Studying mammals: Plant predators
Studying mammals: Plant predators

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Studying mammals: Plant predators

9 Wildebeest migration

The skill of thinking in a scientific way is as much a part of being a scientist as is knowing facts - perhaps more so. In this series of units, you'll not only come across facts about particular techniques, such as radio transmitters and bat detectors, but also the tactics that scientists use to investigate problems. In this section, you learn about the importance of hypotheses and coming to logical conclusions. This way of thinking is hugely important in science, but it isn't the only way scientists operate and things are not always as logical and tidy as they seem here. How does this form of thinking differ from 'commonsense' approaches? Could the same approach be used to help explain other perplexing issues, such as the reasons for the zebra's stripes [p. 106]?

Activity 6

Watch the TV programme from 19.14-23.40 showing the huge numbers of animals, especially wildebeest, migrating across the plains of East Africa. There are some spectacular aerial photographs of this migration in LoM too [pp. 115-121]. What is the most obvious question to ask as you watch this compelling sequence?

Discussion

Millions of animals undertake a dangerous journey of hundreds of miles between the Masai Mara in southern Kenya and the Serengeti to the south in Tanzania. The most striking question is why do they do it?

Possible reasons are given in LoM but an enormous amount of research has been done, starting in the 1950s, to reach these conclusions. Let us consider how this work might have progressed, to gain an insight into the way that scientific investigations can be carried out.

Imagine that you are a newly graduated biologist, with a particular interest in mammals, and you have been given funding to start to investigate the reasons for the wildebeest migration. How might you start the work?

You would first need to discover the details of the migration pattern, perhaps by following the animals in vehicles, or doing an aerial survey from a light aircraft or helium airship. You might also want to equip individual animals with small radio transmitters to follow their progress (radiotelemetry). (If you have been watching all the TV programmes in the series, you will recall from 'A Winning Design' that echidnas were fitted with similar small transmitters to enable them to be tracked.)

SAQ 16

Use the information in LoM p. 115 to write a couple of sentences describing the wildebeest migration. Remember, you are just reporting the observations on the migration pattern, not the explanation.

Discussion

Wildebeest leave the grasslands of the southern Serengeti, Tanzania, in May at the start of the dry season and travel north to the grasslands of the Masai Mara in southern Kenya. When the rains begin again about six months later, the wildebeest return to the Serengeti.

Let us start with the first leg of the journey, from the Serengeti to the Masai Mara at the start of the dry season. It is relatively easy to come up with an explanation for this migration and you have read it in LoM. When the dry season begins, the grass on the dry (well drained) volcanic soils of the Serengeti begins to wither and the water holes dry up. So the wildebeest set off for the Masai Mara where occasional showers of rain allow the continued growth of grass and there is water to drink in the rivers.

However, it is much less obvious why wildebeest return to the Serengeti, just as the rainy season has returned and the grass is apparently growing really well in the Masai Mara. Let us assume that you have not read the explanation advanced in LoM, but that you are intrigued by this part of the migration and decide to study it further.

While making your observations on the migration, you might have thought of some ideas as to why the animals returned to the Serengeti. Such ideas, based on scientific observations, are called 'hypotheses'. A hypothesis can be defined as a tentative explanation, based on available evidence, which accounts for observations or facts. It can be tested by making further observations and then can be modified in the light of new evidence or observations.

Suppose that your hypothesis is that the wildebeest migrate to escape from their predators. Other work has shown that there are around 3000 lions and 9000 spotted hyenas living in the Serengeti and preying on herbivores like the wildebeest. During their breeding season, these predators need to remain close to their dens to feed their young and therefore it seems unlikely that they would follow the migrating wildebeest.

This hypothesis appears to be very reasonable and it was, in fact, one that scientists worked with for some time. It was supported by studies showing that wildebeest numbers were indeed less than the population size theoretically sustainable by the amount of available grass; so it seemed very likely that the numbers were being restricted by predators. However, it did not explain why other animals on the plain, which also fell prey to the lions and hyenas, did not migrate. Furthermore, studies of the hyenas showed that they would travel up to 60 kilometres from their dens to catch their prey, and so they could follow the wildebeest and continue to prey on them for a large part of their migration journey.

Eventually, therefore, this hypothesis had to be rejected since it no longer fitted the observations and facts, and another one had to be chosen and tested. Of course, in the real world of scientific enquiry, progress is not made in this linear fashion. Several groups of researchers are often working on the same hypothesis, collecting evidence which they may or may not share with one another, and other groups may be working on different hypotheses.

Returning to the wildebeest migrations, the hypothesis that has been under consideration for the longest time, is that food is the main factor. Clearly, because the wildebeest migrate away from the Masai Mara just as the grass is apparently at its best there, the hypothesis cannot be as simple as 'wildebeest migrate to where the grass is growing best'. Scientists therefore began to consider a more detailed hypothesis, that 'there is some specific feature of the grass that is different in the two areas and that affects the migration of wildebeest'. How could this hypothesis be tested? It needs some investigation.

SAQ 17

Imagine that scientists have collected fresh grass from the two areas grazed by wildebeest and returned quickly to their laboratory. What features of the grass might they examine?

Answer

They might look at the amount of energy available from the grass. They could measure the amounts of protein and minerals, to determine whether the samples from the two areas are equally nutritious.

In fact, this experiment has been done in a very systematic way. Areas in the Serengeti, the area to which wildebeest return in the wet season, and in the Masai Mara, the area where they graze in the dry season, were fenced off to protect the grass, and samples were taken twice a month. Concentrations of sodium, calcium, phosphorus and protein were checked, and measurements of the energy available were made. These values were compared with the minimum requirements to keep wildebeest healthy and to allow the females to produce sufficient milk for their young. The results showed that for most of the factors, the grass from either area would contain sufficient nutrients for wildebeest. However, in the grass from the Masai Mara, there was insufficient phosphorus in all the samples gathered, while phosphorus levels were above the minimum required in the grass from the Serengeti. Phosphorus (as phosphate) and calcium are essential constituents of bone. Other studies have shown that a lack of phosphorus in the diet of grazing animals, which can occur in cattle and sheep pasture in many parts of the world, results in bone and teeth abnormalities, as well as reduced fertility, slow growth, poor milk yield and increased mortality.

So, the hypothesis that 'there is some specific feature of the grass that is different in the two areas' has received support and can be modified to read 'there is a lack of phosphorus in the grass in the Masai Mara, which leads to a phosphorus deficiency in the wildebeest'. How might this revised hypothesis be tested?

An experiment would need to be set up to collect samples from individual wildebeest at different times of the year to measure the amount of phosphorus present in their bodies. Collecting tooth or bone samples could cause injuries to the animals, but blood and urine samples can be obtained from animals that have been mildly sedated. When this was done for a representative sample of wildebeest from the Masai Mara and from the Serengeti, it was found that the phosphorus levels in animals from the Serengeti were normal, but in animals from the Masai Mara were less than half the critical minimum levels. These data provide good support for the hypothesis - but we are still short of a link with migration.

Then scientists turned to the results of some other experiments. Sheep were experimentally deprived of phosphorus, which reduced their blood phosphorus levels to as low as those found in the wildebeest in the Masai Mara. These sheep were found to show a craving for bone and bird droppings and other natural sources of phosphorus. So there does seem to be a similar situation in which phosphorus deficiency leads to changes in behaviour. We can therefore modify our hypothesis to become 'migration in wildebeest from the Masai Mara is triggered by low levels of phosphorus in their blood'. So, it is now possible to begin to give an answer to question posed at the start - 'Why do wildebeest migrate?'

SAQ 18

Write a few sentences to explain the two parts of the wildebeest annual migration pattern.

Answer

Wildebeest migrate from the Serengeti to the Masai Mara at the start of the dry season, when the grass in the Serengeti dries up and water becomes scarce. The grassland of the Masai Mara is very low in phosphorus, so during the dry season, the wildebeest become increasingly short of this vital element. When the rains return, the wildebeest return to the Serengeti, where the grass has begun to grow again and contains much higher levels of phosphorus. The levels of phosphorus in their blood then return to normal during the wet season.

However there are still many unanswered questions - you may already have thought of some.

  • Exactly how does the low level of phosphorus in the blood trigger the migration?

  • How do the wildebeest know where to go to find grass with higher phosphorous levels?

  • How do other mammals that do not migrate from the Masai Mara manage to survive on the low phosphorous levels available in the grass?

It's often said in science that answering one question does little more than raise a complete set of new questions and uncertainties, as is the case here. Only the passage of time (and the uncovering of more information) will reveal whether the 'phosphorus story' holds up to the scrutiny of future scientists, at which point questions like those above might be better resolved (while at the same time producing a host of new areas of ignorance).

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