Boer, W.F., de

Hoetmer, H.

Kos, M.

English

Name not available

Dung and droppings help explain dry season diet choices

Wageningen Yield 

Kruger Park News : Dry Season Diet Choices : Online News Publication http://www.krugerpark.co.za/krugerpark-times-4-15-diet-choices-2466...
1 of 5 12-11-2008 10:02
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Dung and Droppings Help Explain Dry Season Diet 
Choices
 
By Fred de Boer, Martine Kos and Arno Hoetmer
Most animals face changes in how much food there is for them 
to eat over the year. During the dry season especially, the 
situation can look bleak when the grasses become shorter and 
dry up, and the leaves of deciduous trees are falling, leaving the 
branches bare. This leaves herbivores in a difficult position in 
the dry season, and one would expect that what they choose to 
eat reflects these difficult decisions.
One of the goals of the Tembo (The Elephant Movements and 
Bio-Economic Optimality Programme) research is to understand 
how the forage conditions affect the dietary composition of plant 
eaters, especially elephants and impalas, in the greater Kruger 
region. 
For scientists it is not only a question of biomass (the total
amount of plant matter in the area), or forage availability (the
location of the forage – in riverbeds or hard-to-reach places),
but the quality of the plant material, because sometimes forage
is not used because of its low food quality. 
But why then study these two herbivores? 
One reason is that both elephants and 
impala are so-called mixed feeders - they 
both graze and browse - and are able to 
switch from a diet composed of mainly grasses to one 
dominated by browse as conditions change. Hence, these 
species are ideal study animals to document changes in diet. 
Secondly, both species are abundant in the Kruger region and
impact on the vegetation. While driving through the Kruger
National Park (KNP) or the adjoining APNR (Associated Private
Nature Reserves) one sees elephants and impala regularly, but
how can one be sure about what has been eaten? According to
one of the Tembo scientists, Fred de Boer, “Observations of
foraging animals are possible but it is very difficult to document
changes in diet.
 
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Kruger Park News : Dry Season Diet Choices : Online News Publication http://www.krugerpark.co.za/krugerpark-times-4-15-diet-choices-2466...
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Moreover, how does one know whether one 
bite of grass is the same as a bite from a 
mopane tree? These direct observations 
are no longer used by scientists, and we now use other 
methods, such as faecal analysis. The advantage of this 
method is that you do not need to see the animal in order to be 
able to document its feeding behaviour - a couple of spoonfuls 
of dung are sufficient for a nice analysis. 
Of course we collect various samples and mix these to get a 
more overall Most animals face changes in how much food 
there is for them to eat over the year. During the dry season 
especially, the situation can look bleak when the grasses 
become shorter and dry up, and the leaves of deciduous trees 
are falling, leaving the branches bare.
This leaves herbivores in a difficult position 
in the dry season, and one would expect 
that what they choose to eat reflects these 
difficult decisions. One of the goals of the 
Tembo (The Elephant Movements and Bio-Economic Optimality 
Programme) research is to understand how the forage 
conditions affect the dietary composition of plant eaters, 
especially elephants and impalas, in the greater Kruger region.
For scientists it is not only a question of biomass (the total
amount of plant matter in the area), or forage availability (the
location of the forage – in riverbeds or hard-to-reach places),
but the quality of the plant material, because sometimes forage
is not used because of its low food quality.
But why then study these two herbivores? 
One reason is that both elephants and 
impala are so-called mixed feeders - they 
both graze and browse - and are able to 
switch from a diet composed of mainly grasses to one 
dominated by browse as conditions change. Hence, these 
species are ideal study animals to document changes in diet. 
Secondly, both species are abundant in the Kruger region and 
impact on the vegetation.
While driving through the Kruger National Park (KNP) or the
adjoining APNR (Associated Private Nature Reserves) one
sees elephants and impala regularly, but how can one be sure
about what has been eaten? According to one of the Tembo
scientists, Fred de Boer, “Observations of foraging animals are
possible but it is very difficult to document changes in diet.
Moreover, how does one know whether one bite of grass is the 
same as a bite from a mopane tree? These direct observations 
are no longer used by scientists, and we now use other 
methods, such as faecal analysis. The advantage of this 
method is that you do not need to see the animal in order to be 
able to document its feeding behaviour - a couple of spoonfuls 
of dung are sufficient for a nice analysis. 
Of course we collect various samples and mix these to get a
more overall picture, but you can get a pretty good impression
of what has been eaten this way. “The faecal analysis is quite
time consuming, though.
One needs to treat the samples carefully with several
chemicals, also for health reasons, and then you can study the
fragments under a microscope. “The nice thing about faecal
analysis is that some parts of the plants, such as the top layer
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Kruger Park News : Dry Season Diet Choices : Online News Publication http://www.krugerpark.co.za/krugerpark-times-4-15-diet-choices-2466...
3 of 5 12-11-2008 10:02
of the leaves - the so-called epidermis layer - is composed of a
cell layer that cannot be digested by the animals.
Animals mainly digest the inner tissue of the leaves. However,
the epidermis layer is composed of a nice arrangement of cells,
and the structure of these cells differs among species. So if you
are able to recognise the epidermis layers of the different
species [under the microscope] after learning their
characteristics from taking samples of plants you encounter in
the field, you can calculate what the herbivore species has been
eaten from studying their faeces.” 
For scientists, it is clear that there is much more than only the 
differences between grasses and tree leaves. Leaves of 
different species are different in their composition; some have 
more nitrogen, others have more phosphorus, and yet others 
can be digested better. Moreover, some species have so-called 
"secondary compounds". 
These are bad-tasting components, e.g. tannins, which some 
plants have in order to convince the animals to eat something 
else. These chemicals defend plants against high levels of 
browsing, as the impala will probably want to change to another 
species if a plant does not taste good. However, if that is the 
case, why then do all herbivores not eat the same stuff, with a 
high nitrogen content (which equates to good food for a 
herbivore) and a low tannin content? 
“Well, that is one thing we also want to know. One of the
important topics is that elephants are large animals and need a
large amount of forage, while impala are smaller and need a
smaller volume each day. However, there is another important
issue. If you are smaller, then you need a diet of a relatively
higher quality.
Because an impala's body size is smaller than an elephant’s,
the volume that can be consumed each day is smaller, but the
energetic costs of maintaining the body do not decrease that
much,so an impala can eat less, but should compensate for its
relatively higher energetic costs by eating food that is of a
higher quality than the food of an elephant.”
Why is it so important to know what the animals eat and when?
“Well, I think for two main reasons. The distribution of the
species is an important issue in the management of most
conservation areas. Why are there so many elephants in the
north of Kruger? I think that you can only understand, and
maybe predict, the distribution of a species, if one understands
the diet choices it makes - what is suitable forage and what is
not? 
Secondly, animal densities are another important aspect. Is the
food availability or the food quality maybe limiting the species’
population growth? This is another important issue that can be
studied by these methods." From the wet to the dry season
forage availability and quality changes rapidly, and both the
impala and elephant deal with this by changing their diet rapidly.
Tembo scientists have found that elephants decrease the 
proportion of grass they eat from 70 to 25 percent in only a 
couple of months. However, for impalas this proportion only 
changed from 95 to 70 percent. Why then do elephants eat so 
little grass in the dry season? 
“We think that elephant can tolerate low quality food, because
compared to impala, they can survive relatively well on poor
quality forage. Therefore elephants are able to switch to
Kruger Park News : Dry Season Diet Choices : Online News Publication http://www.krugerpark.co.za/krugerpark-times-4-15-diet-choices-2466...
4 of 5 12-11-2008 10:02
mopane leaves that might have a relatively high nutrient
content, but are known for their high tannin contents.
These tannins probably also decrease the availability of 
nitrogen for the animal while digesting, but this is a smaller 
problem for the elephant than for the impala. Impala use other 
forage species, including other trees, more grass, and herbs. 
“We also found that the nitrogen content was relatively high in
these areas during our study period, and we therefore expect
that it is not the nitrogen content which is triggering the animals’
switch in their diet. 
One of the things we found is that the digestibility of the forage
– that is the proportion of the plant material that can be digested
and used by the animal - is a much better predictor of the diet
changes than the nitrogen content, especially for impala, which
are more limited by their relatively higher energy needs.
“We now hope that we can use these findings to better
understand the choices these animals make and why they
change their distribution over the year." 
So while it might be preferable for the researchers to do animal 
observations through an open car window, with a drink at hand, 
and just watch what the impala and elephants are eating while 
keeping an eye open for other wildlife, for the Tembo scientists 
who spend their time picking up droppings and smelly dung, the 
reward comes after all the dung has been ground up and 
studied, providing clues into what exactly causes animals to 
move from one place to another as the seasons change.
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Dung and droppings help explain dry season diet choices

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