When it comes to the endangered Northern right whale, it seems mother not only knows best, she is best for the species' survival. A study published in the current issue of Nature suggests that saving just two female right whales a year would stop the species' slide toward extinction.

The Northern right whale faced extinction once before, around 1900, as a result of hunting. A subsequent ban on commercial fishing allowed the animals to recover, but since 1990 their numbers have dwindled to the current estimate of only 300 whales. To determine the cause of this decline, Masami Fujiwara and Hal Caswell of the Woods Hole Oceanographic Institution studied more than 10,000 whale sightings since 1980. The whales' distinctive markings allow scientists to recognize individuals and track their movements.

The researchers determined that increased mortality of mother whales is behind the species' diminishing numbers. Females take more than 10 years to reach sexual maturity and usually produce only one calf every three to five years. In 1980 the life expectancy of a female whale was 50 years, but by 1995 it was less than 15 years. Saving just two females a year, the study found, could stop the population decline. This finding is in contrast to previous suggestions that the population was declining because females couldn't find suitable mates.

The new analysis makes the conservation value of single whales compellingly clear, writes Peter Kareiva of The Nature Conservancy in an accompanying commentary in the same issue. "Fujiwara and Caswell," he notes, "have shown us the importance of highlighting what might at first glance seem like insignificant numbers of deaths."