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Migrating blue whales rely on their memory to find their food

Breakfast places, cafes and water points fill the daily commute of modern city dwellers, but we try to remember where we get the best food or drink. If we regularly make longer trips, we also keep track of the best grazing land – a restaurant, a gas station offering the best snacks, etc.

Blue whales, according to a study published in PNAS this week, seem to make similar mental notes. During their annual migration, their path follows the places that have proven to be the most reliable feeding grounds over the years. In doing so, whales can bypass hot spots that appear and fade from year to year, suggesting that they rely heavily on memory to find a solid meal. But in a world where the "normal" situation is changing rapidly, endangered whales may no longer be able to count on the abundance of these ancient, bona fide feeding grounds.

Why do whales go where they go?

Blue whales are the largest animal we know to have lived, which means that they require colossal amounts of food. Despite this, they are difficult to feed and feed almost exclusively on small crustaceans called krill, which they eat while passing through a large, open-mouthed swarm, trapping animals in their mouths while sea water falls . And they manage to find food sources by spending a summer near the poles at a winter past near the equator.

Briana Abrahms and her colleagues wanted to understand the eating habits of migrating blue whales, both to add a growing image of how migratory animals find their food and to better understand the type of threat that climate change could pose to these whales. . Focusing on the blue whales that live in the North Pacific, they looked at years of data on their migrations to try to understand what motivated them.

It was not a simple task. Radio tags can provide amazing data on the movements of the animals themselves, but who can say why they chose to move where they did? To understand this, Abrahms and his colleagues used chlorophyll, a pigment used by plants to absorb energy from sunlight. A high concentration of chlorophyll in the ocean suggests that the spot is home to large amounts of plankton – and therefore large amounts of krill that feed on plankton, fed by blue whales.

As spring moves into the northern hemisphere, plankton bloom to the north, beginning earlier in the season in the south and blooming later in the north. At the same time, blue whales from the North Pacific undertake annual spring migration northward from their breeding grounds in the Gulf of California and off Costa Rica to the coast of British Columbia.

The best krill restaurant, every year

The researchers wanted to know which of two different models correspond to the movements of whales in migration. If whales rely on sensory information to find the best krill spots, they would move where they were most abundantly, thus changing the path of their migration from year to year. If they rely on their memory, they should go year after year to the most productive places.

It turned out that memory was the behavior best supported by the data. Year after year, the whales returned to places that had been proven in the past, the amount of food they produced was little different from one year to the next.

The researchers, however, point out that there is certainly more than that. The memory of the best places could lead the whales to general areas that have always given the most krill. Once on site, it is likely that whales will still use sensory information to fine-tune their movements and find the best krill areas in a region.

In a species as ancient as blue whales, this recourse to memory is not necessarily surprising, but it is important to confirm a behavior that could help environmental advocates better understand new threats to species already in existence. threatened. Driven almost to oblivion for about a century until the 1960s, blue whale populations have recovered to some degree, but now face a host of new dangers: collisions with ships, ocean noise and rising ocean temperatures that may disrupt their temperature – sensitive migration patterns.

Krill, which is abundant in the cold waters of the north, can also see its population decline as the oceans warm up. This research suggests that the problem is not just about how much krill is available, but where it is from year to year. Changes in this situation could cause problems for long-lived blue whales. The memories that bring them back to the same reliable place each year can soon leave them at the mercy of recently capricious oceans.

PNAS, 2018. DOI: 10.1073 / pnas.1819031116 (About the DOIs).

Listing listing by the NOAA photo library

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