A puzzle for many biologists is the occurrence of abnormal sex ratios (those that deviate from ~1:1) at birth in populations with chromosomally-determined sex. Consider the hippopotamuses from the previous page. Given what we know about meiosis, we’d expect half of the hippos conceived to have XY chromosomes (of the male sex) at birth, and half of the hippos to be XX (female) at birth. However, in captive populations of the endangered pygmy hippopotamus, only 41% of the offspring are male. This is a significant deviation from 50%, or from a 1:1 ratio. Studies of the sperm of captive pygmy hippopotamuses have indicated that the lowered sex ratio is due to males biasing X-bearing sperm in their ejaculates, although the mechanism through which that occurs is unknown.
Many other examples of skewed sex ratios, in organisms in which biological sex is determined chromosomally, have been documented. Evolutionary biologists have suggested that selection operates in a way to bias the production of offspring of the more advantageous sex. In some cases, the advantageous sex may be male, in others, it may be female. In the case of the captive pygmy hippos, scientists have suggested that—in the captive environment—the perception of a high population density leads to the production of fewer males. Fewer males means fewer rivals for territory. The following is from an article by Joseph Saragusty and colleagues: