of stress would be taken as a sign of negative emotions," they say.
"The same logic should be applied to insects."
Laughing rats 'solicit' tickles
Similar studies in birds and mammals have produced the same kind of responses.
And though not invertebrates, research into rodents has found a whole host of behaviours that point to emotions beyond simple reflexive responses.
Some evidence suggests that rats laugh and will 'solicit' tickles. Photo: 123RF
A 2016 study found prairie voles display "consolation behaviour" - increased grooming of partners suffering distress - but that they don't elicit the same behaviour for unfamiliar voles in distress.
And research from the 1990s found rats at play emitted distinctive chirping at a frequency of around 50 kilohertz, which scientist Jaak Panksepp argued was a primitive form of laughter.
Even more so than when playing, the researchers found the rats would chirp when subjected to "heterospecific cross-species handplay" - otherwise known as tickling.
This is some of the evidence that Andrews says convinced her of broader animal sentience.
"I was struck by the late Jaak Panksepp's work on rat joy and laughter," she says.
"Rats solicit tickles from preferred researchers."
In addition to soliciting tickles, subsequent research found tickled rats showed behaviour patterns consistent with their having a more optimistic outlook than non-tickled rats.
Painful experience leads to behaviour change
Having also previously studied rat behaviour, US-based marine biologist and physiologist Robyn Crook has also been studying pain responses in octopus.
Last year, Dr Crook published research showing that octopus experience long-term pain, rather than just a simple reflexive withdrawal from damaging stimuli.
Her research also suggested an emotional component to their experience of pain.
Octopus in multi-chambered tanks were injected in one arm with a dilute acid.
Following injection, they showed a significant and ongoing avoidance of the chamber in which they had been injected.
They also tended to the arm long after the acid was administered.
Those given analgesic drugs to numb the pain then showed a preference for the chamber where they were given the drug.
There is significant evidence that octopus feel pain and stress, among other feelings or emotions. Photo: Taputeranga Marine Reserve
The argument is that there must be an emotional experience that goes along with the feeling of pain in order to produce long-term behavioural changes.
Without that experience to trigger the negative connotation between, in this case, the chamber and pain, there is no reason for the octopus to avoid the offending space in future.
Crook's research also showed extensive grooming of the painful site, and she demonstrated a physical pathway for pain to be processed by octopus.
According to philosopher Peter Godfrey-Smith from the University of Sydney, whose research interests include the origins of consciousness, Crook's paper has given us the best demonstration so far of pain in cephalopods.
"I think that's the best paper on octopus pain ever - there are so many lines of evidence that point to pain in there," Professor Godfrey-Smith says.
Pain research 'opens door' to other emotions
In terms of pain alone, late last year the evidence was deemed strong enough by the UK parliament that some invertebrates are now recognised as sentient, with rights to humane treatment under their Animal Welfare Bill.
A large scientific review to inform the Bill conducted by researchers from the London School of Economics overwhelmingly found that the literature points to cephalopod molluscs - like octopus, cuttlefish and squid - and decapod crustaceans - lobsters, crabs, prawns and the like - being capable in varying degrees of experiencing pain and pleasure.
The researchers scored the animals against eight criteria to come up with their findings.
Crustacean brain chemistry has been found to change due to 'stress', and anti-anxiety medication can affect their behaviour. Photo: RNZ / Vinay Ranchhod
These included the possession of receptors (nociceptors) sensitive to a noxious stimulus, integrative brain regions capable of processing that sensory information, neural pathways to connect those nociceptors to the critical brain regions, and response to analgesic or anaesthetic.
Perhaps unsurprisingly, they found the strongest evidence for sentience in octopus, but crabs, crayfish and cuttlefish also scored highly.
Godfrey-Smith says those animals were the right place to start in terms of broadening the scope of sentience.
"They chose cephalopods and crustaceans as the central cases to think about, and I think that was the right move," he says.
So what about other invertebrates? They were beyond the scope of the parliament's commissioned study, but Andrews says studying pain tends to be where research into emotion begins.
"We need more research on positive emotions in invertebrates," she says.
"It seems that the research starts with pain, and once pain is established, it opens the door to study other emotions."
How should our behaviour change?
It's worth stating first off that this issue is not entirely settled and there's still a lot of room for scientific debate, Godfrey-Smith says.
"None of this is for sure," he says.
"There are plenty of people who would disagree; there are qualified people who still don't think fish feel pain."
But if we do get to a point where we recognise that invertebrates are capable of emotional experiences, how, if at all, should that change our behaviour and laws?
First, it would mean that animal welfare laws would need broadening.
But the specifics are up for debate.
In the UK for instance, it was a recommendation of the review into the sentience of crustaceans and cephalopods that "boiling alive, slowly raising the temperature of water, tailing (separation of the abdomen from the thorax, or separation of the head from the thorax), any other form of live dismemberment, and freshwater immersion (osmotic shock)" be banned where other more humane killing methods are available.
But what about their treatment before being dispatched? Is it ethical to keep sentient creatures in small tanks, for example?
There are no blanket rules, according to Andrews.
"We should be including invertebrates in animal protection schemes," she says.
"This doesn't mean that we would treat worms like we treat ravens, though."
Instead, these things will have to evolve as the science continues to catch up.
"Until we know more about animal emotions, we don't know what their interests are," Andrews says.
Whether fish and invertebrates are sentient or not raises questions over their treatment. Photo: 123RF
"Even once we know that, we can't automatically infer anything about how to treat animals.
"The specific ethical conclusions about how to treat animals will come from a collaboration between scientists and philosophers."
And what about our own interactions? Does this mean we should think twice before squashing a mosquito, for instance?
Godfrey-Smith says once we get beyond cephalopods and crustaceans, things start to get a bit murky.
"It would be so massive a shift to think that all the countless insects that are subject to pest control are sentient," he says.
"I don't think the case is nearly as strong for changing our behaviour toward them as octopus and crustaceans."
Andrews also doesn't think considering a few more animals sentient will have life-changing implications for most of us.
"It's interesting to me that so many journalists are assuming (or think readers are assuming) that if we decide an animal is sentient, then we can't kill that animal," she says.
"The laws are about how to house sentient animals in a way that respects their needs, and to kill them humanely."
- ABC