Parasites can take many forms. Just this week, I’ve written about a giant virus that reproduces inside amoebae (and has survived being frozen 30,000 years in permafrost), along with a wasp that performs brain surgery to zombify hosts for its young. Viruses and wasps are radically different organisms–some would say that viruses don’t even deserve the label of organism. And they make use of their hosts in different ways. The virus sits inside a cell, manipulates its biochemistry to build virus proteins and DNA. The wasp, on the other hand, sips fluids inside a still-living roach, and builds its own proteins and DNA–and then becomes a free-living creature that can climb out of its host and fly away.
So why are they both parasites? The answer lies beyond the details of anatomy and molecules. It’s all about relationships.
Species can have all sorts of influences on each other. They can eat or be eaten, they can pollinate or steal pollen. But there’s one yardstick that scientists can use to measure all the variety in these interactions: the change that one species has on how many offspring the other can have. By that measurement, the differences between giant viruses and brain-surgeon wasps melt away. Each one is a disaster for its partner species. The viruses multiply inside amoebae until they burst. The roach lives until its wasp parasite is ready to depart. In each case, the relationship is good for the parasite (more offspring) and bad for the host (fewer).
When scientists look at life with this definition in mind, they can see a lot of parasites that might not look like parasites. We don’t think of birds as parasites–they’re too beautiful and not in the least bit creepy. But when a cuckoo pushes out the eggs of a reed warbler and puts her own in their place, and when the cuckoo chicks use all sorts of tricks to fool the reed warbler to feed them as if they were its own, we are seeing another parasite at work.
In the journal BMC Evolutionary Biology, a team of scientists in Finland describe another kind of parasite–one that doesn’t steal food or protein synthesis or even parental care. In the words of the scientists, these are “information parasites.”
Top: Great tit. Bottom: Pied flycatcher. Flickr: http://flic.kr/p/7o6KjK http://flic.kr/p/dGnDRc
These information parasites are, once again, birds. Lovely birds, in fact, known as pied flycatchers. And their victims are another species of bird, the great tit (twelve-year-olds at heart are allowed a few moments to get sniggers out of their system).
The pied flycatchers and great tits, both found across much of Europe, have evolved to the point where their existence is quite similar. They eat a lot of the same kinds of food, get killed by the same predators, and even choose the same sites for their nests. This similarity leads to a fair amount of competition, sometimes quite violent. If a bird from one species flies into a crevice to check out a potential nest spot, only to find the other species there, the two birds will fight–sometimes to the death.
The two species aren’t identical, though, and there a couple differences that are particularly intriguing.The great tits build their nests earlier in the year, and the pied flycatchers have a habit of paying visits to great tit nests before building their own.
In recent years, the Finnish researchers have found a likely reason for these visits. The pied flycatchers are gathering intel. They inspect the nests of great tits to help them decide where they will make their own nests. One piece of information they’re interested in is the number of eggs are in a great tit’s nest. If a nest is loaded with eggs, it’s probably a good place for a pied flycatcher to make its own nearby.
The great tit suffers for letting the pied flycatcher get this information. Now a rival bird sets up house on the same territory and starts to compete for the same food. The researchers have found that great tits that attract these neighbors end up with fewer nestlings as a result. The pied flycatchers, on the other hand, have more success in reproducing because they build their nests on good real estate. One species benefits, and one suffers. But the benefit doesn’t come from cockroach innards or cell proteins. The pied flycatcher is stealing information.
Once parasites evolve a strategy for taking advantage of a host, the host generally evolves defenses. Immune systems recognize pathogens and destroy them. The hosts of some wasps will fly away or fight off their attacker. If pied flycatchers really were information parasites, then great tits might evolve defenses to safeguard their information.
When great tits are laying eggs, they search for sheep hair and other materials to keep the eggs covered. It’s not clear why they bother. You could imagine that the covering is a blanket to keep the eggs warm. But the birds don’t bother to keep the eggs covered once they’re all laid and the embryos start to develop. So it’s possible that they’re doing something else with the hair.
One thing that the hair does is hide the eggs. The Finnish scientists wondered if the great tits use hair to hide information from flycatchers. To find out, they ran an experiment.
They put a decoy of a pied flycatcher five meters from great tit nests and played a recording of a pied flycatcher singing for five minutes. The next day, they collected the hair in the nests. The scientists then ran the same experiment, but with decoys of cedar waxwings–birds that live alongside great tits but don’t compete with them.
The great tits responded to pied flycatchers by adding over 40% more hair on top of their eggs than they would otherwise. The scientists concluded that the birds hide the eggs when pied flycatchers show up so that the pied flycatchers won’t see just how well the great tits are doing. Seeing what looks like a meager nest, the pied flycatchers will be more likely to move on.
When hosts evolve defenses against parasites, parasites sometimes evolve counter-defenses. When flu viruses infect a cell, for example, the cell can respond by making an anti-viral protein called interferon. The interferon guides the cell to chop up the invading virus genes. But flu viruses have proteins that block interferon.
Do information parasites have their own counter-defenses? The scientists don’t offer any solid scientific evidence in their new report, but they do mention that they’ve seen something odd. They’ve seen pied flycatchers sneak into great nests and pull hair from the eggs. That may seem like a pointless exercise, since pied flycatchers don’t use hair on their own nests. It’s possible that they’re just trying to steal some reliable information.
When I first started writing my book on the triumph of parasites, I burrowed into the science and was stunned at how many ways there were to be a parasite. Eventually, the bottom just fell out. This is the first time that I’ve become aware of the concept of “information parasites,” but I suspect it won’t be the last.
[Update: Correction--reed warblers are hosts of cuckoos, not cowbirds]