Sex Hormones and Reproductive Status of the North Atlantic Fin Whales (<I>Balaenoptera physalus</I>) During the Feeding Season

Abstract

Reproductive fin whales (Balaenoptera physalus) (448 females and 278 males), classified by anatomical/histological methods, were studied for serum sex hormones. Of the 207 females classified as pregnant by anatomical methods, 95% had progesterone (P) levels higher than 9.0 nmol/l with a near symmetrical distribution of log10 P-levels around the mean of 1.55 (35.5 nmol/l geometric mean). More than half of the sexually immature females (n = 157) had P-levels £ 0.1 nmol/l. Nonpregnant mature cows were older on average than pregnant cows, suggesting the beginning of reproductive senescence in females before the age of 30 y. The mean serum testosterone (T) of mature males (3.1 nmol/l) was significantly higher than that of immature males (1.0 nmol/l). In mature males, T-levels were positively associated with testicular size, as well as time (daycount) during the summer whaling season. For fin whales, serum P- and T-levels agreed closely with anatomical studies of reproduction and may be decisive when anatomical indexes fail. Furthermore, the serum T-level appears to be an excellent index for monitoring the latter part of the annual male reproductive cycle.