PRENATAL BIAS IN SEX-RATIOS IN A MARSUPIAL, ANTECHINUS-AGILIS

Biased sex ratios of young of birds and mammals clearly occur and may have an adaptive significance, but we rarely know the stage at which a bias as generated, or the mechanism. If a bias is generated prior to birth, studies of marsupials may be insightful because gestation is sh ort and neonates are relatively undifferentiated. This study investiga ted whether biased sex ratios in Antechinus agilis are generated in th e brief period between birth and the attachment of young to the mother 's teats. When all young born, or just pouch young, or supernumerary y oung were considered, litters were strongly biased towards females (0. 32 males), and there was no significant difference across the groups, so a bias is generated before birth in this species. Evidence from cou nts of corpora lutea suggests that embryo loss during gestation cannot account fully for the level of bias observed. Therefore, prefertiliza tion mechanisms must contribute to the generation of sex-biased litter s in this marsupial.