October 21, 2011
Elaborate Plumage Due To Testosterone?
In female barred buttonquails high testosterone levels correlate with elaborate plumage and good body condition
In many bird species males have a more elaborate plumage than females. This elaborate plumage is often used to signal body condition, to intimidate rivals or to attract potential mates. In many cases plumage coloration also depends on the hormone testosterone. Christina Muck and Wolfgang Goymann from the Max Planck Institute for Ornithology in Seewiesen have now investigated whether this also holds true for sex role reversed bird species. In barred buttonquails that live in Southeast Asia, females are polygamous and pair with several males that incubate the eggs and raise the young. However, not only the behavior, but also secondary sexual ornaments that depend on the male hormone testosterone are reversed between sexes.
Colorful plumage and long feathers allow a male to express its quality and/or condition without further physical demonstration of its strength. With such features they may be able to avoid physical fights which are costly with respect to energy expenditure and the risk of injuries. The size and intensity of some parts of the plumage, for example the so-called black bib in house sparrows, depends on the male sex hormone testosterone; males with high testosterone levels also possess a larger and more intensely colored bib.
There is hardly anything known regarding function and regulation of plumage coloration in female birds: females mostly have a dull plumage with almost no variation between individuals. However, in a few bird species sex roles are reversed: here, the females aggressively defend territories and court males. The latter incubate the eggs and care for the young without any help from the females. Only very few species are known to show such sex role reversal in behavior and the evolutionary background is still unsolved.
Christina Muck and Wolfgang Goymann now found a relationship between plumage coloration, body weight and testosterone concentrations in female barred buttonquail, a bird species that lives in Southeast Asia. The researchers kept the birds in pairs for one year in large breeding boxes and regularly took blood samples to monitor the time course of testosterone levels. In addition they weighed the birds and took photographs of the black throat patch of females to determine its size and color intensity on the computer. Males of this species are smaller than females and do not possess such a patch.
The researchers could first show that testosterone levels were similar in males and females and did not exhibit large seasonal changes. Moreover, testosterone levels were rather low which is common is species that do not show a pronounced seasonality. Nevertheless they found a strong relationship between the size and the intensity of the black throat patch and the testosterone levels in females. Moreover, in females there was a correlation between testosterone levels and female body condition. No such correlations existed in males.
"It is really remarkable", states Christina Muck, "that the sex role reversal in behaviors is accompanied by a reversed hormone dependency in the expression of secondary sexual characters". Thus, female button quails succeed when they not only adopt male behavioral strategies but also use the underlying physiological mechanisms.
Reference: Christina Muck, Wolfgang Goymann. Throat patch size and darkness co-varies with testosterone in females of a sex-role reversed species. Behavioral Ecology, in press
Image Caption: Female barred buttonquails: the size and color intensity of their black throat patch is determined by their testosterone levels. Credit: Cornelia Voigt, MPI f. Ornithology
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