Abstract:
Elaboration of costly sexual traits can reduce investment in other aspects of reproduction, such as parental care or intrasexual competition, which may lead to the evolution of alternative mating tactics. In house finches (Carpodacus mexicanus), less elaborately ornamented (dull) males tend to dominate more elaborated (redder) males, but redder males pair earlier and invest more in parental care. This suggests that males may pursue alternative parental or competitive tactics, depending on the elaboration of their sexual trait. Elevation of testosterone, a hormone that is closely associated with condition in male house finches, influences dominance and sexual behaviors but is antagonistic to parental behaviors. We tested the hypothesis that the higher dominance status of dull males reflects an alternative testosterone-dependent mating tactic. First, we experimentally manipulated the testosterone levels of captive males and measured the effect on dominance rank, and second, we measured the association of testosterone elevation and plumage hue in free-living males. We found that, as predicted, testosterone elevation increased dominance rank in captive males. However, in free-living males, testosterone levels were higher in redder males, suggesting that testosterone is dissociated from dominance status under natural circumstances. This may be because the context of social interactions and the higher motivation of dull males to access food resources have a stronger influence on the outcome of dominance interactions than does the physiological effects of testosterone elevation. In turn, the strong positive correlation between testosterone levels and plumage elaboration likely reflects the common condition dependence of these traits.
1. Behaviour is often assumed to be the most flexible of traits, yet recent studies show a high repeatability of behaviour within individuals even across different functional contexts. Such consistent expression of behaviour may evolve either when selection favours its integration with less flexible components of the phenotype or when pleiotropic effects produce correlations between behaviours that have different optimal timing of expression. Examining the physiological mechanisms underlying correlated expression of behaviours provides powerful insight into the evolution of personalities by establishing the extent to which pleiotropic effects might limit the independent evolution of distinct behaviours. 2. Here, we investigated proximate mechanisms behind aggressive and non-aggressive personality types in western bluebirds, Sialia mexicana, to determine whether consistency in the expression of aggression is because of shared effects of plasma-circulating androgens on aggression and mating behaviour. 3. We found that androgen concentration was unrelated to variation in both intra- and interspecific aggression even though it was closely linked to variation in male mating behaviour. These results suggest that pleiotropic effects of circulating androgens are unlikely to cause consistent differences among individuals in aggression. 4. These findings suggest that decoupling of the activational effects of hormones on behaviour is an important step in the evolution of personality traits.
The evolutionary importance of maternal effects is determined by the interplay of maternal adaptations and strategies, offspring susceptibility to these strategies, and the similarity of selection pressures between the two generations. Interaction among these components, especially in species where males and females differ in the costs and requirements of growth, limits inference about the evolution of maternal strategies from their expression in the offspring phenotype alone. As an alternative approach, we examine divergence in the proximate mechanisms underlying maternal effects across three house finch populations with contrasting patterns of sex allocation: an ancestral population that shows no sex-biased ovulation, and two recently established populations at the northern and southern boundaries of the species range that have opposite sequences of ovulation of male and female eggs. For each population, we examined how oocyte acquisition of hormones, carotenoids and vitamins was affected by oocyte growth and overlap with the same and opposite sexes. Our results suggest that sex-specific acquisition of maternal resources and sex determination of oocytes are linked in this system. We report that acquisition of testosterone by oocytes that become males was not related to growth duration, but instead covaried with temporal exposure to steroids and overlap with other male oocytes. In female oocytes, testosterone acquisition increased with the duration of growth and overlap with male oocytes, but decreased with overlap with female oocytes. By contrast, acquisition of carotenoids and vitamins was mostly determined by organism-wide partitioning among oocytes and oocyte-specific patterns of testosterone accumulation, and these effects did not differ between the sexes. These results provide important insights into three unresolved phenomena in the evolution of maternal effects - (i) the evolution of sex-specific maternal allocation in species with simultaneously developing neonates of both sexes; (ii) the link between sex determination and sex-specific acquisition of maternal products; and (iii) the evolution of context-dependent modulation of maternal effects.
