Final answer:
The primary sex ratio in mammalian populations is often weighted toward the birth of males, with factors like infant mortality and societal influences affecting these ratios. Accurate knowledge of sex determination is also crucial, with XX denoting females and XY males in mammals, different from birds. Sexual dimorphism and reproductive success further impact physical and behavioral traits in species.
Step-by-step explanation:
In most mammalian populations, the primary sex ratio at birth is often weighted toward males. This is due to a variety of biological, environmental, and societal factors. In the case of humans, for example, about 105 boys are born for every 100 girls in the United States, and similarly skewed ratios can be observed in other species. However, due to factors such as higher infant mortality rates for boys, more deaths from violence among young men, and heart and lung disease in older men, the overall number of women might slightly outnumber men in certain populations, such as in the United States.
Societal and cultural practices can further influence the sex ratio. For instance, in some regions, economic opportunities or government policies have led to an imbalanced sex ratio. Notorious examples include China's former One-Child Policy, which, along with traditional preferences for male children, resulted in fewer girls being born, and parts of India with dowry customs that favor the birth of boys over girls.
It is also important to accurately understand sex determination in mammals and birds. In mammals, sex is determined by the presence of XX chromosomes in females and XY in males, contrasting with birds where the ZZ chromosomes are found in males and ZW in females.
In discussing sexual dimorphism and reproductive strategies, it is evident that males and females of certain species exhibit distinct differences in physical characteristics and behaviors. These differences are often driven by the variance in reproductive success among males, leading to the evolution of traits like larger body sizes or more elaborate ornaments to attract females.