Sex ratio and litter size in the guinea-pig.

A significant relationship between sex ratio and litter size at birth was observed in the young of guinea-pigs caged singly from 8 weeks of age and during their first pregnancy; a male was present only at or around oestrus. Small litters (1-2 young) had significantly more males than did large litters (4-5 young). Such a relationship was not apparent in animals housed communally, whether young or older or in their second pregnancy. Litters in late pregnancy (day 63) or at delivery were significantly smaller than at mid-pregnancy (day 35). A significant negative relationship was found between litter size and bodymass at birth and to at least 63 days of age. Growth rates during the period of lactation were lower in animals born in large litters. The possible adaptive significance of a plastic relationship between litter size and sex ratio is considered together with the physiological mechanism that might be involved.