The document is a summary of the theory explained in the lectures of the course Behavioural Ecology and of the recommended chapters of the linked textbook as stated on the course guide.
Behavioral ecology = study that combines different disciplines to study animal behavior based on the
ecological/environmental context. Behavioral ecology also makes used of evolutionary biology to
understand how and why a behavior developed. Behavioral ecology in short is the evolutionary basis for
animal behavior due to the environment.
Tinbergen’s four “why” questions
Tinbergen’s four questions are used to answering questions about behavior: SAPERE PERCHè è UN’EXAM
QUESTION
- Questions about causation or mechanism. These are questions about the mechanisms that
underlie or cause a certain behavior, e.g. what mechanisms or stimuli cause birds to sing? How do
they do it?
- Questions about development or ontogeny. These are questions about how a certain behavior
developed during the lifetime of an animal, and how the behavior is influenced by experience and
learning, or is it influenced by genes. E.g. how do courtship of females improves with age among
male birds?
- Questions about adaptation of function. These are questions that help understand how a certain
behavior increases the fitness of the animal, and why the animal does it/what is the function of this
behavior. E.g. why do birds migrate?
- Questions about evolution or phylogeny. These are questions about how a certain behavior has
evolved in the species. E.g. how did singing in a certain bird species evolve?
Causation and development are referred to as proximate questions/explanations, because they investigate
the immediate mechanical reasons for why a certain behavior is expressed during life. Adaptation and
evolution are called ultimate questions/explanations, because they explain how/why a species evolved a
certain behavior and what is its function.
Example Why do blue tits sing? Possible answers:
- Causation/mechanism because the days become longer in spring and they are stimulated to sing.
- Development because they learned it from their parents
- Adaptation/function because they want to attract females
- Evolution because their common ancestor was a songbird
An example to illustrate these questions is about reproductive behavior in lions lions live in prides
consisting of some adult females, some adult males and the cubs. While the females live in the pride where
they are born, males leave their pride at 3 years old and try to conquer another pride from old males. Two
interesting observations in prides, with their respective causal and functional explanation are:
- Females go in estrus at the same time. The mechanisms/causation is probably the influence of
pheromones. One adaptive advantage of synchrony is that different litters are born at the same
time, and cubs can survive better because they can suckle from multiple females.
- When a new male takes over a new pride they sometimes kill the cubs from the previous male. The
causal explanation could be that the unfamiliar odor induces him to kill them. In addition, the
, benefit of infanticide for the male is that killing the cubs from the previous male brings the female
into reproductive condition much faster, so he can reproduce faster.
Natural selection
The theory of natural selection from Darwin is summarized as:
1. Individuals in a species have different morphology, physiology and behavior, i.e. there is variation.
2. This variation is heritable by the offspring.
3. In a population there is competition between individuals for scarce resources such as food and
mates.
4. Some variants will be better at competing, so they will survive and produce more offspring. Their
offspring will inherit their successful characteristics and so on. Therefore, the species becomes
adapted to the environment through natural selection over the generations.
5. In case the environment changes, other variants may be better at surviving, so natural selection can
lead to evolutionary change.
The theory can be restated in more genetic terms 1) all organisms have genes that code for proteins.
Proteins regulate the development of their organism and consequently influence their behavior. 2) within a
population many genes are present in two or more forms called alleles. These alleles cause differences in
development and function, i.e. variation. 3) any allele that results in more survival of the organism that
possess them compared to the alternative allele will replace the alternative form, i.e. there is a change in
allele frequency.
Natural selection favors individuals that adopt life-history strategies that maximize their gene
contribution to future generations. In other words, it favors individuals that can produce a lot of offspring
successfully.
Genes and behavior
Natural selection can only occur when there is genetic variation in the population, i.e. genetic differences
between individuals. Therefore, for a behavior to evolve: 1) there must be more than one behavioral
alternative in the population (e.g. multiple foraging strategies); 2) the differences must be heritable; and 3)
some behavioral alternatives must result in higher reproductive success than others.
Below some examples on how genetic differences between individuals can lead to differences in behavior
are discussed. Keep in mind that:
- The molecular pathways that link genes and behavior are complicated, as they involve genes,
sensory systems, nervous system etc.
- Genes influence behavior, but behavior can also influence gene expression.
- Behavior is not only influenced by genes, but also by environmental factors. Therefore the
presence of a gene doesn’t automatically mean a certain behavior will be performed; appropriate
environmental stimuli should be present.
Behavior is designed to make individuals fit with the environment. In a population we can see animal
might adopt different strategies, i.e. there is variation. If the animal is successful at adapting in the
environment, it will gain access to resources, survival, reproduction. Individuals that have a competitive
advantage compared to others will do best. This competitive advantage might be heritable to the next
generation if it is at least in part caused by genes. There is also the concept of phenotypic plasticity
(spiegato nelle pagine successive), i.e. animals can adjust their behavior based on the context.
,Example moths moths can have two different morphs/phenotypes: black or white. It was observed that
black moths tend to do better in a polluted environment because it can better mimetize on the dark
environment (a causa dell’inquinamento c’è lo schifo depositato in giro), while the white one does better in
the unpolluted environment.
Example foraging strategies in Drosophila fruit flies have two foraging strategies: rovers fly around to
look for food, while sitters tend to remain in a small area to feed. The difference in strategies is caused by
the foraging gene for. Flies with the forR allele become rovers, and forS flies become sitters. forR flies also
have a better short-term memory for olfactory stimuli, which may benefit rovers as they more around to
look for food. forS flies have better long-term memory, which may benefit sitters because they are more
sedentary. In addition to this gene, the different strategies are also caused by another factor, i.e. food
availability and competition. When food is scarce, competition is more intense between individuals of the
same morph, i.e. sitters compete with sitters and rovers with rovers. This leads to a situation where the
rarer type has an advantage (because there is less competition for it?): rovers do better in a population of
sitters and vice versa.
Example MC1R gene for mate choice and camouflage the MC1R gene encodes for a receptor that is
responsible for regulating the amount of pigment melanin in the skin. Mutations in this gene cause color
variation in several species. In snow geese, individuals that are homozygous for one allele are white, while
individuals homozygous for the other allele are blue. Color influences the choice of the mate: white geese
mate with white geese, and blue with blue. In addition, goslings imprint on their parents’ color and then
choose a mate of the same color.
Example migratory behavior in blackcaps blackcap populations in Germany are highly migratory, while
those in the Canary Islands are sedentary. During experiments, birds from these two populations were
cross-bred and the hybrid offspring had an intermediate migratory behavior, suggesting that migratory
behavior is caused by genes. In addition, central European populations traditionally winter in the
Mediterranean area but during the past 40 years increasing numbers of them started wintering in Britain.
When tested in captivity, it was shown that this new migratory behavior was inherited by the offspring,
again suggesting the role of genes. This new migration direction is probably favored because of milder
winters and more winter food in Britain than in the past. In addition, they have to travel a shorter distance
and they can return back to central Europe earlier and take the best breeding territories.
Selfish individuals or group advantage?
In the past, there was a tendency to think that animals behave for the good of the group, e.g. that male
lions don’t kill each other during fights to avoid endangering the species. However, it is not how it works,
individual animals do not act to “save the species”. Rather, many traits evolve because they are
advantageous for the single individual, even if they may be negative for others in the population. E.g.
infanticide in lions is advantageous for the male lion, but is of course disadvantageous for the killed cubs
and the female.
Example optimal clutch size in birds à there is large evidence that individuals try to maximize their
reproductive success, rather than restricting their reproduction to avoid exploiting too much resources for
the group. Experiments were done in great tits to study optimal clutch size. Most
pairs of great tits lay 8-9 eggs. When they lay larger broods they can still incubate
them successfully, but the parents cannot feed larger brood so well and the
chicks have a lower body weight when they leave the nest. Heavier chicks of
, course would survive better, therefore a parent that lays many eggs will actually have lower reproductive
success because the nestling are not fed adequately and survive less.
The clutch size observed most commonly in a species is slightly lower than the optimal predicted
size. One reason for this is to maximize the number of surviving young, because there is a trade-off
between chick quantity and quality: if more offspring is produced, parents will give less care to each of
them which results in lower quality of the offspring. Another reason is that there is a trade-off between
reproductive effort from the parents and adult survival: females that put more effort in reproduction by
producing more eggs have less chances of survival to the next breeding season. The graph shows that the
costs i.e. adult mortality is increased as the clutch size increases. The benefits of producing more eggs, i.e.
a higher survival of the young, increase until a certain optimum clutch size, but then start decreasing as
the clutch size increases. b1 indicates the clutch size that maximizes the number of young produced per
brood, while b2 is the clutch size that maximizes lifetime reproductive success, where the distance between
costs and benefits is larger. In the short term, the animal might think it is better to maximize the number of
offspring in the current brood (b1), and if you see the graph this number actually gives more benefits than
producing a smaller brood (b2). However, producing a clutch of size b1 gives more costs than producing one
of size b2. b2 is the optimal clutch size to maximize lifetime reproductive success, because the bird gets
benefits while reducing costs as much as possible. Therefore, they produce a limited number of eggs that is
lower than the predicted optimal number, to increase the chance of offspring survival without having too
high costs (i.e. adult mortality). Note that the optimal clutch size also depends on environmental
conditions, e.g. if there is less food available, the optimal clutch size will be smaller.
Other examples of trade-offs is between reproduction and feather growth if birds put too much energy
into reproduction, they have less energy for physiology and production of healthy feathers.
Phenotypic plasticity: climate change and breeding times
The ability of an individual to alter its phenotype in response to environmental changes is called phenotypic
plasticity. When the phenotypic variation is continuous, the relationship between phenotype and the
environment is called reaction norm.
Example phenotypic plasticity in great tits à in the last decades, female great tits in
Britain start laying their eggs about 14 days earlier than usual in spring, in line with the
increase in spring temperatures. The higher temperatures make moth caterpillars develop
earlier, and they are the food given to chicks. For many birds, the main signal to start
breeding is the increase in photoperiod in spring, but can also be regulated by
temperature, food availability and social context. Experiments have shown that the earlier
breeding observed in great tits is due to phenotypic plasticity, but not due to genetic
change. All females studied in Britain had similar reaction norms, i.e. they all responded
strongly to the change in temperature by laying eggs earlier as the spring temperature
increases (grafico a). In the great tit population of the Netherlands however the egg laying
date is not changed despite the same environmental changes occurred as in Britain. The result
is that many birds in the Netherlands are now breeding too late when the food supply is more
scarce, so reproductive success declined. As can be seen from graph b, females in the Dutch
population varied in their phenotypic plasticity: some of them responded very little to the
variation in temperature, while others showed stronger responses (steeper lines). Why do the
Dutch and British tits differ? One possibility is that females in the two populations use different signals to
start laying eggs, e.g. the British population may use temperature as a signal while the Dutch ones use other
signals. Another reason could be that they use the same signals but in Britain these signals can predict the
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