Polygyny

(Who is Polygynous, what is it associated with?)

• Most mammals
• Little male parental care
• Males typically larger than females
• Males typically have more "weapons" than females

Monogamy

(who is monogamous, what is it associated with)

• Most birds
• Males and females relatively equal parental care
• Males and females usually same size and color

• In polygynous bird species, males contribute little parental care
• They are also usually more brightly colored and ornamented than females

Favors traits that attract members of of the opposite sex (usually concerns female choice of males)

"Traits that make you sexy"

• Males have greater reproductive potential; greater opportunity to mate with many females
• Males provide less parental care
• Males compete more for females than vice versa
• Males are relatively unselective in mating whereas females are more choosy 
  • More choosy b/c ore costly for female to mate with the "wrong" male, as females invest much time and energy into offspring
  • Choosiness justifies evolution of traits in males (size, color, song) that enhance competitive ability or signal condition, resources

• Monogamous males, monogamous females, and polygynous females reproductive efforts
  • -->large parental effort, small mating effort
• Polygynous males reproductive effort
  • -->small parental effort, large mating effort
• Polygynous sexual selection
  • --> very intense
• Monogamous sexual selection
  • -->more weak (because of no benefit from seeking more mates)

Constraints on Monogamy vs. on Polygyny

(ecological, social, physiological)

Polygynous:

• Males freed from parental care (b/c can only provide indirect care)
• Intense competition for mates
• Strong sexual selection
• Pronounced sexual dimorphism

Monogamous:

• Male's parental care essential
• Less intense mate competition
• Weaker sexual selection
• Less pronounced sexual dimorphism

• Larger beach domain means more females (8 males fertilized 348 females, a 1:43.5 ratio)
• Breeding season is 3 months long, favoring male endurance
• Males lose 34-41% of body mass (only males of superior condition can endure this cost)

High variance in male reproductive success

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For females, not more reproductive success by mating with more than one male

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• Correlation between male dominance rank and ability to monopolize fertile females
• Friendship may be adaptation against infanticide, but not a benefit to male reproductive success
• Sexual dimorphism (seen in scull differences)
• If there were strict correlation w/ male dominance rank, would be blue curve

• If females are sexually receptive at the same time, consortships with males may be many
• A temporary pair bond b/t a male and a sexually receptive female is characterized by close spatial proximity and mate-guarding
• Usually but not always involves alpha male
• Subordinate males monitor consort status all the time
• Sexual consortships are unstable and transient, lasting from several hours to several days
• Subordinate males try to take advantage of temporary separations between consort pair to mate "sneakily"

• Elvis is eating, place loudspeakers
• When females mate, have highly distinctive copulation call
• One loud speaker signals female copulation call, second immitates her consort partner's grunt from other location (not signaling mating)[image]
• Results show most looks toward female speaker in test condition and more approaches to female speaker in test condition

• Dominant male can coerce females to mate
• But also might be in her best interest because of genetic quality, protection, resources, and paternity confusion (to avoid infanticide most commonly committed by dominant males)

• Slight polygyny
  • Lions
  • 1.3/1 M:F body weight
• More polygyny
  • Gorillas
  • 2.3/1 M:F body weight
• Extreme polygyny
  • elephant seals
  • 3.1/1 M:F body weight

Males with trait may provide:

• direct benefits (paternal care)
  • protection
  • territory
  • feeding
• Indirect (genetic) benefits
  • Sons are attractive ("sexy")--> mating advantage
  • Offspring survive at higher rates--> good genes

• Trait may seem arbitrary
  • condition it indicates is not (bright colors, elaborate song)

• Trait is condition dependent 
  • males in poor condition do not develop trait fully
  • often lose the trait after breeding season

• Took eggs, hand fed babies
• Fed experimental group less than control initially
• After 2 weeks can feed themselves
• Evened out by 4 weeks 
• Played songs for them to hear
• Control group had higher song-copying fidelity than experimental group

• Mate choice and tail length in widowbirds
  • Shortened some tails, used those to paste on to others to give longer tails
  • control tails were cut off then pasted back to ensure flying ability not compromised by glue
  • males with elongated tales had significantly more nests
• Tail length, barn swallows
  • number of days it takes to attract a mate is a measure of "sexiness"
  • males with elongated tales took fewer days to pair
• House finch males, red chest
  • died male hair
  • more females preferred red males
• Male chickadees in dominance hierarchies
  • Aggressive playback: hears intruder over mate, then neighbor over intruder, sees neighbor as more attractive
  • Subordinate playback: mate dominates intruder, intruder dominates neighbor, mate is heroic, most dominant
  • Where mate dominated, about 50% of chicks fathered by neighboring male
  • If mates prove dominance, females stay only with social mate
  • This is social monogamy, as now it can be seen that there is sneaky mating
  • Males will often leave nest if he sees mate mating w/ neighbor

• Threshold: choose first male that meets criteria
• Sequential: sample males as long as they are increasing in quality, then drop back to next to last male
• Sample among a pool: requires memory and ability to compare, revisit
• Copy other females

1) Constraints on females' sampling abilities constrain ___?

2) Do females sample optimally?

3) When is sampling more feasible?

1) strength of sexual selection

2) Tend not to sample as much  as is optimum for perfect mate choice as there are often costs, particularly when males provide direct benefits (territory, paternal care)

3) When males provide no direct benefits

• Prey gifts
• Male nutriens (spermatophores, cannibalism)
• Parasite reduction
• Feeding, carrying
• Territories and other defended resources
• Risk reduction (predators, infanticide)

ex. Praying mantis only allows mating w/ male who lets her eat him while they mate (starts w/ head)

Scarlet-bodied wasp moth male eats and stores toxins in abdomen pouch, then releases a "confetti" stream of toxins on the female while mating, which protects the female and her eggs

• Look at feeding rates as function of plumage brightness (house finches), positive correlation
  • Males correlate brightness w/ father's brightness 
  • Females benefit because offspring bright and well fed
• Male repertoire size correlated with chick size in sedge warblers
• Barn swallows monogamous species where males feed offspring
  • testers swapped eggs to test number of mites on offspring from other nests
  • birds with long tales better at fending off mites
  • offspring better protected when raised by male w/ long tale regardless of natural paternity
• Experiment with poison dart frogs and chirp
  • females prefer males who call at high rate at long chirp duration
  • females that mate with males with better calls rear more eggs and even in lab, young have higher rate of survival
• Male peacocks with longer tails have higher concentration of white blood cells

How does sexual selection operate?

How does it interact with natural selection?

If male trait and female preference are heritable, the trait gene and the preference gene may become linked

Female choice may drive the male trait beyond the optimum point in terms of natural selection

High quality males should be able to overcome at least some of this cost

• Predation drives trait to duller
• Female choice drives trait to brighter
• May be attracted in part b/c can defend themselves even with this apparent cost

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• Can tell guppy fitness just by looking at it
• In Trinidad, som streams have higher predation
• Color and spots are "desired traits" show less predation in streams with fish with higher spot number/brightness

1. Females initially prefer males with a given trait because it signals high quality

2. Females who mate with these males produce sons with the trait and daughters with a preference for the trait

3. Female choice may drive the male trait beyond the optimum point in terms of natural selection

4. A runaway process occurs, where males with the trait are preferred only because they have the trait.

5. ‘Sexy’ sons have high reproductive success, even though they may not be in good condition. 

6. Daughters obtain no benefits other than a preference for the trait.

Runaway--> detached from original function, trait "runs away"

1. Females initially prefer males with a given trait because it signals high quality (like runaway)

2. Females who mate with these males produce sons with the trait and  daughters with a preference for the trait (like runaway)

3. Female choice may drive the male trait beyond the optimum point in terms of natural selection (like runaway)

4. High quality males should be able to overcome at least some of this cost

5. Costly male traits are preferred by female because they indicate fitness (even if it’s not always highly heritable)

6. Females who mate with such males produce both fitter sons and fitter daughters 

• ‘Sexy” sons are primarily advantageous if:
  • the sexy trait is relatively rare 
  • the trait isn’t too negatively correlated with offspring survival (especially of daughters) 
  • female choice isn’t too costly
•  There is more opportunity for a runaway process if males provide no direct benefits 
• If male PI or resources are essential, females don’t benefit from choosing a highly polygynous ‘sexy’ male, especially if he’s less fit.
• If males with extreme versions of the trait are less fit, a female may do better to mate with a slightly less ‘sexy’ male whose offspring are more likely to survive.

Predicted in indicator, runaway, or both models:

Does the trait vary among males?

Predicted in indicator, runaway, or both models:

Is the trait an honest reflection of male quality? Is it costly?

Predicted in indicator, runaway, or both models:

Do females prefer males with extreme versions of the trait?

Predicted in indicator, runaway, or both models:

Do females sample among males?

Predicted in indicator, runaway, or both models:

Is there a genetic correlation between the male trait and female preference?

Predicted in indicator, runaway, or both models:

Do females benefit from mating with males with extreme versions of trait?

Predicted in indicator, runaway, or both models:

Benefits include males with trait providing direct benefits (paternal care), protection territory, feeding

Predicted in indicator, runaway, or both models:

Benefits include males with trait providing indirect (genetic benefits)

Predicted in indicator, runaway, or both models:

Genetic benefit is attractiveness of sons --> mating advantage

Predicted in indicator, runaway, or both models:

Genetic benefit is higher rates of offspring survival and good genes

• Little empirical evidence of runaway model
• Most empirical evidence for runaway sexual selection based on experimental studies conducted in captivity, where environmental influences are held constant
• Most empirical evidence supports indicator model
• However, evidence for indirect (genetic) benefits is still scant

• Male song in birds: broadcast to male for territory defense, directed at females for mate attraction
•  Mane color in lions: signal of status to males, attractive to females

• Important for female choice, but not for male-male competition
• May be genetic correlation b/t female choice and male ornamentation
• Ornamentation may reflect male quality
  • Diet
  • Parasite load
  • Predation risk

* direct examination of resources 

* male traits that are correlated with resources 

* male genetic quality independent of resources

Male ornaments are not arbitrary, but have probably evolved because females use them as reliable indicators of male resources and/or genetic quality 

Inter-sexual selection (female choice) does not always reaffirm the results of intra-sexual selection (male-male competition) or natural selection 

It can lead to the evolution of male traits that are costly in terms of  fighting and/or survival

  How is sexual selection distinguished from general natural selection?  Can it be considered a special case of natural selection?

Yes, it’s a special case.  It is still a competition over resources: in this case, that resource happens to be members of the opposite sex.  Note however in some cases sexual selection acts in the same direction as natural selection (this week’s article on pronghorns) and in other cases it acts in the opposite (handicap models).

What are the pre-zygotic (before fertilization) and post-zygotic (after fertilization) factors that influence how selective an individual is?

Pre-zygotic: males and females seem to differ in the amount of reproductive benefit gained per additional mating (a line steeper for males generally).  This may be traced to anisogamy – differences in gamete size.  All these are factors prior to the actual meeting of gametes.

Post-zygotic: Gestation & parental investment.  In spp. with internal fertilization females bear most of the cost.

Under what conditions should an animal be ‘choosy’ (name at least two)?

• When it has relatively high investment in each offspring.
• When there is substantial variation among members of the opposite sex (i.e. in indirect or direct benefits they are able to offer).

And building on the above, when would you expect to see strong competition for mates within one sex (i.e. strong sexual selection on that sex)?

Why is sexual dimorphism usually greater in polygynous species than in monogamous species?

Monogamy implies symmetric selection pressures on both sexes; hence both should be responding similarly.

 Under what conditions would you expect to see sexual ‘role reversals’?

When the two conditions in (4) above apply to males (i.e. high investment, variation among females [e.g. in fecundity])

• Intra-sexual selection favors traits that affect the ability of members of the same sex to compete with one another for mates.

• Inter-sexual selection favors traits that attract members of the opposite sex.

Indicator model:

1) females prefer trait that is indicative of male health and/or male viability

2) Primary adaptive value to choosy females is avoidance of contagious diseases and parasites for both she and her offspring 

Runaway:

1) Females prefer trait that is sexually attractive

2) Primary adaptive value to choosy females is sons' inheritance of trait that makes them sexually attractive; daughters' inheritance of majority mate preference