The Style Of The Aural Neighborhood

Sparrows from different populations have a genetic predisposition to pay attention to and learn songs from their own region, a preference that presumably evolved to help them focus on the most relevant and useful sounds. Genes make culture possible, by providing a blueprint for animal bodies that are able and eager to learn. Culture, once developed, then shapes genes, favoring the blueprints most suited to their cultural milieu. The most dramatic way that culture might affect genetic evolution is by causing species to split. Songs during the breeding season serve both to connect animals with similar preferences and songs and to exclude those with different tastes and vocal displays. Over time, these many differences can create brand new species. If they do, populations can be broken into cliques that breed among themselves but not with others. For more than half a century, scientists have studied the question of whether song learning can cause speciation. Their work reveals that cultural differences in birdsong types are widespread, but these are only occasionally associated with genetic differences among populations. In northern California and southern Oregon, the resident population of the California coast meets the migratory population of the Pacific Northwest. Each has its own song dialect, with the northerly birds singing with longer whistles and shorter sweeps and trills. Playback experiments show that birds respond more vigorously to songs of their own dialect, suggesting that shared songs unite each population and keep them separate from others.

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But in border areas where the two populations mingle, these behavioral differences were slighter. This suggests that although cultural differences in song do seem to keep populations apart, this force may weaken in areas of extensive contact. Vocal learning also affords a degree of flexibility that connects divergent populations, delaying speciation. Female birds sometimes prefer the song types of their home region, but this preference is not universal and can be erased by exposure to other song varieties. In a neighborhood near San Francisco where song types are uniform, females will therefore likely enforce conformity, preferring familiar songs. Farther north, on the Oregon border, females hear many song types and will have more catholic and flexible tastes, potentially selecting males from other areas. For males, too, culture can smooth over geographic differences. By molding his song into the style of the neighborhood, a young male setting up his first territory can partly break free from his parental inheritance. He’s stuck with his genes but can find a new vocal identity through learning. In addition to its role in promoting or slowing the evolutionary splitting of populations, vocal culture can make endangered species more vulnerable to extinction. If population densities drop too low, it is harder for animals to find one another and young birds fail to learn the species’ full songs. In recent years, many of these birds have started singing atypical songs, including the songs of other species.

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Compared with recordings from previous decades, contemporary birds also sing simpler songs. Males with these malformed and often stunted songs are less attractive to females. At the edge of extinction, then, social learning of song can become a liability. As Hawaiian honeycreepers on the island of Kaua’i declined in numbers, the diversity of their songs plummeted, likely due to the loss of social connections that formerly sustained the cultural richness of song learning. In endangered whales too, it seems that cultural diversity is lost when populations shrink. Such losses have been heard in endangered and declining sperm and orca whales. The decline of vocal diversity may have been severe among those species that were scythed down to 10 percent or less of their former abundance. The species’ geographic variation in sound is obvious even to human ears unaccustomed to parsing the details of birdsong. The species offers us an imaginative window into the possibilities of culture in all vocal learning species, most of them unstudied by science. Wherever vocal learning happens, cultural evolution can unfold, driven either by the creative impulses in animal minds or by the simple accumulation of copying errors as each generation learns from its elders. These cultural changes cause sound to change through time and to fan out in a richly textured geography. Why one patch of ocean should be the origin of so much new whale song is unknown, as are the causes for the sudden spread among whale singers of one song variant and not others.

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Cultural variation in the sounds of toothed whales, like sperm whales, orcas, and dolphins, reveals subtle hierarchies of affiliation within each species, from parent to offspring, to clans, to large regions. Sperm whales, for example, live in matrilineal groups that range over thousands of kilometers. Sperm whales communicate with short bursts of loud clicks. When the whales are close together, these pulses of clicks are like the excited chatter of human friends gathering at the weekend, overlapping one another in a frenzy. This individuality is embedded within a larger spatial and social structure. Matrilines have their own distinctive clicking styles and are themselves part of regional dialects. In the Pacific, these dialect groups overlap in range, but whales within each do not associate with one another, seeming to disdain the company of whales with the wrong way of clicking. In the Atlantic, whales in each dialect group stick to their own nonoverlapping subregions of the ocean. When a sperm whale clicks, other whales presumably can immediately identify its region, family, and individual identity, just as we humans can infer the identity and biography of those we hear speak. Sometimes cultural evolution crosses species boundaries. Parrots, lyrebirds, mockingbirds, and many other birds take snatches of the sounds of other species and weave them into their own sonic creations. In the case of Australian lyrebirds, these sounds are then passed down through the generations culturally. When lyrebirds were introduced by humans to Tasmania in 1934, they remembered and repeated the song of the whipbird as part of their mimicking display, even though whipbirds did not live in their new home.