Even though we
separate them for convenience, all of these topical lectures are
connected. Feeding behavior is very connected and very basic to social
behavior. The two readings that were assigned for this lecture are
examples of how feeding ecology affects social behavior.
Food Supply Available to Primates
The first thing to remember is
that primates are found in the tropics. What kind of food is available in
the tropics? Things which are quite different from what you'd find in
Tropical rainforests have leaves (lots of 'em!),
tree sap and gums (some species make their own holes to get gum while
others take advantage of trees' previous injuries), fruits, flowers,
insects, and small vertebrates (anything small enough to catch). Note
that most of these things are tree products. However, animals are also
important sources of protein. From their food source, primates need two
main things; protein and energy. Their ordinary protein source is trees
Differences in Food Dispersion and
There are two important parameters needed to describe a
food source; how much is there and where it is. In the tropics, food is
usually dispersed patchily. It is often clumped in time as well as in
space. For instance, we have data from an island in Costa Rica; 65% of
the trees in the area occurred less than once per hectare. So there must
be an awful lot of species!! Compare this to a Michigan forest where you
could find hundreds of trees of the same species all together.
better illustrate clumping in time, look at how many months in a year
these food items are available:
As you get closer to the equator, you get less seasonality,
but in some places where primates live, there is a lot of seasonality.
Leaves and stems, obviously, are very abundant. Fruits, on the other
hand, are more clumped and less available overall. They are often locally
abundant, however. Plant material is generally more abundant than animal
material. The distribution and availability of food makes a difference in
Types of Food Used and Prototypical Users
New World Monkey: spider monkey
Old World Monkey:
World Monkey: howler monkey
Old World Monkey:
World Monkey: squirrel monkey (saimiri)
Old World Monkey: none,
Ape: none, really, although chimps eat some
New World Monkey: marmoset
Feeding Behavior and Anatomy
The title of this section
should really be "You eat what you are" because you can look at some of a
primate's physical characteristics and see what its diet is.
Insectivores and gummivores are smallest
Frugivores who also eat insects are bigger
who eat leaves are even bigger
Gramnivores and folivores
are the biggest of all.
(Gramnivore means you eat stems and
Why are they bigger? There are two main
reasons. If you're bigger you need to eat more, right? So if your choice
of food is rare, you can't be too big 'cause then you can't find enough.
Animals who like food that is abundant can be pretty big. A chart
plotting body weight vs. metabolic rate slowly levels off- it's not
linear- so even though bigger animals need more calories, if you actually
measure it per pound they need less. If you graph body mass vs.
relative metabolic rate, you'd get a line with a negative slope.
For example, orangutans and gibbons both eat fruit and live in similar
habitats. The orangutan is about twenty times the size of gibbon but it
doesn't eat 20 times as much 'cause its metabolic rate per pound of body
mass is slower. Orangutans do spend a little more time feeding during a
day than gibbons do, but not that much!
How does this affect what
kind of diet they can eat? Small animals don't need a lot of food but
they need a lot relative to their size. It needs to be moved through the
body pretty quickly so that they can get the nutrients quickly enough to
keep them supplied. This means that they need high quality food that is
easy to digest. Big animals need a lot of food 'cause they're big, but
they don't need to move it through so fast since their metabolic is
To put it another way,
Given their higher
energy needs per unit weight, smaller animals must eat easily-digested
foods that can be processed quickly. Foods don't have to be abundant
since small animals require small amounts of food in absolute terms.
In contrast, large animals with smaller energy requirements per unit
wight can process food more slowly, but their total requirements are
great. Thus food must be abundant but not necessarily easy to
Insects are rare, but are of high quality.
Leaves are abundant but of low quality, and fruit is in between on both
Teeth & Guts:
Let us compare fruiteaters and
leafeaters. (Hereafter called Fruits and Leaves) Fruits have broader,
larger incisors because they tend to do more food processing with their
front teeth before they get it into their mouths. Leaves' molars have
sharp ridges with deep valleys which act like scissors to mash and crush
the leaves and cut them into pieces.
Fruits have simple digestive
systems with a small intestine to absorb sugars. Leaves have large
intestines because leaves have to move their food more slowly since they
take a while to digest. Leaves usually also have an adaptation to ferment
their foliage; the caecum with its bacteria, or just a complex stomach,
as in the colobine, or a larger lower intestine like the howlers and
macaques. The bacteria break down complex carbohydrates like cellulose
and they also take away toxins, breaking them up.
specialized teeth for gouging into trees to get sap. They need to be
clinging to the sides of trees so they have claws for clinging, like the
marmoset, for example. Gummivores also have a long caecum to break down
their carbohydrates. Insect eaters have sharp cusps for breaking the
bugs' exoskeletons. They have short, simple guts since insects are easy
Behavioral Consequences of Food Choice
Time of day: They wake up and are hungry, so
they eat quite a bit. Then around midday they don't eat so much. In the
evenings they need to stock up again for the night, so they feed more
again. So, they have two peaks: morning and evening.
influences: In orangutans, for example, there's a big dip in percentage
of time spent feeding during the months between January and April. This
is because food is more scare at these times. Since they don't feed so
much, they cut down on activities to save calories.
Home range size: Remember the graph from an early reading
with range size plotted against total weight of group. Note that as
groups get bigger they need to wander more widely. However, if you
compare leaf eaters to fruit eaters you will notice that fruit eaters
have slightly larger home ranges since their food source is less
abundant. Example: two species who live in same area, are about the same
body size and group size. The red colobus is folvorous and has a small
home range size. The patas monkey eats more fruit and has a larger home
Day range lengths: You don't have to travel far in a day to
find leaves. If you plot percentage of foliage in diet vs. percentage of
time spent feeding, those with most leaves in their diets spend the least
time moving during day, and they also have shorter day ranges. Species
not depending on leaves at all spend more time moving around looking for
stuff and have larger day ranges.
Behavioral Adaptations of
There's really not much energy available to leaf eaters.
So how do they adapt to having such low energy sources? They cut down on
energy use- like colobus frequently don't move more than a few hundred
meters in a day. Howlers are notoriously boring to study because when
they move, they don't move too far and even when they're staying in one
place they don't spend too much time running or playing or anything- they
just lie there conserving energy. Look at the reading on how monkeys
spend their time: they spend more than twice as much time resting than
any other activity combined.
Medicinal Plant Use
coursepack article on muriquis and red spider monkeys notes that they
have no intestinal parasites, but howler monkeys who live in the same
areas have lots of intestinal parasites. So it seems that some leaves do
kill parasites. It hasn't been shown, however, that that is the reason
they eat those specific leaves.
Chimps, however, show a more
thoughtful choice. Researchers will notice that a chimp will begin to not
feel well. It will begin dragging, and moving more slowly. It will go out
of its way, leaving the group entirely, to find a specific plant. They
eat the leaves in a different way than usual, too. Sometimes they'll just
lick the leaf, and sometimes they fold it up, put it in the center of
their tongue, and swallow it whole without any chewing.
companies research new drugs by looking at native people's medicinal use
of plants. They are now finding that they can learn these kinds of things
by watching the monkeys in the area as well.
An example of a plant
chimps have been seen to use in this way:Vernonia
Used by local people in these cases:
schistosomiasis (blood flukes)
anthelminthic (intestinal worms)
|Other uses; |