The AAH refers to sounds that animals make. In particular, it suggests that these sounds evolved in response to the habitat the animals occupy; that they travel better in the animals’ own habitats (“local") than in others (“foreign"). This probably makes intuitive sense. After all, a parakeet chirps to send a message to other parakeets—a possible mate, a competing male. Obviously, it wants that message to get through. So, it adapts the screech to the surroundings where it lives. Take the same parakeet to an unfamiliar place—the middle of the Sahara Desert, or inside a loud factory—and the sound may not transmit as effectively as it does in this specific Mumbai neighbourhood.
Not so different from us human beings, really. You’d probably be shouting if you were in a factory, probably not if you were sitting at home with your family.
Anyway, biologists have tested the AAH with various animal species that are vocalizers—birds, primates, frogs and others. Many studies support the hypothesis. But some don’t, or at best, are only able to provide “partial concordance" with it.
For example, I recently read two intriguing papers, one about listening to frogs in Chile, the other about three species of mongoose in Africa. The frog researchers found that “in northern localities (of their research area), local calls degraded more than foreign calls, and in southern localities the opposite was observed."
As for the mongooses (mongeese?), their “habitat type strongly affected sound transmission, (but) vocalizations did not always transmit best in their native habitat".
But if there’s only partial support for the AAH in both cases, there’s evidence that these animals can—as the mongoose scientists observed—“adjust their vocal behaviour to compensate for environmental constraints on the transmission of their vocalizations".
Which is to say, they’d probably be shouting, too, if they were in a factory. And that has consequences. The University of Tennessee biologist Elizabeth Derryberry studied white-crowned sparrows in the San Francisco Bay Area. Their singing has to be heard over an almost constant barrage of urban noise—traffic and the like. She found that males, particularly, reacted with what she termed “urban anger": the noise, she suggested, “possibly lead(s) to more territorial conflict in urban areas". (Urban sparrows respond to a sexually selected trait with increased aggression in noise, Elizabeth Derryberry et al., Nature, 14 May 2018).
What if a noisy environment suddenly goes quiet? Well, we’ve all just lived through pretty much exactly such an experience. The world over, the coronavirus pandemic put a halt to human activity in ways few of us had ever experienced. It turned our cities far more silent than they usually are—for months, and to an extent that we almost came to see that relative silence as a new normal. Is there evidence that urban animals changed their vocalizations as a result of this dramatic change, this new normal?
In fact, there is. During the pandemic, the Bay Area saw road traffic, and the related noise, drop to levels that were last known 50 years ago.
Here was a ready-made laboratory for scientists to see if animals’ vocalizing behaviour changed at all. So Derryberry grabbed the chance to study the white-crowned sparrows’ response. (Singing in a silent spring: Birds respond to a half-century soundscape reversion during the covid-19 shutdown, Science, 24 September 2020.
These were good birds to work with for this research endeavour. That’s because the frequency of the usual traffic noise is within “a range that interferes with the highest performance and most effective" sparrow songs. Remove that interference and voilà—the frequency and style of the sparrows’ songs changed. Recordings showed that they were suddenly singing “higher performance" songs—more complex ones, that is—but at noticeably lower volumes. Yet even though they were singing less loudly, the far lower ambient noise level meant their songs were now heard over greater distances.
Softer songs, because the birds no longer had to compete with traffic noise. More complex songs, because, again, they didn’t have to “shout". Thus they could be more expressive, use a wider range of frequencies, even ornament their songs with flourishes and trills. Think of how you might sing if you had to be heard over constant and loud noise—you’d probably lose a lot of nuance, too. And you’d get it back if the noise went away. In the same way, the sparrows seemed to have got it back, too—they were now singing songs as they probably had before cars and the other trappings of modernity altered the soundscape.
The interesting thing about this change was that it wasn’t gradual at all. The sparrows “quickly fill[ed] the most effective song space", Derryberry wrote, showing “that behavioural traits can change rapidly in response to newly favourable conditions". What this suggests is that these birds have a built-in resistance to man-made changes to the environment, such as noise from traffic. Undo the changes and they seem able to swiftly revert to what must be instinct, must be a genetic memory of behaviours—in this case, songs.
There’s evidence that male sparrows use their songs to defend breeding territories from aggressive bachelor sparrows. To that end, more complex songs are more effective.
Does this lead to less “urban anger" among sparrows? That’s not clear yet. And with the pandemic winding down, ambient noise has shot up again. But the sparrows’ songs have not returned to their pre-pandemic forms. Why? That’s not clear yet either; perhaps it’s just that sparrows born during the pandemic picked up the more intricate songs their parents were chirping.
All of which makes me wish I had recorded the parakeets.
Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun.
