Color, Display, Fight, and Song in Sexual Selection:
“The Season of Love”

The upper illustration is the king bird of paradise (now Cicinnurus regius). The lower illustration is the South American cock-of-the-rock (now in the genus Rupicola), which Darwin called “one of the most beautiful birds in the world.” These two birds are striking examples of highly ornamented males. Darwin believed that the females chose the most spectacular males as mates, and as a result, over many generations the male birds developed brighter colors, and longer and more elaborate tails, crests, or wings.

In almost all animals, only the males have horns. Darwin argued that the males’ size, strength, and shape were governed by sexual selection, as were tusks, horns, and canine teeth. They were selected through what he called the “law of battle” in which males fight for females.

Specimens courtesy of the Cornell University Museum of Vertebrates.

Damalis (Strepsiceros) capensis, the greater kudu, is also known as Tragelaphus strepsiceros. Darwin thought perhaps some horns, including the greater kudu’s, were ornamental and a result of sexual selection. He incorporated the head of the kudu from Andrew Smith’s illustration into the Descent of Man as an example.

These feathers, from a large bird of paradise found in New Guinea, were glued together for a hat to adorn a similarly decorated female Homo sapiens. Darwin believed that mate choice was also a strong force in human evolution, and that both males and females purposefully embellished their appearance.

Specimens courtesy of the Cornell University Museum of Vertebrates.

A “Cynocephalus hamadryas,” now called the Hamadryas baboon (Papio hamadryas), is a highly dimorphic mammal, with differences in both fur and facial color between the two sexes. Darwin pointed out that the male baboon’s mane was almost as long as the male lion’s.

Darwin cited the varying growth of fur on some monkeys’ heads as an example of the transmission of decorative characteristics to both sexes. It seemed unlikely to him that these features could be of any use to the animals. He concluded that they must have been produced by sexual selection through the females’ choice of mates, with the colors then partially transferred to the females.

This male “Argus giganteus,” now known as the Great Argus (Argusianus argus), is a pheasant native to the Malay Peninsula and Sumatra. Used as a teaching specimen at Cornell for at least a century, it is a unique artifact. Like the ruff also on display, its feathers have been effected by exposure to light, and by coal dust from the time when campus buildings were heated by coal furnaces.

In ten pages, and with four diagrams, Darwin meticulously traced how he believed the ocelli (“ball and socket ornament”) on the argus’s long, secondary tail feathers had developed through sexual selection. He discovered a “perfect series,” or a gradation, from two simple spots on the secondary wing feathers closest to the body, to the more elaborate ocelli. He believed the evolution of these patterns resulted from the drab females’ continual selection of the most decorated males as mates.

Specimens courtesy of the Cornell University Museum of Vertebrates.