Chimps and other primates may show a rudimentary form of morality, but it’s clear that by and large nature is pretty heartless—as you’d expect given the character of natural selection. Nevertheless, who hasn’t felt like reproving the cheetah who brings down, on film, a cute young Thompson’s gazelle?
In class this week, my students were saddened and dismayed by another of nature’s nefarious wonders, the cuckoo—in particular, the common cuckoo (Cuculus canorus). Here’s one:
The cuckoo has hit on the mother of all adaptations, a phenomenon called brood parasitism. Instead of spending huge amounts of time building a nest, brooding eggs, and feeding the voracious young until they fledge, the female cuckoo recruits an individual of another bird species to do all the work for her—a full-time babysitter. That way, the cuckoo can use its time and resources maximizing its reproductive effort without the enormous expense of childcare.
A female cuckoo simply lays one egg in the nest of another species, say a reed warbler or a dunnock, where there is already a clutch of eggs. The cuckoo’s egg then nestles inconspicuously among the others. Inconspicuously? Yes, for the female cuckoo lays an egg that mimics pretty well the other eggs in the nest, so the foster mother can’t easily detect the intrusion.
The curious thing is that within the common cuckoo species, each female lays an egg precisely patterned and color to mimic the eggs in the nest she will parasitize. And here’s the kicker: within that cuckoo species there are seven different types of females, each laying a different type off egg and each parasitizing only the nests of species producing similarly-colored eggs. The different “types” of cuckoos that lay different eggs are called gentes. The gentes are not different species of cuckoos—they are all members of the same species, but with different types of genes that make different types of eggs. This is an example of a genetic polymorphism (from the Greek meaning “different forms”). Polymorphisms are not rare in animals and plants: our own species has them, including eye color variants and whether your earwax is wet or dry (a trait based on a single gene).
Here are four of the different types of eggs laid by four cuckoo gentes. The species that is parasitized is on the left, the mimetic cuckoo egg on the right. Again, each female lays only one type of egg her whole life.
Here are some actual nests, each containing a single cuckoo egg, indicated by the arrow. Note that the mimicry is very good but not perfect—a human observer (but not the bird) can pick out slight differences, and the cuckoo egg is often larger:
What happens next is sad, but a remarkable adaptation showing nature in all its red toothiness and clawdom. The cuckoo chick hatches and proceeds to destroy all its competitors—the other eggs and chicks—leaving it the sole recipient of foster care (click on the “Watch on YouTube” line).
As Attenborough notes in the video, brood parasitism has evolved in many bird species, including the “cuckoo duck” Heteronetta atricapilla.
The European cuckoo’s adaptive habit raises lots of questions. Why is the foster parent fooled by the mimetic eggs, but can’t seem to recognize a cuckoo chick that is so different from its own? In some species of brood parasites, like indigobirds, the foster mother can recognize foreign chicks, and so the parasite babies have evolved calls and “mouth gape” patterns (coloration on the babies’ mouths that induce feeding by the mother) that mimic those of the non-foster young. Another explanation is that the foster mother’s drive to feed whatever chick it sees in its nest outweighs everything else: that is, there’s simply no genetic variation for the foster mother to respond to an alien-looking chick. While genetic variation is pervasive in nature, it’s not always around when it’s “needed.” Every case of a parasite or predator victimizing another species, for example, represents an absence (perhaps temporary) of genetic variation in the victim to fully overcome the challenge.
The biggest mystery, though, is how the polymorphism for color pattern is maintained. How, for example, does a female “know” where to lay its egg? If it had the genes for producing eggs that mimic reed warblers, and laid its egg in a dunnock nest, that egg would be summarily ejected and its genes would not be passed on.
This problem is overcome by imprinting: a female imprints on the song and appearance of its foster mother, so when it comes time for a cuckoo to lay its own egg, it goes right back to a nest harboring a female on which it’s imprinted.
But what about mating? Males, after all, also carry genes for egg color and pattern—they just don’t express them. (If you’re a male human, you carry genes for making breasts and vaginas, but don’t express those either.) But if a female mates with a male carrying egg-pattern genes different from hers, wouldn’t the eggs that their daughter produce be intermediate, and therefore unable to pass the test of mimicry?
One possible answer is that a female knows to mate only with those males carrying similar egg-pattern genes. But this isn’t the case. First of all, there’s no way a female can detect a male’s genetic endowment for egg pattern. But more important, research has shown that female cuckoos mate randomly—they don’t know or care what a male’s “egg genes” are. So how does the pattern fidelity work?
We’re not sure, but it may involve the birds’ sex chromosomes. In birds, unlike mammals, it is the female who has two different sex chromosomes, called the Z and the W. In humans males are XY and females are XX, but in cuckoos and other birds (and butterflies), females are ZW and males are ZZ. A ZW female produces ZW daughters, so the W chromosome, and the genes it carries, are transmitted matrilinearly. The male makes no genetic contribution to this chromosome.
This, then, is a possible solution to the egg-color polymorphism. If the genes for a specific egg color and shape are carried only on the W chromosome, then in the offspring those genes will not be mixed with any genes from the father. This will enforce a fidelity of egg type between female and daughter, regardless of who the female mates with. That, combined with the tendency of female cuckoos to imprint, explains how the single common cuckoo species can harbor several types of females, each laying a different mimetic egg, and with very few “mistakes.”
The genetic studies verifying the location of egg-mimic genes on the W chromosome have yet to be done: you can imagine how hard it would be to cross different cuckoos in the lab or aviary, for that would also involve providing foster parents! This is a project for a bright and energetic graduate student.
And, of course, this whole story tells you where the word “cuckold” came from.