The famous sex life of praying mantises has important implications for whether they, and other insects, can feel pain. Image credit: 夏爱克 CC-By_2.0
Several lines of evidence suggest that the central nervous system of insects processes pain in a much more similar way to ourselves than anyone has cared to admit.
Scientists once performed what we now consider horrific animal experiments out of curiosity. Nowadays, vertebrate studies have to go before ethics committees to show that the value of the research outweighs any harm to the subjects, even if not everyone agrees on the line to be continued. Cephalopods such as octopus and squid are beginning to benefit from the same protection.
Insects, on the other hand, are generally treated as fair game. Fruit fly researchers are not held to the same standards as those working with mice, let alone monkeys. The rationale – that insects do not feel pain as “higher” animals do – is disputed by a new paper in the journal Proceedings of the Royal Society B.
The authors note that the question has not been studied much, perhaps because people were afraid of the possible answer.
In the parlance of neuroscientists, nociception is the encoding by the nervous system of harmful or unpleasant stimuli such as extreme temperatures, pressure or intense chemical attack. Animals (including insects) respond to these signals to limit damage to their bodies that could hinder their survival. What is debated is whether insects sense pain through the central nervous system, or whether the response is localized, for example in an injured limb.
After all, insects have a much less sophisticated central nervous system than mammals, with only a tiny fraction of brain cells devoted to processing these inputs. In particular, they lack the opioid receptors so essential for pain control in our own brains. However, Queen Mary University PhD student Matilida Gibbons and her co-authors say that doesn’t mean they’re running out of simpler versions of the same ability.
Nociception is closely related to pain, but it is not the same thing. Our bodies can sometimes modulate pain without altering nociceptive reflexes, especially in emergency situations where too much pain can distract us from what we need to do. The pain comes later, forcing us not to use an injured limb, for example. Curiously, the reverse was also observed with improved nociception with no change in pain levels.
Nevertheless, we do not understand how nociception and pain are linked in insects, so the authors explore the ability of insects to control nociception, which they consider indicative, if not evidence.
“Behavioral work shows that insects can modulate harmful behaviors. Such modulation is at least partly controlled by the central nervous system since the information mediating such prioritization is processed by the brain.
The authors identify specific neuropeptides produced in insects during traumatic events that may act as analgesics, similar to the role played by opiates in humans.
Further evidence is how insects, like other animals, can become sensitized to particular threats. If fruit flies are repeatedly exposed to high temperatures, they begin to react more quickly when heat is applied. Some of the molecules involved in this sensitization are the same as those observed in humans. Pathways sending nociceptive messages to the brain have also been identified.
Even one of the most famous insect behaviors – the sexual cannibalism of female praying mantises – can shed light on the matter. Infamously, male mantids react to having their heads chewed off by copulating harder. To do this, the male must suppress his typical attack response.
“This evidence has been suggested to indicate the absence of pain in insects,” the paper notes. “However, it is more likely to demonstrate that insects can prioritize other behavioral needs and reduce noxious-responsive behavior in certain contexts.” This in turn indicates a centralized response, which in turn makes pain sensations more plausible, not less.
We still don’t know how pain is processed in insect brains, if at all, but that’s less important than determining our answer if it turns out to be true. If we find that insects feel pain, can we really continue to treat them as we do?