The Red Queen & Red (and Green and Blue) EggsPublished 2 May, 2012
The problem, if you are an African tawny-flanked prinia (Prinia subflava), is that the cuckoo finch (Anomalospiza imberbis) is designed to lay its eggs in your nest, fooling you into investing in the finch’s offspring at the expense of your own. To defend against this, it’s helpful to have eggs that are distinctive; if you laid only plain white eggs, parasites would similarly lay plain white eggs, making distinguishing yours from theirs difficult.
This doesn’t solve the problem. Once there has been selection for distinctive eggs, there is selection on the finches to produce eggs that are similar to the predominant color and pattern of the eggs in a given prinia population, leading, in turn, to selection on prinias to produce eggs different from the sorts of eggs that the finches are now producing. The advantage of laying eggs that deviate from other’s eggs can produce a co-evolutionary race in which prinia egg color patterns are being selected to run away from the pattern finches are producing, with the cuckoo egg patterns in pursuit.
This evolutionary dynamic is the study of a new paper by Claire N. Spottiswoode and Martin Stevens entitled, “Host-Parasite Arms Races and Rapid Changes in Bird Egg Appearance.” They had access to a number of eggs from the host species, collected between 2007 and 2009, as well as eggs collected in the past, mostly the 70’s and 80’s. They were, then, able to compare these two groups of eggs with eggs from the brood parasite species. Based on the ideas above, they made three predictions:
Our first prediction is that phenotypic diversity in egg appearance should have changed over time in both parties, as host phenotypes diversify or contract under negative frequency-dependent selection and parasitic phenotypes follow. Different aspects of egg appearance may oscillate over time in phenotypic space…
…our second prediction is that if parasite evolution is closely tracking host evolution, then parasitic eggs should be a better phenotypic match to host eggs from the same time period than to host eggs from a different period. We further predict that the latter effect should be most pronounced when comparing historical host eggs to current-day parasites that hosts have not yet encountered in their evolutionary history; in contrast, a smaller effect might be expected when comparing current-day hosts to historical parasites, which have previously imposed selection on the host population.
Finally, we predict that parasites should show less phenotypic diversity than hosts, owing to a time lag between host and parasite adaptation.
So, in sum, they predict changes in appearance, a closer match between modern host eggs than to prior host eggs, and more variation in host eggs.
The eggs, it should be said, are pretty cool looking. See Figure 1. The authors used what is known about the avian visual system to quantify the visual properties of the eggs (since what matters is how much eggs resemble one another from the point of the view of the birds, not from the point of view of people, of course). From these calculations, they could quantify how different eggs appeared to be from one another.
Their predictions fared well. Modern host eggs were more variable than the eggs from prior decades, by more than a factor of five, by one of their metrics. The results for the similarity between parasite eggs and modern versus older eggs were consistent with the second prediction for color; the results for patterns on the eggs were more complex, and varied depending on the element: dispersion, contrast, marking size, and proportion coverage. In terms of the former two, parasites seem to be doing a good job chasing hosts. Pattern dispersion and contrast in current-day parasites better mimic current-day hosts than past hosts, whereas in historical parasites these traits showed poorer mimicry of contemporaneous hosts versus hosts that parasites had not yet encountered. This suggests that for these traits the parasite is evolving quickly, thus improving in mimicry.
Looking at marking size and proportion coverage, hosts seem to have the edge:
By contrast, for marking size and proportion coverage current-day parasites are poorer mimics of current-day hosts than they are of historical hosts, while historical parasites are better mimics of contemporaneous hosts versus hosts that they had not yet encountered. This suggests that with respect to these traits the parasite is evolving more slowly than the host and thus growing poorer in mimicry.
They infer that these different aspects of the egg patterns seem to be evolving independently.
Finally, their third prediction, that more variation in hosts should be observed was borne out by their observations.
In sum, over the course of just a few decades, they find evidence of the battle between host and parasite, with hosts evolving a greater diversity of egg phenotypes, and the parasites chasing them. Because these species are locked into a co-evolutionary race, there’s no reason to think that they will settle into a static equilibrium, so we should expect to see continued changes away from the present phenotypes. Which is cool, since it means more pretty eggs for the foreseeable future.