Sexual selection: Do females follow fads?

Is this male attired in the fashionable look of the season? Based on the reaction of the female in the background, perhaps not. Source: Wikimedia Commons

Timeline, 2008: Sexual selection is a mechanism of evolution that sometimes butts heads with natural selection. Under the tenets of natural selection, nature chooses based on characteristics that confer a competitive edge in a given environment. Under this construct, environment is “the decider.” But in sexual selection, either competition between the same sex or a choice made by the opposite sex determines the traits that persist. Sometimes, such traits aren’t so useful when it comes to the everyday ho-hum activities like foraging for food or avoiding predators, but they can be quite successful at catching the eye of an interested female.

Those female opinions have long been considered unchanging. In the widowbird, for example, having long, flowing black tailfeathers is a great way to attract the lady widow birds. But perhaps they don’t call them widowbirds for nothing: if those male tailfeathers get too long, the bird can’t escape easily from predators and ends up a meal instead of a mate. In these cases, natural selection pushes the tailfeather trait in one direction—shorter—while sexual selection urges it the other way—longer. The upshot is a middling area for tailfeathers length.

This kind of intersexual selection occurs throughout the animal kingdom. Probably the most well-recognized pair that engages in it is the peacock and peahen. Everyone has seen the multicolored baggage any peacock worth his plumage drags around behind him. A peacock will fan out those feathers in an impressive demonstration, strutting back and forth and waving its tail in the wind, showing off for all he’s worth. It’s a successful tactic as long as nothing is around that wants to eat him.

Frogs hoping for a mate find themselves elbow deep in the “paradox of the lek.” The lek is the breeding roundup for frogs, where they all assemble in a sort of amphibian prom. For the males, it’s a tough call, literally. They must call loudly enough to show the females how beautifully androgenized they are—androgens determine the power of their larynx—while at the same time not standing out enough to attract one of the many predators inevitably drawn to a gathering of hundreds of croaking frogs. Trapped in this paradox, the frog does his best, but natural selection and sexual selection again end up stabilizing the trait within expected grooves.

This status quo has become the expectation for many biologists who study sexual selection: natural selection may alter its choices with a shifting environment, but what’s hot to the females stays hot, environmental changes notwithstanding. But the biologists had never taken a close look at the lark bunting.

A male lark bunting has a few traits that may attract females: when it shakes off its drab winter plumage and takes on the glossy black of mating season, the male bird also sports white patches on its wings that flash through the sky and sings a song intended to draw in the ladies. But the ladies appear to be slaves to fashion, not consistently choosing large patches over small, or large bodies over lighter ones. Instead, female lark buntings change their choices with the seasons, selecting a large male one year, a dark-colored male with little in the way of patches the next, and a small-bodied male the next. Lark buntings select a new mate each year, and the choice appears to be linked to how well the male will aid in parenting duties, which both parents share. It may be that a big body is useful in a year of many predators, but a small body might work out better when food supplies are low.

The researchers who uncovered this secret of lark bunting female fickleness watched the birds for five years and based their findings on statistical correlations only. For this reason, they don’t know exactly what drives the females’ annually varying choices, but they speculate that environmental factors play a role. Thus, sexual selection steps away from the realm of the static and becomes more like—possibly almost indistinguishable from—natural selection.

Has the ivory-billed woodpecker left the building?

Watercolor painting of ivory-billed woodpeckers from Audubon's Birds of America, 1826.

Imagine waking up one morning to real film footage of a duckbill dinosaur wandering around the Great Plains. Your reaction might be similar to that of birders around the world when Science magazine reported in 2005 that the ivory-billed woodpecker, thought for 60 years to have been extinct in the United States, still existed.

A forest bird of legend

The woodpecker entered birder and ecologist lore when its numbers declined in the early part of the 20th century. Its habitat was bottomland forest in the southeastern United States and Cuba, and its niche included drilling into mature trees. When people came along, logging away the woodpeckers’ homes, the bird appeared to vanish. By the 1920s, we thought it had disappeared forever, although in 1943, there was a single confirmed sighting of a lone female, flying over the stumps of an old-growth forest. She became a central figure in a PhD thesis in 1944. Then for 60 years, silence.

False calls

Well, not complete silence. There were many reports of sightings, but most were traced to another woodpecker species, the pileated woodpecker. The ivory-billed woodpecker differs distinctly from its pileated cousin in beak color, in having white patches on its back when perched, and in its size and the solid-black crest of the female. It has a three-foot wing span, which is huge for a woodpecker, and can grow as large as 20 inches long. It is a big, beautiful, and surprising bird, with a bright red crest on the males that must be startling to see among the cypress of a bottomland forest.

A mesmerizing obsession

Birders, possibly the most obsessive of any taxon fan club, had long wandered into the swampy bottomlands of Arkansas and Louisiana, trying to find ivory-billed woodpeckers. There was a confirmed sighting in Cuba in the ‘80s, and over the decades, people have claimed sightings or reported having heard the ivory-billed’s call. Professionals and amateurs alike have waded among snakes and fought off bugs, playing tapes of the call and listening for a response. At one point, searchers found a nest that had an ivory-billed look to it and trained a remote-sensing camera on it, but saw nothing.

And then in 1999, a kayaker thought that he had seen a pair of the birds. His report received serious attention from the government, local papers, and academic groups interested in the woodpecker both for its inherent beauty and for its status as a symbol of the price of our destructive tendencies. Soon, the old forests of the southeast were crawling with ornithologists, all hoping to catch a glimpse, take a picture, and emerge with definitive proof that a bird long thought to be extinct had survived.

The beat of the forest, revived?

Some people heard the drumming sounds the woodpecker is known to make. A handful of people who really knew their woodpeckers reported sightings. But it was a four-second video of the shy, reclusive bird that clinched it. The video is short and blurry, taken from a kayak in late April of 2004 on a camcorder. But even its poor quality couldn’t hide the distinctive markings and features of the ivory-billed woodpecker.

The confirmation set the world of ornithology astir, but it also reverberates among ecologists and environmentalists. The fact that at least one male ivory-billed woodpecker exists indicates that at least one breeding pair must have survived into the 1990s because the birds live 15 to 20 years at most. And it also might have meant a second chance for us and the woodpecker. Unfortunately, according to a recent report from Cornell researchers who have spent five years looking for more signs of the bird, “it’s unlikely that there are recoverable populations” of the bird where they’ve been searching.

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