Studying mammals: Plant predators

Introduction

The plant predators, or herbivores, are a varied group, but they share certain characteristics. Many of them are large; among the smallest is the chevrotain (or mouse-deer) at about two kilograms weight, and the elephant is the largest, with a typical bull male weighing around six tonnes. In this course we'll be looking in more detail at some of the problems and consequences of adopting a plant-eating way of life. Leaves are a much less nutritious food than most kinds of animal material, so large herbivores have to eat large quantities of plants and they have special ways to digest their food. We will look at how leaves work and the ways in which herbivores are adapted to survive on their plant-based diets.

To get the most out of this course you will need access to a copy of The Life of Mammals (2002) by David Attenborough, BBC Books (ISBN 0563534230), and The Life of Mammals (2002) on DVD, which contains the associated series of ten BBC TV programmes. You should begin each course by watching the relevant TV programme on the DVD and reading the corresponding chapter in The Life of Mammals. You will be asked to rewatch specific sequences from the programme as you work through the course.

This OpenLearn course provides a sample of level 1 study in Environment & Development

Learning outcomes

After studying this course, you should be able to:

• describe the particular problems in digesting plant material

• give examples of the ways in which teeth are modified for a herbivorous diet

• explain the importance of digestive enzymes

• explain the importance of microbes in digesting plant material

• compare the main features of the digestive systems of ruminants and hindgut fermenters.

1 The herbivores

The plant predators, or herbivores, are a varied group, but they share certain characteristics. Many of them are large; among the smallest is the chevrotain (or mouse-deer) at about two kilograms weight, and the elephant is the largest, with a typical bull male weighing around six tonnes. Most herbivores can run fast and they usually have eyes at the sides of their heads, rather than at the front. These laterally pointing eyes provide wrap-around vision and enable them to feed while, at the same time, keeping watch for the animal predators that are likely to be a threat (the carnivores). For some, speedy locomotion enables them, more often than not, to make good their escape, as you will see towards the end of the TV programme 'Plant Predators'.

Most of these plant predators belong to a group generally referred to as the ungulates, whose identifying characteristic is that they have hooves, rather than claws. Within the ungulates, there are two orders, the odd-toed ungulates, or Perissodactyla, and the even-toed ungulates, the Artiodactyla. As you might guess, these groupings depend on the structure of their feet. The odd-toed ungulates are the horses and their relatives, like the zebras, with one toe, and the 'nail' modified as a hoof, and the tapirs and rhinoceroses which typically have three toes on each foot. You may already have spotted that there are some exceptions to these rules; the Brazilian tapir does have three toes on its hind feet, but its front feet have four - three functional ones plus a vestigial fourth toe. The even-toed ungulates have two or four toes, often with a gap between them, giving a cloven-hoofed appearance. They include the camels, deer, antelopes, cattle, sheep and goats. Additionally, included in this loose grouping of plant predators are the elephants, or subungulates, and the sloths, which belong to the order Xenartha.

In this course I'll be looking in more detail at some of the problems and consequences of adopting a plant-eating way of life, building on what you know already from reading LoM and watching the TV programme. As you know from LoM, the plants that make up the diet of many herbivores are grasses and broadleaved plants that are comparable in leaf structure to the familiar garden plants. Herbivores that eat grasses and other herbaceous (non-woody) plants are generally termed grazers and many of the animals I refer to in the early sections of this course are grazers.

2 The herbivore lifestyle - living on leaves

Leaves are a much less nutritious food than most kinds of animal material, so large herbivores have to eat large quantities of plants and they have special ways to digest their food. As author David Attenborough (DA) says, 'Leaves are extremely poor food' [p. 89]. To find out why living on a diet of leaves is particularly difficult, we need to know something about how leaves work.

Figure 1 Diagram of a vertical section though a leaf. Both surfaces are covered by a protective cuticle, which reduces water loss from the leaf. Beneath the upper epidermis are the palisade cells, so called because they are lined up like the fence of stakes around a military establishment

Plants, unlike animals, can make their own food. The process by which they do so is called photosynthesis; 'photo' means light and 'synthesis' means building up. Plants harness sunlight as their energy source for the process that combines water (taken up through their roots) and carbon dioxide (taken in from the air through pores, called stomata, in their leaves) to make sugars.

Plants can use these sugars, together with minerals which are dissolved in the water in the soil, as the raw materials for all the other types of molecule that the plant needs for its healthy growth. A crucial role in photosynthesis is played by the green pigment, chlorophyll, which is located in tiny structures called chloroplasts, inside the cells of the leaf.

Each cell in the leaf is surrounded by a wall of cellulose. Chemically, cellulose is made up of strings of sugar molecules, joined together as threads, like strings of minute beads; these threads are criss-crossed to make a strong lattice around each cell. How can herbivores get access to the potential nutrients around and within the plant cells? Initially, plant material is subject to physical breakdown, grinding it up into small paricles using specially adapted teeth. This phase is followed by chemical breakdown of the cellulose lattice, to release the sugar molecules and allow access to the nutrients within the plant cells.

3 Herbivore teeth

If you are working through all the units in this series, you will recall the basic design and arrangement of mammalian teeth from course S182_2 Studying mammals: the insect hunters.

The molars of herbivores become worn down by constant grinding of plant material between them. The teeth are composed of softer and harder materials, notably dentine and enamel. The overlying enamel is a hard, resistant layer, especially important in the incisor teeth of rodents. When they first erupt, the molar teeth of herbivores often have an extra coating layer of cement (similar to the layer surrounding the root). So in herbivores, cement, enamel and dentine are worn down unevenly, such that differential wear usually produces ridges, especially in the molars and premolars, which make chewing more efficient. These cutting ridges of hard enamel are evident in a vertical section through the single functional molar tooth from the lower jaw of an elephant (Figure 2a, below).

Maximise

Figure 2a: adapted from: Hildebrand, M. (1974) Analysis of Vertebrate Structure, copyright © 1974, John Wiley & Sons, Inc. This material is used by permission of John Wiley & Sons, Inc.; Figure 2b: adapted from: Courtesy of Caroline Pond ©

Figure 2a: adapted from: Hildebrand, M. (1974) Analysis of Vertebrate Structure, copyright © 1974, John Wiley & Sons, Inc. This material is used by permission of John Wiley & Sons, Inc.; Figure 2b: adapted from: Courtesy of Caroline Pond

Figure 2 (a) Vertical section a single molar tooth of an elephant, with enamel ridges providing a serrated surface. (b) A view from above of the lower jaw of an elephant, showing a single molar. (To the left of this tooth, you can see a small part of the unerupted tooth behind, next in line to replace it.)

In all mammals, including ourselves, the molar teeth do not all erupt through the surface of the gum at the same time, but in mature adults all can be functional simultaneously.

The ridges on the teeth of elephants and horses lie across the jaw, and the jaws move mostly backwards and forwards while chewing. Those on sheep and deer run from front to back, and the jaw movement is mostly side to side. You might be able to see this movement for yourself if you have the opportunity to watch some of these herbivores eating.

Now use the information you've entered in Table 1 to answer the following questions.

Biting off and chewing up the plant material is the start of the process of breaking it down. These actions break the food into smaller pieces and squash the pieces to give a larger surface area for the chemical part of the breakdown, called digestion, to begin. The real process of extracting the nutritious material from inside the cells happens after the food is swallowed.

4 Digesting plant material

4.1 A brief digression about digestion

You will probably already know from the descriptions on food packaging that the important dietary components are protein, fat and carbohydrate, together with fibre, minerals, vitamins and water. You may already know the importance of the first three listed ingredients as sources of energy and as the constituent 'building blocks' for synthesis. The food that you eat, or that any mammal eats, has to be digested before the nourishment in it can be absorbed from the intestines into the bloodstream. The nourishment is then carried round the body in the blood to wherever it is needed for energy (mainly the muscles) or to where it is required to construct new body tissue or repair or replace worn parts. The process of digestion breaks down the molecules of protein, fat and carbohydrate into their building blocks.

The building blocks of proteins are called amino acids; they are joined together in long strings, which are broken apart when the protein food is digested. The amino acids are then absorbed and recombined inside the cells of the mammal's body in different, but very precisely determined, sequences to make different types of protein, including the 'structural proteins' that any particular species will need for its own muscle, skin, hair, etc. The order of the amino acids in the various proteins of the body is determined by the genes that the individual has inherited from its parents.

Fat is broken down into its two components (glycerol and fatty acids) which are absorbed into the bloodstream, and then may be reconstituted as fat when they reach the storage tissues of the body. Fat is mostly used to provide energy, and can be stored in various locations. If you have worked through course S182_2, you will recall the importance of fat cells (as WAT and BAT) and the role of the latter in arousal from hibernation.

Many carbohydrates are composed of strings of sugar molecules, so when they are digested, sugars are released, which again are primarily used to provide energy.

The process of chemically breaking down proteins, fat and carbohydrates is achieved by the action of enzymes.

The great majority of enzymes are themselves protein molecules whose function is to speed up processes, such as digestion, which without them would go impossibly slowly. Just like the structural proteins mentioned above, enzymes have to be built up using the amino acids obtained from the breakdown of proteins in an animal's diet.

I've already mentioned that cellulose is the structural component of plant cell walls. It is a rather special type of carbohydrate, difficult to digest. In the human diet, cellulose is mostly considered to be 'fibre' or 'roughage', which passes straight through the digestive system, where it is important for the system's correct functioning, but provides no nourishment. Clearly, for a herbivorous mammal that eats only plant material, there must be a way to digest cellulose - and that is the topic of the next section.

4.2 Digesting cellulose

Maximise

Figure 3: adapted from: Macdonald, D. (ed.) (2001) The New Encyclopedia of Mammals, Oxford University Press ©

Figure 3: adapted from: Macdonald, D. (ed.) (2001) The New Encyclopedia of Mammals, Oxford University Press

Figure 3 Simplified representation of the digestive systems of hindgut fermenters and ruminants

The enzyme needed to digest cellulose is called cellulase. Enzymes are often named like this, i.e. by taking part of the name of the substance that they digest and replacing the end of the word by '-ase'. For example, proteins are digested by types of enzymes called proteases and the sugar in milk, lactose, is digested by the enzyme lactase. Animals that live on plant material have detectable cellulase activity in their digestive systems. However, detailed genetic studies have shown that the vast majority of animals do not possess the gene for making cellulase, so the enzyme is clearly not synthesised by the animal itself. The gene for cellulase does appear to exist in a few animals, namely some insects (such as vine weevil larvae, cockroaches, termites and silkworms), some earthworms, a wood-boring clam, and a fish called the grass carp. This very odd assemblage of creatures has no close taxonomic relationship, indicating that the ability to produce cellulase probably evolved independently several times. Why it has not evolved more frequently is mysterious - it appears never to have evolved in mammals. Mammals that need cellulase have to make use of some additional species that do possess the gene - namely microbes, mostly bacteria and fungi, but also some protoctists (tiny organisms, most of them composed of just a single cell with a nucleus). Parts of the mammal's digestive system are modified to form large sacs where huge colonies of these microbes break down, or ferment, the cellulose.

The two groups are known respectively, as the hindgut fermenters, for obvious reasons, and the ruminants, for reasons that are clear when you look at Figure 3.

You may have been wondering how young herbivores obtain an initial colony of appropriate microbes. They are not, of course, needed for digesting milk - the diet of newborn herbivores. But during the weaning period, it appears that young animals ingest these microbes from the adults around them. They may do so accidentally by eating plants contaminated by adults' faeces. In some cases, it appears that the young deliberately eat the faeces of adults. This habit has been observed in elephants and, as you may have read in LoM Chapter 3 [p. 78], in the mole-rat.

5 Digestive processes

5.1 Ruminants

The earliest ruminant was probably an ancestor of the present-day chevrotain. The chevrotain skeleton appears to have remained virtually unchanged for the past 30 million years and, although there are now only four species confined to the jungles of Africa and Southeast Asia, they once had a worldwide distribution. So, chevrotains are placed in the suborder Ruminantia within the order Artiodactyla, to which other deer, antelopes, cattle, sheep and goats also belong. A second suborder, the Tylopoda, contains the camels and llamas; these also have a fermentation chamber within a complex stomach, and are therefore ruminants. So you can now transfer camels from the 'Not known' column of your Table 2 to the 'Ruminants' column. We'll be returning to camels later, too.

The word 'ruminant' is derived from the Latin word ruminare, to chew again. To allow for the food to be chewed twice, the stomach of ruminants is complex, with four separate sections.

When a ruminant is feeding, it does so in a series of quick bites, giving the food no more than a cursory chew between its molar teeth, mixing it with large quantities of saliva (several hundred litres per day in domestic cattle) and then swallowing it into the first of the chambers, the rumen. Here powerful muscles churn it with the microbes that start the fermentation process. The food ferments, generating methane and carbon dioxide which are eructed (burped!). The microbes start to break down the cellulose of the cell walls into sugars, thereby releasing other nutrients from inside the cells. The microbes use some of these nutrients for their own metabolism, and in doing so generate fatty acids, which the ruminant can absorb into its blood through the wall of the rumen and can use in its own metabolism. Large pieces of plant material float on top of the fluid in the rumen and are passed to the reticulum, which has honeycomb partitions in its walls. Here the food is formed into balls called 'cuds'.

Eventually the animal takes a break from feeding, selects a resting place where it can keep watch for predators and spends some time ruminating - the cuds are regurgitated and the animal chews the material again, mixing it with saliva and breaking it down into smaller particles. This process gives a bigger surface area for the microbes to continue digestion of the food when it is swallowed again.

Only when the material is very finely ground, does it pass into the omasum, where strong muscular contractions churn it up further. Finally, it enters the true stomach or abomasum, where the normal digestive enzymes get to work to break down the remains of the food and also to digest many of the microbes that have continued along with the food. Digestion continues in the small intestine, and absorption of the digested food into the blood begins through the wall of the small intestine. If any tough plant material has still not been broken down, there is a further opportunity for fermentation, and absorption in the caecum (the bulge from the side of the lower part of the digestive tract). Any material that still remains undigested is expelled from the body as faeces.

5.2 Pseudo-ruminants

Animals in the third suborder of the Artiodactlya, the pigs, peccaries and (according to most authorities) the hippopotamuses (suborder Suina), use a slight variant on the ruminant method, and are often referred to as pseudo-ruminants. You might like to add this information to your version of Table 2. These animals do have stomachs with several chambers, similar to the true ruminants, and the first of these chambers houses colonies of cellulose-digesting microbes. However, the pseudo-ruminants do not gain maximum benefit from this arrangement because they do not regurgitate the food and 'chew the cud'. So they are not able to extract as much of the nutrition from a diet of leafy plant material as a similarly-sized ruminant.

Pigs and peccaries overcome the problem by eating a varied diet, not exclusively composed of leafy plant material. As well as leaves and grasses, they eat fungi, ferns, roots, bulbs and corms, and fruit (all of which are easier to digest) and even some animals such as insect larvae, earthworms and sometimes small vertebrates like frogs and mice, which they come across while rooting in leaf litter and moist earth. The hippopotamus has evolved a different strategy to counterbalance its less efficient digestion - it reduces its energy expenditure.

So hippos have a relatively inefficient digestive system, compared with true ruminants, but also have a low energy requirement. The herbivorous mammals that have taken the low-energy lifestyle to its extreme are, of course, the sloths. You might like to reread LoM pp. 87-90 to remind yourself about these animals (and perhaps watch them again in the first few minutes of the TV programme).

5.3 Hindgut fermenters

The odd-toed ungulates (comprising the order Perissodactyla), the horses, tapirs and rhinoceroses, are hindgut fermenters, as are elephants. Update Table 2 with this information. These animals have a relatively simple, small undivided stomach, but this time an even larger caecum and colon where the microbes are housed and where fermentation takes place, as you can see in Figure 3. One disadvantage of this method is that by the time the food reaches the caecum and colon, it has passed the main absorptive region of the gut (the small intestine) and so less absorption of the products of digestion is possible. Secondly, the large numbers of dead microbes also cannot be digested, as they can in ruminants, where they pass from the rumen into the small intestine with the remainder of the partly digested food. Here they are located in the part of the gut beyond the region of digestion.

Horses and their relatives overcome this situation in an ingenious way. Remind yourself of the position of the caecum as shown in Figure 3. It projects from the side of the gut, rather than being part of the main passage through. Food can therefore leave the caecum and instead of moving onwards into the colon, it can be moved back into the small intestine by reverse peristaltic movements (peristaltic movements are contractions of the muscles in the gut wall which propel the contents along). This process, known as intestinal reflux, allows further digestion and absorption to take place, before the food continues back again in its normal direction through the gut.

Investigations show that this behaviour of rabbits, known as coprophagy, is common in other species of the order to which rabbits belong (the lagomorphs, LoM p. 95) and it has also been reported in rodents such as capybaras and coypus. Add these hindgut fermenters to complete your version of Table 2 and then compare it with Table 3, below.

6 Grazers and browsers

A good deal of the discussion so far has been related to animals that eat leaves in the form of grass and other herbaceous plants, the grazers, but this is not the only type of plant food. Also available as food are the leaves of trees and bushes. These form the diet of the browsers.

There are many types of tree in Africa that provide food for a variety of species of browser, of which one is the acacia.

Despite the seed-dispersal benefit, the acacia tree is under constant risk of losing its life-support, the leaves, which are vital for producing its own nutrients by means of photosynthesis. How plants protect themselves against attack by the plant predators is the topic of the next section.

7 Plant defences

Figure 4: adapted from: Courtesy of Audrey Brown.

Figure 4 Leaves and spines of the acacia tree

8 Shortage of nutrients

8.1 Introduction

You know by now that plants can synthesise all the complex molecules that make up their tissues and seeds from very simple molecules - water, carbon dioxide and minerals from the soil. Mammals, on the other hand, need to take in many complex molecules ready-made, and some foods do not contain the right amounts or the right mix of nutrients. They have evolved various strategies to overcome the shortfalls, some of which are described in this section.

8.2 Protein shortage

Most plant leaves and stems contain very low levels of protein and so herbivores need to make maximum use of the amount available. You will recall that proteins in the diet are digested to amino acids, which are then absorbed into the bloodstream. Some of those amino acids are used to produce the mammal's own proteins (Section 4.1), but the proportions of different amino acids can be crucial. Most mammals are able to interconvert some types of amino acid into others, so the amino acids that are essential (i.e. have to be supplied in the diet) may be relatively few. Nevertheless, even in a diet low in protein, there are likely to be amino acids that cannot be used at that time and these extra ones cannot be stored. They must therefore be removed from the body. They are first converted by the liver into urea, which (in most species) is excreted by being dissolved in significant quantities of water and then passed out in urine. However, ruminants have a remarkable ability to recycle urea, via the microbes in their gut. The urea is extracted from the blood by the salivary glands and is reintroduced into the gut in the saliva. (You may recall from Section 5.1 that ruminants produce particularly large volumes of saliva.) The microbes use the urea to synthesise their own amino acids (a synthesis that mammals cannot carry out) and, when the gut microbes die, their proteins can be digested and used again by the ruminant. This recycling has another advantage for the ruminant: it reduces significantly the volume of urine produced and so conserves water.

The contrast between the camel and other desert mammals is striking. Camels need to drink only occasionally, whereas desert hindgut fermenters, like asses, have to drink daily to take in enough water to make urine.

8.3 Shortage of minerals

You may be familiar with salt licks that are provided for domesticated cattle. In the wild, grass is also often low in minerals (e.g. it has almost no sodium and very little calcium), so grazers may have to go to extraordinary lengths to supplement their diet with additional minerals obtained from the most unlikely places. LoM gives some examples, but the most impressive activity takes place in the caves of Mount Elgon in Kenya [pp. 113-114]. You'll probably recall this spectacular footage from the TV programme (14.00-18.00), which showed bushbuck, buffalo and elephants in the cave, all obtaining salts from the cave. Indeed, elephants have an enormous requirement for sodium, having excavated many millions of tonnes of sodium-containing rock over a period of more than two million years. Sodium has a variety of important functions in the body, and in many herbivores - perhaps elephants included - it is thought to be especially important in the synthesis of chemicals that counteract the effect of ingested toxins.

9 Wildebeest migration

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.)

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.

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?'

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).

10 Living in herds

Wildebeest are only one of the species of plant predator that live in herds. Many others do too.

However, since so many plant predators do live in herds, on balance it must be beneficial. If you are studying the next course in this series, you will be looking at the lives of some of those meat eaters that are dependent for their food on the plant predators, and at some of the consequences of living in groups that these carnivorous mammals face.

Conclusion

This free course provided an introduction to studying Environment & Development. It took you through a series of exercises designed to develop your approach to study and learning at a distance, and helped to improve your confidence as an independent learner.

Acknowledgements

The content acknowledged below is Proprietary (see terms and conditions). This content is made available under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 Licence

Course image: Mathias Erhart in Flickr made available under Creative Commons Attribution-NonCommercial-ShareAlike 2.0 Licence.

Grateful acknowledgement is made to the following sources for permission to reproduce material in this course:

The content acknowledged below is Proprietary and is used under licence.

Figure 2a adapted from: Hildebrand, M. (1974) Analysis of Vertebrate Structure, copyright © 1974, John Wiley & Sons, Inc. This material is used by permission of John Wiley & Sons, Inc.;

Figure 2b adapted from: Courtesy of Caroline Pond;

Figure 3 adapted from: Macdonald, D. (ed.) (2001) The New Encyclopedia of Mammals, Oxford University Press;

Figure 4 adapted from: Courtesy of Audrey Brown.

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