Answer:
Explanation:
The real issue is not why most males have more blood than most females, which is easily answered (as others have already done) based on differences in body size. The real issue is why males have more higher blood values relative to body size than females do. Here, things get more interesting—and it’s not just the overall quantity of blood, but also hemoglobin level, red blood cell count, and hematocrit that are higher in men. Reasons include:
(1) These values are inversely proportional to body fat, and females have a higher percentage of body fat than males (an adaptation to preparedness for pregnancy and nursing).
(2) In their reproductive years, women periodically lose blood by menstruation and are constantly making up the loss, so on average, these values are lowered a by by those losses.
(3) Testosterone stimulates hemoglobin synthesis and red blood cell production, and men have much higher testosterone levels than women do. Men also have faster clotting times than women do. This is thought to be an evolutionary adaptation to the fact that throughout the vertebrates, males are more aggressive than females on average, with greater incidence of fighting and risk of injury and bleeding. There are a few species with sex-role reversal where females have higher testosterone levels (and are more aggressive) than males, and there the females of some have higher blood values than the males do.
