North Atlantic right whale reproduction is characterized by a low reproduction rate, great annual variation in the number of calves produced, and the number of adult females that have never been known to give birth. The possible factors responsible for these features could include the low genetic variation found in this very small population, climatic or oceanographic changes that affect the food availability, and the unknown effects of toxic contaminants or marine biotoxins.
Female right whales give birth to a single calf between December and March in the shallow coastal waters of the southeastern United States. The peak in calving occurs in January and February, by late March mother and calves migrate north to feeding areas in the Gulf of Maine and beyond.
Aerial view of right whale mother and calf in the southeast
United States calving ground.
Right whale calves, like other cetaceans, are born tail-first. They measure 4 to 5 m in length and weigh between 700 and 1,000 kg. For the first year, they suckle rich milk from two nipples hidden in slits along their mother's belly. Newborns gain 900 kg in a little more than a month and by the time they are weaned at about 11 months they are about 9 to 10 m in length and weigh over 12,000 kg. Weaning occurs after 10 to 12 months although some calves stay with their mother for up to 17 months 1. The mean age for a female's first calving is 10 years (range from 5 to 20 years for known aged females). The reproductive lifespan of a female right whale is believed to be at least 31 years. Sexual maturity in males is believed to be around the age of 10, if not earlier, but genetic data suggest that mating competition prevents young males from being successful at reproducing before at least the age of 15.
Researchers aboard the R/V Nereid in the Bay of Fundy are observing an impressive surface active group (SAG) of more than 45 different individuals.
Although the location of the breeding or mating ground for North Atlantic right whales is unknown, researchers believe mating takes place in the winter months during large courtship groups called Surface Active Groups or SAGs. Although SAGs are seen on the spring, summer and fall feeding grounds throughout the whales' range, it is unlikely that these SAGs result in conception since females give birth between December and early March after a gestation period of 12-13 months 2. Therefore, actual mating must take place from November through February. Females give birth after a year, suckle their young for another year then rest for a year to recover and restore their blubber layer after severe depletion from the large energetic demand of nursing a fast growing calf. Although right whales are capable of giving birth every three years, the calving interval for North Atlantic right whales ranges from one calf every 3.7 years 3 to over 5 years 4. This reproductive rate is lower than that for right whales in the southern hemisphere who calve on average every 3.1 years.
This 8 month old right whale calf was seen
in the Bay of Fundy nursery ground after
its 2250 km migration from the southeast
Courtship groups on the feeding grounds have been known to last up to 6 hours. They include as many as 50 animals of which only one or two are females 5. Courtship groups are believed to be initiated by a focal female whose calls attract the males 6. The female then makes mating difficult by swimming on her back or diving away from the group. Males compete to reach her, actively pushing others away. When the focal female rolls upright to breathe, a male will attempt to copulate with her although copulations can also occur at the surface with the female upside down. Since multiple copulations take place during courtship groups, it is speculated that sperm competition plays a role in right whale reproduction 7. Supporting that theory are the size of the male testes and penis: at >800 kg, the testes are the largest in the world and the penis is among the longest, up to 3 m. In both categories they are the largest relative to body size among baleen whales 8. Genetic studies have shown that female right whales mate and produce calves with several different partners during their reproductive lifespan.
As with other mammals, the mortality rate is
higher in newborns and juveniles than in adults.
This neonate was found stranded on a beach of
Florida. The cause of death is unknown.
Despite the protection measures from hunting in place since 1935, the North Atlantic right whale population is not showing significant signs of recovery 9, 10. Population growth rates have ranged from 2.5 percent per year in the early 1980s 11 to minus 2 percent per year in the late 1990s 12. These growth rates are lower than their close relative the southern right whale whose numbers are growing at 5-7 percent per year 13. The North Atlantic right whale is on a trajectory toward extinction in 200 years if human related mortalities are not reduced. The possible reasons for this slower recovery could be due to mortality and serious injury from interactions with human activities, low reproductive rate, low genetic variability, diseases, biotoxins, sub lethal effects of pollutants, and oceanographic or climatic changes which would affect the food supply. International recovery efforts are focused on reducing right whale mortality and serious injury from interactions with human activities i.e. vessel traffic and fishing.
1 Hamilton, P. K., M. K. Marx, and S. D. Kraus. 1995. Weaning in North Atlantic right whales. Marine Mammal Science 11(3):386-390.
2 Best, P. B. 1994. Seasonality of reproduction and the length of gestation in southern right whales, Eubalaena australis. Journal of Zoology 232:175-189.
3, 11 Knowlton, A. R., S. D. Kraus, and R. D. Kenney. 1994. Reproduction in North Atlantic right whales (Eubalaena glacialis). Canadian Journal of Zoology 72:1297-1305.
4 Kraus, S. D., P. K. Hamilton, R. D. Kenney, A. R. Knowlton, and C. K. Slay. 2001. Status and trends in reproduction of the North Atlantic right whale. Journal of Cetacean Research and Management Special Issue 2:231-236.
5 Kraus, S. D., and J. J. Hatch. 2001. Mating strategies in the North Atlantic right whale (Eubalaena glacialis). Journal of Cetacean Research and Management Special Issue 2:237-244.
6 Parks, S. E. 2003. Response of North Atlantic right whales (Eubalaena glacialis) to playback of calls recorded from surface active groups in both the North and South Atlantic. Marine Mammal Science 19:563-580.
7, 8 Brownell, R. L., and K. Ralls. 1986. Potential for sperm competition in baleen whales. Pages 97-112 in G. Donovan, ed. Behavior of Whales in Relation to Management. International Whaling Commission, Cambridge, UK.
9 Caswell, H., M. Fujiwara, and S. Brault. 1999. Declining survival probability threatens the North Atlantic right whale. Proceedings of the National Academy of Sciences 96:3308-3313.
10 Fujiwara, M., and H. Caswell. 2001. Demography of the endangered North Atlantic right whale. Nature 414:537-541.
12 IWC. 2001b. Report on the workshop on status and trends of western North Atlantic right whales. Pages 61-87 in P. B. Best, J. L. Bannister, R. L. Brownell, and G. P. Donovan, eds. Right Whales: Worldwide Status. International Whaling Commission, Cambridge, UK.
13 IWC. 2001a. Report on the workshop on the comprehensive assessment of right whales. Pages 1-60 in P.B. Best, J.L. Banister, R.L. Brownell, and G.P. Donovan, eds. Right Whales: Worldwide Status. International Whaling Commission, Cambridge, UK.