Class PP for Friday April 30 (Cl. #39)

What Determines the Sex RatioThis traces to the idea that a parent only has so much energy to invest in offspring. What is the best ratio of males to females?

Depends on:

•Relatedness of chooser to offspring

•Relative costs of the sexes

•What others are doing.

If costs are the same and choosers are equally related to both sexes, is a, for example, a 4F : 1M sex ratio an ESS?

More Background to Fisher’s Allocation Theorem

The choosing sex gains its fitness through both sexes of offspring.

What if both sexes, for instance:

•cost the same to produce

•are equally related to the chooser, and

•the sex ratio is the local area where eggs will be deposited is 50:50

In this case the female and males have the same average fitness. Since fitness is gained through both sexes, the chooser should produce females to males in a 1:1 ratio.

Same Example -- But What if the Local Sex Ratio is Not 1:1

Let’s assume that both sexes cost the same and are equally related to the chooser as before but this time the local sex ratio is biased towards females -- say 3:1. In this case the average fitness of a male is 3X that of a female (1M mates with 3F on the average).

Selection would strongly favor a chooser that could produce mostly males under these conditions.

The Role of Cost• What if it costs 2X as much to make females as males. Is a 50:50 sex ratio stable?

• Suppose a mutant appears where the cost of F/M is still 2X but she produces offspring in the ratio of 1F/2M. Can this strategy invade?

• There is only so much energy/material available for reproduction. In the present population (1:1), females are only half as successful as males per unit cost the chooser pays.

• The population can be invaded by any chooser who makes more cheap males than females. This will eventually move the payoff per investment back towards 1:1.

Fisher’s Equations

#F *CF =#M * CMri, f * #F *CF =ri,m * #M * CM

#F * ri, f#M * ri,m

=CM

CF

#F#M

=CM

CF

Genetic benefits per cost should be the same in both sexes.

Graphic Representation of Fisher’s Model of Sex Allocation

00.51.02.01.50.51.02.03.02.501.52.5Cf / Cm#m / #fThis figure shows all possible cost and sex ratio combinations for the case where the mother is 0.5 related to her offspring.

Fisher’s Ratio and Mother/Daughter Conflict

00.51.02.01.50.51.02.03.02.501.52.5Cf / Cm#m / #f3.54.04.55.0ri,j = 0.5

for both sexes

ri,j = 0.75 for sisters and 0.25 for

sisters to brothersWhat if the daughters determine the sex ratio?

Different ratios are predicted for haplo-diploid speciesAs compared to more typical diploid/diploid species

What If You Can Change Sex?

Some groups this is relatively easy to accomplish. Fish are a good example do to the relative lack of differentiation of the reproductive tracts and external fertilization.

Female to Male -- Protogynous Hermaphroditism

Largest male may mate 40 times in one day -- Sex change is socially controlled

Male to Female (Rarer) --Protandrous Hermaphroditism

Habitat forces monogamy; pair is more successful if larger fish is female. Change is socially controlled.What really happened to Nemo’s father?

Reproductive Effort and the Sexes

males

ME

PE

PE

ME

females

PEPE

ME

ME

promiscuous or polygamous monogamy

intensity of sexual selection

very strongless strong

knp

Reproductive Effort = Mating Effort + Parental Effort

Mating Systems

Monogamy

Polygamous Systems

• Polygyny

• Polyandry

Promiscuous

(including

Polygynandry) The pair bond notion?

Monogamyhttp://en.wikipedia.org/wiki/File:Northern_Cardinal_Pair-27527.jpg

http://wapedia.mobi/thumb/932414725/en/max/1440/900/Female_Cardinal_feeding_her_baby_2.JPG?format=jpg%2Cpng%2Cgif&ctf=0?format=jpg,png,gif&loadexternal=1

Polygyny and Polyandry

How would these graphs look in polyandry?

Polyandry -- Female Viewpoint

Why Polyandry? Thornhill and Alcock

Sperm replenishment.

• Adds to depleted supply

• Avoids costs of storing sperm

Material Benefits

• Nutrients

• Reduced predation

• Protection from other males

Genetic benefits

Convenience and lowered costs

Polyandry -- Males

Males may themselves not be monogamous (ie., monopolized by a single female as males often do in polygyny)-- they might just be "doing their thing" trying to obtain as many mates as possible.

In other cases, polyandry is the result of association with resources needed by young (either held by a male or female) or selection for the best female or the only available female (skewed operational sex ratio).

PolyandryMales provide most care within their territories.

Territories are very productive and are defended by females who compete to woo males.

Sequential matings.

Males become pre-occupied with brooding.

Wattled jacana

Polyandry in Bees

Polygyny

• Female defense polygyny

• Resource defense polygyny

• Lek polygyny

• Scramble competition polygyny

Female Defense in Mammals?

Female Defense Polygynyin Insects

Short-lived and low fecundity?

-- one male provides sufficient sperm

Females mate shortly after become adults.

Females are grouped closely together.

-- therefore, easy to defend.

Female Defense Polygyny in Birds

www.avianweb.com

Oropendola birds (members of the blackbird family)

QuickTime™ and a decompressor

are needed to see this picture.

Territory Quality and Female RS

The Cost of Polygyny

Polygyny Threshold Defined

The polygyny threshold is the decrease in territory quality associated monogamous males that would equal the decrease in material benefit associated with with mating with an already mated male.

Polygyny Not FavoredAssume that a female has two choices -- either enter in a bigamous relationship with A or monogamous relationship with B (who owns a poorer quality territory).

Polygyny Favored

The polygyny threshold was exceeded; polygyny is favored.

How Will Females Distribute Themselves?

Polygyny Threshold Models Based On Relative Direct

Benefits

Lark buntings and shade.

Gain shade but no help from male in rearing young

Experimental Induction of Polygyny in Warblers

Normally monogamous, cavity nesters.

Manipulated cavities.

Prefer floodplain

RS in Warblers According To Mating System

Polygyny

Monogamous Primary Secondary

Laying Date 26 May 28 May 9 June

Overall nesting success

Clutch size 4.0 0.9 4.1 0.4 4.1 0.9

# young fledged 2.1 1.9 2.9 1.9 2.0 1.9

% eggs fledged 51 46 68 45 50 47

Nest success excluding predation

Clutch size 4.2 09 4.2 0.4 4.1 0.9

# young fledged 3.6 1.3 4.0 0.5 2.4 1.8

% eggs fledged 86 27 96 10 61 44

Fledgling wt. (g) 11.5 0.8 11.7 0.7 10.8 0.9

No significant differences in RS -- no cost to polygyny in this case

Very Unlikely to Get to the Remaining Slides

Variable Mating Systems: Dunnocks

Sexual Conflict and Dunnocks

Males have most success in polygyny but females have lowest success due to less male parental effort. Females fight with each other to avoid polygyny.

Females do best with cooperative polyandry -- one female and several males sharing rearing duties. Males fair most poorly in this system due to shared paternity. Males fight with each other.

Monogamy and polygynandry -- intermediate for both

http://luis.casiano.oiseaux.net/accenteur.mouchet.3.html#monde

Sperm Competition

Sperm Age

Dungfly Sex

Second Male Advantage

Mate Guarding

Parental Conflict and Mating Systems

Parental conflict is the notion that the fitness interests of members of a reproductive pair do not coincide.

A Simple Model of Whether or Not to Desert Ones Mate

€

RS = V

1

+ pV

2 If you desert, your payoff is:

V1 + pV2 >V2Desertion favored:

If good chance offspring does well with one parent and if there is a decent chance of finding a second mate.

Assume that in this species, parental care is important such that the number of surviving offspring:

0 if both desert

V1 if one parent leaves but other remains

V2 if both remain and jointly rear

V2 ≥V1