Interspecific variation in egg testosterone levels: implications for the evolution of bird song

Although interspecific variation in maternal effects via testosterone levels can be mediated by natural selection, little is known about the evolutionary consequences of egg testosterone for sexual selection. However, two nonexclusive evolutionary hypotheses predict an interspecific relationship between egg testosterone levels and the elaboration of sexual traits. First, maternal investment may be particularly enhanced in sexually selected species, which should generate a positive relationship. Secondly, high prenatal testosterone levels may constrain the development of sexual characters, which should result in a negative relationship. Here we investigated these hypotheses by exploring the relationship between yolk testosterone levels and features of song in a phylogenetic study of 36 passerine species. We found that song duration and syllable repertoire size were significantly negatively related to testosterone levels in the egg, even if potentially confounding factors were held constant. These relationships imply that high testosterone levels during early development of songs may be detrimental, thus supporting the developmental constraints hypothesis. By contrast, we found significant evidence that song-post exposure relative to the height of the vegetation is positively related to egg testosterone levels. These results support the hypothesis that high levels of maternal testosterone have evolved in species with intense sexual selection acting on the location of song-posts. We found nonsignificant effects for intersong interval and song type repertoire size, which may suggest that none of the above hypothesis apply to these traits, or they act simultaneously and have opposing effects

Sex differences in the response to environmental cues regulating seasonal reproduction in birds

Although it is axiomatic that males and females differ in relation to many aspects of reproduction related to physiology, morphology and behaviour, relatively little is known about possible sex differences in the response to cues from the environment that control the timing of seasonal breeding. This review concerns the environmental regulation of seasonal reproduction in birds and how this process might differ between males and females. From an evolutionary perspective, the sexes can be expected to differ in the cues they use to time reproduction. Female reproductive fitness typically varies more as a function of fecundity selection, while male reproductive fitness varies more as a function sexual selection. Consequently, variation in the precision of the timing of egg laying is likely to have more serious fitness consequences for females than for males, while variation in the timing of recrudescence of the male testes and accompanying territory establishment and courtship are likely to have more serious fitness consequences for males. From the proximate perspective, sex differences in the control of reproduction could be regulated via the response to photoperiod or in the relative importance and action of supplementary factors (such as temperature, food supply, nesting sites and behavioural interactions) that adjust the timing of reproduction so that it is in step with local conditions. For example, there is clear evidence in several temperate zone avian species that females require both supplementary factors and long photoperiods in order for follicles to develop, while males can attain full gonadal size based on photoperiodic stimulation alone. The neuroendocrine basis of these sex differences is not well understood, though there are many candidate mechanisms in the brain as well as throughout the entire hypothalamo–pituitary–gonadal axis that might be important

Parasite levels in blue-black grassquits correlate with male displays but not female mate preference

The handicap principle proposes that male sexual ornaments and displays provide honest indicators of quality. Female preference for high-quality males, however, may be driven not only by genetic benefits but also by indirect benefits. We investigated the impact of parasitism on morphological, ornamental, and behavioral characteristics of male and female blue-black grassquits (Volatinia jacarina) in captivity. First, we tested whether male displays and morphology were influenced by parasitism. Second, we assessed if females were attentive to variation in male morphology and displays linked to parasitism. Third, we tested whether parasitism in females influenced health and mate preferences. We maintained 2 groups of birds in captivity: nonmedicated birds developed high levels of coccidian parasitism, whereas medicated birds were free of parasitism. Parasitized males developed, relative to nonparasitized males, lower weight/tarsus indices and mass. They also showed relative deficiencies in their displays, with less persistence and lower rates. Despite the negative effects of parasitism on males, females did not prefer nonparasitized males. This held for both parasitized and nonparasitized females. Our data suggest that coccidian parasitism has adverse effects on morphological condition and expression of displays. These effects, however, appear not to be attended to by females; moreover, female mate preferences appear not to be impacted by the threat of parasitism. It thus seems that female mate preferences may not depend only on sexual characters affected by parasitism in this species. Copyright 2008, Oxford University Press.

The evolution of hippocampus volume and brain size in relation to food hoarding in birds

Food-hoarding birds frequently use spatial memory to relocate their caches, thus they may evolve a larger hippocampus in their brain than non-hoarder species. However, previous studies testing for such interspecific relationships provided conflicting results. In addition, food hoarding may be a cognitively complex task involving elaboration of a variety of brain regions, even outside of the hippocampus. Hence, specialization to food hoarding may also result in the enlargement of the overall brain. In a phylogenetic analysis of distantly related birds, we studied the interspecific association between food hoarding and the size of different brain regions, each reflecting different resolutions. After adjusting for allometric effects, the relative volume of the hippocampus and the relative size of the entire brain were each positively related to the degree of food-hoarding specialization, even after controlling for migration and brood parasitism. We also found some significant evidence for the relative volume of the telencephalon being associated with food hoarding, but this relationship was dependent on the approach we used. Hence, neural adaptation to food hoarding may favour the evolution of different brain structures