Let us now lean back and see what picture of sex in the city we have painted. First, like anywhere else, animals do their best to advertise themselves as desirable partners. In principle, the city’s dating network employs the same channels for this as in the animals’ original habitat: beautiful sounds, striking colors, impressive actions. But when we look closely, we see that the urban animal appreciates different things in a partner. So the sexual preferences evolve, as well as the qualities that are valued in the other sex. A desperate dark-eyed junco writing in the personals section of the Campus Newsletter for Birds would perhaps say, “Caring male with almost no white in tail seeks acquaintance with female to raise several nests in comfortable bicycle helmet. No fighter but excellent at catching lots of small mosquitoes”—something that the butch readers of the Highland Junco Journal might find laughable.
Secondly, the way these sexual signals are broadcast may change, because of interference with the city environment. Noise and light pollution could reduce or shift the bandwidth at which auditory or visual signals work. Or one kind of signal might even take over from another. For example, using a (rather hilarious-looking) remote-controlled “robo-squirrel,” a team of researchers from Hampshire College in Massachusetts discovered that when urban gray squirrels (Sciurus carolinensis) alert each other of potential danger, they respond better to the sight of tail flicks than to the sound of alarm squeaks. In rural squirrels, they found, it is the other way around. Presumably, this is another result of urban noise, and you could imagine that the same shift from sound to sight may be happening in sexual messaging in some animals.
Or, for that matter, a shift from smell to sight. In their natural habitat, Indian gerbils (Tatera indica) encounter one another so rarely that they rely on scents left as chemical “flags” scattered in the environment. But in cities these animals live tightly packed, and they seem no longer to have a need for such long-distance olfactory communication. As a result, urban gerbils are losing their scent-marking glands.
Some interference by the urban environment with animal sex messaging may be even more insidious. Chemical pollutants with such exotic names as organochlorines, phthalates, alkylphenolic compounds, polychlorinated biphenyls, and polychlorinated dibenzodioxins are released into the environment from a variety of sources. They can be pesticides, additives to plastics, and industrial wastes. What these substances have in common is that they persist in the environment. Many have already been banned but since they can have a half-life of centuries or more, these pollutants will be a fixed feature of the urban chemical landscape for a long time to come. Another common property is that they interfere with sexuality. This is because chemically, they mimic certain sex hormones that animals use to fine-tune their sexual development, both physically and in terms of behavior. The result is the kind of sexual aberrations that we find, for example, in male alligators in lakes polluted with DDT, which have small penises and low testosterone. Conversely, female mosquitofish living in the run-off of a paper mill were found to have male body features and were hyper-aggressive and dominant. You can only guess at how evolution by sexual selection will be struggling to adapt to an interference of this magnitude—if that is at all possible.
Another type of anthropogenic insinuation into the delicate workings of an animal’s sex life is called an “evolutionary trap.” Unwittingly, we humans sometimes create stuff that latches on perfectly to the traditional ways in which a certain animal’s courtship is conducted. Take the Australian satin bowerbird (Ptilonorhynchus violaceus), for example. Like all bowerbirds, the males of this species build amazing works of art to persuade females to mate with them. They create something akin to an ornamental garden, complete with passageways, entry lanes, and arrangements of beautifully shaped and colored objects that they harvest from their environment. Before that environment became dominated by people, the satin bowerbird would use rocks, shells, flowers, butterfly wings, and beetle elytra for this purpose. But, these days, humans supply an endless range of attractive artificial objects to add to his display. He is especially taken by bright blue things—such as those rings that snap off the caps of milk bottles when you open them.
Sadly, those rings turn out to be—quite literally—an evolutionary trap. Sometimes, when the male, excited about such a prize find, is carrying it around in his beak, the ring flips backward and becomes trapped behind the nape of the neck. Thus permanently muzzled, the male then strangles himself or slowly starves. Our mindless interference with his pure aesthetic intentions brought on his downfall.
Curiously, yet another Australian drinking habit wreaks havoc on an animal’s love life. In 1983, two Australian entomologists published a short article in the Journal of the Australian Entomological Society, entitled, “Beetles on the bottle.” Centerpiece of the article were two photographs of a large, yellowish-brown jewel beetle, Julodimorpha bakewelli, trying to copulate with a particular type of beer bottle, locally known as a “stubby.” The animal had mounted the round bottom of the bottle and was furiously but unsuccessfully trying to penetrate the glass surface with its long, brown penis. The stubby and its lover had been found on the sand along a highway outside of the town of Dongara. When the two authors searched the vicinity, they found several other bottles with similarly infatuated male jewel beetles clinging on to them.
It is not likely that the insects were attracted by any beer in the bottle because, as the authors remind us, no Australian would ever throw away a bottle that still has beer in it. Instead, the attraction probably lay in the color and gloss of the glass, its curvature and, crucially, its texture: a rim of small, evenly spaced tubercles that had been pressed into the lower part of the bottle during the manufacturing process. This combination of features brought enough similarity with the backside of a female Julodimorpha bakewelli to make this discarded glassware irresistible to the male beetles. When the pair of entomologists laid out several new bottles in the area, male beetles did indeed descend on them within minutes.
Stubbies are an evolutionary trap not because they kill the beetles, but because they distract the males from their job of mating with real females. It is even possible that the males think they have found a particularly large, shiny, desirable female, by which all real females pale in comparison. If there are a lot of such super-females (super-sexy, but, of course, also super-infertile) lying about, we would expect that the only hope for a way out of this evolutionary trap would be males that are genetically predisposed not to be turned on by beer bottles (maybe focusing on other female features, like scent). Only they will be able to reproduce and this could, eventually, cause the beetles and their sexual signals and preferences to evolve. It would not be the first time that a marriage was saved by weaning the male off his beer bottles.
18
TURDUS URBANICUS
The Galápagos: a handful of cinders tossed into the Pacific Ocean, where evolution has cooked up unique ecosystems from the few ingredients of flora and fauna that the South-American mainland has thrown it. A world unto itself, with home-grown evolutionary trees of tortoises, giant and dwarf cacti, mockingbirds, iguanas, Bulimulus snails, darkling beetles, and, of course, the islands’ most famous inhabitants: the fourteen species of Darwin’s finches, each with a beak shape to suit its particular walk of life.
They’re not finches actually, but either tanagers or buntings. (Ornithologists aren’t quite sure.) They were only named “Darwin’s finches” in 1936, more than a century after the great naturalist discovered them on his voyage on HMS Beagle. Nonetheless, Darwin’s finches have become the poster children for evolution. Not only did Darwin push them as one of the prime examples of his theory—“One might really fancy that, from an original paucity of birds in this archipelago, one species had been taken and modified for different ends,” he wrote—they have also taken center stage in the past forty-five years of cutting-edge evolutionary research.
Since the early 1970s, a whole dynasty of scientists, mostly working from the Charles Dar
win Research Station on Santa Cruz, the archipelago’s second largest island, have studied the birds and have revealed, in amazing detail, how Darwin’s finches continue to evolve. The teams keep track of the birds’ births and deaths, trysts and quarrels, food fads and nesting sites, and measure the sizes and shapes of their bills and bodies, year in, year out. They take blood samples, record songs, and run DNA tests. All this hard labor enables the biologists to watch, and even predict, the finches changing shape in real time. Each turn in the harshness of the climate, or the availability of a particular type of food, results in an evolutionary shift in the birds’ physique. It’s often a matter of fractions of millimeters, but it’s measurable and real.
For example, on the island of Santa Cruz, the medium ground finch (Geospiza fortis) is in the process of splitting into two. You can tell by looking at their beaks. There are a lot of birds with small beaks, a lot with much larger beaks (up to almost twice as big), but not so many intermediate ones. Beak size translates directly to the kinds of seeds the animals can crack. Big-billed G. fortis finches have a bite that is more than three times as strong as small-billed G. fortis. So, they can handle the heavy-duty seeds like caltrop, whereas the weak-beaked ones are good at feeding on smaller and softer seeds of, say, grasses. The intermediate-billed birds, however, fall between two stools: their beaks are not strong enough to crack the large seeds and not small and delicate enough to efficiently manipulate the tiny seeds. So, during lean times, they will be more likely to starve, and natural selection grimly removes them. What’s more, the beak size difference has sexual knock-on effects: large-beaked males sing a different song from small-beaked ones, large-beaked females prefer to mate with large-beaked males and small-beaked females with ditto males, so there is less genetic exchange across the beak-size divide. In other words, there is speciation going on: the evolution of two new, separate species where previously there was only one.
While the Darwin’s finch has become the emblem of speciation in the wild, another bird has taken up that role for urban speciation: the blackbird, Turdus merula.
In 1828, the same year that Darwin befriended John Stevens Henslow, the Cambridge don who arranged for him to go on his Beagle cruise, a small book was published in Italy, entitled, Specchio Comparativo delle Ornitologie di Roma e di Filadelfia. The author was Charles Lucien Bonaparte. A wayward nephew of the better-known member of the Bonaparte family, Charles Lucien lived an incorrigibly zoological life. Having spent his youth in Rome and, after his marriage, the better part of the 1820s in Philadelphia, he published this “mirror” (specchio) of the avifauna of both cities.
His mirror consists of two columns. The birds of Rome are on the left, those of Philadelphia on the right, and everything is carefully arranged along the lines of the official bird classification of that day (in which Charles Lucien himself was one of the main authorities). On page 32, in the column for Rome, we find the following entry:
“69. TURDUS MERULA, L. Merlo, Merla. Comunissimo. Permanente; alcuni individui migratori. Se ne fa caccia. Cantore.”
(Or, in English: “Blackbird. Very common. Resident; some individuals migratory. It is a hunter. A singer.”)
So Napoleon’s nephew saw resident blackbirds in Rome. Is that really such a big deal? After rock pigeons and sparrows, the sleek, sharp-billed birds (females: all-brown plumage and ditto beak, males: black plumage, yellow beak and eye-ring; don’t confuse them with the American birds that go by the same name) are probably the most abundant birds in cities—at least in Europe and western Asia. In China and North America, close relatives, the Chinese blackbird (Turdus mandarinus) and the American robin (Turdus migratorius), take its place and behave pretty much the same.
The reason that this short entry in Bonaparte’s “Specchio” is so pivotal is that it is the oldest record of blackbirds nesting and wintering in a city that we know of. In the Bavarian cities of Bamberg and Erlangen, blackbirds also frequented the town center in the 1820s, but they were not yet nesting there. And everywhere else in Europe, blackbirds at that time were still doing what they had been doing for times immemorial: leading unobtrusive lives deep in dark forests, so shy that they would be mortified rather than be found in the company of humans, and, when the breeding season was over, migrating down to the Mediterranean for the winter.
But, in the following two centuries all that changed. Slowly, at first: by the end of the nineteenth century, urban blackbirds were common sights only in the central parts of Europe. Throughout the twentieth century, however, the city fad spread more rapidly, reaching London in the 1920s and Iceland and parts of northern Scandinavia only by the 1980s. Eventually, nearly every town and city in Europe succumbed, except for a few small resistant areas in southern France, Russia, and the Baltic states. Over the entire period, the urban tendency progressed at an average speed of five miles per year.
Still, this is not to say that blackbirds started to become urban in Rome and from there city-hopped their way across Europe. For a start, some of the blackbird’s subspecies on islands in the Atlantic Ocean, far away from the species’ stronghold in Europe, have independently made the same move into cities. Of their own accord, sometime in the middle of the twentieth century, Turdus merula azorensis and Turdus merula cabrerae, two smaller, darker and glossier versions of the blackbird that live only on the Azores (azorensis), and Madeira and the Canary Islands (cabrerae), also have begun hopping around in the islands’ towns and cities. And the same happened even earlier in the north African subspecies Turdus merula mauritanicus: in the mid-nineteenth century, they were already city-dwellers in downtown Tunis. So, just like great tits across England becoming milk-bottle-top-savvy independently, all over Europe more and more cities obtained their own resident urban Turdus merula population.
Nobody is really sure why the city-dwelling streak spread among Europe’s blackbirds so slowly but relentlessly. Why did it begin in the 1820s and not before or later, and what was it about Rome, Bamberg, and Erlangen that made those cities suitable a century earlier than, say, London or Brussels? And why do cities like Marseille and Moscow remain blackbird-free even today?
Obviously, deeper into the past, most cities may have been too small to constitute a viable habitat in their own right. Still, that cannot be the only answer: in the early nineteenth century, blackbird-free London already covered four and a half square miles, much larger than many smaller towns in Germany where blackbirds were already unflappably nesting in garden sheds and hopping around on the pavements. Parks and other green spaces are important, but many cities had a generous helping of those long before blackbirds dared go there. Probably, the growth of cities and urban green spaces, the balminess of urban heat islands, a greater affluence of city-dwellers (leading to more, and year-round, food surplus), as well as greater safety from hunters, predators, disease and parasites, all acted together to create an urban niche snugly fit for blackbirds.
What is clear is that the process mostly happened in two steps. First, blackbirds began wintering in a city. Then, sometimes only many decades later, a few winter visitors stuck around for spring and eventually took up breeding with one another, gave up migration altogether and turned into resident city-birds. Just like what happened in the Californian dark-eyed juncos of the previous chapter.
That’s about as much as one can glean from field guides and birders’ reports. To really get a feel for what makes the new urban blackbird different from the ancestral forest blackbird, we have to look at the work that an entire cottage industry of urban blackbird researchers has been doing over the past twenty years. Many threads of urban evolution research that we have been following throughout this book become tangled in this one urban bird. The constellation of European cities has become urban evolution’s Galápagos, and Turdus merula its Darwin’s finch. In almost every European country, a team of urban blackbird biologists has stepped up and jointly built a veritable urban evolution fest around this one bird species—one of the world’s oldest and best studied urb
an animals. All this research seems to point in one direction: that the urban blackbird is evolving into a separate species: a case of true speciation.
We speak of speciation when a lot of different aspects of an animal or plant, either simultaneously or successively, evolve away from the original type—to the extent that a taxonomist (the kind of biologist who circumscribes and classifies biodiversity) would consider it a different species. This usually means that its body shape, sexual strategy, and timing of major life events all begin to differ from the ancestor. In other words, when a complete overhaul of its genome takes place. But that alone is not enough. At least some of those changes must cause the original and the new gene pool to become separated and keep from blending into each other—something we call “reproductive isolation.” (You can read much more about this in my book Frogs, Flies, and Dandelions; the Making of Species.)
Darwin Comes to Town Page 18