Listen to part of a lecture in a biology class.So we\'ve been talking about cetaceans, marine mammals like whales and dolphins.Are there any questions before we leave the topic?Yeah, I\'ve been reading about these times when cetaceans are found stuck on ocean beaches. They call it Stranding. Right, right? Stranding.I mean, like sometimes 100 or more animals are found stranded on beaches.So what causes that?Yes, strandings are often in the news, especially mass stranding like you mentioned.We\'re not really sure why this happens, but several hypotheses have been proposed.The first hypothesis has to do with changes in the direction of Earth\'s magnetic field in parts of the ocean floor,Earth\'s magnetic field can change direction.I thought it\'s just oriented south to north.It generally is. But in some areas, the magnetic fields affected by local conditions or by geological processes on the ocean floor.But getting back to cetaceansin the outer layer of the cetacean brain,there\'s a substance called magnetite.We think that magnetite makes it possible for cetaceans to sense Earth\'s magnetic fieldand so figure out which way to swim.Researchers wondered if there could be a link between strandings and areas near coastlines where the magnetic field suddenly changes direction.And one European study did indeed show a connection between regions with magnetic variations and mass strandings.Another hypothesis that\'s been put forthhas to do with the shape of the coastline.Time and again,mass strandings occur in areas where the coastline juts out into the seawith deep water on either side. You mean, likeCape Cod in the northeastern United States.You know the way it\'s a peninsula, like a long arm extending out from the mainland.I\'ve seen stuff on the news about strand of whales there.Cape Cod is a good example.Areas like this are risky for cetaceans, because they\'re more accustomed to deep water.So when the animals suddenly find themselves in shallow water,or if they get lost in a storm,they can end up on the beach.Weather can cause strandings, even when it doesn\'t seem too bad.That\'s what sometimes happens with upwellings.Upwellings form as a result of wind blowing steadily in the same direction,the wind displaces the warmer surface water in an area,allowing colder, deeper water to rise to the surface,which is what we call upwelling.This deeper water is particularly rich in nutrients, because after sea animals die, they usually sink to the ocean floor where they decompose,so a layer of nutrients builds up on the sea bottom.But upwellings bring those nutrient rich areas to the surface,and these areas attract many feeders.Wherever an upwelling occurs, animals will follow.But when certain types of winds are present,upwellings can be pushed closer to the coast than where they normally occur.Animals who follow these zones could find themselves in unfamiliar, shallower water,where they are at a high risk of getting stranded.I thought the whales that get stranded are usually sick.Are you saying strandings are always the result of environmental or geographical causes?No, you\'re right.Cetacean strandings are sometimes caused by sickness too.But even in those cases, there\'s often more to the story.In mass strandings, only one animal may actually be sick,often an older member of the group that others may have seen as a leader.You see many cetacean species have a very stable group structureand rarely abandon other group members.So if a leader were to become sick or disoriented and move dangerously close to land,other group members would be likely to followsome whale species form familial groups that stay together for life.It\'s these species tend to strand on masse.