Social and reproductive biology of North Atlantic humpback whales (Megaptera novaeangliae)
The social and reproductive biology of individually identified humpback whales, Megaptera novaeangliae , was studied in the Gulf of Maine (a summer feeding ground), and in Samana Bay, Dominican Republic (a winter mating and calving area). In the Gulf of Maine, humpback whales showed high rates of seasonal occurrence and annual return, strongly suggesting that this species exhibits foraging area philopatry. The mean crude birth rate was 0.083; an alternative measured was 0.42 calves per mature female per year. Eighty-four of 96 unbroken calving intervals were of two or three years (mean &'61 2.3 years). Separation between mother and calf occurred after one year in 96&'37 of cases. Resightings of independent calves indicate that the composition of feeding stocks is determined matrilineally. The mean age at sexual maturity (determined from observations of calves born to 12 females of known age) was five years; observations from Samana Bay suggest that males attain maturity at this age also. The sex ratio of the offspring of female humpback whales in superior condition (inferred from calving interval) was biased towards sons (the sex with the greater variance in reproductive success). Social organization was characterized by small, unstable groups, with stable associations and associations between relatives being rare, territoriality and dominance were apparently absent. Immature whales become socially adult at sexual maturity. In Samana Bay, molecular analysis revealed that no competitive group contained more than one female, although several consisted entirely of males, perhaps reflecting dominance sorting. Observations of aggressive females may represent repulsion of subadult males. Males may occasionally form coalitions. The occurrence of whales from different high-latitude feeding grounds in these groups suggests that the North Atlantic population is panmictic.