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Listen to part of a lecture in a biology class.

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So we\'ve been talking about cetaceans, marine mammals like whales and dolphins.

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Are there any questions before we leave the topic?

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Yeah, I\'ve been reading about these times when cetaceans are found stuck on ocean beaches. They call it Stranding. Right, right? Stranding.

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I mean, like sometimes 100 or more animals are found stranded on beaches.

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So what causes that?

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Yes, strandings are often in the news, especially mass stranding like you mentioned.

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We\'re not really sure why this happens, but several hypotheses have been proposed.

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The first hypothesis has to do with changes in the direction of Earth\'s magnetic field in parts of the ocean floor,

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Earth\'s magnetic field can change direction.

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I thought it\'s just oriented south to north.

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It generally is. But in some areas, the magnetic fields affected by local conditions or by geological processes on the ocean floor.

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But getting back to cetaceans

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in the outer layer of the cetacean brain,

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there\'s a substance called magnetite.

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We think that magnetite makes it possible for cetaceans to sense Earth\'s magnetic field

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and so figure out which way to swim.

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Researchers wondered if there could be a link between strandings and areas near coastlines where the magnetic field suddenly changes direction.

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And one European study did indeed show a connection between regions with magnetic variations and mass strandings.

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Another hypothesis that\'s been put forth

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has to do with the shape of the coastline.

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Time and again,

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mass strandings occur in areas where the coastline juts out into the sea

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with deep water on either side. You mean, like

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Cape Cod in the northeastern United States.

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You know the way it\'s a peninsula, like a long arm extending out from the mainland.

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I\'ve seen stuff on the news about strand of whales there.

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Cape Cod is a good example.

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Areas like this are risky for cetaceans, because they\'re more accustomed to deep water.

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So when the animals suddenly find themselves in shallow water,

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or if they get lost in a storm,

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they can end up on the beach.

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Weather can cause strandings, even when it doesn\'t seem too bad.

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That\'s what sometimes happens with upwellings.

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Upwellings form as a result of wind blowing steadily in the same direction,

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the wind displaces the warmer surface water in an area,

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allowing colder, deeper water to rise to the surface,

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which is what we call upwelling.

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This deeper water is particularly rich in nutrients, because after sea animals die, they usually sink to the ocean floor where they decompose,

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so a layer of nutrients builds up on the sea bottom.

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But upwellings bring those nutrient rich areas to the surface,

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and these areas attract many feeders.

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Wherever an upwelling occurs, animals will follow.

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But when certain types of winds are present,

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upwellings can be pushed closer to the coast than where they normally occur.

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Animals who follow these zones could find themselves in unfamiliar, shallower water,

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where they are at a high risk of getting stranded.

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I thought the whales that get stranded are usually sick.

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Are you saying strandings are always the result of environmental or geographical causes?

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No, you\'re right.

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Cetacean strandings are sometimes caused by sickness too.

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But even in those cases, there\'s often more to the story.

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In mass strandings, only one animal may actually be sick,

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often an older member of the group that others may have seen as a leader.

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You see many cetacean species have a very stable group structure

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and rarely abandon other group members.

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So if a leader were to become sick or disoriented and move dangerously close to land,

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other group members would be likely to follow

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some whale species form familial groups that stay together for life.

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It\'s these species tend to strand on masse.

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