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Is it time for insect researchers to consider their subjects’ welfare?

April 08, 2024 3 min read

Recent evidence suggests that at least some insect species might plausibly feel pain. These findings should prompt researchers to think about the welfare implications of insect experiments.

Recent evidence suggests that at least some insect species might plausibly feel pain. This Perspective argues that researchers need to rethink the welfare implications of these findings for experiments that use insects.

Do insects feel pain? Pain is an unpleasant subjective experience that often accompanies injury. This feeling is distinct from nociception: the unconscious process of detecting and avoiding harmful stimuli. Pain is hard to identify in nonhuman animals because unpleasant feelings might not accompany simple withdrawal and other behavioral responses. While we can determine that humans feel pain when they describe their experiences, this approach is obviously impossible for animals.

To understand whether an animal feels pain, researchers search for clues in their nervous systems and behavior. These clues can shift the probability of pain, even without delivering decisive proof. For example, imagine an injured dog with neuroanatomy and behavior similar to the human case. The dog winces, limps, licks their wound, avoids the place where they were injured, and seeks out analgesic drugs. None of these lines of evidence formally prove that the dog experiences pain, but collectively, they make this highly likely. Common sense dictates that such probability-shifting evidence is sufficient for us to consider the dog’s welfare.

What, then, can we say about the probability of pain in insects—and the potential importance of insect welfare? Some scientists have expressed skepticism about insect pain, pointing to their small nervous systems and apparently reflexive nocifensive behaviors [1]. Nonetheless, recent work has undermined these assertions

In a comprehensive review, we evaluated over 300 studies to understand the current state of evidence relevant to pain in 6 major insect groups [3]. We adopted a contemporary framework for assessing evidence relevant to pain, which has previously guided welfare policy [4]. The framework includes 8 criteria, encompassing both whether the animal’s nervous system might support pain and whether their behavior likely involves centralized, integrative processing with the sensory and affective aspects of pain (Box 1). Each criterion adds to the case for pain: the more criteria fulfilled, the higher the likelihood.

Box 1. Eight criteria for assessing evidence relevant to pain in animals

Nociceptors. The animal possesses receptors sensitive to noxious stimuli (nociceptors).
Integrative brain regions. The animal possesses integrative brain regions capable of integrating information from different sensory sources.
Integrated nociception. The animal possesses neural pathways connecting the nociceptors to the integrative brain regions.
Analgesia. The animal’s behavioral response to a noxious stimulus is modulated by chemical compounds affecting the nervous system in either or both of the following ways:
1. The animal possesses an endogenous neurotransmitter system that modulates (in a way consistent with the experience of pain, distress, or harm) their responses to threatened or actual noxious stimuli.
2. Putative local anesthetics, analgesics (such as opioids), anxiolytics, or antidepressants modify an animal’s responses to threatened or actually noxious stimuli in a way consistent with the hypothesis that these compounds attenuate the experience of pain, distress, or harm.
Motivational trade-offs. The animal shows motivational trade-offs, in which the disvalue of a noxious or threatening stimulus is weighed (traded-off) against the value of an opportunity for reward, leading to flexible decision-making. Enough flexibility must be shown to indicate centralized, integrative processing of information involving an evaluative common currency.
Flexible self-protection. The animal shows flexible self-protective behavior (e.g., wound tending, guarding, grooming, rubbing) of a type likely to involve representing the bodily location of a noxious stimulus.
Associative learning. The animal shows forms of associative learning in which noxious stimuli become associated with neutral stimuli, or in which novel ways of avoiding noxious stimuli are learned through reinforcement. These forms of associative learning go beyond classical conditioning in which a single conditioned stimulus overlaps temporally with an unconditioned stimulus.
Analgesia preference. The animal shows that they value a putative analgesic or anesthetic when injured in one or more of the following ways:
1. The animal learns to self-administer putative analgesics or anesthetics when injured.
2. The animal learns to prefer, when injured, a location at which analgesics or anesthetics can be accessed.
3. The animal prioritizes obtaining these compounds over other needs (such as food) when injured.