PARENTAL INVESTMENT, SIZE SEXUAL DIMORPHISM, AND SEX-RATIO IN THE RIFLEMAN (ACANTHISITTA-CHLORIS)

R.A. Fisher in 1958 related primary sex ratio to parental investment i n young by predicting that, if one sex was larger, then the higher inv estment in that sex would be offset by an adjustment in the primary se x ratio in favour of the smaller sex. Larger females in the rifleman ( Acanthisitta chloris) provided a prima facae test case for Fisher's th eory. While females seemed most costly to rear, there was no significa nt difference in survival or in numbers that bred in the years followi ng fledging. Parent survival was not affected by rearing more females than males. The sex ratio of both chicks and unhatched eggs was near u nity. I conclude that Fisher's theory is not sufficient explanation of the relationship between size sexual dimorphism, primary sex ratio, a nd the value of each sex to parents.