Description du poste
Animal welfare is an important cause area within effective altruism. Every year, billions of animals suffer, both in factory farms and in the wild. The scale of that suffering means interventions have the potential to measurably improve billions of lives. Yet the organizations most motivated to address these problems have limited resources, so heuristics are required to direct those resources toward the most impactful interventions. One way to measure an intervention’s impact is by how much suffering it reduces, and a critical part of that calculation is an animal’s capacity to feel subjective pain. This is difficult to assess: animals cannot describe their experiences to us, and precise, generalizable neural correlates of subjective pain remain elusive. We are therefore left to rely on heuristics. An obvious heuristic is the number of nociceptors. Nociceptors are sensory neurons that respond to damaging or potentially damaging stimuli. They are distributed throughout the skin, muscles, joints, bones, and viscera, and they form the basis of the sensation of pain. It seems reasonable, then, to expect the number, density, or quality of an animal’s nociceptors to track how much pain it subjectively experiences. So what does research say? I focus on studies in humans, the only animal that can describe its experience of pain.
Nolano et al. (1999) applied capsaicin (the compound that makes chili peppers hot) cream to the forearms of healthy volunteers for three weeks. This reduced sensitivity to painful heat and coincided with the loss of about 80% of the nerve fibers in the outer layer of the skin, fibers that act as nociceptors. After the treatment was stopped, the fibers gradually grew back, and within six weeks both nerve density and pain sensitivity had returned to near-normal levels. Simone et al. (1998) found the same pattern when capsaicin was injected into the skin: pain sensitivity fell as the nociceptors degenerated. Ochoa and Torebjörk (1989) electrically stim