NOAA National Marine Fisheries Service, Northeast Fisheries Science Center.

There is some debate as to whether the sperm whale has a polygynous or a promiscuous mating system, although most would probably now agree that they have overlap promiscuity (Whitehead, 2003).  Older research describes sperm whales in harem groups. Ohsumi, a Japanese Cetologist, proposes that sperm whale nursery groups of females and their offspring become harem groups when the males join them in the mating season.  Dominant mature males compete for possession of the harem as evidenced by male-male fights (Matthews, 1974).Characterized by tooth raking, violent head butting, and biting and locking of the jaws, this male-male competition associates the sperm whale with a “strongly polygynous mating system” (Herman and Tavolga, 1980). However, more recent research has confirmed that male sperm whales only spend a few hours with each group before they move on.  Males have been seen with the same group more than once, but this is usually because they encountered the group again by chance.  There have been observations of one male with the same female but only over a matter of days.  In instances where this was observed, females tended to dive before males, suggesting that the male is seeking after the female. Some of the largest solitary males may not mate at all. Also, two males can be with a group of females at one time (Whitehead, 2003).  Whalers are more likely to kill a large male which reduces the number of available mates for females.  As a result, the reduction in conception has led to a decrease in female fertility (Coakes and Whitehead, 2004).  Males will return to their original clan or go to new ones for breeding.  The preference to choose one clan over another is due to either males being more attracted to a particular clan or females from a particular clan choosing those males (Marcoux, 2005).

For sperm whales off the coast of Chile, the months between June and December seem to be the mating season (Coakes and Whitehead, 2004).  The gestation period is between 14 and 16 months and lactation lasts for 24 months. The typical size for a newborn calf is 4 meters and 1 ton.  Calves are precocial, meaning they are fairly developed and can swim effectively within a few hours of birth.  Calves spend most of their energy keeping up with their mother and suckling. Sexual maturity for males occurs after 10 years and for females after 8 to 11 years.  When a female is sexually mature, she gives birth at a rate of about 1 calf every 4 to 6 years and then the birth rate drops to about 1 every 15 years as older females spend their energy assisting younger members of their unit (Whitehead, 2003). Sperm whales reach full size at about 25 years of age (Minasian et al, 1984).  Males do not participate in the care of the offspring but other females act as babysitters and there are more lactating females to allow for communal nursing (Connor, 2002).  Females also form protective circles known as marguerite formations around their young to protect them from predators. They either put their heads or flukes facing outward to utilize the strength of their flukes or the sharpness of their jaws (Weilgard and Whitehead, 2001).  Predation, especially from killer whales, is most likely the cause of female and offspring groups because predation pressure is a constant threat and groups are not needed to increase their ability to obtain resources.  Because this seems to be the primary purpose of these groups, the mating system probably evolved as a result of this structure (Coakes and Whitehead, 2004).  Since females do not need help from males, thanks to communal nursing and babysitting, there is no reason for males to invest time bonding with females.  Males benefit from promiscuity because they can mate with whomever they want and do not have to take responsibility for their offspring.