Ecology Lecture 10
Life History Patterns 1
Topics covered (both Life
History Lectures)
Sexual selection


What criteria do individuals use to choose mates?
Mating systems



How many mates does an individual have
What factors determine this?
Parental care/parental investment in
offspring
Allocation of resources



Quick reproduction vs. growth & long life (r and K
strategists, etc…)
Relevance to Population
Ecology
Life-history characteristics determine


Organization of individuals in space and time


How the population perpetuates itself



Examples…
discretely vs. continuously; rapidly vs. slowly, etc…
Which characteristics will be selected for and persist
within the population (due to sexual selection)
Conversely, selective pressures (abiotic
factors, distribution of resources, etc..)
will affect the life-history patterns
observed
Sexual selection

Defined:



Selection for characteristics/behaviors that
maximize chances of mating and producing
the most, and highest quality, offspring.
A category within “natural selection”
General pattern: Male-male
competition and female choice

Common pattern with many exceptions!
Example: Satin bowerbird
mating behavior
 Each male build an
elaborate bower
where he conducts
his courtship
display
 Bowers located
near each other
 Each female visits
several times,
finally chooses a
mate
Bowerbird males: # of mates
 Some males much more successful than
others
Bowerbird females: # of
mates
 Only ~1/3 of females have >1 mate
Why do males usually compete,
while females choose?
 Hypothesis 1, A.J. Bateman: “Eggs are
expensive, sperm is cheap!”
 Amount of energy
invested in a
single gamete is
much greater for
females (eggs)
than for males
(sperm)
 Female bird may
invest up to 30% of
body weight in eggs.
“Eggs are expensive, sperm
is cheap.”
 Difference in investment per gamete
 Example: Fairy wrens
 Males have 8 billion sperm in testes at once
 Females lay six eggs maximum per clutch
 Female limited by egg production,
 Male limited by number of mates only
(presumably unlimited sperm)
 Operational sex ratio skewed toward
males
Is sperm really cheap?

It often takes a large number of
sperm to fertilize a single egg, due
to

Hostile environment within female


Acid
Attacks by the immune system
Is sperm really cheap?
 Sperm competition among males
 Occurs when females have multiple mates
 Possibly the predominant situation
 Fitness advantages for female (will explore in next
lecture)
 Some males may actually run out of sperm…
 Garter snakes, zebra finch, blue crabs, rams…
What if there is no sperm
competition?
 In sea horses, eggs
are deposited into
pouches, and there
is not sperm
competition.
 Why not?
 Male sea horses
have relatively low
sperm counts!
Is sperm really cheap?
 Drosophila
bifurca: one sperm
with long tail
 Sperm tail is 20x
length of his body
 His testes make up
11% of his body
mass.
Why do males usually compete,
while females choose?
Hypothesis 2 (R.
Trivers): Competition
vs. choice is based on
individual with the
most total parental
investment



Often the female
(example: mammals)
But in some species, male
makes a greater total
investment
Gulf pipefish
 While male cares for a single brood, a female can
produce two clutches of eggs  male has greater
total parental investment
 Operational sex ratio skewed toward females.
 Males choose large, ( ornamented females over
small, drab ones.
Female
Male
Types of sexual selection


Intrasexual selection: maintenance of
traits that assist in competition within the
gender  successful mating
Intersexual selection: maintenance of
traits that are attractive to the opposite
gender
Intrasexual selection 1:
Adaptations to gain access to females
 Dominance behavior & characteristics
 Example 1: sexual dimorphism in elephant
seals
Male-male competition and sexual
dimorphism (seals)
Male dung beetle, Phanaeus vindex (Rattlebox photography)
Intrasexual selection 1:
Adaptations to gain access to females
 Weaponry for fighting with other
males.
 Example: dung
beetle!
Intrasexual selection 1:
Adaptations to gain access to females
 Sneaker strategies
 Example 1: Plainfin
midshipmen
 Dominant male features
and behavior
 Nest building, singing,
guarding
 Sneaker male features
and behavior
 No nest, no singing, just
sex…
 Small fish, big balls!
Dung beetles: two morphs
 Behavioral and
morphologial differences
similar to midshipmen
 Large, dominant males with
horns defend burrows
 Small, hornless males with “big
balls” sneak
 Midshipmen
 Genetically-based differences
 Dung beetles
 Nutritionally-based differences
Intrasexual selection 2:
Adaptations favoring the use of one’s sperm
 Displacing or inactivating rival sperm
 Damselfly “scooper” penis
Intrasexual selection 2:
Adaptations favoring the use of one’s sperm
 Displacing or inactivating rival sperm
 Example: Chemical sperm inactivation in fruit
flies
Photo: San Francisco Exploratorium
Intrasexual selection 2:
Adaptations favoring the use of one’s sperm
 Mechanisms to
avoid sperm
displacement
 Mate guarding
(Example: many
crab species)
Intrasexual selection 2:
Adaptations favoring the use of one’s sperm
 Mechanisms to avoid sperm displacement
 Prolonged mating and cannibalism (example: redback
spider)
 Female less likely to mate
with another if she eats
him
 He has low likelihood of
finding a new mate (high
predation)
 Andrade, 1996
Intrasexual selection 2:
Adaptations favoring the use of one’s sperm
 Mechanisms to avoid sperm displacement
 Anti-aphrodisiac (Example: Heliconius erato)
Intersexual selection
Focus on female choice
 Material benefits: Nutrition Ex: hangflies)
 Length of mating time depends on quality and size of
“courtship gift”
Intersexual selection
Focus on female choice
 Material benefits: Anti-predator substances
 Defensive compounds in arctiid moths
Photo: butterfly-conservation.org
Intersexual selection
Focus on female choice
 Ability of males to provide sufficient sperm
 Female fruit flies (some species) choose virgin males
Photo: San Francisco Exploratorium
Intersexual selection
Focus on female choice
 Parental ability
 Cannot assess directly
 May be correlated with
other features of the
male
 Example 1: Redwing
blackbird
 “Epaulettes” correlated
with nest defense
 Courtship effort
correlated with feeding
effort
Photo: Vancouverislandbirds.com
Intersexual selection
Focus on female choice
 Parental ability (Ex: sedge warbler)
 Size of song repertoire correlates with chick weight
at fledging
Sedge warbler (cont.)
 Female sedge
warblers choose
on the basis of
repertoire size.
 Thus they choose the
most fit males
Intersexual selection
Focus on female choice
 Health/Genetic quality
 Example: song repertoire in great reed warbler
 Females chose males with larger repertoires
 This was correlated with greater offspring survival
(unrelated to parental care)
Intersexual selection
Focus on female choice
 Health/Genetic quality
 Example: Bright coloration of sticklebacks
negatively correlated with low parasite loads
Intersexual selection
Focus on female choice
 Health/Genetic quality (bright color
negatively correlated with parasite
load)
 Advantages to females choosing these males
 Avoid getting parasites while mating 
 Avoid transferring parasites to young
 Are choosing healthier males; their health status
may be related to “genetic quality”
Satin Bowerbirds: multiple signals of
health and fitness (and good genes?)
Origin/maintenance of mate choice
for “exaggerated” characteristics

In some species, why do males
develop what appear to be extreme
traits that actually can hamper their
survival?

Example: Peacock’s tail
Origin/maintenance of mate choice
for “exaggerated” characteristics

Hypothesis 1: Runaway selection
(R.A. Fisher)

Directional that takes on a life of its own


Starts as an “honest signal”  more extreme.
Mechanism: Females choose 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)
Origin/maintenance of mate choice
for “exaggerated” characteristics

Hypothesis 2: Handicap or “good
genes” hypothesis (R.A. Fisher)

Exaggerated trait might decrease chance of
survival, only males with superior genes can
survive despite the handicap


Example: peacock tail as a handicap.
In this case, a female choosing a male with
these traits would be improving her fitness.
(His signal is an “honest signal” of fitness.)
Distinguishing between Runaway
Selection and “Good Genes” (Petrie)
Methods


Males of different ornamentation/tail length and
randomly bred them with females


Young raised under identical conditions and then
released
Results




Why random?
Offspring of the “attractive” males weighed more at
day 84.
Offspring of the attractive males were more likely to
be alive after two years
Which hypothesis is supported by this
data?
Petrie’s peacock data
Origin/maintenance of mate choice
for “exaggerated” characteristics

How extreme can a characteristic
become? Under what conditions
will directional selection stop?
(Think about costs vs. benefits…)