General Ecology: Lecture 10
Life History Patterns (Part 1)
Related reading in text: pp. 215-225 (we won’t go into the life table calculations), pp. 227-230, 234-
235. You should focus on lecture information, but reading may help you better understand the concepts and provide you with additional examples.
I.
Overview of life-history patterns
A.
Topics covered in life history lectures
B.
Relevance to population ecology
1.
The life-history characteristics of populations will determine a.
the organization of individuals within a population both in space and time. b.
how the population perpetuates itself (i.e. discretely vs. continuously; rapidly vs. slowly) c.
which characteristics will be selected for and persist within the population
(due to sexual selection)
2.
In turn, particular selective pressures, such as distribution of resources, temperature, etc…will affect the life-history patterns observed.
II.
Overview: sexual selection
A.
Defined: Characteristics/behaviors are selected for which maximize chances of mating and producing the most, and highest quality, offspring. Type of “natural selection.”
B.
General pattern: Male-male competition and female choice. (Not universal)
1.
Example: Satin bowerbirds . a.
Basics of mating behavior b.
Comparative success of males and females

Males: Record = One male mated with 33 females in a season while other males were completely unsuccessful.

Only ~1/3 of females have >1 mate
C.
Why do males usually compete, whereas females are the choosy ones?
1.
One hypothesis: “Eggs are expensive; sperm is cheap.” (A.J. Bateman, 1948) a.
Differences in energy investment for one egg vs. one sperm

number of offspring for female limited by egg production, whereas the male is limited by the number of mates/eggs fertilized.

Example: Comparison of male and female fairy wren gametes b.
Operational sex ratio is skewed towards a high proportion of males.
2.
Is sperm really cheap? a.
May take a large number of sperm to fertilize a single egg, due to

Hostile environment within female (acid, immune system attack) b.
Sperm competition among males: more sperm

better odds.

Occurs when females have multiple mates (common?)
 Some males may actually run out of sperm (examples…)

One exception that supports the concept: sea horse example c.
Some males produce relatively large sperm, large energy investment.

Example: Drosophila bifurca :
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3.
Second hypothesis (R. Trivers): Competition vs. choice is based on individual with the most total parental investment a.
This is often the female.

.Example: mammals

Trivers hypothesis predicts that males will be the choosy ones when their parental investment is higher

Example: Gulf pipefish:
 Which gender has greater investment? Explain…

Operational sex ratio skewed toward... ?

Males choose large, ornamented females.
D.
Types of sexual selection
1.
Intrasexual selection: maintenance of traits that assist in competition within the gender that results in successful offspring production
2.
Intersexual selection: maintenance of traits attractive to the opposite gender
III.
Intrasexual selection (focus on male-male competition)
A.
Adaptations that help males successfully compete and gain access to females
1.
Dominance behavior and the large size/strength that accompanies it a.
Sexual dimorphism among many mammals (example: elephant seals)

Sexual dimorphism and dominance of females (see PP figure) b.
Specialized weaponry (example: dung beetle horns)
2.
Sneakers! a.
Example: Plainfin midshipmen (Dr. T., p. 90)

Dominant male features and behavior (know!)

Sneaker male features and behavior (know!)

NOTE: If sneakers are common, the diff. in gonad size isn’t as great.
Why not?
B.
Adaptations favoring the use of a male’s sperm (differs from gaining access…)
1.
Mechanisms to displace or inactivate rival sperm [Fig. 10.26, 10.27] a.
Example: Dragonfly and damselfly “scooper” penis b.
Chemical sperm inactivation (Example: fruit flies)
2.
Mechanisms to avoid sperm displacement a.
Mate guarding (Example: Dungeness crabs) b.
Prolonged mating

Sometimes linked to cannibalism (i.e. redbacked spider) c.
Anti-aphrodisiac ( Heliconius erato , a butterfly species)
IV.
Focus: Intersexual selection (focus on female choice)
A.
Material benefits
1.
Nutrition a.
Example: hangflies. Chose based on upon the quality and size of prey.
2.
Anti-predator substances a.
Defensive compounds in arctiid moths (i.e. tiger moths)

Females are able to determine the levels within the males.
B.
Ability of males to provide sufficient sperm
1.
Female fruit flies choose virgin males to avoid this
C.
Parental ability (not directly assessable)
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1.
Honest signal: May be correlated with aspect of male appearance or behavior a.
Example 1: Epaulettes of redwing blackbird correlated with effort in nest defense, while his courtship effort is correlated with his feeding effort b.
Example 2: Sedge warbler:song repertoire size correlates with weight of chick ad fledging c.
Example 3: Male size/strength may denote his ability to defend his territory and protect the young
D.
Health/genetic quality (difficult to separate)
1.
Example: song repertoire in great reed warblers correlated with greater survival of offspring (unrelated to parental care)
2.
Bright coloration is an indicator of health in heavily parasitized species of birds and fish (examples: sticklebacks and satin bowerbirds)

How does the female benefit by choosing a parasite-free male?
V.
Origin/maintenance of mate choice preference for particular characteristics (especially ones that may seem excessive or exaggerated).
A.
The issue: In some species, why do males develop what appear to be extreme traits that actually can hamper their survival?
1.
Example: A peacock’s tail is large and bright, making them potentially much easier targets for predators.
B.
Hypothesis 1: Runaway selection (R.A. Fisher)
1.
A type of directional selection that takes on a life of its own a.
What starts as an “honest signal” becomes more extreme. b.
Mechanism: Females choose the males with large tails, multiple eyespots.
 the next generation has a higher proportion of these males.

Will work even if his traits are not honest signals of quality. Why?

Evidence of arbitrary choices by females (bird band example)
C.
Hypothesis 2: Handicap (A. Zahavi) or “good genes” hypothesis
1.
Exaggerated trait might decrease chance of survival, only males with superior can survive despite the handicap a.
Example: peacocks with large, conspicuous tails are more likely to be prey.
But if he is superior in his strength or speed, he can escape predators.

In this case, a female choosing a male with these traits would be improving her fitness. (His signal is an “honest signal” of fitness.)
D.
Controlled breeding experiments to distinguish between the two hypotheses: Petrie took males of different ornamentation/tail length and randomly bred them with females
(so the female’s traits wouldn’t come into play). The young were raised under identical conditions and then released back into the park population.
1.
Offspring of the “attractive” males weighed more at day 84.
2.
Offspring of the attractive males were more likely to be alive after two years a.
Which hypothesis is supported by this data?
E.
How extreme can a characteristic become? Under what conditions will directional selection stop? (Think about costs vs. benefits…)
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Study questions (see also questions within body of outline!)
1.
How is an organism’s life history pattern relevant to population ecology? Conversely, how might aspects of an individual’s or population’s environment influence its life history?
2.
Describe some population patterns that are influenced by life-history patterns.
3.
What is sexual selection? How is it related to natural selection?
4.
State, in your own words, the two hypotheses presented as to why males usually compete, whereas females usually are the choosy ones.
5.
What is meant by “operational sex ratio?”
6.
What aspects of eggs and sperm support the “eggs are expensive, sperm is cheap” hypothesis? What types of evidence contradict this hypothesis (or at least make the cost differences between eggs and sperm less extreme than the hypothesis suggests.)
7.
According to Triver’s hypothesis, under what circumstances will females compete and males choose?
Provide an example.
8.
Define “intrasexual selection.” Define “intersexual selection.”
9.
Describe several adaptations that help males successfully compete with other males to gain access to females. Be sure you are familiar with all examples presented in lecture.
10.
Describe the difference between the dominant and sneaker male characteristics and strategies in the plainfin midshipmen fish. Also explain why the difference in testes size/weight between the two types is not as great if there are more sneaker males.
11.
Describe several adaptations favoring the use of a male’s sperm and note how this differs from gaining access to females. Be sure you are familiar with all examples presented in lecture.
12.
List four key criteria females may use to select mates, and provide an example of each. Also, be sure you are familiar with all examples presented in lecture.
13.
What is meant by an “honest signal” of fitness?
14.
Describe runaway selection. Does runaway selection imply a fitness benefit to the female who chooses the exaggerated trait?
15.
Describe the “handicap” or “good genes” hypothesis for the development of exaggerated trait. Does this hypothesis imply a fitness benefit for the female who chooses the exaggerated trait>
16.
Which hypothesis is supported by Petrie’s experiments? Describe her experiments along with the supporting data.
17.
What limits how extreme a characteristic can become via sexual selection? In other words, under what conditions will directional selection stop?
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