# Who cares for the kids?

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Male deserts. Male stays. Female deserts. Offspring fitness not much improved with even 1 parent, or BOTH parents can increase number of offspring by a large amount if they leave the current ones. - PowerPoint PPT Presentation

### Text of Who cares for the kids?

• Who cares for the kids?

• How many offspring to rear?Lacks hypothesis parents lay just enough eggs to maximize total number of surviving offspring. Tradeoff between offspring size and number you can make a lot of offspring, but they will tend to be small and survive poorly, but if you make them too big, you could probably have made a larger number of fairly healthy offspring with the same resources.

• ChicksurvivalNumber of chicksNumber of eggsNumber of surviving chicksN*Lacks hypothesis

• What KIND of offspring to rearHow many males vs. females? The answer is of each, on average. Why? One half of all genes in generation x+1 come from males in gen. X, and from females in gen. X. Imagine there were K times as many males as females in gen. X. Then on average it MUST be true that each male has 1/K the RS of the average female. So a family that produces 4 females will have K times the RS of a family with 4 males, thus selecting for female-biased families. This bias will occur until the sex ratio in the population is 1:1, at which point the average RS per male and per female MUST be the same.

• Sex vs. number of offspringWhat if males are much more costly to produce than females (or vice-versa)? Fishers answer is that total INVESTMENT in each sex should be equal. So if males each cost 2x as much to raise as females, then you should produce only as many males. In primates, in fact, birth sex ratios are biased toward males in many species in which females stay at home and males disperse, because the females eventually compete with their mothers for food, thus leading to a higher cost overall.

• Biased sex investmentThe above rules hold all other things being equal. If a female KNOWS that she is in better or worse condition than average, she may favor one sex or the other. She may do this pre- or post-natally. This can occur even in primates (including man) that have only one offspring at a time. The rule is to favor the sex that will benefit most from the extra resources in its future RS. In species in which males benefit especially from being bigger than other males, a female in very good condition should favor males, but in poor condition, she should favor females. Case of wood rats.

• Parent-offspring conflictThe interests of parent and offspring are not identical.Parent is related to itself by 1.0 (identity) but to its offspring by 0.5; likewise for offspringOffspring will be selected to demand more resources from parent than parent is willing to give why? More effort in raising offspring often comes at cost of higher parental mortality

• Fitness costs or benefitsResources invested in offspringbenefitsCost to parent(Indirect) cost to offspring = cost to parentP*O*

• Hamiltons ruleHow can selfish NS favor individuals that give resources and care to others, even to offspring?Answer seems obvious offspring carry the parents genes, but this principle holds true even for more distant relatives as well.More formally: rb> c, where b=benefit to receiver, c= cost to donor, r=relatedness = chance that 2 individuals share same allele by common descent (how to calculate?)

• ABP1P2GP1GP2R1R2S1CDEFr(AB) = (A P1 B) +(A P2 B) = * + * = Rule is: draw or write ALL pathways that connect two individuals to ANY common ancestor that is NOT already counted (in this example do not connect A and B through GP1 or GP2, as you must pass via P2). Then count the number of links in each pathway, and call this L. Then, r is the sum of (1/2) L across all pathways. For r(B P3), the pathways are (B P2 GP1 P3) and (B P2 GP2 P3); r =1/4P3

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