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including smaller body size, production of fewer eggs per clutch and

shorter lifespan. Intuition suggests that this genotype remains in nature

because it has the benefit of avoiding pathology induced by the parasite.

However, Yanet al. (1997) could detect no fitness costs of infection. This

presents a perplexing question. Why does the genetic polymorphism

continue in natural habitats if resistance to infection has a fitness cost but

no benefit when the mosquito is infected?

More common are circumstantial stories. One such example concerns

an odd anatomical feature found on lizards of at least five families – skin

invaginations on the neck, axilla and other body regions that seem partic-

ularly prone to infestation by mites and ticks. Arnold (1986) suggested

Parasite Virulence 287

Rate of parasite reproduction

Allowed high Held low

Cost to host

Benefit to host

Optimal investment

in defence

Cost or benefif

Fig. 14.1. Curves showing one possible relationship for the cost and benefit of

differing levels of immune attack against a parasite. The horizontal axis shows

the level of the host’s antiparasite effort necessary to hold the parasite’s rate of

replication to low levels, or which will allow the parasite to replicate rapidly. The

‘Cost to host’ curve shows the cost to the host’s fitness from mounting an immune

attack. A low-level immune attack, one that allows the parasite reproductive rate

to remain high, would have low cost, but a potent immune attack that holds the

parasite to a low rate of reproduction would have a high cost. The ‘Benefit to host’

curve shows the benefits of limiting the rate of parasite reproduction. A fairly

low-level immune attack with a marginal reduction in the parasite growth rate would

be highly beneficial, so the benefit curve is steep. However, further reduction in

parasite growth rate would have a declining additional benefit for the host. The

shape of the cost curve shows that the initial small investment in the immune attack

would have low cost, but completely curtailing the parasite would have an extreme

cost to the host. The optimal solution is an intermediate level of immune response.

Parasites with effective methods to evade the host immune system may select for

very limited immune attack. The host may well be able to trump the parasite’s

defences, but the cost would outweigh the benefits.