Onset of sentience: The potential for suffering in fetal and newborn farm animals
Publication Type: |
Journal Article |
Year of Publication: |
2006 |
Authors: |
David J. Mellor, Tamara J. Diesch |
Publication/Journal: |
Applied Animal Behaviour Science |
Keywords: |
birth, fetus, neural development, neuroactivators, neuroinhibitors, newborn, suffering, unconsciousness |
ISBN: |
01681591 |
Abstract:
Sentience and consciousness are prerequisites of suffering. Thus, animals must have sufficiently sophisticated neural mechanisms to receive sensory information and to transduce this information into sensations, and they must also be conscious to be able to perceive those sensations. Moreover, those sensations must be sufficiently noxious or aversive to cause suffering. The neural apparatus of embryos and fetuses of farmanimals is inadequate to support sentience for at least the first half of pregnancy, but the required structures and mechanisms do develop by the time of birth. Thus, although one of the preconditions for suffering is satisfied shortly before birth, the embryo and fetus are apparently never conscious for the following reasons. The embryo-fetus initially does not have brain structures that are functionally capable of supporting consciousness, and subsequently, when the fetal brain might have that capability, it displays electrical activity indicating a continuous state of sleep and therefore unconsciousness. Furthermore, the fetus is apparently actively maintained in sleep-like states by several endogenous neuroinhibitory mechanisms which involve adenosine (a potent neuroinhibitory and sleep inducing agent), allopregnanolone and pregnanolone (two neurosteroidal anaesthetics), prostaglandin D2 (a potent sleep-inducing hormone), a placental neural inhibitor, warmth, buoyancy and cushioned tactile stimulation. Consciousness evidently appears for the first time only after birth. This results from a substantial withdrawal of the neuroinhibitors, especially adenosine, and the involvement of neuroactivators including 17β-oestradiol (a potent neuroactive steroid with widespread excitatory effects in the brain), noradrenaline (released from excitatory locus coeruleus nerves that extend throughout the brain), and a barrage of novel sensory information associated with the newborn’s first exposure to air, gravity, hard surfaces, unlimited space and, usually, to cold ambient conditions. We conclude that the embryo and fetus cannot suffer before or during birth. Furthermore, we conclude that suffering can only occur in the newborn when the onset of breathing oxygenates its tissues sufficiently to substantially reduce the dominant adenosine inhibition of brain electrical activity. The implications of these observations for managing fetuses and newborns in ways that minimise suffering are considered briefly.