Within‐clutch variation in offspring sex determined by differences in sire body size: cryptic mate choice in the wild

Sexual selection theory predicts that paternal quality should drive female investment in progeny. We tested whether polyandrous female side‐blotched lizards, Uta stansburiana , would adjust within‐clutch progeny investment according to sire phenotypes. In two different years, polyandrous females selectively used sperm from larger sires to produce sons and used sperm from smaller sires to produce daughters. This cryptic sperm choice had significant effects on progeny survival to maturity that were consistent with sexually antagonistic effects associated with sire body size. Large sires produced sons with high viability and small sires produced daughters with high viability. These results are consistent with our previous findings that alleles for male body size have different fitness effects in male and female progeny. Breeding experiments in the laboratory indicate that results from the wild are more likely due to female choice than biased sperm production by males. Our results demonstrate highly refined gender‐specific female choice for sperm and indicate that sire body size may signal the quality of sons or daughters that a sire will produce.