Chapter 8 Life History Patterns - Western Oregon Universitywou.edu/~snyderj/Biology 357 -...

Chapter #8 Life History Patterns(pg. 164 – 180)

What are the goals for living organisms?

8.1 – Reproduction May Be Sexual or Asexual

Parthenogenesis – born w/o fertilization

Dioecious – Separate Males and Females

Hermaphroditic –Individual organisms with both male and female organs.

Monoecious – separate male and female flowers on the same plant.

8.2 – Sexual Reproduction Takes Many Forms

8.3 – Mating Systems Describe the Pairing of Males and Females

Mating Systems Range from Monogamy to Promiscuity


Polygamy – the acquisition by another individual of two or more mates, none of which is mated to other individuals. A pair bond exists between the individuals and the individual having multiple mates is generally not involved in caring for the young.

- Size of group depends upon synchrony of fertility and receptivity.

- Small time period – fewer individuals.- Longer time period – more individuals.


Polygyny – Where an individual male pairs with two or more females.

Polyandry – Where an individual female pairs with two or more males (exception rather than the rule)

Access to resources

ParentalInvestment


Relevance to Population Ecology• Life-history characteristics determine

– Organization of individuals in space and time • Examples…

– How the population perpetuates itself • 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 .

8.4 – Acquisition of a Mate Involves Sexual Selection

What is the advantage to the individual and species by this display?

8.4 – Acquisition of a Mate Involves Sexual Selection

Sexual Selection involves:

-Intrasexual selection: male:male competition or female:female competition for access to potential mates. The maintenance of traits that assist in competition within the gender successful mating.

- Intersexual selection: differential attractiveness of males to females and vice versa. The maintenance of traits that are attractive to the opposite gender.

Which gender determines the “winner”?

Intersexual Selection:A Focus on Female Choice

• Material benefits: Nutrition Ex: hangflies)– Length of mating time depends on quality and size of

“courtship gift”

• Material benefits: Anti-predator substances– Defensive compounds in arctiid moths

Photo: butterfly-conservation.org


• Ability of males to provide sufficient sperm– Female fruit flies (some species) choose virgin males.

Photo: San Francisco Exploratorium


• Parental Ability– Cannot assess directly

– May be correlated with other features of the male

– Example 1: Redwing blackbirds

• “Epaulettes” correlated with nest defense

• Courtship effort correlated with feeding effort Photo: Vancouverislandbirds.com


• Parental Ability (Ex: sedge warbler)– Size of song repertoire correlates with chick weight at

fledging


The Sedge Warbler (cont.)

• Female sedge warblers choose on the basis of repertoire size.– Thus they choose the

“most fit” males.

• Health/Genetic Quality– Example: song repertoire in great reed warbler where:

– Females chose males with larger repertoires.

– This was correlated with greater offspring survival (unrelated to parental care).


• Health/Genetic Quality– Example: Bright coloration of sticklebacks

negatively correlated with low parasite loads


• 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”


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

• Hypothesis 1: Runaway selection (R.A. Fisher - early 20th Century)

– 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)


• 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 randomly

bred with females• Why random?

– Young raised under identical conditions and then released

• Results– 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?

Marion Petrie’s Peacock Data

Source: http://blog.lib.umn.edu/denis036/thisweekinevolution/2008/06/guest_blogger_the_peacocks_tal.html

• How extreme can a characteristic become? Under what conditions will directional selection stop? (Think about costs vs. benefits…)


Intrasexual Selection- Adaptations to Gain Access to Females –

• Dominance behavior & characteristics– Example 1: sexual dimorphism in elephant seals

Male-male competition and sexual dimorphism (seals)NOTE: Each point represents a species

Male dung beetle, Phanaeus vindex (Rattlebox photography)

• Weaponry for fighting with other males.

• Example: dung beetle!


• Sneaker strategies– Example: 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 morphological 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- Adaptations Favoring Use of Sperm -

• Displacing or inactivating rival sperm– Damselfly “scooper” penis

• Displacing or inactivating rival sperm– Example: Chemical sperm inactivation in fruit

flies

Photo: San Francisco Exploratorium


• Mechanisms to avoid sperm displacement– Mate guarding

(Example: many crab species)


• 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


• Mechanisms to avoid sperm displacement– Anti-aphrodisiac (Example: Heliconius erato)


8.5 – Females May Acquire Mates Based on Resources

In sexual Selection, the female will select mate(s) based on their physical characteristics, because this is an indirect measure of their health OR their (their mate’s) ability to acquire and defend resources (e.g., food and space).

Territoriality - Monogamy

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 builds 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 are much more successful than others.

Bowerbird Females: # of MatesOnly ~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

But, 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

• 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!

• 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

Overview• A mating system includes

– how members of a particular species (or population) choose and bond with mates

– how many mates per individual– how parental care (if it occurs) takes place.

• Types of mating systems– Monogamy: One male mates with one female– Polygyny: One male mates with several females– Polyandry: One female mates with several males

• “Social” vs. “genetic” monogamy

Key Principles

• The system that evolves depends upon the individual interests of each gender.

• Male and female interests are often in conflict. Why?– Differences in gamete investment and/or total parental

investment– Male “default” system = ____________. Why?– Is there a female default system? Why?

• Interests/behavior of one gender serve to constrain options available to the other gender.

Polygyny• Resource defense polygyny

– Example: African cichlid fish, Lamprologus callipterus

• Defended resource = shells in which females lay eggs

Polygyny• Female defense polygyny

– Example: Elephant seals (females aggregate)

Photo: www.driftersister.com

Polygyny• Female defense polygyny

– Example: Elephant seals (males compete for beachmaster status)

Photo: www.wetasschronicles.com

Polygyny• Lek polygyny

– Males clump, but not due to another resource• Males become the clumped resource!

– Example 1: satin bowerbirds

Satin Bowerbirds: multiple signals of health and fitness (and good genes?)


– Males clump, but not due to another resource• Males become the clumped resource!

– Example 2: sage grouse (filoplumes and sound in central area of lek determines mate preference)


– Example 3: bullfrogs• Females choose males with longest, loudest and deepest calls• But don’t forget the sneaky f--kers

www.tc.umn.edu

Polygyny: benefits/costs• Male:

– number of offspring (+)

• Female: – gets a high-quality male (+)

– gets less of the male’s time and attention for• raising young

• being defended against predators

Monogamy• Common or rare?

• In which group of animals is it most common?

• What hypotheses would account for it?

www.magicmud.com

Monogamy: alternate hypotheses

• Mate assistance: it takes two parents to raise the offspring

– Example: Adelie penguins

– Both parents needed for chick survival

Photo: Karen Haberman


• Mate guarding: guarding assures paternity; not guarding jeopardizes it.

– Especially critical if females are rare or receptive for a limited time

– Example: many crab species


• Female-enforced monogamy

– Similar to mate-guarding, but done by female.

– Example: Burying beetles– A female would lose

resources, and possibly her offspring if she allows her male to mate again. www.royalbertmuseum.ca


• Danger “theory”– Leaving increases

chance of dying if predation rates are high.

– Example: The mantis shrimp Lysiosquilla sulcata

Lysiosquilla sp. Opencage.info

Mantis shrimp (another type)


• Pop ‘em out “theory”– Highly fertile mate

– Not worth time/energy to seek another.

– Example: Djungarian hamsters

bbs.petsky.com.cn

Social Monogamy and extra-pair copulations

• Extra-pair copulations can increase fitness of participants.

• Males: More mates more offspring possible.

• Females: – Historical (not current) ideas: no advantage for

females

– Observational/experimental evidence: clear fitness benefits documented for some species

• Example: Yellow-toothed cavy

Yellow-toothed cavy: Offspring survival as a function of multiple mates for females


• Direct fitness benefits: genetically based– Good genes

• What does this mean?

– Genetic compatibility• What does this mean?

– Genetic variability among offspring• Why important?


• Other benefits that may improve fitness for females:

– More resources hypothesis• Example: Orange-rumped honeyguides swap food

for sex.

– Better protection/care hypothesis• Example: Dunnocks (European song bird)

– Mate with two males both care for young

– Infanticide reduction hypothesis• Example: chimpanzees (who’s dad?)

Polyandry (w/o polygyny)• Spotted sandpipers: near-complete sex-role

reversal– Females arrive on breeding grounds; compete with

other females for territories.– Initial male arrives, mates, cares for her first clutch.– Second male arrives later, mates, and cares for her

second clutch.

What Circumstances Promote Polyandry?

• Female: only lays 4 eggs at once– Add eggs (experimentally) decrease the

total young successfully raised• Related to incubation effort and protection

– Female can ↑ reproductive success by laying a second brood• Needs second mate

• Reproductive success limited by mates rather than gametes in this case

• Why would males “comply?”– Operational sex ratio biased toward males

(related to absolute ratio for this species)– She abandons

• He stays offspring survive• He leaves offspring die

– Male 1: Certain of paternity for clutch 1; possibility of paternity for clutch 2• How is this possible?

– Male 2: Later arrivals less dominant, but still have a chance of paternity if they stay.


• Food fluctuation hypothesis– In food-poor years, females put all energy into

eggs and have no energy left for care of eggs/young.

• Mate assistance (by male) essential monogamy

– In food-rich years (i.e. many mayflies), the female “recovers” her body mass and can lay another batch

• Monogamy Polyandry


• Heavy predation pressure on nests– Multiple nests assure that at least some young

will survive. • Male is needed to prevent predation

• Young will all be lost if he doesn’t stay.


Patterns of Reproductive EffortVariations

• Numbers of young produced at a time– More young = less parental

investment/individual high mortality among young

• Care of eggs/larvae– Variability in parental investment

• Type of young produced– Precocial vs. altricial offspring (What is the

difference?)

• Number of reproductive events in a lifetime– Semelparous: one big reproductive event in

lifetime/many offspring• Many are relatively short-lived (squid, annual plants)• But some are long-lived (periodical cicadas)

– Itoparous: many reproductive events in lifetime/ fewer offspring per event.

• Common especially birds and mammals

• Timing is an issue: – begin early materials/energy into reproduction, – Begin later materials/energy into survival and

growth

Patterns of Reproductive EffortVariations

“r”-strategists “K”-strategistsSemelparous ItoparousMany offspring Few offspringLittle/no parental investment

per individual offspringHigh levels of investment per

individual offspringRelatively short lifespan Relatively long lifespanBegin to reproduce relatively

early in lifeBegin to reproduce relatively

later in lifeGood colonizers of newly

available habitat, but often not effective competitors

Not usually colonizers, but arrive later in succession, compete successfully

“r ” vs. “K” strategists

Chapter 8 Life History Patterns - Western Oregon Universitywou.edu/~snyderj/Biology 357 -...

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