Mammals and birds

Evidence quality

Evidence quality: High for the capacity of mammals and birds to experience pain; variable for precise measurement of pain intensity in a particular animal (National Research Council, 2009; Prunier et al., 2013). A National Research Council expert committee concluded that all vertebrates should be considered capable of experiencing pain and described the likelihood as particularly strong for mammals and birds (National Research Council, 2009). This conclusion is supported by comparative neuroanatomy, physiology, behavior, and responses to interventions rather than by any single marker (National Research Council, 2009; Prunier et al., 2013).

Neurological and physiological evidence

Pain is not identical to nociception: nociception is the neural detection and processing of potentially damaging stimuli, while pain includes an adverse subjective experience (National Research Council, 2009). Because nonhuman animals cannot verbally report that experience, researchers assess whether multiple lines of evidence are consistent with pain rather than relying on a withdrawal reflex alone (National Research Council, 2009).

Birds possess the neurological components involved in transduction, transmission, modulation, projection, and perception of painful stimuli (Douglas et al., 2018). Their nervous systems differ anatomically from mammalian systems in some respects, but the avian review literature concludes that birds likely perceive pain in a manner similar to mammals (Douglas et al., 2018). In farm mammals and birds, pain is also associated with changes in behavior, the hypothalamic-pituitary-adrenal axis, sympathetic activity, immune measures, and performance; the specificity and feasibility of these indicators differ by context (Prunier et al., 2013).

Behavioral and analgesic evidence

Behavioral indicators include changes in posture, movement, activity, social behavior, and use of an affected body part (Prunier et al., 2013). A review covering farm species found numerous behavioral changes associated with pain and concluded that combining indicators generally improves the sensitivity and specificity of assessment (Prunier et al., 2013). Shared neuroanatomy and physiology therefore operate alongside, not in place of, observable behavior (National Research Council, 2009; Prunier et al., 2013).

Responses to analgesia add evidence because a pain-related behavior that changes after pain relief is less readily explained as a fixed reflex (National Research Council, 2009; Danbury et al., 2000). In a controlled study, lame broiler chickens selected more carprofen-containing feed than sound birds; selection increased with lameness severity, and walking ability improved in a dose-dependent manner (Danbury et al., 2000). This does not establish that every behavioral change is caused by pain, but it supports the interpretation of lameness-related behavior as reflecting an aversive state that the birds were motivated to relieve (Danbury et al., 2000).

Sentience and scientific practice

Sentience is broader than pain and refers to subjective states, including positive and negative experiences (Proctor et al., 2013). A systematic review of more than two decades of literature found substantial agreement about many states animals can feel, while also identifying gaps, particularly for positive emotions and less-studied taxa (Proctor et al., 2013). Much of the evidence base is mammal-centered, which reflects both the use of mammalian models and their comparative similarity to humans; this limits equal precision across species but does not create comparable doubt about the general capacity of mammals to experience pain (Proctor et al., 2013).

The evidence is also reflected in routine welfare and veterinary science, where validated or developing pain indicators are used to assess cattle, pigs, sheep, poultry, and other species (Prunier et al., 2013). On the narrow claim in the stub, denying mammal or bird sentience is inconsistent with mainstream scientific assessment (National Research Council, 2009; Proctor et al., 2013).