Since the 1960s, there has been a suspicion among evolutionary biologists that animals can influence the sex of their offspring. According to this theory, mammals manipulate the sex of their offspring in order to maximise their offspring’s future reproductive success.
The hypothesis was supported by a 1984 study, reported in the journal Nature, where a researcher, TH Clutton-Brock from University of Cambridge, found that among wild red deer, dominant mothers produced significantly more sons than those deer that held a subordinate position within the herd. According to the researcher, this was so because parents who were healthy or dominant or had other such traits invested more in producing sons. The inherited strength and health of the sons would stand them in better stead while competing with other males for mates. In comparison, less fortunate parents preferred producing more daughters, since they select mates instead of competing for them. However, the study encompassed only two generations of wild red deer. Hence, it was unable to show whether those deer that produced more sons also gained more grandchildren.
A new study, published in the journal PLOS ONE, which tracked three generations of a number of mammals, found the hypothesis to be valid. The results came from analysing about 90 years’ worth of records for 40,000 mammals, ranging from primates to rhinoceroses, at the San Diego Zoo. The researchers found that females that produced the most number of males went on to have up to 2.7 times the number of grandchildren from those sons in comparison to those mammals that had even numbers of male and female offspring.
Nobody knows for certain how mammals manipulate the sex of their offspring. According to the authors of the study, it is possible that the mother is able to control the levels of glucose or fatty acids in the bloodstream, higher levels of which may make conditions hostile for female embryos. The researchers claim that it is the mothers, in all likelihood, who are determining the sex of their offspring.