Why Head South?

Does it surprise you that the whales would go all the way from Alaska to Mexico when they aren't going to give birth? Why do they go if they're too young to mate? Why are a few stragglers still heading south, when many whales are already going back north? We can only guess. Two experts tell what they think:

Michael H. Smith, Project Director of Gray Whales Count:

Why do they do it? For some, the answers appear easy: food (northbound whales) and mating or birthing (southbound whales). But what about those going south who are too young to mate? My guess is that it's about gaining experience with the group: learning what is need for survival, learning for themselves why they go.

Dr. David Rugh, Wildlife Biologist, National Marine Mammal Laboratory

I believe that evolution has favored gray whales thatmigrated to Baja California for the winter; those that did not bother to travel had a higher probability of being selected out of the gene pool. Gray whales feed on the shallow ocean shelves west of Alaska in the summer. But winter's darkness lowers biological productivity, so there is no real advantage to staying there. Also, the Chukchi and Bering Seas are more or less iced over, making it best for the whales to leave.

As long as the whales are not feeding, they may as well be traveling because they have to swim to the surface to breathe anyway. By going to Baja, they can spend time in warmer waters, lowering thermoenergetic demands, especially for the calves. So the lagoons are useful not only as a nursery, but also as a good resting place for the adults.

We suspect that there is an inconsistent percentage of the population that completes the migration each year. That is why we sometimes have unexpectedly low abundance estimates followed by a year with higher numbers. Hundreds or even a few thousand whales may dally around the Gulf of Alaska or along the Pacific coast north of California, so they don't get counted at our site near Granite Canyon, in central California. Most conceptions happen in late November, which is at the start of the southbound migration; but if a female does not conceive then, she is given a second chance some 40 days later, or at the south end of the migration. This means there is some advantage for males old enough for breeding to follow through with a full migration.

The migration is long and dangerous, but in some ways it may be more dangerous to stay put in one area. Killer whales will eventually take advantage of the predictability of resident whales. Although they do now take advantage of the predictable migration, it is a somewhat random selection from the whole population, whereas a resident group might be taken away totally. Also, the way gray whales feed has a big impact on the substrate, or ocean bottom. Their bottom-feeding means they need to be nomadic to avoid completely wasting a local area."

Try This! Journal Question
  • What are two reasons why a long migration could be less risky than staying in one place?
  • Were the experts' thoughts different than you expected? What questions do you have now? How could you find answers?