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Assignment 2
Due Wednesday October 29th at the beginning of lecture
To write
Behavior is an important way in which animals can respond flexibly and plastically to
different environments. Use the following two papers and material from class to discuss why
behavioral plasticity and morphological diversity can be favored in mating systems. The goal for
this assignment is to discuss the ultimate reasons (the selective forces) why alternative mating
strategies and sex-changing are favored by evolution. While discussing both of the papers, be
sure to elaborate on how the environment influences which behaviors/strategies are expressed.
Describe specific examples from each paper that demonstrate how behavior is plastic. Be sure to
thoroughly describe the relevant findings of each paper in your discussion. Note that there is a lot
of information in each paper. Thus, you will need to focus on the most relevant results that
illustrate how selection can favor animals that can respond flexibly and plastically to different
environments.
Please write an essay about the topic, similar to what you did for the first assignment. The
focus of the essay is on the two assigned papers and demonstrating that you understand them.
However, would be good to have an introductory paragraph that discusses/defines plasticity,
polyphenism, and other relevant topics. Thus, you can use lecture material to help introduce the
topic and put the papers into the broader context of behavioral ecology.
Remember that the writing assignments should be 1.5-2 pages, typed, single-spaced, normal
margins, 12 pt font, and standard essay format. Assignments are due each at the beginning of
lecture on the specified day. Late papers will not be accepted (with exception to valid excuses).
Assignment 2
Due Wednesday October 29th at the beginning of lecture
To read
Diversity and flexibility of sex-change strategies in animals
Philip L. Munday, Peter M. Buston, and Robert R Warner
TRENDS in Ecology and Evolution Vol.21 No.1 February 2006 89-95
You should read Box 2, but do not worry about the details. Concentrate on the rest of the
paper.
Alternative reproductive tactics and male-dimorphism in the horned
beetle Onthophagus acuminatus (Coleoptera: Scarabaeidae)
Douglas J. Emlen, Behav Ecol Sociobiol (1997) 41: 335-341
One thing that may be confusing about this paper is interpreting the graphs in Figure 3 and 4.
The fights won are grouped according to how much of a difference in male size there was. For a
particular size difference, the fights won by larger males are in black bars above the X axis, and
the fights won by smaller males are in white bars below the X axis). For example, at the size
difference of .1 mm in Figure 3, one large male won a fight and one small male won a fight.
Predation risk
1. Foraging = short-term caloric gain; predation = permanent death
a. Balance energy intake with need to remain vigilant
2. Can cause predictions based on optimality models to differ from
observations
3. Shift to less-preferred food when predation risk for preferred
habitats is high
a. But condition not static. Even high predation-risk habitats
can be preferred if pay-off is big enough
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Predation risk
1. Milinski and Heller (1978) examined if predation risk influences choice
of food intake in sticklebacks (Gastersteus aculeatus)
2. Hungry fish preferred high prey densities. However, when a model
kingfisher was flown over the tank, the fish moved to low prey density.
3. Low prey density allows more time for vigilance
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Predation risk
1. Predation risk can also be age/size specific
2. Gilliam (1982) tested habitat preference as bluegill sunfish
(Lepomis macrochirus) grow
3. With no predators, fish foraged on benthic inverts which gave
highest rate of food intake compared to plankton
4. When predatory bass added to ponds smaller sunfish foraged
in reeds where food intake reduced by 1/3 and growth rate by
27%. Larger sunfish were safe and continued to forage in the
benthos.
5. Fish maximized chance of survival by adapting their foraging
strategy with age
Food storing
1. Many birds collect food in the autumn, which they then hide.
The food is retrieved during the winter and spring.
a. A single nutcracker is estimated to store 30000 seeds in
2500-4000 separate hiding places.
2. This helps them deal with environmental variability. Stored food
is similar to body fat. Both are stored during times of plenty and
used in times of scarcity.
3. Food storing requires spatial memory to retrieve the food.
a. Hippocampus is the part of the brain involved in spatial
memory.
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Food storing
4. Species that store food tend to have larger relative hippocampal
volumes than species that do not store food.
a. Light blue = average for families that do not store food.
b. Dark blue = average for familes that do store food.
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Food storing and the evolution of cognition
1. Mental abilities extend beyond spatial memory in some food storing
birds.
2. Nicky Clayton and colleagues tested this with Western scrub jays
(Aphelocoma californica).
3. Test1: Can the jays remember what kind of food stored, as well as
when and where (i.e. can they remember specific events)?
4. Experiment: Jays were allowed to store either nuts or worms and then could
store the other food after 120 hours. Food was retrieved 4 hours later.
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Food storing and the evolution of cognition
5. Worms are preferred, but they go bad by 124 hours, whereas nuts do
not.
6. Birds retrieved the worms if they were stored second but not if they were
stored first.
a. Would do this even if the food was removed from the hiding place, which
eliminated any scent cues.
7. Thus, they can remember what, where, and when they hid food!
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Food storing and the evolution of cognition
1. Test 2: Can scrub jays interpret the knowledge of another
individual? Can they recognize potential thieves
2. When a bird was observed by others when storing food, it
would later on in private move the food cache to a new location.
3. Birds more likely to move food when observed by a dominant
individual than by their partner or subordinate
4. Individuals also more likely to move cache if they stole food from
others
Evolution of cognition
1. Test 3: Can Scrub jays plan for the future?
a. ‘Mental time travel’: ability to project into the future, independent of
current physiological conditions, and plan accordingly.
2. Experiment: Raby et al. (2007) placed birds overnight either in a
compartment where they received food first thing in the morning or in a
compartment where they had to wait 2 hours before receiving food
3. After training period, birds were allowed to obtain nuts from a central
room and store them. They preferentially stored them in the room where
they had to wait for food in the morning.
4. Birds could anticipate in which room they would be hungry if locked in
overnight!
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Optimality Models
Limitations
• What to do when an animal doesn’t behave
“optimally”?
•
Incorrect assumptions
• currency
• incomplete knowledge either by animal or by
scientist
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Summary
1. Optimality models are used to make quantitative, testable
predictions about the choices an animal makes that maximize the
benefits, while minimizing costs
a. The currency for maximum benefit and the constraints will
influence predictions
2. Foragers often face trade-offs in relation to foraging: predation
and/or starvation risk, handling time, etc
3. Foraging is a very important part of life and may lead to the
evolution of higher cognitive abilities.
Outline of Lecture
Understanding Sexual selection
Why compete for mates?
Male-male competition and the traits it affects.
Sexual Selection
Sexual dimorphism
Long tailed widow birds
Elephant seals
Sexual Selection
“When the males and females of any animal
have the same general habits of life, but
differ in structure, colour, or ornament, such
differences have been mainly caused by
sexual selection: that is, by individual males
having had, in successive generations, some
slight advantage over other males, in their
weapons, means of defense, or charms,
which they have transmitted to their male
offspring alone.”
The Origin of the Species
Natural Selection vs Sexual Selection
1. Natural selection: selection to compete well for
resources (food), selection for survival (against
predators/diseases).
2. Sexual selection: selection due to
competition for mates. Sexual selection explains
ornaments, conspicuous displays, and sexual
dimorphism.
“[Sexual Selection] depends, not on a struggle for existence in
relation to other organic beings or to external conditions, but on a struggle
between the individuals of one sex, generally the males, for the possession
of the other sex. The result is not death to the unsuccessful competitor, but
few or no offspring.” from The Origin of the Species
Natural Selection vs Sexual Selection
3. For both natural and sexual selection, the goal
is high reproductive success = high fitness.
4. Often there is a tradeoff between sexual
selection and natural selection. What really counts is
passing on your genes…not just survival (one big lesson
for this lecture.)
Why is it a competition?
1. Robert Trivers proposed in 1972 that: “When one sex
invests considerably more than the other, members of the
latter will compete among themselves to mate with members of
the former.”
2. In general, females are slower at producing eggs than males
are at producing sperm because eggs are more energetically
expensive to make.
a. A cow’s ovum is 20000 times the size of a bull’s sperm.
3. In many cases, females can maximize reproductive
success by converting resources into eggs/offspring whereas
males can maximize reproductive success by fertilizing many
females.
4. This theory was inspired by work by A.J. Bateman.
Experiments of Bateman
1. Bateman put equal numbers of male and female fruit flies
(Drosophila melanogaster) in replicate vials and let them
mate.
2. He scored the number of matings and offspring produced by
each individual using genetic markers = phenotypic
mutations. Each adult was heterozygous dominant for a
particular marker mutation.
Results of Bateman
Male reproductive success increased with the number of
mates, but female reproductive success leveled off.
Conclusions of Bateman
1. Male reproductive success increases with the number of
mates. Female reproductive success does not increase with
the number of mates.
2. Males have greater variance in reproductive success than
females.
a. Some males get few mates, some get many.
3. The sex with greater variance in reproductive success is
likely to be more subject to sexual selection.
Testing Bateman
1. Investment in reproduction in Drosophila is largely
determined by the size of sperm and eggs.
2. In some species of Drosophila males have unusually large
sperm, probably as a result of intense sperm competition.
3. Greater similarity between male and female gamete sizes
should make male and female investment in reproduction
more equal.
a. Isogamy = gametes similar in size.
b. Anisogamy = gametes different in size.
4. This could affect the intensity of sexual selection.
Testing Bateman
1. Bjork and Pitnick (2006) did experiments similar to Bateman
in Drosophila species with different sperm sizes.
2. Drosophila bifurca has very large sperm (58mm). D.
melanogaster sperm = 1.87 mm.
3. Sperm: egg production rate = 29:1 for D. melanogaster.
Sperm: egg production rate = 6:1 for D. bifurca.
Large sperm in Drosophila bifurca
Testing Bateman
1. Male mating success is limited by number of mates in species with
small sperm but this is not the case in species with large sperm.
c. D. melanogaster
sperm length =
1.87 mm
e. D. lummei
sperm length =
7.79 mm
Males = solid line and
circles
Females = dashed
line, open circles
P = 0.0027
P = 0.0005
P = 0.1896
Not significant
P = 0.3388
Not significant
d. D. virilis
sperm length =
5.70 mm
f. D. bifurca
sperm length =
58.29 mm
Mating success in rough skinned newts
1. Bateman’s conclusions have since been tested in other
species like rough-skinned newts (Taricha granulosa).
2. Jones et al (2002) caught all males and females living in
an isolated pond. Eggs were collected from each female.
3. DNA from the adults and hatchlings were analyzed to
determined paternity.
Mating success in rough skinned newts
1. The sexes differ in mating success.
0.8
0.7
frequency
0.6
0.5
0.4
0.3
0.2
0.1
0
0
1
2
3
4
5
Figure 1. Distributions of genetically documented mating
events for male and female newts. Grey bars, males; black
bars, females.
Jones et al 2002 Validation of Bateman’s Principles: ...
Mating success in rough skinned newts
number of offspring reproductive success)
1. Males showed a greater increase in reproductive success
with number of mates than females.
a. Males = diamonds and solid line
b. Females =circles and dashed line
400
350
300
250
200
150
100
50
0
1
3
4
5
Figure 2. A plot of reproductive success versus mating
success for newts from our focal population, showing
Bateman gradients (also known as sexual-selection gradients)
for males (solid line) and females (dashed line).
Mating success in pipefish
1. Pipefish (Syngathus typhle) have reversed sex-roles. During
copulation a female will transfer eggs to a pouch on the male.
The male fertilizes the eggs and carries them until they are born.
2. Males tend to be choosy and females compete for males.
3. Jones et al (2000) hypothesized that this should reverse the
predictions of Bateman. Tested this with experimental groups of
pipefish and paternity tests.
Mating success in pipefish
1. Females showed a greater increase in reproductive success
with number of mates than males.
a. Males = squares and solid line
b. Females = circles and dashed line
0
fertility (no. of progeny)
100
(b)
80
60
40
20
0
0
1
2
3
4
Jones et al 2000
Repetition of Bateman’s Study
1. Given the large impact of Bateman’s study, Gotawy et al
2012 replicated his study using the same stocks of fruit flies.
They uncovered a critical flaw!
2. Bateman assumed that the genetic mutations that he used
were neutral and that Mendelian segregation could be used to
infer matings.
3. Gotawy et al found that often double mutant offspring did
not survive. Thus, Bateman’s method for determining
parentage and number of matings was not correct!
Was Bateman Correct?
1. No: His experiment was flawed so the results of his actual
experiment cannot be trusted.
2. Yes: He had a good idea that has been successfully
tested in other species. Others have even found some
support for his ideas in Drosophila, just not using his
methods.
3. Maybe: While there is some evidence supporting
Bateman’s ideas, there are other theories about sexual
selection. We need to test Bateman’s hypothesis and
alternative hypotheses to really understand what is going on.
Why is it a competition?
1. The sexes differ in reproductive investment.
a. Cost of making eggs vs sperm
b. One parent may invest more in parental care
(pregnancy, nesting, etc.) and thus may be choosier
about mates.
Ex. Pipefish & seahorses, jacanas.
Why is it a competition?
2. Biased sex ratio: The numbers of one sex can be in short
supply. Ex. Penguins
3. Variation in mate quality.