Lecture Notes 5 – Behavioral Ecology
This is a single chapter in the book, but since I am a behavioral ecologist, I really enjoy the material and
have therefore made it a whole lecture.
Introduction to Behavior:
- In 1973 the Nobel prize for medicine / physiology was given to three scientists who founded the field
of animal behavior or “Ethology”. Why would such an award be given to behavioral scientists? Perhaps
because these scientists studied behavior as a physiological process. They were interested in finding
very simple rules behind behaviors, similar to how if a doctor hits your knee with a hammer, your leg
swings forward reflexively.
- In this way Konrad Lorenz studied how young animals like baby birds “imprint” upon the first thing that
they see. If a baby bird sees a human caring for it when it is very young, it will imprint on the human,
and follow that human around.
- Karl von Frisch discovered the “honey bee dance language” whereby bees are able to signal to each
other where to find flowers by dancing in the hive, giving information about the angle to fly (relative to
the sun) and how far to fly.
- Niko Tinbergen worked on what he called “release stimuli”, which is an event that releases or starts a
behavior in an animal. For example, if a baby bird touches the bill of its parent, that stimulates the
parent to regurgitate food.
- Collectively, you can see they concentrated on understanding simple rules of behavior, and they are
considered the fathers of this field.
- One of Tinbergen’s achievements was also to clarify what kinds of studies behaviorists can engage in.
He saw four different kinds of studies: those that explained how a behavior physiologically occurred,
those that explained how it develops through the life of the individual, those that aim to understand
how that behavior is adaptive (fits the environment and contributes to the reproductive success of the
individual), and those that aim to understand how the behavior has evolved in related organisms (its
evolutionary history).
- For example, in studying a behavior such as bird song, one can ask: how does the bird physiologically
produce the sound (physiology), how do birds learn song during their lives (development), how do birds
use song to find a mate (adaptive function), or how has song changed in the diversification of a bird
family (evolutionary history). These are different kinds of questions.
- We will start a little bit by talking about development, because this was a particularly large question in
the beginning of the field of animal behavior: what proportion of a behavior is genetically hard-wired
(encoded in genes), and what proportion is learned?
- As you might have understood in understanding what those early 3 ethologists were interested in,
these scientists viewed much of behavior as genetically encoded (just quite the reflex of the knee). An
example comes from the work of Tinbergen, who should that mother birds, if shown an egg, will
instinctively try to roll in towards their nest and sit on it (a stimulus of a behavior).
- Amazingly, if given a huge egg (what Tinbergen called a “superstimulus”), the birds will get very excited
and continue to try to sit on it. This explains why some birds have become “cheats”: cuckoos are birds
that lay their eggs in other bird species’ nests. Even though the cuckoo chick is very large compared to
the parents’ other offspring, the adoptive parents care for the huge cuckoo chick (which often kills some
of the parents real offspring).
- In contrast to the ethologists, there were another group or school of people interested in animal
behavior who focused on learning. These scientists tended to call themselves “psychologists”. A
famous one of these scientists, by the name of B. F. Skinner, did a lot of experiments with pigeons (not
the smartest birds!). But by carefully rewarding the pigeons with food when they did something
correctly, Skinner could train the birds to do quite complex tasks.
- Hence the debate between people who saw behavior as instinctive (“Nature”) vs those that saw
behavior as learned (“Nurture”).
- Today we see that both these schools were somewhat correct. All behaviors have some innate
components and some learned components. Or in other words, some behaviors are innately more easy
to learn than others. Let’s consider an analogy of an object (the animal) placed in an environment (the
landscape).
- In slide 7 we see behavior as completely genetic. A square is placed in a flat landscape. It does not
move. It stays where it was put.
- But in slide 8 we see behavior as completely learned. The object is now a ball, and can roll easily along
the surface. It is difficult to know where it will end up.
- Finally in slide 9 we see a combination or learned and genetic influences. The objects a ball and can roll
(learn). But there are some places on this landscape where it can roll more easily than other places. This
represents our current view of behavior.
- This idea of “channeled” learning can be illustrated by experiments with rats and dogs. Rats have very
good senses of smell. They can learn to avoid foods with a certain odor. But they cannot learn to
associate a sound with a reward or punishment, something that dogs, which use sound more in their
daily lives, can do. The genetically in built capacities of these animals dictate what they can learn.
- All behaviors are somewhere on this continuum (gradient) between completely instinctive and
completely learned. For an example of human behavior that is highly genetically encoded think of a
human psychiatric disease like schizophrenia. These kinds of illnesses tend to run in a family.
Foraging ecology
- The textbook chapter says that it’s basically impossible to summarize the whole field of behavioral
ecology in one chapter (or one lecture!), so it concentrates on three subjects: foraging, mating and
sociality. We will follow this idea, and now discuss a little bit about foraging ecology. As we shall see,
this organization makes some sense due to how the field of animal behavior historically developed.
- The field of “optimal foraging ecology” is focused on an adaptive question (#3 of Tinbergen’s 4
questions): do animals forage in such a way as to maximize their energy intake? We would expect this
due to natural selection: those animals that behave in such a way would be likely to survive and
reproduce.
- We will look at three questions in turn: do animals select the most profitable prey?, how long do they
stay in one patch before moving to another?, and do they adjust their foraging to minimize the amount
of energy spent? Researchers have used ingenious experimental methods to look at these issues. For
example, in the experiment we’re about to look at John Krebs and colleagues presented birds with
different kinds of prey on a conveyer belt (like that your luggage comes in at the airport) to see if they
could decide which was most profitable.
- The profitability of a food = the energy one gains from eating it over the time it takes to consume.
Krebs gave the birds different kinds of prey that had little pieces of plastic glued to them; the birds
needed to remove these plastic pieces with their feet, thus changing the handling time. Different colors
of plastic were made to be more difficult or more easy to handle. Birds learned quickly which colors
were most profitable, and would disregard (not touch on the conveyer belt) those colors that were not
profitable.
- Another question is how long an animal should stay in a patch of food (an area over which food is
distributed evenly). When a patch is discovered there’s a lot of food and it is very profitable to forage
there. But at some time the patch will begin to get exhausted and then it’s better to move to another
patch. Animals have shown the ability to decide when to abandon a patch based on how much energy it
takes to travel to a new patch: if patches are far apart, then it maximizes the energy gain of the animal
to stay in each one for a longer time.
- A last example is where we can mathematically express in the equation the forces that influence the
amount of energy that an animal expends in capturing some food. For example, crows drop shellfish
called whelks (snail-like animals that live in a hard, difficult-to-break shell) from high in the air onto the
ground to break the shells. The energy gained by such behavior is a function of how high the crow flies
and how many drops the crow can do per minute.
- One can figure out this question mathematically (a mathematical model) and then compare the height
the model predicts the crow to go to what the crow does in nature, and the model predictions have
been shown to be quite good.
- Foraging ecology was one of the first issues studied by behavioral ecologists (the examples in class
were all done in the late 1970’s). The ability to predict complex behaviors with simple mathematical
models was exciting. But although researchers continue to look at these issues, we now appreciate more
the many reasons that animals do not behave “optimally” in the field and why these models often fail to
predict animal behavior.
- Such reasons include that animals are under predation threat themselves and will adjust their foraging
to reduce their risk. They also do not perfectly gather information, and the information itself naturally
varies in the field (thinking of Kreb’s experiments, for example, they are not realistic because not all
samples of a particular food have exactly the same amount of calories).
Sexual selection
- We are now going on to look at two problems that troubled Charles Darwin because they seemed to
represent cases incompatible (in conflict with) natural selection: some sexes of some animals having
very elaborate traits (like the feathers of a peacock), and some animals doing something to help others
at a cost to themselves, with the extreme example being some animals like honeybees being “eusocial”
where worker bees do not reproduce but only help their sister queen.
- Let’s talk about Sexual Selection first. This issue troubled Darwin so much that he wrote a separate
book “Descent of Man and Selection in Relation to Sex in 1871, 12 years after he published the “Origin
of Species”.
- Darwin came to the view that these elaborate traits (ornaments) of one sex could be explained as a
kind of selection he called Sexual Selection, where the ornament helped the animal mate and reproduce.
Compare natural selection to sexual selection (slides 21 and 22).
- Natural selection and sexual selection can collide in that what makes an animal ornamented and
successful at getting mates, can sometimes seemingly impair its ability to survive. Ever seen a male
peacock try to fly?
- There are two different kinds of sexual selection. Some animals have ornaments that allow them to
fight with other individuals of the other sex, usually male, over access to females. This is intra
(“within”)sexual selection. Think the antlers of a deer. And then there are traits that one sex has, usually
male, to attract females (intersexual selection; the peacock’s tail).
- Does one sex (usually female) actually use these elaborate traits to choose members of the other sex?
A famous demonstration of this was done by Malte Andersson in 1982. He worked with an African bird
called the widowbird that has a really long tail. He did a nice experiment where he randomly assigned
male birds to one of 4 treatments: tail cut off, tail added (increasing the length of tail by gluing on more),
unmanipulated, and tail cut off and then added back to the same bird. He then looked at which males
the females preferred and they liked the ones whose tails had been added to.
- Let’s talk about these generalizations: why I say usually the male is the one that has traits or fights for
access to females that choose him (and are thus called “choosy”). Scientists (a famous proponent of this
theory is Robert Trivers) suggest that this comes because females invest more in each reproductive
attempt than males. This starts simply from the differences between sperm and eggs, where eggs are
much larger and require more resources. Hence males can reproduce many times, but females should
be more selective when they choose to reproduce.
- Sometimes the “exceptions prove the rule”. In some species the males invest more in offspring and in
these species often the females are more ornamented. For example, male phalaropes (a kind of bird)
incubate the nest. They are dull plumaged, whereas females are bright and display for males. In another
kind of bird, the jacana, females mate with several males and fight with other females to access to males.
Female jacanas will even kill a male’s offspring with another female in order to remate with it and make
sure it has her offspring. Finally in the pipefish (a type of seahorse) the male actually gets pregnant,
carrying the eggs with him as they mature. Pipefish males choose females that display.
- So what does the choosy sex get from chosing a male? In some cases (s)he actually gets a direct benefit:
the male gives her something. For example, in scorpion flies, males give females a “nuptial gift”. In
many birds and some mammals, the male contributes significantly to offspring rearing, so a female who
choses a good quality male is directly rewarded.
- But in other cases, males do nothing except mate. An example is the mannakin (a bird, remember the
movie), which has an elaborate display, mates with the female, and then never provides any help to the
offspring. What does the female get in this situation? Think about the Petrie 1994 article on peacocks,
which are similar.
- Animal behaviorists see two potential benefits females may get from males that give nothing but
sperm. One is called the “sexy son” hypothesis: if a female mates with a male with good plumage, she
ensures that her sons will also have good plumage. Another hypothesis is called the “good genes /
handicap” hypothesis (suggested by the scientist Amotz Zahavi) and says that the male trait is a signal
which says to the female “look, I’m so good that even with this very long tail I can survive”. In practice,
these hypotheses are difficult to distinguish from each other.
- A problem with these theories is that one would expect well plumages males to always reproduce well,
and therefore why would there continue to be genetic variation in the population (why would some
males not have good plumage)? A scientist by the name of Marlene Zuk came up with a solution: she
said that plumage could indicate resistance to parasites or pathogens (diseases). Since such pathogens
or diseases are always changing (think of a “moving target”) so one set of genes are not perfect.
Plumage can be an “honest signal” of male quality.
- Maybe the (male) ornament might actually not be connected to good genes or good offspring. But the
mathematician Ronald Fisher hypothesized that if a female preference for a trait evolved in connection
with the male trait, the trait could become more and more elaborate over time, a theory known as “runaway” sexual selection.
- There is indeed some evidence that female preferences could evolve with male traits. This is the
example of stalk-eyed flies. Two lines of flies were bred in the laboratory, one in which only the male
flies with the longest eye stalks were allowed to breed (the normal situation in the field) and one in
which only the males with the shortest eyes were allowed to breed. In the population selected for short
eye stalks, the females preferred males with short stalk eyes.
Groups
- We go on to talk about animal sociality: why do animals like to live in groups? We want especially to
talk about why animals might help other group members, even at their own cost. Remember, this
doesn’t seem to fit the idea of natural selection (which says that individuals maximize their own
reproductive fitness).
- Animals can live in many types of groups. Some groups are “aggregations”: groups that form around
stationary food resources (we won’t talk more about this). In other groups, non-related individuals
move together (like a flock of birds, or a herd of buffalo). And also there can be groups of animals in
which the members are all related to each other (a pride of lions).
- We will talk about unrelated groups first. The mathematician William Hamilton first proposed that such
groups can form just from individuals trying to reduce their own risk. If animals are more vulnerable by
themselves or at the edges of groups, they should always try to move to the center of the group, and
this should make the group actually come together (idea known as “selfish herd”).
- Groups have many attributes that reduce their member’s risk of attack. For example, the more eyes in
the group, the more likely the group is to detect a predator (“vigilance”). Also the more members of a
group, the less likely one individual is to be eaten (“dilution” of risk). Groups can also confuse or
threaten a predator. However, one problem here is that potentially larger groups can be more readily
detected by the predator.
- Groups may also have some aspects that increase their members foraging. For example, animals may
copy off each other’s success. This has been shown in experiments with birds in cages, where the more
birds in a cage the more quickly they find hidden food items. Sometimes the activity of one animal can
disturb prey for another animal (egrets – white wading birds – often travel behind cattle for this reason).
But there are problems here, too… the bigger the group the more competition for food, and animals can
be aggressive with each other, lowering their foraging efficiency.
-These ideas can be put together into a mathematical model. Too small a group and one doesn’t get the
advantages of vigilance, risk dilution and social copying. Too big a group and competition is intense and
the animals are always fighting amongst each other. So there should be some optimal size of flocks…
- And indeed researchers have collected evidence for this idea. Think of data about quails.
- Let’s now move on to talk about groups of related individuals.
- In some animals, individuals live in groups where two parents (mother and father) live with their older
offspring. Those offspring help to raise a new group of young, and after which they disperse (go away
from where they were born). This is called “cooperative breeding” and is known especially well in birds.
- Now here’s where Darwin has a problem. Why don’t these older young just leave without helping their
parents? Because natural selection has that individuals need to maximize their own reproductive
potential.
- The biologist William Hamilton came up with a solution to this problem. He suggested that juvenile
birds might help their parents raise their siblings because their siblings and they share genes. Hamilton
calculated something called r which is a measure of relatedness. It is the proportion of genes that a
person shares with another person. Hamilton suggested that animals will help other animals as long as
the following equation is met: rB – C > 0, where B are the benefits of the behavior and C is the Cost.
- In slides 43-45, see that you understand why r between mother and daughter in humans is 0.5.
- People have now researched many animal groups and found that generally group members only
cooperate when they are related.
- What about that extreme behavior: eusociality. A eusocial organism is one in which there are two
types (“castes”) of individuals: those that reproduce and those that do not. Why would an animal ever
not reproduce?
- The most famous animals that are eusocial are the ants, wasps and bees. They have an interesting
lifecycle… and amazingly, it helps explain why they are eusocial. This is because they are haploid/diploid:
males are born from unfertilized eggs whereas females are born from the mating of male and females.
- Now look at slides 48-50 and convince yourself that sisters (like the worker bees and their sister the
queen) have an r of 0.75, higher than the relationship between mothers and daughters.
-This was a very famous result when Hamilton wrote about it in 1965. I do want to note that it doesn’t
explain other eusocial animals such as termites and naked molerats.
Human behavior
- Finally let’s try to apply these ideas to humans.
- For example, is human altruism (help to others given at some cost to ourselves) effected by kin
selection?
- The research on slide 53 tends to support the idea that it is: humans are more willing to save someone
from a fire if they are closely related (high r). And humans tend to save in such situations those who can
reproduce again.
- A more subtle prediction of kin selection in humans is that, because human fertilization is internal,
males are not sure of their paternity. So it can be predicted that maternal grandparents should invest in
their grandchildren more than paternal grandparents, because they are more sure they are really
related to them. Again the evidence supports this idea.
- What about human sexuality? It’s clear that women put more investment into offspring care physically
than do men (pregnancy + lactation). So they should be more choosy about whom they mate with.
- It is a strong result that men are more willing to have sex with someone they don’t know. But I should
caution that this may be influenced by social, cultural factors as well as biological ones (women may be
taught that casual sex is not cultural acceptable, whereas society more accepts this behavior from men).
- So men are having short-term mating partners. But if men are doing this, it means women are too (this
idea captured by the phrase “it takes two to tango”… )! So let’s think about what differences women
might expect in a short-term partner, as opposed to a long-term partner.
- Care by the father is an important part of the success of children, so this is certainly something women
look for in a long-term partner.
- In a short-term partner, perhaps what they look for is “good genes” (like female peacocks chosing
males). Or perhaps they would to test their partner to see if he would be a good long-term partner.
- A body of literature has shown that females are more receptive to masculine characteristics of men
(physical strength, symmetry in features, which is correlated closely with attractiveness) when they are
fertile – the point in their estrus cycle when they can conceive.
- Perhaps the strangest test of this showed that women appear to be able to smell something about
attractive men: when given T-shirts of men, women can predict which men are more symmetrical. This
wild result (published in 1998) is a bit controversial and I will assign it as homework. Read the paper and
think about whether it is convincing to you, and then we can discuss this in class.
- Some parts of female behavior also support the idea that in choosing short-term relationships they are
evaluating how good a partner their mate may be. For example, symmetrical men are better dancers.
And women are more perceptive about how good their partner is in dancing than are men.
- Which means, guys, that if you can’t dance well, don’t ask a girl to dance if you like the girl!