Abstract
When mimicry imposes costs on models, selection may drive the model's phenotype to evolve away from its mimic. For example, brood parasitism often drives hosts to diversify in egg appearance among females within a species, making mimetic parasitic eggs easier to detect. However, when a single parasite species exploits multiple host species, parasitism could also drive host egg evolution away from other co-occurring hosts, to escape susceptibility to their respective mimics. This hypothesis predicts that sympatric hosts of the same parasite should partition egg phenotypic space (defined by egg colour, luminance and pattern) among species to avoid one another. We show that eggs of warbler species parasitized by the cuckoo finch Anomalospiza imberbis in Zambia partition phenotypic space much more distinctly than do eggs of sympatric but unparasitized warblers. Correspondingly, cuckoo finch host-races better match their own specialist host than other local host species. In the weaver family, parasitized by the diederik cuckoo Chrysococcyx caprius, by contrast, parasitized species were more closely related and overlapped extensively in phenotypic space; correspondingly, cuckoos did not match their own host better than others. These results suggest that coevolutionary arms races between hosts and parasites may be shaped by the wider community context in which they unfold.
Highlights
When mimicry is costly to models, selection should drive a model’s phenotype to evolve away from that of its mimic [1]
Distinguishing self from non-self is paramount to the hosts of avian brood parasites, as it is to the victims of many other aggressive mimics
To improve their chances of detecting a parasitic mimic, hosts can diversify their own eggs into a multi-dimensional phenotypic space comprised of such traits as egg colour, luminance and pattern [8,9,14]
Summary
When mimicry is costly to models, selection should drive a model’s phenotype to evolve away from that of its mimic [1]. We conducted a ‘pairwise’ analysis in which we compared the accuracy (i.e. discriminant rate) of phenotypic partitioning among all possible pairs of parasitized species, to that among all possible pairs of unparasitized species These analyses provide a less realistic picture of how a community of species responds to parasitism pressure, but have the advantage of removing any bias introduced by differences between groups made up of different sample sizes of species. We calculated a null hypothesis of classification rates based solely on the relative sample sizes of species within each group; this ‘expected accuracy’ was the probability that a species would be correctly assigned due to chance alone This tested the assumption that DFA performs better than chance in classifying species. Complete results of the phylogenetic signal analyses are in 4 electronic supplementary material, table S4
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