Evolution And Hygiene

Parasites are an incredibly diverse and widespread form of life. Ranging from sub-microscopic viruses to large multi-cellular organisms like tape worm, parasites share a strategy of using another organisms for energy and/or shelter.

As you might expect, parasitic infection is not in the interest of the host organism, and indeed all hosts have sophisticated mechanisms for preventing parasites from entering the body, and for eliminating infection when it does occur. In the last few decades the sister fields of parasitology and immunology have tracked how this ancient conflict has shaped the physiology of both hosts and their parasites. More recently, scholars of both human psychology and animal behaviour have realised that this conflict has shaped not only the physiology, but also the brains and behaviour of host organisms.

Immune responses are costly and sometimes ineffective: energy is needed to make disease fighting cells, some parasites are very difficult to eradicate, and there is often a chance that the infection will spread on to other family members. A better strategy is to behave in a way that reduces the chances of any contact with the parasite. If organisms could recognise and avoid the parts of their environment that are particularly parasite rich then they minimise the chances of getting infected in the first place. And such ‘hygienic’ behaviour is just what we see in many different species: Sheep graze selectively, avoiding patches where other sheep have defecated, thus avoiding eating - and being infected by - the parasites in and around faeces.

Similarly, some species of monkey have specific ‘defecation grounds’ where they avoid foraging. Lobsters will avoid infected lobsters, and social insects have an impressive array of behavioural strategies that limit infection: dead ants and waste matter are carried out of the nest, caste structures maintain divisions between parasite rich forager ants and rest of the colony, and infected individuals are ejected from the nest. Time and time again, natural selection has favoured behavioural strategies that limit the chances of infection.

Of course, our real species of interest at the Hygiene Central is humans. And indeed the behaviour of humans appears to have been shaped by history of interaction with parasites, just as the behaviour of sheep, ants and lobsters has. We are decidedly repulsed by many of the more parasite rich elements of our environment – faeces, decaying flesh, vomit, infected open wounds… Researchers at the Hygiene Centre (Dr. Curtis, Dr. Aunger and Dr. Biran) have argued that disgust is an emotion that has been shaped by natural selection in order to limit the chances of infection.

Disgust operates in many domains now – we are disgusted by corrupt politicians and deviant sexual behaviour too – but its primary evolutionary function has been to keep us away from that which will make us ill. In future research Dr. Val Curtis and PhD candidate Micheal de Barra hope to focus on a number of topics relating to evolution and hygiene behaviour.

1. The relationship between human and non-human disease avoidance strategies and their psychological foundation.
2. How disgust relates to hygiene behaviour, and how it can be utilised in the context of hygiene behaviour change programs.
3. Integrating an evolutionary understanding of disgust with its development across the life span.

Further reading:

Social Immunity. Current Biology, 17(16), R693-R702
Cremer, S., Armitage, S. A. O., & Schmid-Hempel, P. (2007).

Evidence that disgust evolved to protect from risk of disease. Proceedings of the Royal Society of London Series B-Biological Sciences, 271, S131-S133
Curtis, V., Aunger, R., & Rabie, T. (2004).

The Behavioral Immune System. Its Evolution and Social Psychological Implications. In J. P. Forgas, M. G. Haselton & W. von Hippel (Eds.), Evolution of the social mind: Evolutionary psychology and social cognition(pp. 293-307). New York: Psychological Press
Schaller, M., & Duncan, L. A. (2007).