A New Look at Darwinian Sexual Selection
Keywords: Darwin, male ornamentation, fancy male species, food-courtship theory, peafowls, bower birds.
MERLE JACOBS Note 1Goshen College, Goshen, IN 46526, USA, firstname.lastname@example.org
Received February 17, 1999, published March 10, 1999
To explain the origin of the ostentatious plumage of the males of fancy-male bird species and the bright coloration of males of non-avian species, Charles Darwin proposed the theory of sexual selection (1). He argued that the ornate features of males are a consequence of female mate selection based on an abstract aesthetic sense. Darwin likened this process to animal breeders producing fancy-male varieties of pigeons by a process of conscious artificial selection.
Alfred Wallace, Darwin's collaborator and a student of wildlife behavior, suggested an alternative explanation. Male adornment, he noted, was associated with heightened physical activity and he proposed that because of their greater energy the most highly adorned males are able to win out in the competition with rival males (2).
In the 1930s, Julian Huxley (3) and R.W.G. Hingston (4) pointed out that male adornment is itself instrumental in establishing dominance relationships among males. When so employed, adornment actually reduces the physical activity necessary to intimidate rivals.
In the early 1950s, as a graduate student, I examined the process of female choice in studies of the dragonflies Perithemis tenera and Plathemis lydia (5), fruit flies Drosophila melanogaster (6) and other organisms (7). My observations led me to the conclusion that what appeared to be choice of an adorned male by a female was really a mutual attraction of both sexes to a particular type of reproductive site. Thus mate selection required not an aesthetic sense, only an awareness of features characteristic of a suitable breeding site, which might be mirrored in the ornamentation of the male. According to this view, mate selection is related directly to adaptive niche specialization. From this insight, I went on to a career in field and laboratory studies leading to the development of a food-courtship theory of mate selection.
The food-courtship theory
Life on earth is driven by solar energy as found in food molecules in various ecological niches. Organisms compete for these niches. That population most efficient in use of the energy available in a particular niche will be the fittest to survive there. Through natural selection, organisms will tend to become specialized to form isolated populations, each adapted to make the most efficient use of energy available in a particular niche. This process of segregation and specialization of populations is facilitated by employing in the mating process samples of the food available in the preferred niche. In particular cases, the male may display the food to the female or feed it to her in the courtship ceremony.
Among birds, the male may bear permanent representations of specific foods on his plumage. In this case, the female may be attracted to the male because she is attracted to these representations of the territorial foods. This process, bringing together males and females of similar tastes and physiologies, may lead to speciation. Some of the male display features may come to be involved in species identification. Male adornment could have a dual function, repelling rival males as well as attracting females. Clarification of the role of male ornamentation thus presents a huge challenge to investigators.
The food-courtship theory may be applied to the case for mate choice among peafowls Pavo cristatus. The "eyespots" on the tail feathers of the male bear a striking resemblance to blue berries. Thus, according to the food-courtship theory, it is because their plumage bears representations of food that peacocks attract peahens. This is consistent with the finding of Petrie et al. (8) that males with the most "eyespots" on their tail have the greatest mating success. Furthermore, it is not inconsistent with a possible role of the "eyespots" in reproductive competition among males (9). Moreover, this explanation seems more plausible than the suggestion that by selecting mates according to the perfection of their tail-feather "eyespots," peahens are able to identify mates with the greatest "fitness" (10).
Because male bowerbirds place fruit and flower arrangements in their courtyards and paint the walls of their bowers with colored fruit pulp, they are considered to have the most highly developed aesthetic sense among the birds. But according to my observations, it is more plausible to explain the mating behavior of this species in terms of the food-courtship theory.
The male satin bowerbird Ptilinorhynchus violaceus, which has blue plumage and blue eye irises, places blue fruits and flowers (sometimes facsimiles of such blue foods) in his courtyard. He also paints the inner walls of his bower with bluish regurgitated fruit residues. Is it not possible, therefore, that the colored objects in the court represent favorite fruits of the species, and the chewed materials painted on the walls represent such foods hung out to dry in a more permanent form?
In northeastern Australia's Mount Lewis, I studied the satin bowerbird. Early in the day, the male chewed up materials in the courtyard, then immediately regurgitated a viscous substance and applied it with his bill onto the inner walls of his bower. He later occasionally pecked at this as if tasting it. When a female arrived she fed on materials in the male's court. The male then chewed foods in the court while making loud coughing sounds while regurgitating the viscous substance from his beak in the presence of the female. When the female approached him and started to enter his bower, he charged her. She fled, only to return later, and the entire performance was repeated (7).
A published videotape (11), shows both male and female satin bowerbirds pecking at the "painted" walls of the bower within which mating occurs. Bull dog ants swarm over these walls as if finding something edible thereon. The same videotape shows the regent bowerbird, Sericulus chrysocephalis, which has yellow patches on his plumage and yellow eye irises, using yellow fruits much as the satin bowerbird uses blue fruits.
The bowerbird habit of spreading food remnants around the nest may be observed among other birds. It has been reported that among the bird-of-paradise, Parotia lawsii, the male in his court, rubs chalk from a chalk cliff over his display perch. The visiting females eat the chalk, which is rich in calcium (12).
The North American white-breasted nuthatch Sitta carolinensis performs similar food-smearing behavior at the nesting hole. These birds swipe their bills against the bark around their nesting holes while holding food items in their bills. Examination of the bark around the holes reveals parts of insects and spiders (7). Some of these materials are retrieved later and used in mate-feeding or feeding young.
An implication of the food-courtship theory is that the selective pressures associated with food preference, habitat choice, courtship behavior, and male coloration will tend to be mutually reinforcing. This may explain why so many fruit-eating birds bear "florid" or "fruity" plumage. For example, the crested guan Penelope purpurascens of Costa Rica has a red throat pouch, giving the impression that the throat contains a red fruit of the type produced by the palms on which the birds feed. The keel-billed toucan Ramphastos sulfuratus and chestnut mandibled toucan Ramphastos swainsonii feeding on the same fruits have red undertails and bill markings again closely resembling these fruits. The Montezuma oropendolas Psarocolius montezuma feeding on these fruits have red tips on their bills. When the male feeds the female during courtship, it is difficult to know whether he has, or has not, red fruit pulp in his beak (7).
David Snow, an avid student of tropical American birds, was one of the first to link colorful plumage of certain birds with the fruit-eating habit (13). Another such student, Steven Hilty, agrees with Snow's assessment: "By almost any measure, the most colorful tropical birds are usually fruit-eating and nectar-feeding species. These are followed in colorfulness ranking by partly fruit-eating birds, while the legions of dull-colored species are drawn mostly from the ranks of insect-eating birds." (14).
Not only tropical birds, but also those of temperate regions, demonstrate a correlation between colorful plumage and the fruit-eating habit. Among the finch-like birds in the vicinity of my home in Indiana, those bedecked in red are frequently seen eating red berries, such as staghorn sumac Rhus typhina, scarlet elder Sabucus racemosa, and hawthorn Crataegus sp. These birds include the cardinal Richmonena cardinalis, rose-breasted grosbeak "Pheucticus ludovicianus", purple finch Carpodacus purpureus, and house finch Carpodacus mexicanus. On the other hand, black-brown-streaked finch-like birds, such as song sparrows Melospiza melodia are seldom if ever seen feeding on red berries.
(1) Darwin, C.A. 1871. The descent of man and selection in relation to sex. John Murray, London; A.L. Burt Publ., 2nd Ed., 1874, New York.
(2) Wallace, A.R. 1889. Darwinism. 3rd Edn. Macmillan, London.
(3) Huxley, J.S. 1938. Darwin's theory of sexual selection and the data subsumed by it in the light of recent research. American Naturalist 72:416-433.
(4) Hingston, R.W.G. 1933. The meaning of animal colour and adornment. Edward Arnold, London.
(5) Jacobs, M.E. 1955. Studies on territorialism and sexual selection in dragonflies. Ecology 36:566-586.
(6) Jacobs, M.E. 1978. Influence of beta-alanine on mating and territorialism in Drosophila melanogaster. Behavior Genetics 8:487-502.
(7) Jacobs, M.E. 1999. Mr. Darwin misread Miss Peacock's mind: a new look at mate selection in light of lessons from nature. NatureBooks.
(8) Petrie, M., T. Halliday and C. Sanders. 1991. Peahens prefer peacocks with elaborate trains. Animal Behaviour 41:323-331.
(9) Andersson, M. 1994. Sexual selection. Princeton University Press, Princeton, New Jersey.
(10) Zahavi, A. and A. Zahavi. 1997. The handicap principle. Oxford University Press, Oxford and New York.
(11) Kaufman, F. 1997. Bower bird blues. (Videotape) Video Nature Library, Thirteen/WNET (for BBC TV in association with Partridge Films, Executive Producers John Sparks and Michael Rosenberg, Produced by Clive Bromhall, Filmed by Glen Threflo and Neil Bromhall, Written by Barry Paine).
(12) Pruett-Jones, S.G. and M.A. Pruett-Jones. 1988. The use of court objects by Lawes' Parotia. Condor 90:539-545.
(13) Snow, D.W. 1976. The web of adaptation: bird studies in the American tropics. The New York Times Book Co., New York.
(14) Hilty, S. 1994. Birds of tropical America. Chapters Publ. Ltd., Shelburne, Vermont.
1 Merle E. Jacobs is Research Professor Emeritus in Zoology, Goshen College, Goshen, Indiana. His book Mr. Darwin Misread Miss Peacock's Mind: A New Look at Mate Selection in Light of Lessons From Nature was published this year by NatureBooks. Information about this book (ISBN: 0-9665916-1-5, 248 pages, hardcover) is available from the author at 2214 South Main Street,Goshen, IN 46526, USA (email@example.com, phone: 219 533-2726) or from the distributor: BookMasters Inc., Ashland, OH 44805-0388, (firstname.lastname@example.org, phone 800 247 6553, fax 419 281 6883).