Module 1: Ecological Interactions and Energetics

Study Reminders
Text Version

Study of Behavior and Behavioral Ecology

Set your study reminders

We will email you at these times to remind you to study.
  • Monday




























Today we begin our next lecture which is the Study of behaviours and behavioural
(Refer Slide Time: 00:17)

Before we begin let us look at the definitions. What is behavioural ecology? Behavioural
ecology is the study of the evolutionary basis for animal behaviour due to ecological
pressures. Essentially what we are saying when we say behavioural ecology is that, we say
that there is an ecological pressure which is governing the behaviour of animals and we
are trying to understand what is the relationship between both of these and how that also
plays a role in the evolutionary process.
Behaviour refers to the ways that organisms respond to each other and to particular cues
in the environment.

(Refer Slide Time: 00:58)

Essentially, if you have a stimulus and that stimulus is resulting in a response. The way in
which the organism is utilizing or sensing this is stimulus making all the computations
about what to do and giving out this response, all this, is known as behavior. The study of
behaviours; the scientific study of animal behaviour goes by the name of ethology.
(Refer Slide Time: 01:25)

There are a number of topics that we study in the case of behavioural ecology. We study
foraging behaviours which means how do animals eat, how do they decide, which thing to
eat, what not to eat and so on. Anti-predator behaviours: so, how do animals try to avoid

their own predators, how do they try to save themselves. Social behaviours: how do
animals behave in a group and also the mating behaviours and so on.
(Refer Slide Time: 01:54)

One fundamental approach through which we understand behaviours goes by the name of
the cost benefit approach. Now the cost benefit analysis is an assessment to determine
whether the cost of an activity is less than the benefit that can be expected from the activity.
To give a simple example, suppose I consider that the cost of this pen is say 10 rupees and
the benefit that I will derive by using this pen or by getting this pen is say 15 rupees. So,
in that case, I will go and get this pen, so that will be a behavior. On the other hand, if I
think that the cost of this pen is again 10 rupees, but the benefit that I will get out of getting
this pen is say 5 rupees. In that case, why should I go for this pen? Maybe I will not get to
go for this pen.
Similarly an animal also has to make a number of choices. For instance there is a tiger and
that tiger sees a sambar in front of it. This tiger has to make some calculations, how far is
that sambar, what is the terrain that is between the tiger and the sambar. If that terrain is
extremely rugged, if this tiger has to say climb up a mound and then get back. Probably it
will not go for hunting this sambar because the activity by which it will go and hunt the
sambar will also entail quite a lot of cost in terms of energetics. The tiger will be expending
its energy to get to that sambar to chase that sambar and to kill that sambar. The benefit

that the tiger will get if it goes for this attempt is the energy or the nutrition that will it will
get by eating that sambar.
If the cost is more than the benefit. If the terrain is extremely rugged and there is a very
low probability that this tiger will be able to eat the sambar. In that case when the cost is
more than the benefit, the tiger will probably not go for this; for chasing this sambar. On
the other hand if the benefit is much greater than the cost. For instance this tiger sees that
it was resting somewhere and this sambar came near itself. It does not have to run a very
great distance. At the same time this tiger is hungry. Essentially it requires these nutrients.
It puts a very high price or a very high value on nutrition. In that case, this tiger will go
and attack the sambar and maybe kill it and eat it.
(Refer Slide Time: 04:32)

Essentially we perform something when the cost less than the benefit so, in that case you
have the behaviour that will happen. If the cost is greater than the benefit, then you will
probably have some other behaviour which will refer to as by B‘. In this case the second
behaviour in the case of our tiger would be to do nothing or maybe to take some bit more
of rest. In this context, this curve becomes important. Here we are trying to put, so in this
case we are trying to understand what is the territory size that will be used by an organism?
By putting this territory size here and the cost or benefit as the territory size changes on
the y axis.

The y axis is cost of benefit, the x axis is the territory size. For instance if there is a tiger
that has a small territory. If you have a small territory, you have a small amount of benefit
because you can only hunt “n” number of animals that are present in your territory and at
the same time the cost of defending that territory is also less because there is a very small
area that needs to be defended. If the territory size increases so, the cost increases.
The tiger will have to expend much more amount of energy to defend that territory. The
cost of defense increases like this. Earlier it was very less and then it is increasing
exponentially and then, when you have a very large size territory, any small bit of increase
in the size will entail quite a lot of cost to defend that territory. On the other hand if you
look at the benefits that are gained; if you have a very small territory, you have a very little
amount of benefit. As territory size increases your benefits increase, but then they start
getting saturated after a while because one instance if a tiger is able to kill only 1 prey
every week. It is killing somewhere around 50 preys in a year. If it is able to get 50 preys
in this much size of territory that is good enough for it.
Now, if that size increases so, at this size you have like 100 animals that are available to
be killed; at this size we have around 200 animals that are available to be killed, but then
a tiger is only going to eat somewhere around 50 animals. So, what is it going to do by
having 200 animals in its territory? That does not make any sense. In that case we will say
that the benefit increases, but after a while it starts saturating, which is why we are saying
that this curve is becoming more and more parallel to the x axis.
As territory size increases, so, in this area we will say that the benefit is greater than the
cost in this area. So, all of these territory sizes are those that the tiger will go for. If we
look at this area, so, at this particular size of territory we have the cost of defense is greater
than the benefit that is being provided by that size of territory.
The tiger may not go for that size of a territory. So, all of these, the curve that is from here
to here, now this is the area in which the tiger is going to operate. Amongst all of these
different sizes there would be one size that is the most optimum size. If the tiger goes for
this size, so, the benefit minus the cost which is the profit is with very large. If for instance
the territory size is this much, so, let us draw this curve.

(Refer Slide Time: 08:27)

Here we have the territory size and here we have the cost or benefit. Now in this case, the
cost increases like this and the benefit goes like this. Now this is the benefit; now if we are
considering a territory size of this much. At this territory size, the cost and benefit are
equal. So, the profit, net profit is 0 and similarly at this particular point you have the net
profit is equal to 0. Now at this size of the territory, we have a profit that is given by this
much which is the benefit minus the cost. And at this size you have a profit that is given
by this much size which is the benefit minus cost and if we take another area to the left,
again the profit reduces.
If we say that we have these profits of P1, P2, P3, so, in that case, we have P1 is less than
P2 and P3 is also less than P2 so, P2 is the maximum. In this case, this territory size will
be called as the most optimum size of the territory because here your benefits are much
greater than the cost. If we consider a territory size like this. So, at this particular size, we
have the benefit that is given by this much. This is the benefit and the cost is given by this
much. So, the benefit is less than the cost. Actually the tiger will be expending much more
amount of energy and is getting very little in return. So, probably it will not go for that
particular size. By doing such an analysis we can compute or try to understand why certain
behaviours are preferred by animals and why certain behaviours are not preferred by the
Now, we will look at another example.

(Refer Slide Time: 10:44)

This example is about why do carnivores live in groups? Now here we are considering a
theoretical case in which you have lions and the x axis is showing you the group size of
the lions and the y axis is showing you the group hunting success. Essentially, group
hunting success means that if the group of a particular size is going for a kill, so, it has
spotted a prey animal and it is going for a kill so, what is the probability that this group
will be able to kill the prey and get the food? Now if there is only 1 lion in the group, so,
if you have the point here. So, the group hunting success is only about 0.3 which means
that only three out of 10 animals or roughly 1 out of 3 animals that is being hunted gets
Now, if you have more number of lions in the group. Suppose you have 2 number of lions,
so, the hunting efficiency increases point from 0.3 to 0.5. In the first case you had a
situation that the lion was able to get food only one in three cases and the second case it
gets food roughly 50 percent of the time. And as the size of the group increases and if they
are able to cooperate fully, so will have a situation in which the group hunting success will
become closer and closer to 1. Essentially if you have like 10 lions in the group and these
10 lions are cooperatively trying to hunt an animal so, there is a very good chance that they
will actually be able to hunt that animal, but that only happens if the lions are cooperating

Whereas, if the lions are trying to cheat or if the lions are trying to hunt by themselves and
they are not cooperating. So, one lion is not cooperating with the next lion; so the hunting
efficiency will not increase. So, this is what our theoretical model states. Now if you look
at actual field situations of the lions and if we see these lions of Uganda so, we will observe
that the actual situation is somewhere in between. Actually you have lions that cooperate
for some time and they also try to cheat for some. This is a way in which we can understand
why the lions are living in groups.
(Refer Slide Time: 13:13)

We can even dissect it further by looking at the cost and benefits of group living for lions.
Now we consider the cases of males and females separately, now if you are living in a
group and if you are a male lion, so, there will be a sharing of paternity which means that
if there is only 1 lion and or say, in the group you only have 1 male lion. All the cubs that
are born in this particular pride will be fathered by only that particular lion, but if that lion
is allowing another male lion to live inside that group so that male lion may also give rise
to the progeny.
In that case the paternity of the cubs is being shared by both of these lions. If you have say
3 or 4 male lions that are living together so, all 4 of them will be sharing the paternity or
in other words the lion, the 1 male lion that was earlier having an exclusive access to all
the females, now has to share the females between the other lions.

So, that is a cost of grouping for the male lion. The benefit of grouping for the main lion
is that it has an increased access to mates. For instance if you had only a single lion so, it
would not be able to compete with the already existing prides. It will not be able to make
a space for itself, but then if it combines with another male lion and if they work
cooperatively, in that case they will be able to topple the other prides and take their
females. So, the benefit of grouping is that it increases the access to mates and also it
provides a protection to the offspring against infanticide.
What is that mean? If you have a pride in which you have male lions and female lionesses
and if that pride is toppled over so, essentially the male lions have been defeated by an
incoming group. What this incoming group will do is that, it will kill off all the cubs that
are there in this pride, why? Because the male lions that are coming from outside they also
want to mate and the females will not get into heed, if they are having cubs with them.
So, the incoming lions will try to kill off the existing offsprings to make way for their own
offsprings. As we saw in one of the earlier lectures, we always talk about fitness. Now
fitness does not mean that you give rise to more number of progeny. It means that you give
rise to more number of progeny and more and more of them are able to survive and able
to reproduce to the next generation as well.
If the lions are able to produce cubs, but if those cubs get eaten up or are killed by incoming
lions, that does not make much sense evolutionarily. Which is why we have this behaviour
that the lions, even though they have this cost of sharing the paternity even then they will
try to remain in group, they will also allow other lions to come and cooperate with
themselves. By having an increased access to mates and by providing a united front, it is
much more plausible that the pride in which this cooperating lions are there will not be
toppled and so their offsprings will have protection against infanticide.
Now that is for the main lions. What is the benefit for the female lions or the lionesses?
Why do lionesses also live in the group? Lionesses too lives in groups, so again here we
have the cost and benefits. The cost of living in the group is a lower rate of food intake
because the larger the size of the group, the lesser the amount of food that you will have
from every hunt whereas, it also has some benefits; there is help from the kin and there is
territorial defense.

In protection of their cubs and in performing the grooming of the cubs they also get help
from their siblings. And then it also helps in territorial defense because you have more
number of lions and lionesses that are defending the same amount of territory. As we saw
before, there is a cost, there is a benefit both for males and females. So, the males and the
females will normally go for the cost benefit approach and wherever the benefit minus the
cost is the greatest, that would be the optimal number of males and the optimum number
of females that will come together to form the group.
(Refer Slide Time: 18:05)

Now, similarly herbivores also live in groups, why?

(Refer Slide Time: 18:09)

This paper demonstrated that when you have bisons that are living together and there is
this pack of wolves. Now, if again, if you have a big sized pack of wolves so, they will be
able to hunt much more efficiently if they are able to cooperate with each other. Similarly,
if the bisons are there in the group, in that case the wolves will find it extremely difficult
to hunt the bisons because they will put up a united front for defense.
(Refer Slide Time: 18:38)

If the bisons becomes solitary, if one of the bison lags behind, then that case that particular
bison will be attacked by the wolves from all sides and then it will get eaten up.

(Refer Slide Time: 18:51)

(Refer Slide Time: 18:53)

We can see that in the case of group living animals if you look at the cost and benefits the
benefits are increased foraging efficiency. Essentially if you are hunting together, if you
are living together, if you are hunting together so, there is a greater chance that you will
be able to hunt somebody. In the case of these wolves they were able to hunt because they
are of a larger size. If there was a single wolf, probably, it would not be able to hunt the
bison. On the other hand, the cost is that there is competition for food and there is an
increased risk of diseases or parasites.

Now because this bison was hunted by so many wolves, so, all these wolves will have to
share the flesh. In that case that is a cost that is associated with the group living and at the
same time because they are living together, if one of these wolves gets sick, so, that will
also spread diseases to the other wolves. Hence, by living together there is a greater
propensity that you become prey to diseases.
The other potential benefit is that of reduced predation whereas, the cost as the attraction
of predators. What do we mean by this? If you are living together so, for instance; if this
bison was not a part of this group, but was living alone. In that case this bison would
normally have been hunted long back, but because you have all of these bisons that are
putting together a united front, so, only one of them will get hunted and the others will be
able to survive.
So, that is a benefit. The cost is that if you have a larger sized group, so, more number of
predators might get attracted to hunt this particular group. Here again, you have cost and
benefits. Other potential benefits are increased access to mates whereas, the potential cost
are loss of paternity and brood parasitism. Now what does that mean? If you are living
together in a big group so, the animal has got a greater access to the mates because you
have more number of males and females that are living together; so, it is much more likely
that you have an access to a mate.
The potential cost is loss of paternity as we saw before because the mates are not
exclusively available to any particular organism and also there is a chance of brood
parasitism in which you can have a situation where an outsider can come into your group
mate with the females and then move out.
Similarly, you have the potential benefit of help from the kin if you are trying to raise your
kids and the potential cost is loss of individual reproduction.

(Refer Slide Time: 21:49)

Now by looking at different cost and benefit analysis, we can also understand various
ecological behaviours like this community interactions that we saw before. You have this
langur, here, you have the chitals here and they are interacting because the chitals are
getting access to the leaves and the fruits from these trees when the langur is dropping
them. The chital is also having a benefit of having these eyes; the eyes of the langurs that
are on top of the tree and are able to see the predators.
On the other hand, the langurs are also getting some amount of benefit from the chitals
because the chitals are able to sense the predators from a lower height. These cost and
benefit analysis can help us understand different community interactions in ecology.

(Refer Slide Time: 22:37)

Another example is, if you are living in a group, then there is a chance that you will find a
predator nearby and you have to make a decision whether or not you should give out an
alarm call.
What is an alarm call? This is a ground dwelling squirrel. This is a species of squirrels that
lives on the ground, it lives as part of a group and whenever it sees a predator it gives out
this alarm call. If you give out an alarm call, you are alerting everybody in the group that
there is a predator nearby, but then the predator will also hear you alarm call. In that case
you are making yourself much more visible to the predator. There is a chance that the
predator in place of hunting anybody else will go after you because you have made your
position extremely clear to the predator by giving out this alarm call. So, again you have
the cost and benefits of giving out an alarm call.

(Refer Slide Time: 23:57)

You have the cost and you have to benefit. The cost is that you are using up your energy
to make the alarm call and the other cost is that you are making yourself visible to the
predators. The benefit is that you are able to save the group. Now the question here is, the
cost is something that you are entailing to yourself. You can get killed if you make the
alarm call. The benefit is something that you are making to the group because you are
saving not yourself, but the other organisms that are there in the group.
Here we are looking at one question of population dynamics. We have this population of
ground squirrels and every squirrel has to make this decision whether or not to give an
alarm call. If there is an alarm call, it is putting itself at risk, it is using up its own energy
and the benefits are not coming to itself, but are coming to the other members that are there
in the group.
In the first instance it might look like any squirrel that is giving out an alarm call is
sacrificing itself for the group. Is it a case of altruism or are we observing some other
points that are working here or are at play here to give up this behaviours?

(Refer Slide Time: 25:31)

So, this phenomena was studied. Scientists tried to understand, when do you make a call.
So, here we have a graph of the callers regardless of residence to a predatory mammal.
Here we see that if you have these adult females and here we have an expected rate of
calling and here we have the observed rate of calling. Now expected rate of calling is given
by computing – the expected values are computed by assuming that animals call randomly
in direct proportion to the number of times they are present when a predatory mammal
appears. Essentially if you have more number of females or if you have situations in which
the females were there, when there was this predatory animal that had come there. So, we
put a random chance that every animal is going to make a call every “n” number of times.
So, from that we get this expected rate of calling and here you observe the observed rate
of calling. This is what actually happens in the ground. Now you can see here that the
females were expected to call say 41 percent of time, but they actually called around 65
percent of time. So, their calling was much greater than what we had expected.
In the case of adult males, we had expected them to call around 25 percent of times, but
they actually called just around 8 percent of time. Now in the case of 1 year females, we
had expected them to call say around 15 percent of times, but they also called somewhere
around 38 percent of times. So, in the case of 1 year females as well as in the case of the
adult females, we observe that they are observed rates of calling are much greater than the
expected rates of calling.

In the case of 1 year males, we had expected them to call around 10 percent of time and
they actually call say around 5 percent of 9. Here also the observed rate was much less
than what was expected. And in the case of juveniles it was very much less than what was
(Refer Slide Time: 27:51)

Now the question is so why do females call more than expected and why do males called
less than expected? What is there that is making the females put themselves more at risk
to alert their group? This question brings us to the concept of kin selection. In the case of
these ground dwelling squirrels it is observed that the females in generally live together
and the males after they have reached some stage of maturity they move out.
Essentially, if a female is calling up, is giving out this alarm call, it is alerting most of its
own relatives whereas, the male does not have much to lose in terms of genes if one of the
members or more of the members of the group gets killed. Because the male wants to
protect its own genes because it is in any case going to move somewhere else. The male
puts much more price on saving itself than saving the group whereas, the female puts a
much greater price on saving the group than on saving itself because it has more number
of genes that are common with that of the group. Here we have the concept of kin selection
and kin selection see is the evolution of traits that increase the survival and ultimately the
reproductive success of one’s relatives.

(Refer Slide Time: 29:18)

Now, in the case of kin selection, if we are observing this behaviour that you have
organisms that are even willing to sacrifice themselves to save somebody else from their
own kin which are relatives. If this behaviour is being observed, it means that this
behaviour must have evolved over time.
Now if there is a behaviour that has evolved over time, then this behaviour must be
providing more fitness. Because if you have a situation in which there is a behaviour that
gives you more amount of fitness only then that behaviour will be selected through the
process of natural selection and only then such a behaviour will be evolved and which is
why we are observing this behavior. Now the question is, how is it possible that by
sacrificing yourself you are getting a fitness? and by you; I mean not just you, but your
genes how are your genes getting to a situation that they are more fit to survival if they are
Here we also have another corollary which is known as group selection. Natural selection
for traits that favor groups rather than individuals because group selection operates much
more slowly than the individual selection so, it is a much weaker selective force in most
circumstances. Now in this case, what we are seeing is that when we have the kin selection,
we are also observing the group selection. Kin selection is only working when you are
working in a large size group and in this group your own fitness is not that important as is
the fitness of the whole of the group. Now why is that so?

(Refer Slide Time: 31:18)

Hamilton gave us this rule which says that genes increase in frequency when we have this
formula it works. So, we have r into B is greater than C where r is the genetic relatedness
of the recipient to the actor; often defined as the probability that a gene picked randomly
from each at the same locus is identical by descent. So, r is the genetic relatedness between
the recipient and the actor. In this case the actor is the ground squirrel that is giving out
the alarm call and the recipients are the other members of the group. Now B is a additional
reproductive benefit that is gained by the recipient of the altruistic act.
Now, reproductive benefit that is gained by the recipient; by this we mean that when the
ground squirrel made out the alarm call the other members of the group, they were saved
and they were able to reproduce and their progeny was able to survive. So, that is the
additional reproductive benefit that it got because there was one squirrel that sacrificed
If that squirrel had not sacrificed itself or had not given out the alarm call which increases
the chances of it getting sacrificed so in that case more and more members of the group
would have been killed and so their progeny would also be less. And C is the reproductive
cost to the individual performing the act which is the reproductive cost of the squirrel that
gave out the alarm call.

(Refer Slide Time: 32:58)

Basically what we are seeing here is that if you have an individual and this individual has
a number of other relatives. Now, if you have to make a choice whether you should kill
yourself or whether you should have the whole of this group killed; so how do you make
that choice? It will depend on how much are you related to the big group. So, for instance
if the level of relatedness is very less. So, basically this individual is related to this
individual and all the other members are not related to the actor individual or A.
So, the other members are not related to A only B is related to A. So, in that case because
A and B have different have different genes. So, A would try to protect itself even at the
cost of the whole of the group because it does not have much to lose if the group dies off.
On the other hand, if A has a relative here, here, here so, if all of these are related to A. So,
in that case, if a sacrifices itself and is able to save so many members of the group, so,
more and more of the genes that were present in A are getting a survival because the group
gets the survival or to put it in the words of Haldane. So, Haldane give up this statement,

(Refer Slide Time: 34:35)

If an individual loses its life to save two siblings. Now you have a situation.
(Refer Slide Time: 34:41)

You have a mom and a dad and then you have say 3 siblings. These are the paternal
chromosomes. So, if this is paternal, this is maternal. This individual has 50 percent genes
from P and 50 percent genes from M and similarly this individual has 50 percent from P,
50 percent from M and similarly this individual has 50 percent from P and 50 percent from

Now, if this individual, if individual A sacrifices itself to save B and C together. So, in
that case by losing 50 percent of genes of P, it is saving more than 50 percent genes of P.
So, by losing 50 percent of genes of P by sacrificing itself, it is able to save more than 50
percent of genes of P and by losing these 50 percent of genes of M, it is able to save more
than 50 percent genes of M.
Essentially in genetic terms, it is a benefit. So, if an individual loses its life to save 2
siblings or 4 nephews or 8 cousins, it is termed to be a fair deal in evolutionary terms as
siblings are on average 50 percent identical by descent, nephews are 25 percent identical
by descent and cousins are 12.5 percent identical by descent.
(Refer Slide Time: 36:20)

Another example of how behaviour plays a role in ecology is the case of territoriality.
Territoriality is a type of intraspecific or interspecific competition; intraspecific is within
the same species, interspecific is between two or more species; that results from the
behavioural exclusion of others from a specific space that is defended as a territory.