How did you picture your scientific career unfolding after taking Paynter’s class?
At that point, the science I had the most view of was ecology: what birds eat, where they migrate, how they feed their young. But in my sophomore year, I learned there was a debate raging about classification.
I already knew the Linnaean classification of birds — the classical system of taxonomy that placed chickadees and titmice in one family in an order with thrushes, while ducks were in another family in a different order. The traditional view of most of the 20th century was that classification was a convenient filing system.
But this new theory, called phylogenetic systematics, proposed that the classification of organisms should reflect their actual evolutionary history. This made this rather arcane art form into a new discovery-based science, the goal of which was to discover phylogeny, or what Darwin called the “tree of life,” in all its details. It got me excited about evolutionary biology because people were saying, “You can be part of this scientific revolution.” That was heady stuff.
After you graduated from Harvard, you went to Suriname for six months, and then elsewhere in South America to study the courtship display behaviors of manakins — short, stubby South American birds. Watching the manakins, what stood out for you?
One of my favorite manakins is the golden-winged manakin, Masius chrysopterus. The male is velvety black, except his crown is yellow and red, and the inner webs of his wing and tail feathers are bright golden yellow. So when he flies, you see these flashes of bright yellow. In his courtship display, he flies through the forest and lands down on a mossy, fallen log, then rebounds, turns around in midair and lands with his bill down and his tail up.
Often he’ll then fluff up his plumage like a little ball, cock his tail and bow from side to side, like a little windup toy. All of this is associated with a series of vocalizations — as he flies, he gives a single long “ceeeeeeeeee” note, and then as he jumps off a log, he goes, “ceee ceee aaak,” then ends with a sort of froggy “nerk” note.
What did you learn from studying such elaborate displays of
different South American birds?
When I looked at the different species of manakins and the associated male behaviors, I saw a history of what females preferred in various lineages. It’s sort of an evolutionary version of Freud’s classic question, “What do women want?” In some bird species, like manakins, females do all the parental care, so they can mate with whichever males they prefer.
In these species, what often evolves is a courtship display arena, or what ornithologists call a lek, where males aggregate to do their dances and females choose among the available males. Under these conditions, the males’ display behaviors, songs and plumages are important in female mate choice.
Sexual selection is about who gets the opportunity to reproduce. There are a lot of males that fail every breeding season, and whatever it is about them that females don’t prefer doesn’t get represented in the next population. As a result, those features that females use in order to choose their mates evolve very rapidly.
If the females like long tails, tails are going to evolve to be longer. If the females like colorful plumage, the plumage is going to be more colorful. If they like this or that kind of movement, then that’s what is going to happen.
As the male plumage and displays and songs diversify, the mating preferences of the females are also evolving and diversifying among species. Darwin described sexual selection by mate choice specifically: Each species evolves its own standard of beauty by which it chooses mates.
What you are suggesting, then, is that birds’ sexual displays are not about solving an explicit environmental challenge to aid survival, like avoiding a predator or cracking open a seed.
Right. The beak of the finch is marvelous in that it can crack open seeds. As the seed changed and evolved in size and hardness in the environment, the beak also has to evolve in order to face that challenge. But the sexual display of a manakin functions in the mind of female birds, not in the outside world.
To understand how these aspects of biodiversity evolve, we must understand that the challenge is one of seducing a mind that has the capacity to evolve nearly infinite preferences. We see in sexually selected traits a much greater diversity than we see in those traits that are under strict natural selection.