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Chapter 24
Obtaining Food in Animals
Foraging: Animals show distinct behaviour patterns, organised to gain
maximum energy when they go searching for food ( foraging).
Problems they Face:
availability of food
competition
predation
physical barriers
economy
Examples of Foraging Behaviour
Planaria: complete
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Search Patterns:
1) Bees:
2) Ants:
3) Higher animals:
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Economics of Foraging Behaviour
Foraging itself is very costly. The costs will include energy losses due to
handling, digestion and foraging for food as well as fighting to overcome prey,
avoiding conflict with competitors and so on.
Predators have to weigh up these losses against potential gains and select
their options.
Optimal foraging
To be economic the animal must consume those food items which will give it
the best return for the time and energy spent.
If the energy gained from the food that it finds is less than the energy
expended on the search, then the animal suffers a net loss of energy.
Factors Affecting Foraging Behaviour and Subsequent Choice of Food
Items
1. Time
Search time - amount of time spent locating the food.
Pursuit time - time spent obtaining the food once located.
Where search time is short and pursuit time long e.g. lion and gazelles, then it
is economic to be selective i.e. the lion picks off an old or weak gazelle.
Where search time is long and pursuit time short e.g. a bird searching in
dense foliage and finding a rich supply of easily captured insects, then it is
economic to be non selective and eat as many prey items as possible since it
may take a long time to find a similar haul.
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2. Unproductive versus Productive Ecosystem
(a) In a poor ecosystem, the forager cannot afford to be choosy.
If it ate only desirable prey, too much time and energy would be
spent on the search. If it ate only poor prey, the predator would
not gain enough energy. To be economical, it has to settle for
a mixture of items with those of intermediate quality giving the
best net energy return for the time spent on the search.
(b) In a rich ecosystem, search time is reduced and the forager
can afford to be more selective since it can obtain all of its
energy requirements from a few types of choice food items.
However, in some cases, it is economical for a herbivore to settle for the food
available in a poorer ecosystem if foraging in a food-rich area exposes it
unduly to the risk of attack by predators.
Copy
Fig 24.4 p 191 Effect of poor ecosystem on economics of foraging
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3. Size of Prey
At first, net energy gain increases with increasing size of prey since larger
items contain more energy than smaller ones. However, an optimum prey
size is eventually reached.
Beyond this point, net energy gain decreases because the very largest prey is
the scarcest (hence involves a longer search time) and it tends to put up the
best fight. It is not economical if the predator has to expend much energy
subduing the prey.
Copy
Fig 24.5 p 191 Effect of food size on economics of foraging
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Competition
1) Interspecific Competition
Arises when 2 organisms of different species occupy the same ecological
niche i.e. the same position in a food web.
Interspecific competition or a limited resource may become so fierce that one
species ousts the other.
Example 1. Paramecium: complete including graphs p192
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Example 2. Squirrels : complete
Interspecific competition can be reduced when different species of animals in
a community eat different food, nest in slightly different habitats or seek food
at different times of the day etc.
Example 3. Cormorants : complete
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2)
Intraspecific Competition
Arises when competition exists between members of the same resources in
areas of overcrowding. Intraspecific competition is therefore even more
intense than intraspecific competition when there is a scarcity of some
resource e.g. food
This will result in behavioural responses such as migration and aggression
e.g. bird colonies, lemmings.
Territorial Behaviour - Intraspecific competition often takes the form of
behaviour which involves competition between members of the same species
for territory.
The size of this area defended (territory) will depend on a number of factors,
one of the most important of which is the availability of food.
Territorial behaviour can be considered in terms of costs and benefits.
The major factors are:
a) If a territory is too small, not enough food will be obtained.
b) If a territory is too big, it costs too much energy too defend it.
c) As food quality goes up, the size of the territory which needs to be
defended goes down.
d) Territories which are settled, result in reduced aggression, with
consequent energy saving.
Territorial Behaviour in the Robin complete
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Chapter 24
Social Groups
Dominance Hierarchy
Complete:
This is a system where …..
The linear form of social organisation in birds is called a …………. ………….
Make note on dominance hierarchy in mammals
List the advantages of dominance hierarchy
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Co-operative Hunting
List 4 examples of animals which rely on co-operation between members of
the social group to hunt their prey.
Describe what is meant by:
Ambush strategy
Running down
The dominant members of the social group are likely to get the biggest share
of the kill. Explain the advantages of co-operative hunting to the social group
as a whole which result in subordinate members of the group being willing to
participate in co-operative hunting.
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