By Colleen Bryan, April Cook and Sarah Geoffrion

Three question come to mind when migration occurs in animal populations. Scientist have tried to determine why mysticetes migrate?; how do they migrate?; and why do some have significantly shorter or no migration patterns at all?

Mysticetes migrate for several reasons including mating, calving, feeding, and heat maintenance. Above all, however, one thing must be maintained; the benefits must outweigh the costs.

Mysticete females have synchronized two-year reproductive cycles. At about the same time every year they will begin a migration from the colder feeding grounds to the warmer calving areas. This migration is timed so that the animals will be in the calving grounds for the duration of the winter. The first females to leave the feeding areas will be the females who have already weaned their calves and are ready to become pregnant again. The newly pregnant females will be the last to leave these feeding areas. Regardless of their departure time, however, all of the females will arrive in the calving grounds within a month of each other (Baker, 1980).

Male Mysticetes will also migrate to the warmer calving grounds at the same time as the females. During this migration the males show no real signs of aggression. Instead, they use songs to space themselves throughout the journey. The males, as well as, the females will fast throughout the winter while in the calving grounds and for a portion of their migrations (Baker, 1980). For this reason any activity, such as aggression, that would tax their energy reserves is considered wasteful and is avoided. Once again, the benefits must outweigh the costs.

Once the Mysticetes reach the warm calving grounds the females about to give birth will find inland lagoons or near-shore bays that will provide a safe environment for, as well as, a good place to teach their newly born young. The remainder of the animals will aggregate along the perimeter of these areas and perform their mating activities.

Once the calves are born they will begin nursing. Calves suffer greater heat loss than adults as a result of smaller size and hence greater surface to volume ratio. Therefore, it is essential that their mother's milk has a very high fat content which promotes the formation of blubber. The warm water also aids in this process because the calves can devote a greater amount of energy to growth rather than maintaining body temperature against an extreme thermal gradient, such as that which would be experienced in the colder feeding areas (Dingle, 1980). This thick layer of blubber will be essential for the calf to survive the migration to the cold summer feeding areas.

Heat maintenance is an important part of the migration for the adults, as well as, the calves. The reduction in heat loss must more than compensate for the energetic cost of the migration itself. Or, more simply put, the benefits must outweigh the costs. The large extremities of these migrating Mysticetes have a large surface area. This amount of surface area helps keep the whales from overheating in the warm water. In the colder feeding areas the animals are able to restrict the blood flow from these extremities to retain heat within the body (Dingle, 1980). This process helps to ensure that all of their important internal organs stay warm.

When the winter comes to an end the Mysticetes will begin the migration to the summer feeding areas. During the summer the higher latitudes experience longer days which produce more phytoplankton and zooplankton. The amphipods which eat the plankton will then grow in abundance as well, and hence provide a greater food supply for the whales. The newly pregnant females will be the first to leave the breeding grounds because they want to spend the most time in the feeding grounds building up their energy reserves. The lactating females will be the last to leave the breeding area with their calves. They will make a slower progression towards the feeding areas (Winn, 1985). Once having reached the feeding areas the whales will concentrate where the largest abundance of krill lies. The males and the females will go their own ways during this time in order to search for food.

There are several factors that contribute to how and where a mysticete migrates to annually. The four main reasons that they know how to migrate are photoperiods, spyhopping, navigation and genetic inclinations.

Photoperiods are the seasonal changes in day length. As winter approaches days become shorter and water temperatures begin to cool. This triggers the whales to start their migration to lower latitudes. The changes in day length generally constricts whales of the same species, but from different populations to stay in that hemisphere. This does not mean that they do not occasionally cross the equator and sometimes they will transfer from a northern population to a southern population (Matthews 1978).

Spyhopping a behavioral activity of mysticetes that is exceptionally useful in migration. The whale pushes itself vertically out of the water so that its eyes are out of the water. This activity lasts approximately 15-30 seconds. While their eyes are out of the water the whale is able to look around like a periscope. Spyhopping is useful in identifying landmarks and using the sun as an orientation cue (Gauthreaux 1980). Grey whales and humpback whales are thought to have a coastal migration pattern so that can follow the coastline and recognize landmarks from year to year (Matthews 1978).

Navigation is a tool that all animals rely on in order to migrate. Mysticetes have few different ways that they can navigate. If they are not the first to leave on the migration route, they can heavily rely on direct sound signals from other mysticetes. The whales that are in their population ahead of them can send sound signals to those whales that are behind to indicate their location. This is possible through low frequency soundwaves that are able to travel miles (Matthews 1978). Biomagnetism is also thought to possibly be a navigational instrument. The magnetite in the whales head is able to help direct the whale by the earth's magnetic bands (Baker 1918). Underwater topography is also useful for mysticetes in remembering physical features such and trenches and peaks.

Genetic inclination is another major way that mysticetes know how to migrate. Migration is thought to be built to the genetic code. Just as the genetic code builds a humpback whale, it also tells the whale migrate. Even though migration may be built into the genetic code, it is an innate behavior that needs to be stimulated either externally or internally. Internally it may be stimulated by hormone levels or degree of blubber thickness. Externally it can be stimulated social by their mother as a learned behavior. Normally different gene pools lead to different migratory destinations (Matthews 1978).

There are two species of Mysticetes that do not make long migrations between high and low latitudes. These two species are the Bowhead whale and the Bryde's whale.

The Bowhead whale (Balaena mysticetus) is named for its bow shaped head. They are extremely large, robust whales that swim at a very slow speed of 2-4 mph. They grow to a maximum length of 60 ft. and weigh up to 120,000 lbs. (Carroll 1994). The females are larger than the males. The distribution of Bowheads was nearly circumpolar until they were heavily hunted and are now endangered species with only five populations remaining (Discovering Whales, Bowhead 1999).

Bowheads are very well adapted to the arctic and spend their entire lives near the ice. Like all whales, a layer of blubber insulates Bowheads. Their special adaptation to the extreme cold is that their layer of blubber is 1-2 ft. thick. They also have a very heavy bone structure in their skull to break through ice up to 2 ft. thick in order to breathe. They have no dorsal fin that enables them to swim under the ice for food or to escape from predators. Their only natural predator, other than humans, is the killer whale. Bowhead whales feed all year round on copepods, amphipods, and euphausiids. These small planktonic creatures are especially abundant in the north during the summer due to the extended photoperiod and phytoplankton blooms (American Cetacean Society 1996).

The Bowheads mate during late spring/early summer and their gestation period is about ten months. They only calve every 2-3 years. Their calves are born shorter and heavier than most other baleen whales with an extra thick layer of blubber. Bowhead whales do not migrate to breed, feed, or calve. They do migrate with the advancing and retreating ice always keeping close to the ice edge (Carroll 1994). If Bowheads tried to migrate south, they would overheat from their thick blubber and small surface area to volume ratio.

The second species of Mysticetes that does not migrate is the Bryde's whale (Balaenoptera edeni). These whales are distinguishable by their three longitudinal grooves on top of their head. They are rorqual whales with 40-70 throat grooves. They grow to be about 46 ft. long and weigh 26 tons. The Bryde's whales prefer warm water above 20^o Celsius. They are found in tropical and subtropical waters of all seas between 40^o N and 40^o S. Their populations are basically sedentary migrating only very short distances with no known long-distance migrations to higher latitudes. They mate and breed all year long and their lack of migrations is due to their feeding habits (Discovering whales, Bryde's whale 1999). Unlike many Mysticetes the Bryde's whale feeds on larger fish that are always abundant in the tropical waters.

The difference in migration patterns among Mysticetes is due to availability or preference of food, protection of calves, and special adaptations to their environments. If the benefits out way the costs, it is in the whales best interest to migrate.

American Cetacean Society Fact Sheet. 1996. Bowhead Whale. http://www.acsonline.org/.

Baker, R. 1981. The Mystery of Migration. New York: Viking Press.

Carroll, G. 1994. Bowhead Whales. http://www.state.ak.us//oc…s/FISH.GAME/notebook/marine/bowhead.

Discovering Whales. 11/10/99. Bryde's Whale. http://whales.magna.com.au/DISCOVER/BRYDES/brydes.

Discovering Whales. 10/13/99. Bowhead Whale. http://whales.magna.com.au/DISCOVER/BOWHEAD/bowg.

Gauthreaux, Sindney A. 1980. Animal Migration, Orientation, and Navigation. New York: Academic Press.

Matthews, L. H. 1978. The Natural History of the Whale. New York: Columbia University Press.

Winn, L. K. and Winn, H. E. 1985. Wings in the Sea the Humpback Whale. Hanover, RI: University Press of New England.

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