Feeding Behavior


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.

Nature of 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 temperate climates.
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 and vertebrates.

Differences in Food Dispersion and Availability

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.
To better illustrate clumping in time, look at how many months in a year these food items are available:
young leaves6.8
flowers2.7
ripe fruit1.1

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 behavior.



Types of Food Used and Prototypical Users

Frugivores
Prosimian: ring-tailed lemur
New World Monkey: spider monkey
Old World Monkey: red-tailed monkey
Ape: gibbon
Folivores
Prosimian: indri
New World Monkey: howler monkey
Old World Monkey: leafmonkey/colobine/langur
Ape: gorilla
Insectivores
Prosimian: tarsier
New World Monkey: squirrel monkey (saimiri)
Old World Monkey: none, really
Ape: none, really, although chimps eat some
Gummivores
Prosimian: bushbaby
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.

Body size:

Insectivores and gummivores are smallest

Frugivores who also eat insects are bigger

Furgivores who eat leaves are even bigger


Gramnivores and folivores are the biggest of all.

(Gramnivore means you eat stems and leaves)


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 slower.

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 digest.

Insects are rare, but are of high quality. Leaves are abundant but of low quality, and fruit is in between on both axes.

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.
Gummivores have 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 to digest.

Behavioral Consequences of Food Choice

Temporal factors

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.
Seasonal 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.

Spatial factors

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 range.
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 Folivores

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

The 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.

Some drug 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 amygdalina
Used by local people in these cases:

Parasites;
schistosomiasis (blood flukes)
malaria
anthelminthic (intestinal worms)
amebic dysentery
intestinal disease
constipation
diarrhea 		Other uses;
scurvy
coughing
rheumatism
pms