Michael Shirber in Live Science:
The tendency of some mothers to coddle their sons may be ingrained, at least in species where males compete for mates.
A recent study of Iberian red deer on a research farm has shown that mothers produce more milk of higher growth-potential for a male calf than a female one. The reason appears to be a genetic advantage, seeing as healthier, stronger males will mate more often – spreading the mother’s genes.
The researchers studied 60 female deer, which – over five years – produced 44 male and 47 female offspring. In collecting milk samples, the researchers took account of the weights of mother and fawn in their analysis.
In equal circumstances, sons were not only given more milk, but the milk was also three percent higher in protein concentration than what females were given.
More here.