New study suggests house crickets may feel pain

Evidence that insects may be capable of feeling pain is growing, and a new study has added another finding to the case.

Researchers at the University of Sydney in Australia have documented how the apparently simple house cricket protects and grooms its antennae after being touched with a heated probe. The insect is already known to possess the equipment needed to feel pain.

How researchers assess pain and sentience

Determining whether an animal experiences pain is difficult. Even in people, pain is subjective, so the most direct way to measure it is usually to ask someone to rate it on a scale from 0 to 10.

Yet the subjective experience of pain is an important consideration when deciding whether an animal or other entity is sentient: capable of feelings and emotions rather than merely physiological reactions.

A cricket cannot simply be asked, “Does it hurt?” Instead, researchers must find ways to interpret its responses to potentially painful stimuli.

In 2022, a team of experts in animal behaviour and cognition did just that, establishing eight criteria for assessing whether an animal could be considered sentient on the basis of its subjective experience of pain.

The researchers developed the criteria for crabs, lobsters and other decapod crustaceans, but they have since become a benchmark applicable to many other animals.

Meeting any one criterion counts as evidence that an animal might experience pain beyond a simple withdrawal response and could therefore be considered sentient.

The work also led the UK government to include decapod crustaceans, such as crabs, lobsters and prawns, and cephalopods, such as octopuses and squid, in the Animal Welfare (Sentience) Act 2022.

The eight criteria are:

1. Nociception: The animal possesses the nerve receptors needed to detect harmful stimuli.
2. Sensory integration: Regions of the animal’s brain can combine information from different sources.
3. Integrated nociception: Neural pathways connect the animal’s harm receptors to brain regions that integrate information.
4. Analgesia: Substances known to relieve pain in humans alter the animal’s response to harmful stimuli.
5. Motivational trade-offs: The animal shows sufficient evidence of weighing potential harm against potential reward, demonstrating flexible decision-making.
6. Flexible self-protection: After a harmful stimulus, the animal behaves in ways such as tending a wound, guarding, grooming or rubbing.
7. Associative learning: The animal shows signs of linking harmful stimuli with, for example, neutral stimuli, or perhaps learns through reinforcement how to avoid harm.
8. Analgesia preference: An animal learns, for example, to administer painkilling compounds to itself, or even chooses them over other needs, such as food, when injured.

Some other insects have already met several of these criteria. Flies and cockroaches have met six, while bees, wasps and ants have qualified for four.

Insects in the order Orthoptera, which includes crickets, have not been tested sufficiently. This is especially notable because the house cricket, Acheta domesticus, is among the world’s most widely farmed insects and is eaten by humans, pets and research animals.

How house crickets responded to heat

The University of Sydney team carried out an experiment to investigate whether house crickets tend an injury after heat is applied to their antennae.

“We found that crickets didn’t just reflexively flinch and recover,” entomologist Thomas White and philosopher-biologist Kate Lynch explain in an article for The Conversation.

“They nursed the harm, returning again and again to groom the affected site, much as we rub a burned hand.”

Male and female crickets both displayed this behaviour. They were twice as likely to do so when the probe touching an antenna was heated to 65 °C as when it was unheated or did not touch the cricket at all.

That temperature is sufficient to activate the cricket’s ‘harm’ receptors, but not to cause lasting damage. The researchers observed the insects for ten minutes after each prod.

“Grooming was directed specifically at the heated side, not spread evenly across both antennae as it was after gentle touch or no contact,” White and Lynch explain.

“And the behaviour wasn’t a brief, reflexive reaction. It was elevated from the outset and tapered gradually over minutes, much like rubbing a burned hand as the felt sting slowly fades.”

What the findings mean for cricket sentience

The study shows that this pet-shop staple meets the criterion for flexible self-protection. The finding could have ethical implications for the way these insects are farmed, handled and used in experiments.

“Crickets have already been shown to possess nociceptors, centralised sensory integration, have the capacity to learn from aversive events, and moderate their responses to noxious stimuli following analgesia,” White, Lynch and their team write.

With flexible self-protection now added to that list, crickets meet five of the eight criteria, the researchers say.

“Together, these lines of evidence suggest that orthopterans exhibit the same conjunction of features – nociception, integrative processing, learning, and targeted self-protection – that are taken to warrant serious consideration of sentience,” the team concludes.

The research was published in Proceedings of the Royal Society B.