Dispersal and the sex ratio at birth in primates

The females of many species of primates settle for life within the home ranges of their mothers, whereas males disperse as immatures. According to the theory of sex allocation, the costs incurred by mothers through local competition for resources with their philopatric daughters should favour the evolution of male-biased sex ratios at birth. I report here two tests of this hypothesis based on data from 15 genera of primates. First, I show that the intensity of competition for resources within kin groups is strongly and positively correlated with sex ratios at birth. Second, I show that sex ratios at birth are higher in genera with female-biased philopatry than in genera in which philopatry is not female-biased. These analyses suggest that local resource competition among kin powerfully influences the evolution of sex ratios in primates.     

安徽省出生人口性别比失衡的现状及对策——基于第六次全国人口普查数据

自二十世纪八十年代中国政府实行计划生育政策后,安徽省出生人口性别比就开始出现失衡现象,这是多种因素综合产生的结果。它造成了人口结构的不合理,同时还严重影响婚姻市场的配对、人口再生产以及家庭和社会的和谐。必须采取加快经济发展步伐、建立健全社会保障体系、运用社会公共政策等措施来影响人们生育观念的转变,弱化人们的性别偏好,使出生性别比失衡从根本上得到解决。     

The optimal sex ratio in cooperatively breeding populations

In some cooperatively breeding species, elder siblings remain within their home ranges to assist their parents in raising their younger siblings by providing protection or food. Previous attempts to model the popu- lation sex ratios of such species have assumed that helping is cost-free and only involves benefits to parents, resulting in offspring sex ratio biases towards the helping sex. However, parents may also incur costs in maintaining helpers. We incorporated costs and benefits of helping to predict the sex ratio of offspring in a theoretical model. Our model showed that the evolutionary stable strategy （ESS） sex ratio strongly depends on three parameters： （1） the average number of helpers per female; （2） the benefits that a helper brings to its mother＇s offspring production （MOP）; and （3） the cost-benefit ratio of helping. When one sex of elder siblings provides help, the ESS sex ratio is biased towards the helping sex if MOP costs are less than the benefits, （i.e., the cost-benefit ratio 〈1）. However, the ESS sex ratio becomes biased towards the non-helping sex if MOP costs exceed the benefits （i.e., the cost-benefit ratio 〉1）. Additionally, our model shows that the ESS sex ratio becomes biased in favor of the more helpful sex when both male and female elder siblings provide help. These results explain why sex ratios may become biased towards the helping sex, as well as the non-helping sex, in some species.     

Population density affects sex ratio variation in red deer.

Many mammal populations show significant deviations from an equal sex ratio at birth, but these effects are notoriously inconsistent. This may be because more than one mechanism affects the sex ratio and the action of these mechanisms depends on environmental conditions. Here we show that the adaptive relationship between maternal dominance and offspring sex ratio previously demonstrated in red deer (Cervus elaphus), where dominant females produced more males, disappeared at high population density. The proportion of males born each year declined with increasing population density and with winter rainfall, both of which are environmental variables associated with nutritional stress during pregnancy. These changes in the sex ratio corresponded to reductions in fecundity, suggesting that they were caused by differential fetal loss. In contrast, the earlier association with maternal dominance is presumed to have been generated pre-implantation. The effects of one source of variation superseded the other within about two generations. Comparison with other ungulate studies indicates that positive associations between maternal quality and the proportion of male offspring born have only been documented in populations below carrying capacity.     

空间结构下性比率的稳定机制研究

分别利用基于个体的斑块占据模型（ IBM）和偶对近似方法（ PA）构造了一个空间结构下的两性集合种群模型,通过计算机仿真和数值模拟,研究了空间结构对性比率的稳定性的影响。结果表明：空间结构下由于Allee 效应的强作用,两性种群只有性比率维持稳定在1∶1附近时种群才能够续存;偶对近似方法可以很好的反映出元胞自动机的研究结果,这对进一步研究空间结构下两性集合种群的时空动态提供了有效的理论工具。     

An extremely luminous X-ray outburst at the birth of a supernova

Massive stars end their short lives in spectacular explosions--supernovae--that synthesize new elements and drive galaxy evolution. Historically, supernovae were discovered mainly through their 'delayed' optical light (some days after the burst of neutrinos that marks the actual event), preventing observations in the first moments following the explosion. As a result, the progenitors of some supernovae and the events leading up to their violent demise remain intensely debated. Here we report the serendipitous discovery of a supernova at the time of the explosion, marked by an extremely luminous X-ray outburst. We attribute the outburst to the 'break-out' of the supernova shock wave from the progenitor star, and show that the inferred rate of such events agrees with that of all core-collapse supernovae. We predict that future wide-field X-ray surveys will catch each year hundreds of supernovae in the act of exploding.