Sexual Dimorphism, the Big-Mother Hypothesis, and Analyses of Hormones, Minerals, and Stable Isotopes to Determine Reproductive Events in Bowhead Whales (Balaena mysticetus)

Abstract

Baleen from bowhead whales (Balaena mysticetus) provided an opportunity to study reproduction and sexual dimorphism via analyses of hormones, minerals, and stable-isotopes. Concentration of progesterone in baleen was significantly different in nonpregnant and pregnant females, and correlated with size of fetus in pregnant females. Analyses of progesterone of serial samples (every 1 cm) along the length of baleen from a sexually-mature (16.9 m in length), nonpregnant female were conducted to determine short-term fluctuations of progesterone. Preliminary results indicated that reproductive events could be estimated using concentrations of progesterone in baleen obtained from sexually mature females. Stable-isotope analyses conducted previously were used as a timeline in conjunction with analyses of progesterone to estimate frequency of pregnancy in 15 mature females. Correlation between length of body and estimates of frequency of pregnancy did not support the “big mother” hypothesis. A stronger relationship between age and estimates of frequency of pregnancy indicated that age appears to be a greater contributing factor than length of body in determining frequency of pregnancy. Using data from historical whaling records, comparisons of length of body between males and females of several species of mysticete whales did not support Rensch’s Rule.