Brothers delay menarche and the onset of sexual activity in their sisters

• Published in: Proceedings of the Royal Society. B, Biological sciences Biological sciences, 2011-02, Vol.278 (1704), p.417-423
• Main Authors: Milne, Fritha H., Judge, Debra S.
• Format: Article
• Language: English
• Subjects: Adolescent; Adult; Age Factors; Aged; Birth Order; Brothers; Children; Cohort Studies; Costs Of Brothers; Daughters; Female; Helper-At-The-Nest; Humans; Male; Menarche; Menarche - physiology; Menarche - psychology; Middle Aged; Mothers; Onset Of Sexual Activity; Pregnancy; Proportional Hazards Models; Reproduction; Reproductive success; Retrospective Studies; Sexual Behavior - physiology; Sexual Behavior - psychology; Sexual intercourse; Siblings; Sisters; Sons; Western Australia; Young Adult
• ISSN: 0962-8452; 1471-2945; 1471-2954
• DOI: 10.1098/rspb.2010.1377

The higher costs of sons compared with daughters extends to a negative effect of brothers on the lifetime reproductive success of their siblings in subsistence and preindustrial societies. In societies with fewer resource constraints, one might expect that these effects would be limited or non-existent. This study investigates the costs of brothers and sisters in a contemporary western society of adult Australians. Girls with elder brothers had a delayed age at menarche. Younger brothers were associated with delayed onset of sexual activity in sisters, but not in brothers. Neither younger nor elder brothers influenced fitness parameters (number of pregnancies, number of children, age at first pregnancy or age at first birth) in siblings of either sex. This study provides evidence that brothers negatively affect their sisters' onset of reproductive maturity and sexual activity; however, this delay is not associated with a fitness cost in contemporary Australia. We suggest this is due to the long period of independence prior to child bearing.