Download Sexual Selection: Why does sex matter

Lab 5- Sexual Selection: Why does sex matter?1
Before the lab
- Read the background information provided in this lab, pp 523-527 in Chapter 24 and pp 260-263
in Chapter 12 of Freeman.
- Read and analyze the three case studies on sexual selection provided in this lab. Create
notes for yourself, or highlight important information that you will use to answer questions with
your group during lab.
- Complete prelab questions 1-4 individually before coming to class.
Evaluation (6%)
1% Pre-lab questions
5% In-class group assignments (one mark for the group-though TA will deduct marks for
noncontributing members)
Objectives
By the end of this lab session students should be able to…
1. Understand the relationship between natural selection and sexual selection, and how they
compare when determining the fitness of an individual.
2. Demonstrate how the reproductive investment of each gender affects their strategies to
increase mating success.
3. Define intersexual and intrasexual competition in terms of differential success of
individuals in obtaining mates.
4. Explain why intersexual and intrasexual competition affects the asymmetry in morphology
and behavior between sexes.
5. Evaluate the direct and indirect benefits for females when choosing a mate through female
choice.
6. Discuss the importance of sex.
Time Schedule
2:10
Hand in completed pre-lab questions
2:15 - 2:30 Class discussion on what students will be doing within their own experiments.
2:30 - 3:40 Design & conduct experiment on the intrasexual competition between male house
crickets. Write up your results and give to TA
3:40 – 5:00 Complete assignment for the three case studies.
Introduction
Most individuals have one main goal in life, survive in order to breed and sire offspring (Sparks,
1999). The fitness of an individual is an important concept to grasp when trying to understand
both natural and sexual selection. The fittest individual is not necessarily the strongest, fastest, or
biggest. An individual’s fitness includes its ability to survive, find a mate, produce offspring—and
ultimately pass its genes onto the next generation.
Bright colours, enlarged fins, feather plumes, songs, horns, antlers, and tusks are often
exaggerated conspicuous displays that are seen on certain animals. In addition to recognizing
natural selection as a mechanism for evolutionary change, Charles Darwin had questioned the
morphological extravagance of certain species and genders. Why have males in many species
evolved more conspicuous ornaments, signals, and weapons than females? How can such traits
1
This lab was written for a BIO299 research project by a former BIO152H5 student.
BIO152H5F 2006 University of Toronto at Mississauga
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evolve when they may reduce male survival? Such questions prompted Darwin to propose
perhaps his most scientifically controversial idea—the theory of sexual selection.
Sexual selection is a more specific form of natural selection. Sexual selection acts on traits that
influence an individual’s ability of find a mate within a population.
Sexual selection is a form of natural selection: similarities with "regular" natural selection:
• must be variation in a trait or traits within a population .
• variation must be heritable.
• selection operates when individuals with certain variations have more surviving offspring
than individuals with other variations.
• selection operates between individuals of the same species.
Sexual selection is different from "regular" natural selection in some ways:
• individual’s differ in their ability to mate, which means that individuals of the same sex
affect each other's reproductive success.
• members of one sex compete against other members of the same sex, instead of against
all other individuals in the population.
BATTLE OF THE SEXES
When defining “success” in evolutionary terms, it means producing offspring who will survive and
carry on their parents’ genes to their own offspring. The different strategies males and females
use to achieve success is what drives sexual selection (Sparks, 1999). This can be demonstrated
through the differences in reproductive allocation - the time, effort and resources a parent
devotes to each of their offspring - between males and females.
Bateman and Trivers explain the differential mating strategies between males and females by
stating “eggs are expensive, but sperm are cheap” (Freeman, 2005). Males produce many tiny
gametes (sperm), whereas females produce fewer, large gametes (eggs) in most organisms.
This dramatic difference in gamete size is of crucial importance to sex allocation theory. Due to
the difference in gamete size, males and females devote different amounts of their bodily
resources to each gamete.
The male can easily make thousands of sperm. Since sperm are cheap (or easy) to make, males
will maximize their fitness by trying to mate with as many different females as possible. He often
goes for quantity of matings, not quality of mates.
However, the female is quite limited in how many eggs she can make, because each egg
depletes her of a significant amount of resources. Mating with lots of different males will not
necessarily increase the female’s fitness, because one mating can usually provide her with all the
sperm she needs to fertilize her eggs; she usually goes for quality of mates.
In comparison, males in general spend more energy finding mates then females. Therefore,
sexual selection acts more on male traits because; the ability of find a mate influences the fitness
of the male more than the female. As a result, traits in males are usually elaborated. This leads to
sexual dimorphism, which is demonstrated within a species when a trait differs between males
and females.(Freeman, 2005)
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Table 1: Comparing the challenges males and females face when finding a mate
(Buss, 1994)
Short Term
Long Term
Male’s reproductive challenges
- Number of partners
- Identifying females who are
sexually accessible and fertile
- Minimizing cost and risk
- Paternity confidence
- Commitment of female
- Identifying women with good
parenting skills
Female’s reproductive challenges
- Immediate resource extraction
- Physical protection from other
aggressive males
- Having potential backup mates
- Evaluate mates for commitment
- Attaining men with high quality
genes
Monogamy vs. Promiscuity
During the mating seasons, both males and females have developed strategies to increase their
reproductive success. Monogamy and promiscuity are two different strategies questioning
whether the number of mates an individual has affects their reproductive success.
Monogamy is the exclusive mating relationship between a male and a female, where the two
individuals only mate with each other. Monogamous relationships may only last throughout the
mating season in some species, or extend through the adult life of both individuals. (Allaby, 1998)
The advantages of having a committed for relationship both individuals is that the male provides
protection and resources to the female and her offspring, and the male is insured that he is the
father of her offspring.
Promiscuity is also a form of polygyny, were either a male or female mates with more than one
individual of the other sex. These relationships are usually short, and end after mating. (Allaby,
1998) The advantages of lack of commitment allows the male to mate with as many females he is
able to access; this increases his chances of siring some of the female’s offspring. Promiscuous
females are able to receive sperm from many different mates; increasing the chance that some of
her offspring will have fit genes.
Sex ratio and Operational sex ratio
The "sex ratio" in a population is the ratio of females to males. If there are equal numbers of
males and females, the ratio is 1:1.
However, although there may be equal numbers of males and females, the "operational sex
ratio" (ratio of sexually receptive males to sexually receptive females) can be quite different. If
half the females have already mated, or if half the males are sexually immature and can't mate
yet, the operational sex ratio will be quite different from the actual sex ratio. Males have enough
sperm to inseminate many females (with sperm to spare). Females can only produce a given
number of eggs at a time, and one mating can easily fertilize all the eggs.
Therefore, because the potential rate of reproduction is quite different between the sexes, at any
one time there are many more sexually receptive males than sexually receptive females. It makes
sense for a female to choose a mate based on traits that would help her survive (direct benefit)
and to help her offspring survive (indirect benefit). For example, a female bird would do well to
choose a strong-looking, disease-free mate. That male likely carries “good” genes that allow him
to resist disease and get sufficient food - he will pass those genes on to his offspring.
See a great website on Jumping Spiders, sexual selection, and their discussion of operational sex
ratio: http://eebweb.arizona.edu/Animal_Behavior/spiders/spider4.htm
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MALE COMPETITION AND FEMALE CHOICE
Due to the biased operational sex ratio, males and females have different strategies for
maximizing their fitness.
Intrasexual selection - Same Sex Competition
• Often there are many more males than receptive females; therefore, males compete against
each other for access to females.
• Individuals that show greater strength, size, agility, confidence, or cunningness are usually
more successful during fights.
• Males compete for access to females, the amount of time spent mating with females, and
even whose sperm gets to fertilize her eggs.
• This type of competition is popular among species that hold territories; for food and females.
Question: How does male-male competition cause large body size, or weapon like features to
evolve in some males?
Intersexual selection - Females Select Among Mates
• A female doesn't need many matings to fertilize her eggs. She can produce better offspring if
she selects a particularly "good" mate (some males can be preferable to other males in a
variety of ways).
• The outcome of female choice may directly benefit her and her offspring or indirectly benefit
her offspring.
• Females choose which males to mate with, how long to mate, and even whose sperm will
fertilize her eggs. Some females can eject sperm from an undesirable mate.
Question: Why are males usually the sex with exaggerated traits used in courtship?
However, there are many examples of females choosing mates based on less useful traits (e.g.,
song complexity) or even traits detrimental to survival (e.g., brightly coloured plumage, as in the
case of the peacock). How did these preferences arise? If a female chooses a male with bright
feathers, her sons will have bright feathers, which are likely to attract predators. A gene for
choosing brightly coloured males would seem to be disadvantageous. How do such genes spread
through a population?
Good genes (direct & indirect benefit to female and her offspring)
Consider a bird population (e.g., pea hens and pea cocks) in which females choose mates at
random. Some males in the population have
better genes for survival than others, but it is
difficult to tell whether a male has good genes or
not. In this scenario, long tails make it more
difficult to survive—they are costly to produce
and maintain. Since they are so costly, only
males with good genes have the extra resources
to produce them. In this situation, a long tail is an
indicator of good genes. A gene for female
choosiness (longer tail = sexier) will be favoured,
since—by choosing a long-tailed/good gene
male—she will have sons with good genes. This
trait will spread through the population until most
females choose long-tailed mates and males that
are able to produce long tails are favoured.
[http://www.markfunk.com/cartoons/peacock.jpg]
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Part A: Cricket Experiment
Within this section, students will break into half benches where they are expected to observe the
morphology and the competitive male behaviour of the Acheta domesticus. Students will design
an experiment that will analyze the morphology and behaviour of male crickets and how these two
features relate to an individual’s fitness.
Students are expected to develop an experimental design, including a hypothesis, recorded
observations, and a short discussion paragraph. A handout is provided, indicating the specific
components needed for each experiment. Each half bench will break up into a two groups, who
will have similar experimental designs, but will perform their own experiment and record their own
observations. Within the group, both members will hand in one completed handout by the end of
the lab.
Background Information
Acheta domesticus are commonly known as house crickets. This species of cricket are usually
found inside buildings and houses; areas where there are warm, moist conditions and where a
they have access to food. Since these crickets are omnivores, they feed on a range of resources.
If they live outside, they will feed on leaves, plants, or other insects. However, when living indoors
they feed on fabrics.
Male house crickets are territorial with space and resources. They will stand in a straight, upright
position to indicate their dominance. They use aggressive behaviour, by pushing other crickets
with their hind legs, if they feel threatened. Males also have the ability to produce chirping sound,
called stridulation. Only males can stridulate. They use it to warn other males, mark their territory,
or attract females.
The mating behaviours of house crickets demonstrate both intra and intersexual selection. It has
been observed that male crickets compete for territory. Male-male competition results in one male
dominating a certain area, where he not only holds resources but has access to females. In order
to attract a female, a male will produce a chirping sound through his wings. This is one male
characteristic, used by females to assess the quality of the male, as well as locate him. (Gray,
1997)
When comparing the morphology of the male and female, it is evident that the house cricket is a
sexually dimorphic species. One morphological characteristic that distinguishes the male from the
female is that the female has a long ovipositor, used during mating. Males also have elaborate
designs on their wings, compared to the plain wings seen on females. The elaborated designs on
male wings are used to produce different pitches in sound. Depending on who they are trying to
communicate with, males produce different types of chirps. It is also important to consider the fact
that house crickets sense their environment by using their antenna; used to sense chemical
messages that would be sent by other crickets.
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Procedure
Each bench will be divided into two sections, where every half bench will have approximately four
students. Within this group, students are to complete tasks 1 and task 2 together; designing the
experiment. However, this group will be split up into groups of two, where each pair will perform
their own experiment. This will allow students to compare two sets of results, in their discussion.
Students will be shown a short video clip of two male house crickets fighting. This will illustrate the
certain behavioural characteristics students will need to look for when observing their own male
crickets.
Task 1: Choosing a Cricket
Each half bench will be performing two experiments testing the same hypothesis. Therefore, two
students from each half bench must go and pick two pairs of male crickets from the TA, and bring
it to their bench. Each student must,
- Pick two male house crickets for their own experiment.
- Compare the two male crickets they have chosen and see if they show some variability to
compare in task 1.
- Compare the pair of males they picked to their bench member’s males to see if the crickets
they chose had similar variations. (This insures the experiments are as similar as possible.)
Task 2: Comparing individuals
From their understanding of the morphology and behaviour of male house crickets (from the video
clip and background information), students, in their group of four, will compare the differences
observed within each pair of male crickets they have chosen. Students can compare the
morphology, (differences in body size, color, pattern on their backs, etc) or behaviour, (activity
level before fighting, differences in sounds produced, the initiator of the fight or the defender, etc.)
of the two males. In order to develop a hypothesis, each group will need to determine one
morphological or behavioural characteristic they think will affect a male’s fitness. Students are
expected to write this information down on the handout that they will hand in at the end of the lab.
Ex. Morphological/behavioural characteristic being measured: Body Color
Description of Cricket A: Dark brown body color
Description of Cricket B: Light brown body color
The two pairs of male crickets picked for each experiment should show the same variation for the
one morphological or behavioural characteristic chosen.
Task 3: Develop a Hypothesis, Null Hypothesis, and Prediction
Each group will develop a hypothesis based on the morphological or behavioural characteristic
they chose in the previous task, and explain how it will affect the fitness of a male house cricket.
Ex. If the trait being test was body color, then one hypothesis could be; darker body coloration
shows greater fitness in male house crickets.
The null hypothesis is exactly opposite to the hypothesis the student first developed. If the student
rejects their hypothesis, they support their null hypothesis.
Ex. Darker body coloration shows less fitness in male house crickets.
The prediction takes your hypothesis, and states what results the student would expect to see if
the hypothesis was supported. Predictions are always in an IF … THEN format.
Ex. If darker body coloration shows greater fitness in male house crickets, then Cricket A will
become the dominating male.
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Task 4: Setup the Experiment
Two transparent boxes, with opaque dividers
are placed at every half bench. Once the
group of four has agreed upon a hypothesis,
and indicated the variations between the two
crickets, each group of two can start their own
experiment.
With the pair of crickets they had observed
previously, the two group members can place
the two male house
crickets in the separate sections of the box.
Figure 2: Setup for experiment
This does not require the student to have any contact with crickets, since they can just empty the
containers into each section. Students should wait a couple of minutes before lifting the divider, so
that each cricket can adjust to the new environment.
Task 5: Record Observations
Once the divider has been lifted, every member of the group is expected to note any behaviour
they observe. Students should write these observations on the handout that they are expected to
hand in at the end of the lab. Students should consider the behaviours they saw during the short
video clip: how do they compare? Record observations on the relative distance between each
male, how long it takes them to interact with each other, who initiated the fight, what body parts
were being used, etc. Once the fight is over, record which male was dominant (refer to the
behaviours seen in the video clip). Sometimes a dominant male is hard to distinguish, therefore
students should evaluate each male’s previous behaviour to see who was more successful
overall; which male pushed or wrestled the other one more, or kept on running away? The TA will
present a list of the criteria they will be expecting to see within the recorded observations of each
group.
Once both groups have finished recording their observations, they will need to share results with
their other group. Each group must make sure they have written down all the information they
need to compare and discuss both results.
Task 6: Discuss Results
In this section students, will assess whether or not the results from their observations supported
their hypothesis. If the results of the experiment supported their hypothesis, students are
expected to explain how the observed behaviours from their experiment illustrate what they
hypothesized. These students should also explain how their morphological/behavioural
characteristic is advantageous during mate competition. If their hypothesis was rejected, students
should explain why their results did not support their hypothesis and provide an alternate
hypothesis or explanation describing their results.
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Part B: Case Study Assignment
Before coming to class, students are expected to have read and highlighted key points and
concepts within the three case studies provided. Students will form groups of three or four, where
they will discuss what they read within each case study. Each group will be given 3 questions for
each of the three cases and hand in one set of answers for all group members.
Learning how to analyze and answer questions from the case studies will be good practice for the
exam.
Case study #1: Intersexual selection
It has been hypothesized that female choice influences the mating displays of males within a
population, through ornamental or behavioral characteristics. However, Jean-Guy J. Godin and
Stephanie E. Briggs are two biologists who have designed an experiment which hypothesized that
natural selection also influences sexually selected traits, where females become less attracted to
males who are at higher risk of predation due to their costly traits. Certain costs that are
associated with expensive traits are: increased risk of predation, higher requirement of energy,
greater risk of disease or parasites, and less effort for male parental care.
The experiment focused on the Trinidadian guppy, Poecilia reticulata, which was used to
investigate the effect of predation on female choice. Body color pattern is the most common
variable trait in guppies that influences female choice. Two natural populations with different
levels of predation were investigated in the experiment. The predator used within the population of
guppies was a cichlid fish, which prefers eating colourful male guppies. Females who are found
around colourful males have a higher chance of being eaten by a predator as well. When the
predator in both populations was removed, the female guppies preferred males with more
colourful patterns. When the predators were placed back in both populations, it was observed that
females preferred to mate with less colourful males. By comparing their results for female
preference when predators were both present and absent, Godin and Briggs observed that female
guppies can change their preferences for male traits depending on the predation risk. The
experiment also supported the hypothesis Godin and Griggs had proposed, illustrating the
relationship between natural and sexual selection.
Case Study #2: Intrasexual selection
When observing the extravagant designs of certain species, it is widely assumed that these
sexually selective traits have evolved due to female choice. Extensive studies have been done to
test this hypothesis. However, the ring-necked pheasant, Phasianus colhicus, is one species that
uses its colored feathers, long tail, red wattle, and ear tuft to not only catch a female’s attention
but to warn other males of his fitness. Through an experiment done by Concha Mateos and Juan
Carranza, two biologists from the University of Extremadura, the morphology of the ring-necked
pheasant was explained using behavioral observations seen during male-male competition.
Within the species, ring-necked pheasants are sexually dimorphic, where the female has no
ornamentation. Since males hold territories, females are will prefer mating with the male who
dominates that territory. Therefore, male-male competition for territory plays a strong role in the
mating success of male pheasants. During competition, males will first assess each other using
signals shown by spreading their tail and feathers, raising their ear tufts, and inflating their wattle,
before fighting. The ornamental features of the ring-necked pheasant were manipulated in size
and color. This range in variation was used within the experiment to test what traits were desirable
for male competitiveness. Characteristics like the color of the wattle, length of tuft, body size, tail
length, and feather color were all considered. Within their experiment, Mateos and Carranza
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observed that the wattle size and the length of the ear tuft were the most significant signals males
would display between each other. As the size of the wattle or length of the tuft became more
extreme, the male demonstrated a greater chance of winning if provoked to fight. Males would not
fight if they knew they wouldn’t win. In addition to using signals to assess another male’s fitness, it
was also observed that signals were sent between males to identify rank within the group. The
dominant male would maintain a straight tuft and wattle. Consequently, it would usually be the
dominant male with the straight turf and wattle that attracted the females.
Case Study #3: Monogamy vs. Promiscuity
When assessing the reproductive success of an individual, it has been propose that there is a
correlation between the number of mates it has. In some species the act of promiscuity is a
mating strategy in which the goal of the female or male is to mate with as many individuals of the
opposite sex as possible. For females, the more males she mates with, the greater the chances
that her eggs will be fertilized by the fittest sperm, and the more males there will be to protect and
provide for her. For males, the more females they mate with, the greater the chances they will sire
offspring. However, it has been questioned that if promiscuity is so advantageous for both males
and females, then why is monogamy still present within some species. In a study done by three
biologists, Nicole E. Poirier, Linda A. Whittingham, and Peter O. Dunn, from the University of
Wisconsin, a comparison was made between males that were monogamous, sequentially
monogamous, and polygynous (promiscuous).
The study was based on an experiment using house wrens. House wrens are a sexually
monomorphic species, where both the female and male have a similar morphological appearance.
A male’s ability to sing is used to defend its territory. However, during the mating season males
will build nests out of twigs to attract the female. When the female chooses a male, she lines his
nest with grass. In their experiment, Poirier, Whittingham and Dunn captured, marked, and
recorded the physical characteristics of male house wrens. Within their observation on the mating
behavior of each male, they recorded the number of females each male mated with. Males were
categorized as either monogamous, mated with one female, sequentially monogamous, mated
with two females but one at a time during the season, or polygynous, mated with more than two
females simultaneously. By analyzing every male horn wren’s DNA, and comparing it with the
DNA of all the offspring each female produced, the reproductive success of each male was
measured. From the results, it was calculated that overall polygynous males were fitter then
monogamous males, yet, sequentially monogamous males were equally fit to polygynous males.
It was observed that even though a monogamous male produced less offspring on the whole, it
still sired all the offspring of the one female it mated with. However, most of the polygynous males
that mated with three females only sired a few of the offspring produced by the females they
mated with, even though they mated with more females then monogamous males did.
Sequentially monogamous males also sired all the offspring from the two females they mated
with. However, polygynous males had a third female that would replace the number of offspring it
didn’t sire from its other two mates.
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References
Allaby, M. (Ed.). (1998). A dicitionary of ecologyOxford University Press. Retrieved August 2,
2005, from
http://www.oxfordreference.com.myaccess.library.utoronto.ca/views/BOOK_SEARCH.html?b
ook=t14&subview=BookHomePage
Buss M., D. (1994). The strategies of human mating. American Scientist, 82, 238.
Fox, R. (2004). Invertabrate zoology lab exercise. Retrieved July 25, 2005 from Godin J., J., &
Briggs E., S. (1996).
Female mate choice under predation risk in the guppy. Animal Behaviour, 51, 117. Retrieved
June 7, 2005,
Gray A., D. (1997). Female house crickets, Acheta domesticus, prefer the chirps of large males.
Animal Behaviour, 54, 1553.
Mateos, C., & Carranza, J. (1997). The role of bright plumage in male–male interactions in the
ring-necked pheasant. Animal Behaviour, 54, 1205. Retrieved July 8, 2005.
Poirier E., N., Whittingham A., L., & Dunn O., P. (2004). Males achieve greater reproductive
success through multiple broods than through extrapair mating in house wrens. Animal
Behaviour, 67, 1109. Retrieved July 10, 2005,
Poirier, N. E., Whittingham, L. A., & Dunn, P. O. (2004). Males achieve greater reproductive
success through multiple broods than through extrapair mating in house wrens. Animal
Behaviour, 67(6), 1109-1116.
Sparks, J. (1999). Battle of the sexes in the animal world. London: BBC Worldwide Ltd.
McIntosh, M. and Papaj, D.
http://eebweb.arizona.edu/Animal_Behavior/spiders/spider0.htm Retrieved July 10,
2005.
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