Sexual dimorphism in human skulls. A comparison of sexual dimorphism in different populations

Application and comparison of sex discriminant functions in different populations led to the conclusion that a certain combination and weighting of a few sex dimorphism variables (in this study we only used craniometric variables) can give a good discrimination between male and female individuals, independent of the racial group to which this function is applied. In our study, the sex-discriminatory power of five discriminant functions which were based on different ordination and selection procedures (e.g. professional knowledge, stepwise discriminant analysis, literature) of the cranial variables is compared. These discriminant functions were applied to three different data sets, the first being skull measurements from an Amsterdam series (Europids), the second skull measurements of a Zulu series (Negrids) and the third skull measurements of a Japan series (Mongolids). Our decision as to whether a function is a good or less good sex-discriminating function is determined by the Dt values (these values give an idea about the discriminatory value of the discriminant function when applied to a new test sample), the number of variables necessary to obtain this Dt and the location of the sectioning point (i.e. comparison between the estimation of the sectioning point and the ”real” sectioning point). These discriminant functions were compared withGiles Elliot's (1962, 1963) “race-independent” sex function.     

The evolution of sexual dimorphism in parasitic cuckoos: sexual selection or coevolution?

Sexual dimorphism is ubiquitous in animals and can result from selection pressure on one or both sexes. Sexual selection has become the predominant explanation for the evolution of sexual dimorphism, with strong selection on size-related mating success in males being the most common situation. The cuckoos (family Cuculidae) provide an exceptional case in which both sexes of many species are freed from the burden of parental care but where coevolution between parasitic cuckoos and their hosts also results in intense selection. Here, we show that size and plumage differences between the sexes in parasitic cuckoos are more likely the result of coevolution than sexual selection. While both sexes changed in size as brood parasitism evolved, we find no evidence for selection on males to become larger. Rather, our analysis indicates stronger selection on parasitic females to become smaller, resulting in a shift from dimorphism with larger females in cuckoos with parental care to dimorphism with larger males in parasitic species. In addition, the evolution of brood parasitism was associated with more cryptic plumage in both sexes, but especially in females, a result that contrasts with the strong plumage dimorphism seen in some other parasitic birds. Examination of the three independent origins of brood parasitism suggests that different parasitic cuckoo lineages followed divergent evolutionary pathways to successful brood parasitism. These results argue for the powerful role of parasite-host coevolution in shaping cuckoo life histories in general and sexual dimorphism in particular.     

棕背伯劳羽色多态与MC1R基因的相关性

黑素皮质素受体1 (melanocortin-1 receptor, MC1R)基因是控制动物黑色素合成的重要基因, 鸟类羽色的变异与MC1R基因的变异有密切关系。棕背伯劳(Lanius schach)在我国东部沿海多地存在羽色多态现象, 有棕色型、黑色型和黑色白边型的分化。为了探究MC1R基因与棕背伯劳色型分化的关系, 本研究对分布于广东省的3种色型共计11只棕背伯劳的MC1R基因编码区进行单核苷酸多态性(SNPs)分析和氨基酸多态性分析。结果表明: (1) 11个实验个体的MC1R基因序列共有4种单倍型, 其中黑色型和黑色白边型共享单倍型H3。(2) 3种色型棕背伯劳MC1R基因编码区的第34-931位的899个碱基中共有47个碱基变异位点, 相对应的氨基酸序列共有18个变异位点, 这些变异位点与黑色表型无对应关系。(3)黑色型与黑色白边型个体基因型在第268-303位编码区出现了36个碱基的缺失, 对应着12个氨基酸的缺失, 该缺失与黑色表型相对应。因此推测棕背伯劳的黑化与MC1R基因碱基片段的缺失密切相关。     

Delayed maturation of plumage coloration and plumage spottedness in the Barn Owl (Tyto alba)

Summary The Barn Owl (Tyto alba) varies in plumage from dark reddish-brown to white, and from heavily marked with black spots to immaculate. Males are commonly lighter coloured and less spotted than females. I assessed whether male and female Barn Owls delay the full expression of plumage coloration and spottedness to the second year of life. In Switzerland, I quantified the two traits of birds captured at the nestling stage, first, second and third year of life. Males and females became lighter coloured only from the first to the second year. Males became less spotted only from the first to the second year, and females less spotted from the nestling stage to the first year but more spotted from the first to the second year. Females were also similarly spotted at the second and third year of age. By cutting off small pieces of feathers of females I could recognize which feathers had later been renewed. After a complete moult old females did not change in plumage characteristics.

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Verzögerter Wechsel vom Juvenil- zum Adultgefieder bei der Schleiereule(Tyto alba)

Zusammenfassung Das Gefieder der Schleiereule (Tyto alba) variiert von rostbraun bis weiss und von dicht gefleckt bis fleckenlos. Die Männchen sind eher hell und weniger gefleckt als die Weibchen. Es wurde untersucht, ob männliche und weibliche Schleiereulen die volle Entfaltung ihrer Gefiederfarbe und -struktur auf das zweite Lebensjahr verlegen. In der Schweiz habe ich beide Gefiederpolymorphismen bei Jungvögeln, ein-, zwei- und dreijährigen Vögeln quantifiziert. Bei Männchen und Weibchen wurde das Auslichten der Gefiederfarbe nur zwischen dem 1. und 2. Lebensjahr beobachtet. Männchen wurden weniger dicht gefleckt nur zwischen dem 1. und 2. Lebensjahr, während Weibchen zwischen dem Nestlingstadium und dem 1. Lebensjahr weniger gefleckt wurden und zwischen dem 1. und 2. Lebensjahr wieder mehr Flecken hatten. Weibchen wurden also gleich gefleckt im 2. und 3. Lebensjahr. Um die erneuerten Federn zu erkennen, wurden bei Weibchen kleine Federstücke herausgeschnitten. Nach einer vollständigen Erneuerung der Brustfedern hatte sich die Farbe und die Anzahl Punkte bei alten Weibchen nicht geändert.

     

The genetics of sex determination in stinging nettle (<Emphasis Type="Italic">Urtica dioica</Emphasis>)

Urtica dioica (“stinging nettle”) includes both dioecious and monoecious forms. In most sexually dimorphic angiosperm species, the genetic mechanisms of sex determination are completely unknown. The few species that include both monoecious and dioecious forms provide an unusual opportunity to examine the genetic mechanisms that underlie the separation of sexual functions, through crossing experiments and analysis of progeny segregation. Our focus is on the genetic mechanisms distinguishing monoecious and dioecious forms of U. dioica. A complicated picture of sex determination in this species has resulted from crosses between dioecious and monoecious subspecies, as well as between dioecious and monoecious forms of the same subspecies. Most significant is evidence for a maternal influence on sex determination and for the possibility of gynodioecy as an intermediate stage in the evolutionary pathway to dioecy. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

What was the ancestral sex‐determining mechanism in amniote vertebrates?

Amniote vertebrates, the group consisting of mammals and reptiles including birds, possess various mechanisms of sex determination. Under environmental sex determination (ESD), the sex of individuals depends on the environmental conditions occurring during their development and therefore there are no sexual differences present in their genotypes. Alternatively, through the mode of genotypic sex determination (GSD), sex is determined by a sex‐specific genotype, i.e. by the combination of sex chromosomes at various stages of differentiation at conception. As well as influencing sex determination, sex‐specific parts of genomes may, and often do, develop specific reproductive or ecological roles in their bearers. Accordingly, an individual with a mismatch between phenotypic (gonadal) and genotypic sex, for example an individual sex‐reversed by environmental effects, should have a lower fitness due to the lack of specialized, sex‐specific parts of their genome. In this case, evolutionary transitions from GSD to ESD should be less likely than transitions in the opposite direction. This prediction contrasts with the view that GSD was the ancestral sex‐determining mechanism for amniote vertebrates. Ancestral GSD would require several transitions from GSD to ESD associated with an independent dedifferentiation of sex chromosomes, at least in the ancestors of crocodiles, turtles, and lepidosaurs (tuataras and squamate reptiles). In this review, we argue that the alternative theory postulating ESD as ancestral in amniotes is more parsimonious and is largely concordant with the theoretical expectations and current knowledge of the phylogenetic distribution and homology of sex‐determining mechanisms.     

Patterns of growth and sexual size dimorphism in two species of box turtles with environmental sex determination

An adaptive explanation for environmental sex determination is that it promotes sexual size dimorphism when larger size benefits one sex more than the other. That is, if growth rates are determined by environment during development, then it is beneficial to match developmental environment to the sex that benefits more from larger size. However, larger size may also be a consequence of larger size at hatching or growing for a longer time, i.e., delayed age at first reproduction. Therefore, the adaptive significance of sexual size dimorphism and environmental sex determination can only be interpreted within the context of both growth and maturation. In addition, in those animals that continue to grow after maturation, sexual size dimorphism at age of first reproduction could differ from sexual size dimorphism at later ages as growth competes for energy with reproduction and maintenance. I compared growth using annuli on carapace scales in two species of box turtles (Terrapene carolina and T. ornata) that have similar patterns of environmental sex determination but, reportedly, have different patterns of sexual size dimorphism. In the populations I studied, sexual size dimorphism was in the same direction in both species; adult females were, on average, larger than adult males. This was due in part to males maturing earlier and therefore at smaller sizes than females. In spite of similar patterns of environmental sex determination, patterns of growth differed between the species. In T. carolina, males grew faster than females as juveniles but females had the larger asymptotic size. In T. ornata, males and females grew at similar rates and had similar asymptotic sizes. Sexual size dimorphism was greatest at maturation because, although males matured younger and smaller, they grew more as adults. There was, therefore, no consistent pattern of faster growth for females that may be ascribed to developmental temperature. Received: 20 March 1996 / Accepted: 10 March 1998