Behavioural defences in animals against pathogens and parasites: parallels with the pillars of medicine in humans

Abstract

No other theme in animal biology seems to be more central than the concept of employing strategies to survive and successfully reproduce. In nature, controlling or avoiding pathogens and parasites is an essential fitness strategy because of the ever-present disease-causing organisms. The disease-control strategies discussed here are: physical avoidance and removal of pathogens and parasites; quarantine or peripheralization of conspecifics that could be carrying potential pathogens; herbal medicine, animal style, to prevent or treat an infection; potentiation of the immune system; and care of sick or injured group members. These strategies are seen as also encompassing the pillars of human medicine: (i) quarantine; (ii) immune-boosting vaccinations; (iii) use of medicinal products; and (iv) caring or nursing. In contrast to animals, in humans, the disease-control strategies have been consolidated into a consistent and extensive medical system. A hypothesis that explains some of this difference between animals and humans is that humans are sick more often than animals. This increase in sickness in humans leading to an extensive, cognitively driven medical system is attributed to an evolutionary dietary transition from mostly natural vegetation to a meat-based diet, with an increase in health-eroding free radicals and a dietary reduction of free-radical-scavenging antioxidants.

Figure 1.

Fly switching by captive elephants, as in wild elephants, increases when flies are present and the behaviour significantly reduces the number of blood-sucking flies [18]. (Photo by the author.)

Figure 2.

Dogs, like cats, regularly eat plants, especially grass. Contrary to common beliefs the behaviour is not typically preceded by signs of illness nor followed by vomiting; the dogs are usually normal in behaviour, as reported by the owners. Plant eating would appear to be a behaviour inherited from wild ancestors that, in nature, had a role in maintaining low intestinal parasite loads [38]. Photo courtesy of Sheila D'Arpino, of her dog Cinco.

Figure 3.

A comparison of plant eating by dogs up to 1 year of age and 3–13 years of age. (a) The younger dogs eat plants more frequently. (b) Not only do younger dogs appear ill beforehand less frequently, but they also rarely vomit afterwards. These differences reflect an apparent innate developmental adaptation to the young being more vulnerable to the costs of intestinal parasites and eating the parasite-purging plants more frequently [38].

Figure 4.

In vitro experiments on the effects of bay leaves, which are brought into the nests of dusky-footed wood rats, in controlling fleas by killing the larvae or repelling adult fleas. (a) Effects of incubation of torn bay leaves, and leaves from control plants, in killing flea larvae. Bay leaves killed significantly more larvae than control plants [45]. (b) Effects of extracts of bay leaves, compared with the commercial repellant, DEET, and control plant, Toyon, in repelling adult fleas. Bay leaf extract did not differ significantly from DEET in repelling fleas, whereas Toyon did significantly repel fewer fleas than DEET (A. Alvarado & B. L. Hart 2005, unpublished data).

Figure 5.

Example of social-empathic behaviour in elephants showing the grieving-like, or standing-vigilant behaviour over a recently deceased elephant. In this instance, prior to the severely injured matriarch's death, repeated attempts were made to help her get up. After her death her body was visited on a daily basis by different elephants [56]. (Photo courtesy of Shivani Bhalla—Save the Elephants.)

Figure 6.

A diagrammatic illustration of the unique von Economo neurons (VENs), in the human and elephant cerebral cortex. The information-processing pyramidal neurons, profiled here as black triangles are layers 3 and 5. The blue (grey) spindle-shaped VENs, which are much less dense, are believed to mediate social-empathic behaviours, such as caring and grieving over deceased conspecifics. Illustration by the author is based on published data [57,58].