Ellen Ketterson

We approach conceptual issues in evolutionary biology from an endocrinological perspective, noting that single hormones typically act on several target tissues and thereby mediate suites of correlated phenotypic traits. When several components of such a suite are beneficial, an important evolutionary question is whether all are adaptations or some are exaptations. The answer may depend on whether the traits arose in response to selection on variation in systemic levels of the hormone on variation in responsiveness of target tissues to invariant levels of the hormone. If the former, selection probably acted directly on fewer than all traits; beneficial traits arising indirectly would be exaptations. In contrast, multiple beneficial traits that arose out of independent changes in target‐tissue sensitivity to invariant hormone levels could all be adaptations. Knowledge of specific hormonal mechanisms as well as of historical …

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 …

Hormones influence many aspects of organismal behavior, physiology, and morphology, and thus hormones may lie at the root of many life-history trade-offs. By manipulating hormones we can create novel phenotypes (i.e., perform phenotypic engineering) and attempt to relate phenotypic variation to fitness. We report the effect of testosterone treatment on parental behavior and vocal behavior of adult male dark-eyed juncos. Testosterone partially suppressed paternal behavior and increased the frequency of song. When we compared treated males and controls for nine potential correlates of fitness (offspring growth and survival to the age of 10 d, condition of females, length of the interval between consecutive nestings, size of subsequent clutches and broods, mate retention within and between breeding seasons, and survival rate), we found no statistical differences. In some measures treated males outperformed …

The past 30 years of immunological research have revealed much about the proximate mechanisms of maternal antibody transmission and utilization, but have not adequately addressed how these issues are related to evolutionary and ecological theory. Much remains to be learned about individual differences within a species in maternal antibody transmission as well as differences among species in transmission or utilization of antibodies. Similarly, maternal-effects theory has generally neglected the mechanisms by which mothers influence offspring phenotype. Although the environmental cues that generate maternal effects and the consequent effects for offspring phenotype are often well characterized, the intermediary physiological and developmental steps through which the maternal effect is transmitted are generally unknown. Integration of the proximate mechanisms of maternal antibody transmission with …

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 …

The evolution of bird migration and the role of migration in life history have long been matters of general interest, and the volume of recent literature on these subjects (Baker, 1978; Dingle, 1980; Gauthreaux, 1978, 1979, 1982; Fretwell, 1980; Greenberg, 1980; Greenwood, 1980; Myers, 1981a; Ketterson and Nolan, 1982) reflects their continuing importance to students of avian ecology and evolutionary biology.

A salivary conditioning experiment with dogs is described. Not only was the conventional conditioned response studied, but also the general overt behavior of the animal during the training and testing periods." The constant occurrence of biologically significant co-variation of the secretory and overt components of behavior is considered incompatible with a response element theory of conditioning such as that of Pavlov. The lack of identity in effector terms of conditioned and unconditioned behavior even in the training situation, but especially when the situation is varied, is considered to be incompatible with all stimulus substitute theories of conditioning… salivation reveals itself as a dependent component within a complex, widespread, organized, goal-directed behavioral act, which varies adaptively with changes in the experimental situation…. It is felt that the data here presented require a more complex …

Hatching asynchrony in avian species often leads to the formation of a size hierarchy that places last–hatched nestlings at a significant disadvantage. The hatching muscle (musculus complexus) is responsible for breaking the shell during hatching and for dorsal flexion of the neck during begging. An increase in its strength in last–hatched nestlings could mitigate the effects of hatching asynchrony by reducing the time required for hatching or enhancing the effectiveness of begging for parentally delivered food or both. We have previously found that yolk testosterone concentration increases with laying order in the red–winged blackbird Agelaius phoeniceus. In this study, we investigated the hypothesis that yolk testosterone has anabolic effects on the development of the complexus, thereby influencing competition among asynchronously hatched nestlings. We found that both yolk testosterone concentration and …

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 …

Monogamous and polygynous male songbirds generally differ in their breeding season profiles of circulating testosterone. Testosterone level spikes early in the breeding season of monogamists and then declines, but it remains high in polygynists. Male dark‐eyed juncos (Junco hyemalis) are socially monogamous and exhibit the usual pattern, but experimental maintenance of high testosterone throughout the breeding season alters normal behavior and physiology and affects various components of annual reproductive success but not overall annual success. Because stabilizing selection predicts that alteration of naturally existing phenotypes should reduce lifetime reproductive success, we asked whether prolonged testosterone exposure might impair immune function and perhaps thereby reduce life span. We assessed immune function in captive and wild male juncos that we treated with either testosterone …

Perhaps the best way to determine whether and how traits of organisms are currently adaptive is to alter them experimentally and compare the relative fitness of altered and unaltered individuals. We call this method phenotypic engineering. To the extent that natural selection moulds organisms on a trait‐by‐trait basis, we would expect fitness of unmanipulated (control) individuals to be higher than that of experimentally altered individuals. However, other outcomes are possible and of interest. If, for example, a single trait were altered and the fitness of manipulated and unmanipulated organisms were found to be similar, we might conclude that selection is not currently operating on the altered trait. Phenotypic engineering with hormones describes an experimental approach to the study of adaptive variation in suites of traits that are hormonally mediated and correlated in their expression. A likely outcome of such …

Novel or changing environments expose animals to diverse stressors that likely require coordinated hormonal and behavioral adaptations. Predicted adaptations to urban environments include attenuated physiological responses to stressors and bolder exploratory behaviors, but few studies to date have evaluated the impact of urban life on codivergence of these hormonal and behavioral traits in natural systems. Here, we demonstrate rapid adaptive shifts in both stress physiology and correlated boldness behaviors in a songbird, the dark-eyed junco, following its colonization of a novel urban environment. We compared elevation in corticosterone (CORT) in response to handling and flight initiation distances in birds from a recently established urban population in San Diego, California to birds from a nearby wildland population in the species' ancestral montane breeding range. We also measured CORT and …

To assess whether alterations in the normal pattern of testosterone (T) secretion might be beneficial or detrimental, we studied a breeding population of darkeyed juncos in which we elevated T experimentally and measured its effect on potential correlates of fitness. We treated both free-living and captive males with implants that were either empty (C-males, controls) or packed with T (T-males, experimentals). Timing of implant varied and was designed to mimic natural peak breeding levels except that peaks were either prolonged or premature. We bled the birds at recapture and analyzed their plasma, and that of their female mates, for T and corticosterone (B). We also measured body mass and fat score in free-living T- and C-males. In the field, T-implants elevated T and kept it elevated for at least a month. Experimental males also had higher B than controls. In captives, the effect of the implants on plasma T was …

Male birds frequently face a trade‐off between acquiring mates and caring for offspring. Hormone manipulation studies indicate that testosterone often mediates this trade‐off, increasing mating effort while decreasing parental effort. Little is known, however, about individual covariation between testosterone and relevant behavior on which selection might act. Using wild, male dark‐eyed juncos (Junco hyemalis), we measured individual variation in testosterone levels before and after standardized injections of gonadotropin‐releasing hormone (GnRH). The GnRH challenges have been shown to produce short‐term testosterone increases that are similar to those produced naturally in response to social stimuli, repeatable in magnitude, and greater in males with more attractive ornaments. We correlated these testosterone increases with behavioral measures of mating and parental effort (aggressive response to a …