VARIATION IN FETAL AND POSTNATAL SEX-RATIOS IN LONG-FINNED PILOT WHALES

Variation in foetal and postnatal sex ratios was examined jointly with the 1986-1988 Faroese international research programme on the ecology and status of the long-finned pilot whale (Globicephala melas). Data were obtained from 58 schools of whales landed in the Faroe Islands fr om 1958 to 1992. The sample includes 505 embryos and foetuses. Variati on in foetal sex ratios was analysed according to gestational age, mot her's age and years. Foetal and postnatal sex ratios from pilot whales off the Faroe Islands also were compared with those obtained off Newf oundland by Sergeant (1962). The overall foetal sex ratio was biased s ignificantly towards more females than males. The proportion of male f oetuses declined as the size of foetuses increased. Females older than 25 yrs of age in comparison to younger females bore more female than male foetuses. From birth, through the three first years of life, sele ction acted mainly against females and parity was observed again in th e age group 3 and maintained until the age group 10. Annual variation may occur. For similar season, at a 30-year interval, foetal sex ratio differed significantly between the Faroes and Newfoundland (46.9% and 58.8%, respectively, more male foetuses). The relative higher mortali ty of male than female foetuses suggest that a significant overall foe tal mortality occurs, which has to be taken into account when estimati ng fecundity.