Over the years, experimental biologists have had a field day (literally) devising experiments to test urban animals’ neophilia. What is more fun than cobbling together weird and wonderful objects that don’t look like anything and confronting unsuspecting experimental animals with them? It’s like candid camera for biologists. In the name of urban behavioral biology, common mynahs in Australia have been exposed to green hair brushes and yellow rolls of tape, English crows to works of modern art pieced together from chip bags, jam jars, and polystyrene fast-food containers, and chickadees in Tennessee to wonderful towers made from Duplo blocks. And in virtually all cases, urban birds approached these weird objects quicker and with more interest than wary rural birds.
One particularly thorough study is worth mentioning here. Piotr Tryjanowski and colleagues studied 160 bird feeders in and around Polish cities. Half of the feeders they adorned with an object “brightly green and made out of gum with a tuft of hair.” “We never observed anything even vaguely similar in the field making it highly unlikely that local birds responded to this object as anything but a novel object,” they write in their Scientific Reports article. Other feeders were left unmodified. Lo and behold, for the four bird species that visited the feeders most (great tit, blue tit, greenfinch, and tree sparrow), the rural visitors were neophobic: they avoided bird feeders with the weird green thing on the roof, whereas in the cities, the reverse took place: here, birds actually flocked to the pimped-up feeding tables.
After problem-solving and neophilia, the third and final personality trait that the urban environment will select for is tolerance: a reduced fear of humans. In a 2016 article in Frontiers in Ecology and Evolution, a team led by Matthew Symonds of Deakin University in Australia compared forty-two different bird species for their so-called FID: flight initiation distance, the average distance a human needs to approach a bird before it takes off.
They found that, across all these bird species, the urban version was more tolerant than the one from the countryside. Not only that, but the difference was greater the longer the birds had been residing in cities. For example, jackdaws (Corvus monedula) in cities (which they had already colonized by the 1880s) only get spooked if humans come closer than 8 yards, whereas in rural areas they will take off at 30 yards distance. Great spotted woodpeckers (Dendrocopos major), on the other hand, urbanized only since the 1970s, still have similar FIDs in city and countryside: 8 and 12 yards, respectively.
That positive relation with the time since first city-dwelling is important, because it shows that tolerance has actually evolved. It is unlikely that over the generations, wariness would wane by each bird generation learning to be a tad less careful around people than their parents were—you would expect this to happen more quickly. Rather, if there is a benefit to being more tolerant, genes for tolerance can gradually accumulate and the species’ personality will evolve. Such an explanation is particularly likely, since the same researchers also found that a bird’s tolerance has nothing to do with the size of their brains: brainy birds did not become tolerant of humans more quickly than, well, bird-brained birds.
It seems likely that problem-solving, neophilia, and tolerance all are prone to urban evolution, and we will actually see examples of this later, when I will present you with some of urban evolution’s pièces de résistance. For the time being, let’s remember that an important aspect of the evolutionary pressure urban animals are under is their continual arms race with urban humans over gaining access to their food and other resources.
The evolutionary landscape of the city is now nearly completely revealed to us. There are close encounters of the first kind—the tough but static physical and chemical structure of the city (heat, light, pollution, impenetrable surfaces and all the other urban features we saw in Section II of this book). Evolution as a result of such encounters may come to a standstill when the perfect adaptation is reached. Then there are the even more exciting close encounters of the second kind. These happen where urban animals and plants interact with aspects of the city that are not static, namely where they involve other animals and plants, including humans—all of which could, in principle, respond by changing themselves. This kind of encounter is all the more exciting because it may lead to “Red Queen” evolution: evolutionary arms races where both partners keep finding new ways to gain the upper hand. In theory, such evolution never stops.
Yet there is one final part of this urban evolutionary landscape that we have so far skirted around. In the previous chapters, we have seen close encounters of the second kind involving interactions between species. But what about that particularly close encounter within a species? Males and females of the same species also evolve to adapt to each other—we call this sexual selection. It would be naïve to think that there is no urban impact on the amorous animal.
16
SONGS OF THE CITY
Every year in September, I organize a general orientation course for the new Leiden University biology students entering our master’s program in Evolutionary Biology. In the first week, we always do some urban ecology and evolution. We go searching for grove snails using a smartphone app that records whether snail shells are more brightly colored in the urban heat island (I’ll say more about that toward the end of this book). And we do an unusual field practical. Most of the students view the entry in the day’s program with a puzzled frown. “Erm … What is ‘urban … acoustic … ecology’?” they ask. Just wait and see. We gather outside the biology building waiting for the afternoon’s instructor, my colleague Hans Slabbekoorn, who is—you guessed it—an urban acoustic ecologist.
At 1:30 p.m., he emerges. Khaki shirt and shorts, with long graying hair and, somewhat uncharacteristically (for an acoustic ecologist), a pair of binoculars around his neck. He hoists his bag, adorned with a Pacific Northwest native pattern, onto his shoulder, stands himself in front of the expectant group of thirty or so students, and outlines the afternoon’s activity. We humans are very visually oriented animals, he explains. We view our surroundings first and foremost with our eyes. But for a biologist, it is also very important to be aware of the acoustic landscape, as many animals communicate by sound. As an exercise to stimulate that auditory awareness, Slabbekoorn explains, “We’re going on a silent walk. We will walk in a line, not talking, in total stillness, to become more aware of all the sounds around us.” It would be even better, he adds, to also close our eyes, but that would pose too many navigational challenges.
So, off he heads, through the residential area near the university, and then into a nearby urban park. The students have little option but to follow him, and I bring up the rear. After some initial giggles and shooshings, the group of students morphs into a taciturn chain gang walking along the main road. Cars appearing from side-streets stop for us with revving engines, and pedestrians stand still to look at this strange long file of people walking in complete silence along a busy thoroughfare. Some passersby make duck-quacking noises in ridicule. But we manage to maintain our silence and do as Slabbekoorn wanted us to do: take in the soundscape of the city.
And it works. We hear things we would not have heard otherwise. The different types of growl of diesel- versus petrol-powered vehicles, the grating, clangy sounds of old battered bicycles passing by, jetliners flying overhead, the incessant crashes from a nearby building that is being demolished … But also the wind rustling in the reeds, the leaves of poplar trees rattling, a robin doing its little waterfall-like song, a hammering woodpecker, the bubbling sounds of a nuthatch, the screams of ring-necked parakeets flying over … Subtle details, too: our footsteps changing in character when we move from pavement to shell-lined paths in the park, and a grasshopper’s papery song falling silent as we pass by.
In the end, we gather on an open field, surrounded by tall trees, with the old university hospital dorm being torn down in the background. Slabbekoorn: “This used to be one of the quietest places in Leiden. The tall hostel shielded the din from the busy road behind it, and
we’d be far away from the urban center.” These days, the area has become noisier: the dorm building is being demolished and on top of that, the city sounds now penetrate the park unhindered. Upon prompting by Slabbekoorn, the students reveal how much they picked up. One student remarks how the traffic actually seemed to become louder as we entered the forest. “That is temperature inversion,” Slabbekoorn explains. On the forest floor it is cooler than out on the street, and the traffic sound becomes trapped in the layer of cool air at ear level.
“Let’s close our eyes for a second,” Slabbekoorn says, “and listen to the urban sounds.” Of course, we initially hear only the fits and starts of the heavy machinery gnawing away at the carcass of the student dorm, and the staccato plops of a heavy motorcycle passing by, but Slabbekoorn asks us to ignore those and focus instead on the low-level background sounds of the city. And indeed, when we train our ears to it, in between all those discrete urban sounds, there is a nearly imperceptible carpet of constant, low-pitched rumbles, waxing and waning irregularly. It is the city’s breath: a cacophony composed from the combined sound waves emanating from the engines, brakes, and horns of countless motorcycles and automobiles, the steel on steel of passing trains, the jet engines of airplanes, air-conditioning compressors and other machinery, construction pilings, voices and shouts, music from loudspeakers, and so on, all mixed into the grayish porridge that we call noise, muffled and channeled via the labyrinth of buildings and streets. In Europe, 65 percent of the human population is exposed to urban background noise louder than that of constant rainfall. And the animals in the city that try to make themselves heard have to contend with all this as well.
Dealing with background noise is not unheard of, of course. Natural habitats can be loud, too: frogs that live near streams or waterfalls, or birds in rocky canyons where every sound is amplified by echoing, know this problem all too well. Or think of a cricket trying to make itself heard by another cricket in a tropical jungle full of animals’ yells, whoops, buzzes, and whirrs. Some of the solutions from those pristine environments are surprisingly similar to the ones that urban animals come up with, Slabbekoorn explains, and he points to the wheezy, high-pitched “bicycle-pump” melody of a male great tit singing from a poplar tree behind us, “dee-du, dee-du, dee-du!”—crystal clear despite the backdrop of urban rumble.
It is the great tit’s “dee-du,” and all the variations on that theme, uttered to attract females and to repel other males, that brought Slabbekoorn his early fame, when, in spring 2002, he and his student Margriet Peet began recording Parus major all over Leiden. From April to July of that year, they became a familiar sight to the city’s residents, hauling recording equipment, a directional microphone, and an omnidirectional microphone on a five-yard-long pole from neighborhood to neighborhood, like a pair of traveling acrobats. At thirty-two places, ranging from quiet residential areas like the park where our acoustic ecology lab took place, to busy city center crossings or highway roadsides, they set up their microphones to record the songs of territory-defending male great tits (females don’t sing) with the directional microphone, and pick up ambient urban rumble with the omnidirectional one (from the tits’ vantage point, hence the five-yard pole). And to average out the effects of the time of day, they paid each bird three visits: to record sounds before, during, and after the rush hour.
The results, which Slabbekoorn and Peet published in a very influential one-page article in Nature in 2003 (more than 700 other publications have cited it since), revealed the birds’ struggles to make themselves heard above the din of traffic. Pitch played a crucial role in that. Most urban noise is concentrated in a low-frequency band of up to 3 kilohertz. The repertoire of the great tit spans a range from 2.5 to 7 kilohertz, the lowest notes overlapping with the urban noise. Slabbekoorn and Peet discovered that tits in noisy areas of Leiden deal with this by raising the pitch of their songs to above 3 kilohertz to avoid being drowned out by the sounds of the city, whereas those in quiet neighborhoods also use tones that go down to below 2.5 kilohertz.
As early as the 1970s, zoologists studying the famous great tits of Wytham Woods had discovered that the birds adjust their song to their surroundings: birds in open woodland sing higher-pitched songs than in forests because dense vegetation tends to muffle the higher notes too much. But Slabbekoorn was the first to discover that the birds apply the same musical strategy in our urban habitat. Since his ground-breaking studies, dozens of bird species in cities in many different countries have been seen (or rather, heard) to do the same: Chinese bulbuls (Pycnonotus sinensis), in Asia; song sparrows (Melospiza melodia), in North America; rufous-collared sparrows (Zonotrichia capensis), in South America; silvereyes (Zosterops lateralis), in Australia … All over the world, urban birds sing higher, and probably also louder, than the same species in a quiet rural setting. And not only birds: also the Australian Southern brown tree frog (Litoria ewingii) croaks higher in Melbourne than in the surrounding countryside, and grasshoppers of the species Chorthippus biguttulus along noisy German roads sing a shriller song than in quiet meadows.
While he is pleased that his work has spawned so much new research, Slabbekoorn says that many questions have surfaced along the way. Do song-determining genes evolve in the city because males with inaudible baritone genes can’t woo any females, while the tenors get all the girls? Or do they learn to drop the deeper-voiced songs from their repertoire? And, if they learn, do they do so by imitating their fathers or rival males, or by keeping track of which songs have the most impact? And what about plasticity? Could it be that animals growing up in a noisier place automatically grow squeakier voices? Slabbekoorn and his fellow urban acoustic ecologists are still grappling with these questions, but they seem to depend on the kind of animal.
One of Slabbekoorn’s students, Machteld Verzijden, took the lab’s sturdy microphones to the busy Rotterdam-Amsterdam A4 highway just outside of Leiden. Here, despite the din, lots of male chiffchaffs (Phylloscopus collybita, a sleek brownish-gray warbler) advertise their territories by singing their characteristic monotonous “chiff-chaff” song throughout the breeding season. Like in the great tits, her recordings revealed that the frequencies of every “chiff” or “chaff” were, for the birds singing close to the highway, about 0.25 kilohertz higher than for those along a quiet river a half mile away. But Verzijden did not stop there. She brought a boom box to her riverside site and, while the rural chiffchaffs were singing, gave them a taste of the noise level that their hard-shoulder brethren had to cope with by playing them loud traffic noise at close range. The results? An individual chiffchaff will immediately raise the pitch of his song if there is a lot of noise around: her rural chiffchaffs also increased their chiffs and chaffs by about 0.25 kilohertz as soon as she flipped the boom box switch.
Clearly, no evolution there: the deep-voiced riverside and high-voiced highway chiffchaffs do not differ genetically; they simply adjust their song to the ambient noise. But in some other animals, it is not that simple. The songs of frogs and of many non-songbirds like flycatchers and doves, for example, are much more stereotyped. They are hard-wired from birth and cannot easily be changed simply because humans are creating a racket. And the same goes for songbird calls (short utterances for expressing alarm or keeping in touch with one another). And yet, the songs of frogs and non-songbirds, as well as the calls of songbirds, are all higher-pitched in cities—while they are less likely to be simply adjusted by the animal itself.
The results from the Department of Evolutionary Biology at the University of Bielefeld, where they study those Chorthippus biguttulus grasshoppers in Autobahn roadside verges, are even cuter. When PhD student Ulrike Lampe took immature males (which do not yet sing) from the bustling roadsides and from the sleepy countryside to her lab, put them in separate boxes and let them grow until they were mature and ready to sing, the songs of the ones that had been brought from the roadsides were about 0.35 kilohertz higher. This would seem like insurmountable proof of urban evoluti
on, since these insects had never had the chance to learn about urban noise and yet as soon as they hatched into adult grasshoppers, they began singing at the perfect pitch. But even here, the reality is probably a bit more complicated. For when Lampe split the immature insects into two groups, and let one group grow up in a quiet lab, and one in a lab where she played constant recordings of traffic noise, the ones from the noisy labs grew up to sing at a slightly higher pitch than the ones from quiet labs—regardless of whether their original habitat had been roadside or meadow. In other words, the grasshoppers’ urban acoustics are a bit of both: part evolution (nature), part plasticity (nurture).
Now this chapter is about sex, so talking about the urban acoustics of only the sex that sends a sexy signal is only half the story. In fact, it is no story at all, if we do not take into consideration how this affects the receivers for whom all these love songs are intended.
For a male great tit singing in his territory, those receivers come in multiple kinds. First, there are the neighboring rival males, always eager to step on your turf or have illicit affairs with your female. Secondly, there are the females. Ah, the female Parus major! She needs to be wooed into building a nest with you. Then, every day she needs to be persuaded to have her daily egg inseminated by you and not by one of the neighboring males; and then there are all the neighboring females which you might entice into having a quickie with you. This whole theater of great tit socio-sexual opportunities, threats, decisions, and interactions is played out during the dawn chorus, when territorial males are flitting about nervously, loudly broadcasting their “dee-du” calls, and meanwhile keeping an eye on their own females and on rival males, as well as ogling female neighbors.
Darwin Comes to Town Page 16