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What is sexual dimorphism based on

Differences in reproductive potential. Dimorphism can be both anatomical and behavioral.


Why are females choosy

Have higher direct reproductive expenses (higher parental investment - time/energy/parental care/egg production)
- compared to males: produce fewer, more expensive gametes
- provide more parental care
- therefore, low reproductive potential


Why do males tend to compete for females

Produce lots of inexpensive sperm; usually don't care for offspring
- therefore- high reproductive potential limited only by # of mates
- more males than females available to pursue additional mates: lead to competition for females. Resources channeled into competition and mating tactics instead of production of gametes.


Consequence of male high reproductive potential

- more males than females ready to produce at any time (operational sex ratios: OSRs. Are skewed towards males)
-- sets p competition among males for limited available females. And males have it mess incentive to increase # of mates.
-- general pattern: competitive males, choosy females (theory behind sexual selection)


When would you expect competitive females, choosy males?

- when there is high investment by males

Ex: Mormon crickets. Males produce expensive spermatophore - lots of nutrients, maintenance involved with it. Spermatophore presented to females, who uses it to fertilize eggs and eats some of it to produce more eggs after saving sperm.
- cost prevents males from mating more than once. Females compete for access to males, males reject lighter females. Correlation between body mass and egg production.
-- males let females crawl onto their backs, kick certain ones off and give spermatophore to one they've chosen


What leads to variation in success?

Sexual selection: selection for traits that confer mating advantage

Intra: selection for traits that improve ability to interfere with mating opportunities of other members of same sex
Inter: selection for traits that increase attractiveness to opposite sex

- strength of selection depends on skewedness of OSRs, amount of male parental effort. Species tending towards monogamy are less dimorphic


What kinds of intrasexually selected traits evolve?

Display-related: honest signals of fighting ability
Combat-related: weaponry, large size
Fertilization-related: features that facilitate sperm competition


Do alternatives to direct confrontation exist?

Yes. Game theory (alternative mating tactics)
- may be part of behavioral conditional strategy
-males can change tactics depending on what others are doing
- may be hereditary alternative phenotype (males tactic innate and relatively inflexible - accompanied by appropriate morphology)
-- phenotype maintained by frequency-dependent selection. Seen a lot in fish, some reptiles/amphibians


Example of conditional strategy

Scorpionfly mate acquisition
- flexible based on females preferred resources. Females like insect gifts. If males can't get an insect, is capable of producing a nutrient-rich ball of saliva to present instead. May also try to force self on female if energy for spitball not available and don't have an insect.

How do you test for conditional strategy? Rule out alternative phenotype.


Examples of alternative mating phenotype

- coho salmon jacks vs. hook noses
- marine isopod males - alpha is largest, sponge defenders. Beta is a female impersonator. Gamma is small, sneaky.
--alphas defend sponges where females lay eggs. Betas ignored by alphas, end up fertilizing some of the eggs. Gamma almost completely undetected by alpha males.


What if females have potential access to other males?

- 1. Prevent insemination through mate guarding. Males physically guard female. May reduce attractiveness of female (ex-insects produce pheromones sprayed on females that are "anti-aphrodisiac). Copulatory plugs from males seal off reproductive tract to block sperm entry from other males.

2. Compete internally for fertilization (sperm competition)
- increase copulation rate, increase pint of sperm delivered, remove sperm of other males.
- ex: damsel fly females can store sperm for a long timer. Males have hooks at end of penis, capable of scraping out sperm from previous males and replace it with their own


Case study that shows mate guarding works

Seychelles warbler males lost paternity when they reduced mate-guarding. Put fake egg in nests to make male think she had already laid, did not need to defend her anymore - successful EPCs and intrusions went may up because mate guarding went down.


Intersexual selection

- traits evolve that females use to assess male quality


Why are females choosy?

Female preferences drive male ornamentation - both behavioral and physical.

Direct benefits
1. Food for egg production (nuptial gifts). Spermatophores, prey, male himself (Ex: red back spider males offer themselves to get eaten to help feed his future offspring. Theory is that their potential to mate again is so low that their fitness is increased more by allowing cannibalism.)
2. Territorial resources (males are typically the territory-holders): food, breeding sites, protection
3. Parental care: defense of young and eggs, feeding of young (courtship may advertise potential parental quality directly or indirectly), ability to fertilize eggs, reduce risk of STDs, other parasites


What if no direct/material benefits are evident?

Fishers runaway selection theory
Good genes (Hamilton-Zuk hypothesis)


Fishers runaway selection theory

Evolutionary positive feedback between male trait and female preference
Both the trait and the preference become more extreme each generation
Sensory processing bias - if preference and trait become linked, then it snowballs until selective pressures like predation shut down further development of the trait in the population over time


Good genes

- females choose males based on their genetic quality
- Zahavi's handicap principle: costly structures/displays are honest indicators of male genetic quality
- predictions of good genes hypothesis: showiest males are best quality, male quality is heritable


Support for Zahavi's handicap theory predictions

- Male guppies with largest orange spots have longer swimming stamina
- Grey tree frog song duration correlates with his offspring's viability
--used half siblings. Fertilized 1/2 of each female's eggs with short-call male, 1/2 with long call male
-- offspring fertilized with longer call male had higher fitness
-- why use same mother? Maternal effects may cause increase in viability. Ex: zebra finches prefer males with re leg bands. Secrete more testosterone into eggs. Directly controlling viability of offspring just by thinking about who they were mating with.


Hamilton-Zuk hypothesis

Sexual ornamentation/displays are reliable indicators of heritable disease resistance
Predictions: showier males are disease-resistant. Offspring of showier males are more disease-resistant
Ex: may depend on species whether this is true. Found that yellow feather pigmentation is later if male is infected with >1 blood parasite species. Bird


How do you know which hypothesis is correct? (Runaway or good genes)

Must show that male trait is not related to heritable viability
- ex: female zebra finches prefer males with artificial crests
- hypotheses may not be mutually exclusive


Female choice after/during copulation

Cryptic female choice
- selective sperm storage, use for fertilization
- females can be selective at any point
-- coupling - intro mission - insemination - sperm transport
- can mate with multiple males, decide which sperm to use


Further evidence of "the battles if the sexes" (competition among and between sexes)

- males reduce parental care if multiple mates
- forced copulation by males
- females sometimes resist infanticide
- false estrous


Mating systems

Based on # of breeding partners

Monogamy: 1 male 1 female
Polygyny: 1 male and >1 female
Polyandry: 1 female and >1 male
Promiscuity: >1 female and >1 male


When is monogamy favored

1 receptive females are widely distributed (ex: dik-diks. Females are solitary. Large home range. Monogamy driven by ecological factors. Males could not possibly form home range big enough to defend paternity with multiple females.) Tend to see monogamy when ecological factors drive wide distribution of females.
2. Male parental care required. Feeding, brooding, protection. (Ex: birds. Chicks need to be fed by both parents and get protection from both. More chicks of yellow-eyed junco survive when male is present.)
3. Monogamy is enforced by male or female.
--mate guarding. Female burying beetles attack male if he releases as pheromone. Shuts down production. Resource-wise, would have to share food item with another female's offspring if one is attracted.
- female refusal to mate with mates males
- aggression toward mate (intrapair aggression)


Monogamy is common in

Birds (over 90%)
Rare in mammals and fish (except when male parental care provided or females not defendable)

However, extra-pair copulations are common even among monogamous species (genetic mating system may not be consistent with social mating system) - will take opportunities to increase personal fitness


Advantages of multiple partners/EPMs for females

- Direct benefits: more nuptial gifts, more parental care, access to >1 territory (ex: female megacillid bees permitted access to pollen nectar only if they mate with territorial males), infanticide reduction

- genetic benefits: fertilization insurance, chance for higher quality genes, more diverse offspring


Polygyny is favored when

Males free of parental care, receptive females are clumped in space, receptivity of females is asynchronous

Males either defend females or resources females need or beat other males in race to mate with in defended females (scramble competition)


Female defense polygyny

Evolves when female movements unpredictable
Ex: plains zebra (resources abundant). Females could be anywhere so males better off traveling with them


Resource defense polygyny

Evolves when females predictably visit resources, but are defendable

Ex- Grevy's zebra, resources space out


Scramble competition polygyny

When females are widely dispersed and undefendable