The following are some differences between males and females related to sex ratios at conception, birth and later in life; rapidity of development, and some physiological features at birth and later in life, including puberty.
Research based on aborted and miscarried human fetuses indicates that males are conceived at substantially higher rates than females, with the difference being as high as 160 to 100 at conception, and around 130 to 100 by the end of the first trimester. In animals where there is reliable data, for hamsters this ratio is 180 to 100, and 122 to 100 for rabbits.
When it comes to sex at birth, almost all studies indicate that the number of males born is around 103-106 compared to every 100 females.
Most studies show that pre-birth and birth complications are more common among males.
Cases of more females born than males have been associated with the exposure of one or both parents to toxic substances or environmental pollutants during the pregnancy or at the time of birth. The hypothesis is that male fetuses are more vulnerable to miscarriage the further away they are from optimal pregnancy conditions.
It is interesting to note that while sex determination in mammals seems to be genetic, in some other animals, environmental factors seem to have a high influence on the sex of the animal. When it gets too hot or too cold during the incubation period, the hatchlings of amphibians are mostly females, while at optimal temperatures, males are hatched at greatest numbers.
Sex ratios are virtually equal in early adulthood, but given higher rates of death throughout childhood, adolescence, and adulthood in males, ratios increasingly favour females from early adulthood.
Across all species studied including humans, males average greater weight and length at delivery. Two studies in the Andes and Bolivia on persistent oxygen deprivation among mothers showed male fetuses being more affected and seeing their weight drop more than female fetuses. Males also exhibit greater weight and length variability than females at birth.
Human sub-adolescent females mature more rapidly (skeletal growth) than do males and reach behavioural milestones earlier, such as walking and talking.
Most studies on the onset of puberty in humans indicate that females reach puberty (menarche or first menstruation, pubic hair) 1,5 years before males (lowering of the voice, pubic hair), and growth spurts associated with puberty also take place earlier in females.
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