A striking and major difference between free-living and parasitic species is that free-living organisms evolve in response to selection pressures exerted by both abiotic and biotic factors whereas parasites almost uniquely respond to the selection pressure exerted by their hosts (although this does not apply to parasitic species that spend a considerable part of their life outside, or not in contact with, a host). This means that parasite evolution cannot be envisaged other than in the light of host evolution. In other words, hosts and parasites are involved in a process of coevolution where the emergence and spread of a trait in the parasite (i.e., a trait that confers a better ability to exploit the host) select for a specific response in the host and, vice versa, the emergence and spread of a trait in the host (i.e., a defense mechanism) select for a specific trait in the parasite (see Coevolution).
One particular group of parasites provides, probably, the best illustration of the coevolutionary process. As mentioned above, parasitism in not restricted to microorganisms and invertebrates. Some birds, such as cuckoos and cowbirds, have also adopted a parasitic life style. Of course, cuckoos do not develop inside a host and do not consume host resources as a microorganism would do. Cuckoos and cowbirds exploit a particular resource of the host: parental care. Cuckoos cannot reproduce unless they find an appropriate host (another bird species) that takes care of their eggs and nestlings. If the cuckoo egg is incubated by the female host, a cuckoo nestling will hatch,
Table 2 Adaptations and counter-adaptations involved in the coevolutionary process between brood parasites and their hosts
Parasite exploitation strategy
Finding a suitable host nest in the appropriate breeding phenology. Laying an egg
Monopolize host resources (parental care). Incubation time is shorter in the parasite than the host. As soon as the parasite hatches, it ejects host eggs or kills/outcompetes host nestlings (nestlings of the European cuckoo have evolved a particular structure in the back that, like a spoon, allows them to eject host eggs out of the nest rim). Cuckoo nestlings provide superstimuli (visual and vocal) to the foster parents to obtain sufficient food
Defending the nest against the intruder
Spotting any strange egg in the clutch and reject or destroy any egg differing from other eggs of the clutch
Dropping the egg in the nest within few seconds when the host has left the nest unattended Laying mimetic eggs that match as close as possible host eggs in size and color Lay thicker eggs that resist dropping in the nest and host puncturing Nestling mimicry®
aHost defense against parasitic nestlings has been reported for the interaction between the superb fairy-wren (Malurus cyaneus) and the Horsfield's bronze-cuckoo (Chrysococcyx basalis).
usually before any other host nestlings. The impact of cuckoo parasitism on host reproductive success is dramatic as the cuckoo nestling ejects all host eggs and nestlings out from the nest, reducing the host brood to a single parasitic nestling. Given the cost for host fitness, it is straightforward to expect that hosts have evolved a set of traits aiming at reducing the risk of brood parasitism and, in turn, brood parasites have evolved a series of strategies to overcome host defense. Table 2 summarizes the most prominent host adaptations and parasite counter-adaptations involved in the coevolutionary process between brood parasites and their hosts. Why is this example particularly relevant to illustrate the coevolutionary process? The specificity (usually brood parasite species or host races exploit a single host species, even though exceptions exist) and the nature of the traits involved (traits that do not have any possible function other than exploiting the host or resisting the parasite) limit the chances that the presumed adaptation has arisen because of indirect selection exerted by any other source not involved in the interaction.
Even though more diffuse, coevolution is a major and pervasive characteristic of host-parasite interactions. Understanding the evolution of parasitic strategies and lifestyle is, therefore, a particularly tough task as it has responded and currently still responds to the selection pressures exerted by other living and evolving organisms, the hosts.
See also: Coevolution; Competition and Behavior; Life-History Patterns; Parasites.
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