Parasitoid - Influence On Host Behaviour

Influence On Host Behaviour

In another strategy, some parasitoids influence the host's behaviour in ways that favour the propagation of the parasitoid, often at the cost of the host's own life. A spectacular example is the endoparasitoid Dicrocoelium dendriticum, the Lancet Liver Fluke that causes host ants to die clinging to grass stalks where grazers or birds may be expected to eat them and complete the parasitoidal fluke's life cycle in its definitive host. Similarly, as strepsipteran parasitoids of ants mature, they cause the hosts to dawdle high on grass stalks, positions that are risky, but favour the emergence of the Strepsipterans. Other species of endoparasitoids cause infected woodlice and land Amphipoda such as Talitroidesto run about in the open by day, where predators such as birds can catch them and continue the cycle.

Returning to the case of the rabies virus and the disease, one could rationalise the death of the host similarly. The virus affects the host's central nervous system with eventually fatal effects. That could be seen as a consequence of the strategy for dissemination of the virus by affecting the host behaviour. Similar principles might apply to, for example, Vibrio cholerae, the cholera bacterium and other, often fatal, enteric pathogens that induce diarrhoea and spread by contagion.

For the soft ticks the benefit of the paralysis they inflict might be seen as influencing behaviour in that it prevents the host from wandering away while they feed, which they do very quickly and in large numbers, some species emerging from their hiding places at night. Other species hide in sandy patches in the shade of trees in semi-desert such as the Kalahari, and emerge to feed as soon as any large animal settles down in the shade during the heat of the day.

In parasitic birds such as cuckoos, the young often are adapted to act as "super solicitors", with loud, persistent voices and with large, vividly coloured gapes and behaviour that stimulate the feeding instincts of the foster parents to the utmost. Consequently the legitimate chicks, even if they are not evicted, often starve because they are less well-equipped for soliciting for food.