Wednesday, April 9, 2014

Why care for your kids when someone else will do it for you?

Its been a while since I wrote a blog post, and this mainly because I have been lost in my own thoughts during my 'sabbatical' in Sydney. But I managed to raise myself from self-indulgence and go to a really nice seminar on Friday by Ros Gloag. Ros is doing a Postdoc in Madeleine and Ben's social insect group in Sydney, after finishing her PhD together with Alex Kacelnik in Oxford.

Ros spent her PhD studying shiny cowbirds and chalk-browed mockingbirds in South America. These cowbirds pose something of an evolutionary puzzle, because they are such blatant parasites. What the cowbirds do is hop in to the mockingbirds nest, lay an egg and then fly away. The mockingbirds don't like this. If they catch the cowbirds (which they often do) they attack them pretty fiercely (see a bird get beaten up but still lay her egg in a video below). This doesn't seem to stop the cowbirds laying an egg, although it does stop the cowbirds damaging any of the mockingbirds own eggs.


video

A mockingbird nest with three of
 its own turquoise eggs and four parasitic
cowbird eggs. 
But the strange thing is that the egg left by the cowbird looks very different from the mockingbird's own eggs. Why doesn't the mockingbird just destroy it and throw it out? Ros showed that mockingbird eggs in nests containing cowbird eggs were less likely to be damaged or destroyed. This is because when other cowbirds come along and attack the nests, they end up damaging the cowbird eggs which are already there rather than the mockingbird's own eggs.

Once the all the eggs hatch, host birds continue to look after both the cowbird and their own chicks. One clue to why the hosts do this can be found in the vocal recordings of the cowbird chicks. The cowbird appears to mimic the calling of groups of hungry chicks, producing a general enough sound to fool a range of different host species. This causes the parents to collect even more food than they usually would, which the cowbird laps up.

Ros' thesis is a beautiful example of how the behavioral ecology approach helps us understand the evolution of host and parasite interactions. She has nicely combined the cycle of experiment, thinking about what the experiment implies, building a theory, and then testing it with a new experiment.

There is a danger in the behavioral ecology approach that experiments are done until one of them identifies an advantage for the host, at which point we stop and congratulate natural selection on its amazing power of finding a balance. This can make me a bit suspicious.  In Ros' case there are still a few open ends, but I think this is to her credit. Instead of finding a full answer, her research documents the subtleties of hosts and parasite relationships.

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