Such biological differences should never be used to justify discrimination or sexism. That is, brain sex differences uniquely affect biochemical processes, may contribute to the susceptibility to specific diseases, and may influence specific behaviors. Although the brains of men and women are highly similar, they show consistent differences that have important implications for each sex. Of particular interest are sex differences that have been identified in the brain. Yet, there are fundamental differences between the sexes that are rooted in biology. Some of these differences are influenced by environmental factors. These differences include both biological phenotypes and psychological traits. Men and women are different in many ways. Ultimately, the study of the biological basis for sex differences will improve healthcare for both men and women. As science continues to advance our understanding of biological sex differences, a new field is emerging that is aimed at better addressing the needs of both sexes: gender-based biology and medicine. We also discuss novel research being done on unique populations including people attracted to the same sex and people with a cross-gender identity. We highlight the `four core genotypes' model and sex differences in the midbrain dopaminergic system, specifically focusing on the role of Sry. This paper reviews the evidence for direct genetic effects in behavioral and brain sex differences. However, emerging research has shown that some differences are mediated by mechanisms other than the action of these hormone secretions and in particular by products of genes located on the X and Y chromosomes, which we refer to as direct genetic effects. Historically, it was argued that such differences were largely, if not exclusively, due to gonadal hormone secretions. Instead, brood care may be optimal if partners choose traits that ensure behavioural compatibility rather than ornamental traits.Biological differences between men and women contribute to many sex-specific illnesses and disorders. „The pronounced individuality of mate preferences in zebra finches clearly does not result in strong selection for attractive ornaments“, says Bart Kempenaers, director of the department. This may be because the costs of competition can easily exceed the benefits of choosiness when only one female can get the most beautiful male in the end. As zebra finches are socially monogamous and nearly every female ends up with exactly one male partner, strong preferences for highly ornamented individuals may not be favoured by selection. “A closer look at the statistics of the previously published studies revealed that positive results were only reported when sample sizes were small, suggesting that the results may be artefacts.” For the researchers in Seewiesen, this finding did not come as a big surprise: a previous study from the same group found that female zebra finches chose compatible partners, and not necessarily the ones that were highly ornamented. “The large sample size clearly showed that there is no effect at all”, says Wolfgang Forstmeier, who led the study. The researchers, however, did not find any effect of band colour on reproductive success of either males or females across all populations. This sample size exceeds the combined sample of all 23 publications that clearly supported the “colour-band effect” hypothesis. The current study from the Department of Behavioural Ecology and Evolutionary Genetics at the Max Planck Institute for Ornithology in Seewiesen analysed the results of eight experiments in which the researchers quantified reproductive success of a total of 730 colour-banded individuals from four different captive populations. Twenty-three studies confirmed that red-banded males are superior to green-banded ones, while an additional eight studies found at least some effect on one of the “success” parameters studied. Since the first claim, 39 studies have been published in which zebra finch males had been fitted with such colour bands. The idea sounds plausible: zebra finch males decorated with red bands are more attractive to females than males wearing green bands, because a red band amplifies the ornamental effect of the male’s beak, which is coloured deep red. More than 35 years ago, a study reported that leg bands of certain colours have major effects on the attractiveness of an individual and hence on mate choice. This result of a study from the Max Planck Institute for Ornithology in Seewiesen contradicts long established text-book “knowledge” and questions whether ornaments play a major role in mate choice of monogamous species. The colour of bands attached to the legs of birds for individual identification does not have an effect on the birds’ behaviour, physiology, life-history or fitness.
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