Though advertising for sex often results in a jail sentence in most areas of the United States, these advertisements are highly conspicuous and visible daily in the animal world. You may not often think about it, but sexual innuendo is all around you. If you are one of many who glance up from the morning paper to enjoy the Cardinals in your backyard, you are only enjoying the result of a combination of sex and time. The bright red plumage of the male Northern Cardinal does not just “happen” to exist, but is an example of a “secondary sexual characteristic, a trait (in this case, sexually dichromatic coloration) which has evolved over time due to sexual selection.
For Darwin neophytes, sexual selection is a process where females choose males (in most cases) based on a certain characteristic which denotes “fitness”, i.e., the male’s ability to provide “competent” sperm. Since the genetic variability of any reasonably sized population is fairly great, there is a wide range of variability in the fitness of males. In many species, gaudy coloration is the trait that females look for. Since bright colors are detrimental to make survivorship (because of predation pressure), a surviving male with bright colors must be a superior individual–one that female would want to father her offspring. The evolutionary end result to this process is brightly colored males.
Many people are surprised to discover that physical appearance is not necessarily the only trait that can come under sex-selective pressure. Some species of animals have lek breeding systems, where males congregate and display for females. This system is taken to the extreme in manakins, a Neotropical bird family of some forty species whose makes typically utilize complex dance rituals on permanent display grounds.
One species I have enjoyed spending hours of time over several months observing is the White-collared Manakin (Manacus candei), a fairly common resident of low elevation moist forest along the Caribbean slope. M. candei is typical of Manacus species in that individual males will clear the ground of any debris in an oval approximately 1.5 meters long and 0.5 meters wide. In the center of this oval are two sapling trees about 1.5 meters tall and stripped bare of all leaves except at the tip. Ten to twenty males will arrange these ground courts within visual contact of one another.
Most manakin dance displays are little known as they don’t often go to completion when the prying eyes of researchers are witnessing them. In July of 1997, I videotaped manakin displays, which I provoked by playing the taped noise of another male. I refer to it as “noise” because the birds are not very vocal, but create loud whirring and snapping sounds with special feathers on their wings. This taped noise leads the real manakins to believe that a female must be in the area, so they all begin their displays in earnest.
Upon seeing a female, the male immediately begins hopping back and forth–either on the ground or between the two saplings–at such a speed it appears almost a blur. At the start of each hop, it creates a loud “pop” with the modified secondary feathers. The spectacle is really quite entertaining to watch, as it is easy to imagine that the bird continually comes to rest on a land mine and an explosion (the popping sound) sends it flying back in the other direction. The rest of the dance seems to consist of head bobbing movements and wing fluttering that creates a buzzing sound. A related Manacus species is reported to slide sideways down the face of one of the saplings onto the top of a receptive female. Unfortunately for the male manakin, unless he is the alpha male (who gets most of the mating opportunities) his dances normally are for naught.
Even more fascinating in many respects is the dance of the Long-tailed Manakin (Chiroxiphia linearis), a species found throughout high elevations in Costa Rica such as the famous Monte Verde Cloud Forest Preserve. Dave McDonald of Wyoming University spent many years studying this species, resulting in a feature article in the journal Science in 1995. He found that males of this species work cooperatively in their dance displays, forming two-member groups. The dance consists of a number of coordinated movements including a display in which the two males hop over each other like a game of leapfrog. The dance is finished by the dominant alpha male who gets most of the mating opportunities. This system allows young beta males to learn how the dance is done, and even occasionally sneak in a copulation or two in the process. The alpha male cannot successfully mate without a beta helper, and the beta male always becomes an alpha male when the original one dies.
It is quite obvious that the element of female choice of particular male traits has been powerful in the evolution of many bird species. Here, I have only scratched the surface of a fascinating subject, one which could result in hours of interesting reading for the curious. For such readers, I propose the book Arena Birds by Paul A. Johnsgard which will lead you into the fascinating world of the mating displays of such diverse groups as waterfowl, cotingas, bowerbirds, and gamebirds. For the rest of you, just continue to glance up from your morning paper and enjoy the scarlet.