Elephant seals, both northern and southern, spend most of the year foraging alone on the high seas. Each year during the breeding season, they arrive at their breeding sites in the northern and southern hemispheres within a few days to give birth to their young shortly thereafter. This raises the question of how the seals know when it is time to leave for the breeding grounds. A California research team came closer to finding the answer in a new study of northern elephant seals (Mirounga angustirostris) published in the journal Current Biology.
Like many other marine animals, elephant seals migrate between their feeding grounds and breeding sites year after year. They cover very long distances of several thousand kilometers. Tracking data from 108 adult female northern elephant seals used for the study show that the animals spend about 240 days foraging in the northeast Pacific, traveling up to 10,000 kilometers before returning to their breeding beaches.
Although elephant seals spend time in the open ocean very widely dispersed at different longitudes and latitudes, with different day lengths, and foraging independently, they arrive at their breeding beach in a very narrow window of a few days during the breeding season. To date, it is not known what causes females to start the return migration months before giving birth to their young.
Using the tracking data, the research team determined the time when the females turned around and began the return trip to the breeding grounds. Accordingly, females that were farther from the breeding beach turned back earlier. This means that the animals know the distance to the breeding site and plan accordingly more time for the migration. In addition, the researchers said, the data show that reproductive synchrony in migratory animals is possible at the population level.
After the females arrived at the beach, the researchers noted the respective time of birth, which occurred about five days after arrival. The calculations indicated that females began their return migration from the open ocean about 98 days before parturition when they averaged about 2,800 kilometers from the breeding beach.
While it remains a mystery how elephant seals determine their position — geomagnetically, celestially, acoustically or olfactorily — the researchers’ findings suggest that the large seals have a map sense that allows them to adjust their migration based on their current position relative to their destination, the authors said. At least this could be an explanation for the finely timed arrival at the breeding beach shortly before giving birth.
Julia Hager, PolarJournal
