Overlapping Generations Chapter 10

First Thoughts Potentially important Limited theoretical and empirical work Weak selection / confounding factors / alternative explanations

Basic Idea If standard age distribution is disturbed (by predictable / unpredictable events) and there are overlapping generations, facultative adjustment of the sex ratio may be adaptive. RS of males and females vary during their life (males often peak, females more even)

Two types of Theoretical model Stable-age distribution disturbed by: Exceptionally high/low mortality (or recruitment) Degree of generational overlap varies cyclically (by season) and differs between the sexes. Can lead to temporary difference between sexes in competition for resources

1. Exceptionally high/low mortality A. Impacting one Sex (Werren & Charnov, 1978) Favours individuals producing excess of the affected sex Need to parameterise the level of generation overlap and fitness benefits needed for bias to be favoured. Assessment mechanisms? Empirical work mixed, but supports prediction – alternative explanations?

B. Impacting both sexes equally (West, 1997) Can occur if reproductive value is distributed differently in each sex. After mortality peak, there is selection for the sex with highest old-age RV. But: if parent can predict mortality and adjust sex allocation beforehand, favours sex with highest young-age RV. (Effect less/negated if life history is one with RV concentrated in old- age) No empirical work. Exceptionally high/low mortality

C. Temporary exceptional recruitment Affecting a single cohort (generation) Above average: sex with most evenly distributed RV favoured. Below average: sex with most uneven RV? Bias reverses in the subsequent generations If females mate once (and store sperm) male RV more evenly spread.

2. Degree of generational overlap varies cyclically E.g. Spring/Autumn broods. Spring birth: breed autumn and die in winter. Autumn birth: overwinter (some die), the breed spring and some survive to breed autumn then die. Favours males in autumn, females in spring. Generally: sex which has greatest overlap with future generations favoured

Seger’s models (Seger, 1983) Sphecid Spring and summer generations. Overlap as spring males mate with summer females and some offspring of spring females overwinter. As overlap increases, bias for males in spring and females in summer.

Halcitine Females hibernate, lay eggs in spring, hatch in summer, second generation emerge in autumn. No males survive winter. Some hibernating females delay development until autumn, some summer males survive to mate with autumn females. Empirical evidence mixed. Often males do not survive to mate with next generation. Seger’s models (Seger, 1983)

Cyclical models and Eusociality Need female biased sex ratio (haplodiploidy) Segler’s models show possible – female bias in first generation favoured. Offspring can choose to help mother reproduce Other explanations: differential diapause, or variation in cost/benefit of producing sexes (if offspring from overlapping generations breed together) or virginity

Concluding remarks Theory limited, empirical testing even more limited Relevance and prevalence of OG unclear. Cyclical patterns in sex allocation may be unrelated to overlapping generations. Weak selection? assessment mechanism? Need species-specific models Very interesting idea, but its unproven.