No abstract

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Abstract.-This article, which focuses on hormones and the diverse effects they have on behavior and physiology, raises evolutionary questions that hormonal studies appear especially well suited to address. These include the endocrine basis for life-history trade-offs, the role of hormones in adaptive alterations in social organization and mating systems, and whether natural selection acts on traits or organisms. The article also shows how phenotypic engineering by hormonal manipulations can reveal the evolutionary significance of phenotypic variation. By generating rare or novel phenotypes, we can attempt to determine the shape of fitness profiles in nature. To illustrate phenotypic engineering, we manipulated plasma testosterone in a freeliving bird, the dark-eyed junco (Junco hyemalis), and measured the effects of the treatment on behavior, including allocation of time to mate attraction and parental behavior as well as use of space, and on physiology, including regulation of body mass, corticosterone, and molt. We compared treated males to controls for various correlates of fitness, including territory acquisition, mate acquisition, mate retention, physical condition of the mate, apparent reproductive success, extrapair fertilizations, and survival. The results to date appear to indicate that selection is relatively indifferent to a broad range of phenotypes, while extreme deviations from the norm are selected against.The objective of this article is to describe the potential that studies of hormones have to increase understanding of the consequences of variation in behavior and physiology and thus understanding of the evolution of life histories, social organization, and mating systems. We begin by reviewing some basic facts about the action of one hormone, testosterone, in birds. Our aim is to illustrate a few principles, including (1) that hormones are secreted in response to stimuli that are generated both internally and externally, (2) that hormones have multiple targets and diverse effects, and (3) that the complexity of hormonal action leaves ample room for variation on which natural selection can act.We then pose four interconnected questions that relate hormones to the evolutionary process and that provide fruitful avenues for research: (1) How might hormones form the mechanistic bases for trade-offs in life histories? (2) What is the role of hormones in the evolution of inter-and intraspecific variation in social organization and mating systems? (3) Does natural selection act on organisms or on traits? (4) What is the evolutionary significance of phenotypic variation in hormonally mediated traits? From the recent literature we give examples of studies that address o...

When selection on males and females differs, the sexes may diverge in phenotype. Hormones serve as a proximate regulator of sex differences by mediating sex-biased trait expression. To integrate these perspectives, we consider how suites of traits mediated by the same hormone in both sexes might respond to selection. In male birds, plasma testosterone (T) varies seasonally and among species according to mating system. When elevated experimentally, it is known to enhance some components of fitness and to decrease others. We report that female T also varies seasonally and co-varies with male T. Female T is higher in relation to male T in sexually monomorphic species and is higher absolutely in females of species with socially monogamous mating systems, which suggests adaptation. We also consider the effect of experimentally elevated T on females and whether traits are sensitive to altered T. We hypothesize that sensitive traits could become subject to selection after a natural change in T and that traits with opposing fitness consequences in males and females could constrain dimorphism. Results from birds, including the dark-eyed junco (Junco hyemalis), reveal many sensitive traits, some of which appear costly and may help to account for observed levels of sexual dimorphism.

The sex ratio of Dark—eyed Juncos wintering in the eastern and central United States and Canada varies clinally along a latitudinal gradient. The percentage of @V @V among both museum skins and live—caught birds is °70% in the south, 20% in the north. When abundance according to latitude is also considered, an average ♀ appears to winter farther south than an average ♂ and hence probably tends to migrate farther. Latitude alone is an excellent predictor of sex ratio (r2 = 85%), and latitude plus 13 other measures of climate explain virtually all the variation (r2 = 96.6%). Extreme measures of climate, as compared to mean measures, are equally predictive. Principal component analysis indicates that snowfall, temperature, and latitude are the most important climatic variables associated with sex ratio. Because ♂ ♂ average larger than ♀ ♀ and are concentrated northward, mean wing length increases with latitude and is significantly correlated with climatic measures that vary with latitude. Further, larger birds within each sex may select higher altitudes as wintering sites. Sex ratio does not vary measurably with date in wintering populations. Among possible explanations for clinal variation in sex ratio are sex—associated differences in (1) advantages of early arrival on the breeding or wintering grounds, (2) impacts of inter— and intrasexual competition, and (3) effects of low temperature and intermittent food availability. Comparison of ♂ ♂ and ♀ ♀ with respect to potential fasting endurance, a size—related metabolic parameter, indicates that at 0 degrees C an average ♂ should be able to fast 4% longer (1.6 h) than an average ♀ at standard metabolic rates. An extremely heavy 7 ♂ might endured fasting up to 29% (10.7 h) longer than a very light ♀. These differences may confer greater survival ability upon the ♂ at latitudes where snow cover can often preclude feeding.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org..

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org..

No abstract

Monogamous male birds typically allocate less e¡ort to courtship and more to parental behaviour than males of polygynous species. The seasonal pattern of testosterone (T) secretion varies accordingly. Monogamous males exhibit a spring peak in plasma T followed by lower levels during the parental phase, while males of polygynous species continue to court females and maintain T at higher levels. To determine whether testosterone underlies the trade-o¡ between mating and parental e¡ort, we treated male darkeyed juncos (Junco hyemalis) with exogenous T and compared the reproductive success (RS) of T-treated males (T-males) to that of controls. T-males had lower apparent annual RS than controls, probably because elevated T reduced parental care. Nevertheless, annual genetic RS of the treatment groups was similar because (i) T-males su¡ered fewer losses in genetic RS due to extra-pair fertilizations (EPFs), and (ii) T-males gained more genetic RS through their own EPFs. This is the ¢rst hormonal manipulation of an avian phenotype shown to in£uence male RS through EPFs. Together with other studies, it suggests that testosterone may have mediated the evolution of inter-and intraspeci¢c di¡erences in allocation of reproductive e¡ort to mate attraction and parental care.