Pain, Suffering, and Behavior | 399 What Animals? Only animals that have the neurological development and capability to experience adverse states are the subjects of concern here More primitive forms such as amoebae, simple multi-cellular organisms that lack a complex organized nervous system are unlikely to feel, although they may well react and even possess simple programming mechanisms However, they are unable to interpret novel circumstances When animals possess a level of consciousness that allows them to assimilate new information and to apply general learned principles to novel circumstances, they are more likely to anticipate the future and therefore possibly suffer more than an animal that does not have these faculties As far as we are know, this level of awareness is generally found in vertebrates, but not in invertebrates However, new evidence and a reinterpretation of existing data for invertebrates suggest that this is not entirely true For example, the octopus seems to have an ability to recall adverse experiences and use avoidance behaviors Moreover, the ability to feel pain and other adverse states varies between different phyla and, if two key questions regarding sentience are phrased differently, a different view emerges Specifically, does the animal possess similar or homologous neural pathways, neuropeptides, and hormones that might indicate sentience? And does it behave as it if is sentient in response to what would be a noxious stimulus to vertebrates? Not surprisingly, sentience progressively develops throughout gestation or incubation In humans this seems to be somewhere between 18 and 26 weeks, probably later rather than earlier Furthermore, the development of the nervous system in some mammalian species has been shown not to stop at birth For example, the descending inhibitory pathways that control the passage of nociceptive impulses up to the brain, which is known as gating, because it serves as an obstacle to the continuing passage of impulses up to the brain, continue to develop for several weeks after birth This has led to the speculation that neonatal and young animals feel more pain than they will later in life when their nervous systems have fully matured and the gating mechanism is fully developed Finally, the development of self-awareness and therefore the ability to reflect on one’s own circumstances, could add another dimension to any experience of suffering, and appears to develop at around two years of age in humans, but there is little data in animals other than that it may be present or absent in adults This entry discusses those beings that are sentient, that is, capable of feelings such as pain, fear, frustration, boredom, and possibly other feelings such as happiness, pleasure, grief, and guilt “A Negative Emotional State ” Animals that are sentient can feel positive and negative emotions, and suffering may occur when these feelings are overwhelmingly negative In some situations there may be a mixture of positive and negative states Obtaining food at the price of an electric shock may still be an overall positive experience, and an animal may return to such a situation to maintain its homeostasis, that is, to satisfy its inner feeling of hunger It is obvious that animals can experience a range of emotions, from those indicating pleasure and happiness in some way (dogs wagging tails, cows eager to get out to grass even though they have ample food before them, cats