You will hear a lecturer discussing possible causes of mass strandings of whales and dolphins.
First, you have some time to look at questions 31 to 40.
Now listen carefully and answer questions 31 to 40.
Since some of you will soon begin fieldwork, today's topic is especially relevant.
I want to consider why groups of whales and dolphins sometimes leave the sea, move onto a beach and, in most cases, die.
These events are known as mass strandings.
Sadly, strandings occur frequently in several of the places you will visit.
In some such locations, the tide can change very quickly, leaving the animals confused.
However, researchers have proposed a number of other explanations.
One theory connects strandings with parasites.
Large infestations are often discovered in stranded animals.
For example, worms are commonly found in the ears of dead whales.
Marine mammals depend heavily on their hearing to navigate, so parasites affecting it can cause serious harm.
A second explanation concerns toxins, or poisons, which have contributed to many marine animal deaths.
Such toxins may originate in plants or animals and are then consumed by whales during normal feeding.
Whether they lead directly or indirectly to stranding seems to depend on the particular poison.
In 1988, 14 humpback whales that stranded along Cape Cod were examined.
They had eaten tuna containing saxetoxin, a substance that can also be fatal to people.
Some researchers have suggested that animals may strand by accident while pursuing prey close to shore.
In 1995, David Thurston observed pilot whales beaching after they had followed squid.
Yet, this explanation is unlikely to account for most mass strandings.
Analysis of stomach contents shows that the majority of the animals were not feeding at the time.
More recent theories connect strandings with human activity.
One increasing concern is that powerful noise in the ocean can cause such events.
Sounds generated during military tests have been linked in particular with several recent strandings.
A case in the Bahamas in 2000 occurred at almost exactly the same time as trials of a new submarine detection system.
Several unusual features led scientists to search for a new cause.
First, every stranded whale was healthy.
Second, the animals were distributed along 38 kilometres of coastline rather than being together in a group, as normally happens in a mass stranding.
The final theory involves social behaviour.
It proposes that marine mammals cannot tell whether a leader is healthy or sick and will continue following an ill leader even when doing so is fatal.
This explanation is particularly interesting because the whale species considered most social, namely toothed whales, are also those that strand most often.
Evidence from a dolphin stranding in 1994 supports the theory.
Examinations showed that only the leader had been ill while all the other dolphins were healthy.
Because no single explanation accounts for every case, our immediate options are limited to helping animals whenever possible.
Stranding networks now operate around the world, rescuing survivors and collecting samples from animals that cannot be saved.
For anyone wanting to learn more about these networks, or perhaps establish one, John Connor's book Marine Mammals Assure is an excellent place to begin.
That is the end of part four.
You now have one minute to check your answers to part four.