Concentrations of gonadal steroids such as testosterone (T) often vary widely in natural populations, but the causes and particularly the consistency of this variation is relatively unexplored. In ...breeding males of a wild population of the dark-eyed junco (
Junco hyemalis), we investigated seasonal and individual variation in circulating T during two breeding seasons by measuring the responsiveness of the HPG axis to a standardized injection of gonadotropin-releasing hormone (GnRH). Individuals were bled prior to and 30
min after injection. Pre- and post-challenge levels of T were measured using EIA. Many subjects were sampled repeatedly across multiple breeding stages. Plasma T concentrations nearly doubled in response to GnRH during early spring, but showed significantly smaller increases in later breeding stages. When controlling for seasonal variation in response to challenge, we also found repeatable differences among individuals, indicating individual consistency in the release of T in response to a standardized stimulus. These seasonal and individual differences may arise from comparable variation in responsiveness of the pituitary or a decline in gonadal sensitivity to downstream gonadotropins. In contrast, pre-challenge T showed almost no seasonal changes and did not differ consistently among individuals. To our knowledge, this is the first demonstration of individual repeatability of short-term hormonal changes in a wild population. Such repeatability suggests that hormonal plasticity might evolve in response to changing selection pressures.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Because testosterone (T) often mediates the expression of attractive displays and ornaments, in the absence of constraints sexual selection should lead to an evolutionary increase in male T levels. ...One candidate constraint would be a genetic correlation between the sexes that leads to a correlated response in females. If increased T in females were to have deleterious effects on mate choice, the effect of sexual selection on male T would be weakened. Using female dark-eyed juncos (Junco hyemalis), we tested whether experimentally enhancing female T would lead to a decrease in discrimination between two classes of males, one treated with T (T-males) and one control (C-males). The two female treatments (T-implanted and C-females) spent equal amounts of time with both classes of males, but T-treated females failed to show a preference for either male treatment, whereas C-females showed a significant preference, albeit in an unexpected direction (for C-males). T-females were less discriminating than C-females, irrespective of the direction of their preference. To our knowledge, this is the first study to show that circulating hormones can alter female choosiness without reducing sexual motivation. Our results suggest that hormonal correlations between the sexes have the potential to constrain sexual selection on males.
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1. Despite considerable recent interest in plasma and yolk testosterone (T) in female birds, relatively little is known about environmental regulation of female T, individual variation in female T or ...the relationship between plasma and yolk T. 2. In breeding females of a wild population of dark-eyed junco (Junco hyemalis), we assessed variation in the responsiveness of the hypothalamo-pituitary-gonadal (HPG) axis to a challenge with gonadotropin-releasing hormone (GnRH) by measuring circulating T before and 30 min after a standardized injection of GnRH. We asked whether response to challenge varied seasonally or with stage of reproduction and whether it was repeatable within individuals or related to T deposited in eggs. 3. Initial and post-challenge levels of T were measured using enzyme immunoassay. In a subset of these females, luteinising hormone (LH) was measured using radioimmunoassay (RIA). In addition, eggs were collected from nests of 15 females that had received a GnRH challenge, and yolk T was measured using RIA. 4. During most of the breeding season, plasma T did not increase in response to GnRH. GnRH consistently caused increases in plasma T only during the 7 days before oviposition, when females were rapidly depositing yolk in eggs but had not yet begun to lay them. Among a small subset of females we found a positive correlation between the magnitude of this increase in plasma T in response to GnRH during egg development and the amount of T deposited in the yolk of eggs collected at a later time. 5. These results suggest that ovarian response to GnRH-induced increases in LH is greatest when females are actively depositing yolk into eggs. Factors that stimulate the release of GnRH during egg formation may result in higher levels of plasma T which could influence adult female behaviour. Further, because plasma T was correlated with later yolk T, factors that stimulate GnRH release may also lead to higher levels of yolk T potentially influencing offspring development or behaviour.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NMLJ, NUK, OILJ, PNG, SAZU, SBCE, SBMB, UL, UM, UPUK
Ectothermic species are particularly sensitive to changes in temperature and may adapt to changes in thermal environments through evolutionary shifts in thermal physiology or thermoregulatory ...behaviour. Nevertheless, the heritability of thermal traits, which sets a limit on evolutionary potential, remains largely unexplored. In this study, we captured brown anole lizards (Anolis sagrei) from two populations that occur in contrasting thermal environments. We raised offspring from these populations in a laboratory common garden and compared the shape of their thermal performance curves to test for genetic divergence in thermal physiology. Thermal performance curves differed between populations in a common garden in ways partially consistent with divergent patterns of natural selection experienced by the source populations, implying that they had evolved in response to selection. Next, we estimated the heritability of thermal performance curves and of several traits related to thermoregulatory behaviour. We did not detect significant heritability in most components of the thermal performance curve or in several aspects of thermoregulatory behaviour, suggesting that contemporary selection is unlikely to result in rapid evolution. Our results indicate that the response to selection may be slow in the brown anole and that evolutionary change is unlikely to keep pace with current rates of environmental change.
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Hamilton's theory of inclusive fitness revolutionized our understanding of the evolution of social interactions. Surprisingly, an incorporation of Hamilton's perspective into the quantitative genetic ...theory of phenotypic evolution has been slow, despite the popularity of quantitative genetics in evolutionary studies. Here, we discuss several versions of Hamilton's rule for social evolution from a quantitative genetic perspective, emphasizing its utility in empirical applications. Although evolutionary quantitative genetics offers methods to measure each of the critical parameters of Hamilton's rule, empirical work has lagged behind theory. In particular, we lack studies of selection on altruistic traits in the wild. Fitness costs and benefits of altruism can be estimated using a simple extension of phenotypic selection analysis that incorporates the traits of social interactants. We also discuss the importance of considering the genetic influence of the social environment, or indirect genetic effects (IGEs), in the context of Hamilton's rule. Research in social evolution has generated an extensive body of empirical work focusing—with good reason—almost solely on relatedness. We argue that quantifying the roles of social and non-social components of selection and IGEs, in addition to relatedness, is now timely and should provide unique additional insights into social evolution.
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