Table 1.

Examples of evidence in mammals for higher cost of sons over daughters.

species	nature of cost demonstrated	study
bighorn sheep, Ovis canadensis	ewes had higher faecal lungworm count after producing a son	Festa-Bianchet (1989)
	ewes experienced a delay in return to oestrus after producing a son	Hogg et al. (1992)
	ewes more likely to have a daughter after producing a son	Bérubé et al. (1996)
	ewes produced lighter offspring after producing a son	Bérubé et al. (1996)
red deer, Cervus elaphus	hinds less likely to reproduce in the year following a son; delayed calving in those who did	Clutton-Brock et al. (1981)
	lower post-reproductive maternal survival after weaning a son (subordinate hinds only)	Gomendio et al. (1990)
	milk composition dependent on sex, e.g. higher protein percentage in milk provisioned for sons	Landete-Castillejos et al. (2004)
fallow deer, Dama dama	lower maternal weight accumulation during the period after gestation of offspring, when a son is produced (old hinds only)	Birgersson (1998)
humans	birth interval longer after the birth of a son (low-parity women only)	Mace & Sear (1997)
	maternal longevity associated negatively with the number of sons produced	Helle et al. (2002)
	lower birth weight of offspring produced after sons	Trotnow et al. (1976); Magnus et al. (1985); Blanchard & Ellis (2001); Côté et al. (2003)
	higher maternal energy intake in women carrying sons	Tamimi et al. (2003)