When pups suckle during pregnancy, the proportion of males in the next litter is higher than when there are no suckling pups present. Size of litter. Super Mom S ome mechanisms for sex ratio biasing may originate in the generation prior to the one that needs them. A former NSF grantee John Vandenbergh of North Carolina State University found that sex ratio biasing in house mice is influenced by the experiences of the mother herself in utero . In most animals, humans included, the female brain is the basic model.
Males develop their own distinct brain circuitry only after being bathed with testosterone.
Females surrounded in the womb by male siblings also get hit by testosterone. Masculinized females also have more sons, relative to daughters, than do other females. But when populations are large and food is scarce, the advantage goes to mice ready to fight for food, and the masculinized female comes out on top. She has learned that top-ranking female baboons have first choice of best foods and reproduce at the youngest ages. For females, rank and its privileges are crucial.