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How We Understand Sexual Selection and Male Phenotype Among the Birds of Paradise

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Blood smears were taken from mist-net- ted birds which were individually banded prior to release. The blood was taken from a clipped toenail and 1 or 2 thin smears were made. After air drying, fixing of the smears in 95% ethanol was completed within 24 hr, and they were stained with Giemsa 3 to 5 wk after collection. All sam- ples were examined by HIJ using an Olym- pus BA 211 microscope for 15 min with a 40 x objective and for a minimum of 15 min under oil immersion

The smears were examined for blood protozoa as well as microfilarial larvae of filariid nematodes. Haemoproteus spp. and Plasmodium spp. were counted as the number of protozoa per 10,000 red blood cells (RBC). Leucocytozoon spp., Trypanosoma spp., and microfilaria were counted as the total number of each type on the slide. About one half of the samples were scored for parasites withoutknowledge of the identity of the host.

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There are close to forty species of birds of paradise in New Guinea and no two look alike. The Red bird of paradise (Paradisaea rubra) is deep crimson with yellow highlights and has a pair of long wiry quills sprouting from its tail. The Goldie’s bird of paradise (Paradisaea decora) has a yellow and dark green body and large, fluffed crimson plumes on its back. The Ribbon-tailed Astrapia (Astrapia mayeri) is covered in metallic blues and greens with a “streamer” tail that extends over three times the length of its body. The spectacular plumage that has come to typify the Paradisaeidae family is generally reserved for only the males in the species. Females sport a far more conservative look of lackluster grays and browns. Females may be drab but we have them to thank for the males’ saturated colors, fuzzy feathers, and streamer tails. Females choose mates based on the condition and color of the males’ plumage. Bright and flashy coloring signals that the male is healthy and will produce healthy offspring. Males puff their feathers, vibrate and buzz – whatever it takes to show off their more colorful assets to a potential mate. Females will choose the most impressive and attractive males based on their displays

Over time, genes associated with these aesthetically pleasing features are passed down and the attributes become more prominent within the species. This process is known as sexual selection. Natural selection is usually about competition for resources, but sexual selection is about competition for mates. On an island with abundant resources and few predators, females are the number one decider in how males evolve. Sexual dimorphism, strong physical distinctions between the sexes, is not found in all birds of paradise. All five birds in the Manucodia genus are monomorphic, with almost no plumage variation between the sexes. These species are also monogamous. One possible explanation for this is related to diet. Species where males and females share coloring tend to feed on less nutritious simple fruits like figs. Males must work with females to forage and successfully raise young, leaving no time for outside courtships. This isn’t an issue for species that eat a diverse and reliable diet of fruits and insects. Females are able to raise hatchlings alone, freeing up the male to court other females with his magnificent and much-needed plumage. Carola Parotia (Parotia carolae), Superb bird of paradise (Lophorina superba), and Blue bird of paradise (Paradisaea rudolphi) all have wildly different displays they perform for their female counterparts. During the Carola Parotia dance, males lift their flank feathers to form a tutu-like skirt and hop and waggle across the forest floor. Superb bird of paradise snap their breast shield and back plumes into a complete physical transformation, creating an ellipse of blue and black feathers around their head. Resembling nothing like a bird, they hop around the females, rhythmically snapping their tail feathers as they go. Blue bird of paradise hang upside down from the branches, pulsating with their blue and violet feathers spread out like a fan. During the show, Blue bird of paradise produce a low, soft buzzing sound.

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Explaining the evolution of exaggerated ornamentation and weaponry (hereafter ‘ornamental traits’) in male animals has preoccupied biologists since Darwin published his ideas on selection in relation to sex (Price J. J., Lanyon S. M., 2009). Darwin realized these traits must often arise via socially mediated mechanisms because they appear to impose costs without delivering any survival benefits. The central mechanisms he proposed—male–male competition and female choice—remain the foundation of sexual selection theory today. However, because Darwin's ideas were framed around competitive processes in males, they leave many questions unanswered about the nature of selection in females. Recent research has increasingly focused on addressing this issue, but instead of clarifying how selection operates within and between the sexes, the discussion has mainly generated controversy

In particular, the analysis of selection on females has highlighted a growing uncertainty about whether classical sexual selection can explain the evolution of ornamental traits in both sexes. This debate has major implications for our understanding of general patterns in biology, such as sexual dimorphism, sexual dichromatism and the expression of ornamental traits in both males and females of a species (e.g. mutual ornamentation). The evolution of ornamental traits in males is almost universally ascribed to sexual selection, presumably because male–male competition for sexual resources is usually so overt. By contrast, similar traits expressed in females have been explained in three different ways. The original view, favoured by Darwin, is that ornamental traits in females arise from selection acting on males and are simply the result of correlated inheritance—what we might today call ‘shared genetic architecture’. One of the main points of controversy relates to the array of social mechanisms influencing ornamental traits, and whether it is so broad that a more general theory than sexual selection is required to account for patterns of phenotypic evolution. Such questions have resurfaced periodically over the last century, generally inspired by conflicting views about how selection operates on ornamental traits in females (Huxley J. S. 1938).

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In the final analysis, using these five bird-of-paradise datasets, Prost and colleagues identified genes that show signs of past influence of selection and evolution, some of which appear to be important for coloration, morphology, and feather and eye development. For example, they identified a gene called ADAMTS20 that is potentially involved in producing the exquisite birds-of-paradise colorful feathers. ADAMTS20 is known to influence the development of melanocytes, specialized cells for the production of pigmentation patterns.

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Keller L., Ross K. G. 1998. Selfish genes: a green beard in the red fire ant. Nature 394, 573–57510.1038/29064

Huxley J. S. 1938. Darwin's theory of sexual selection and the data subsumed by it, in the light of recent research. Am. Nat. 72, 416–

Langmore N. E. 1998. Functions of duet and solo songs of female birds. Trends Ecol. Evol. 13, 136–14010.1016/S0169-5347(97)01241-X

Filardi C. E., Smith C. E. 2008. Social selection and geographic variation in two monarch flycatchers from the Solomon Islands. Condor 110, 24–

Price J. J., Lanyon S. M., Omland K. E. 2009. Losses of female song with changes from tropical to temperate breeding in the New World blackbirds. Proc. R. Soc. B 276, 1971–198010.1098/rspb.2008

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