In the image above, all the eggs in the top row are laid by cuckoos and those in the bottom row belong to their victims. These uncanny similarities help cuckoos to fob off their parental duties by laying their eggs in the nests of other species. If the hosts can’t tell the difference between their eggs and the foreign ones, they’ll end up raising the cuckoo chick as their own. And they pay a hefty price for their gullibility, since cuckoo chicks often kill or outcompete their foster siblings.
The relationship between cuckoos and their hosts is a classic example of an evolutionary arms race. Cuckoos, should evolve eggs that more closely match those of their hosts, while the hosts should evolve keener senses to discriminate between their own eggs and a cuckoo’s.
But in Africa, this classic story takes an unusual twist.
The greater honeyguide isn’t a cuckoo but uses the same tactics—it parasitises the nests of little bee-eaters by laying eggs of the same size and shape. But this mimicry doesn’t help it to fool the bee-eaters, which seem to accept any old egg no matter how different it looks. Instead, Claire Spottiswoode from the University of Cambridge has found that the parasitic honeyguides are fighting an evolutionary arms race against… each other.
Bee-eaters build their nests underground, usually within abandoned aardvark burrows. When honeyguides invade, they’ll puncture the bee-eater’s eggs before laying their own. This kills some of the eggs outright and weakens others. If any chicks survive to hatching, they’re finished off by the honeyguide chick, which stabs its foster siblings to death with a vicious hooked bill.
Spottiswoode filmed this brutal behaviour in 2011, and she showed that honeyguides are a huge problem. Two-thirds of the nests are parasitised, and a third of these contain eggs from more than one honeyguide. That’s important—it means that honeyguides aren’t just trying to dupe the bee-eaters, but are also competing with each other.
The honeyguide eggs are reasonably similar to those of the bee-eaters in both size and shape. But when Spottiswoode added very dissimilar eggs to the clutches, including those from doves, woodpeckers or kingfishers, the bee-eaters almost never noticed. The honeyguide eggs can’t have evolved to fool the bee-eaters, since the bee-eaters apparently have the discriminatory prowess of a brick.
But the honeyguides are more savvy. Whenever they visited a nest where Spottiswoode had added an extra egg, they inflicted at least twice as many punctures upon it as they did to the others. They could tell if an egg looked different to the usual bee-eater shape, and concentrated their efforts on destroying it.
Spottiswoode thinks that the honeyguides have evolved bee-eater-esque eggs to fool each other, because they go easier on such eggs than on those that feel very different. After all, honeyguides can’t puncture everything. If they go overboard, they might cause so much damage that the hosts abandon their nests altogether. So, their best play is to puncture the bee-eater eggs a little bit, enough to give their murderous chick an advantage when it hatches but not so much that the bee-eaters desert.
If a honeyguide egg was obviously different to a bee-eater’s one, it would be thoroughly destroyed by other honeyguides that arrived at the same nest. If its mimicry is good, it gets a chance at surviving.
This story reminds me of some recent discoveries in bacteria. In 2010, I wrote about how the harmless gut bacterium Escherichia coli can sometimes make us ill because disease-causing strains are better at fending off hungry amoebas. The bacteria evolve in response to the threat of predators and by complete coincidence, they also become deadlier to us. Along similar lines, the normally harmless nose bacterium Streptococcus pneumonia can become infectious when it competes with another species called Hameophilius influenzae. It defends itself against this rival by producing a thicker coat, which also happens to shield it from our immune system.
We have this knee-jerk tendency to view parasites in terms of their hosts (since we ourselves are host to legions). But as the honeyguide, E.coli and S.pneumonia show, parasites also have to compete with each other. This competition can drive their evolution just as readily as the need to outfox a host.
Reference: Spottiswoode. 2013. A brood parasite selects for its own egg traits. Biology Letters http://dx.doi.org/10.1098/rsbl.2013.0573