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

What Determines the Sex Ratio This 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 Genetic benefits per cost should be the same in both sexes.

Graphic Representation of Fisher’s Model of Sex Allocation This 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 What if the daughters determine the sex ratio? Different ratios are predicted for haplo-diploid species As 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

Mating Systems Monogamy Polygamous Systems Polygyny Polyandry Promiscuous ( including Polygynandry) The pair bond notion?

Monogamy

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).

Polyandry Males 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 Polygyny in 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 Oropendol a birds (members of the blackbird family)

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 Favored Assume that a female has two choices -- either enter in a bigamous relationship with A or monogamou s 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 MonogamousPrimarySecondary Laying Date26 May28 May9 June Overall nesting success Clutch size4.0    0.9 # young fledged2.1    1.9 % eggs fledged51    47 Nest success excluding predation Clutch size4.2    0.9 # young fledged3.6    1.8 % eggs fledged86    44 Fledgling wt. (g)11.5    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

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 If you desert, your payoff is: Desertion 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