Now that we know all vertebrates and probably the majority of invertebrates feel pain what about young animals? Until an animal (or human) reaches a certain stage of development its brain is presumably not sufficiently mature enough to recognise and register that a painful event is occurring. This does not mean painful events do not occur or that the body does not respond to them in some way. It just means that the brain is not able to consciously register the event. This happens during surgery when an animal (or human) is anaesthetised. The surgical event is still painful but the part of the brain that recognises and responds to this fact has been switched off by virtue of the anaesthetic. Extrapolating from this a foetus is effectively unconscious and unaware of painful stimuli until its brain has become complex enough to develop consciousness.
The electrical activity in the brain can be measured via an electroencephalogram (EEG). Very early on in development the EEG is absent i.e. there is no detectable brain activity. As the animal develops EEG activity begins to appear. The time when this occurs relative to an animal’s birth depends on how mature it is after it is born. Lambs are born fairly well developed and are able to stand and walk shortly after birth. They develop EEG readings and therefore conscious perception after about 80% of the pregnancy has elapsed (similar to humans) and are born more or less fully conscious, although their EEG shows significant continued maturation during the first week of life. Lamb EEG responses to castration are not as great at one to two days after birth as they after one week post birth.
Rats are reasonably immature when born and have no detectable EEG. EEG signals don’t appear until 12 to 18 days after birth. Rats whose tails are clamped five to seven days after birth do not respond, while those clamped after 12 days do.
Marsupials are an interesting case because they are born at a very immature stage and crawl into their mother’s pouch where they complete most of their development. Interestingly they are able to complete that task even though their brain consists of only two layers of cells. In the tammar wallaby EEG activity does not appear until after 120 days of pouch life (total pouch time is approximately 250 days). Earlier than this and there is no response to toe clamping (Diesch et al 2007).
The main assumption here is that EEG activity correlates with consciousness. While this seems valid it is impossible to be certain and, even if the young animal does not consciously experience pain, the body can still react to painful stimuli by releasing stress hormones, withdrawal reflexes and changes in brain blood flow. There is some suggestion that while the animal cannot consciously perceive the pain it becomes sensitised to it such that it develops an increased perception of pain after birth that could become permanent (http://www.daff.gov.au/__data/assets/pdf_file/0019/1046431/25-craig-johnson.pdf). As always it seems prudent to err on the side of caution and avoid causing pain wherever possible.
Dr. F. Bunny
Diesch, T.J., D.J. Mellor, C.B. Johnson, and R.G. Lentle. 2007. Responsiveness to painful stimuli in anaesthetised newborn and young animals of varying neurological maturity (wallaby joeys, rat pups and lambs). Proceedings of the 6th World Congress on Alternatives and Animal Use in the Life Sciences, Tokyo. Pp. 549-552.