While my committee is happy for my thesis to be a pile of loosely connected papers, the university still expects it to be a single unified document with a single topic, a single Abstract, and transitions. This is a bit of a stretch, considering how far afield some of my chapters are (chapters 2 and 4 for example have almost nothing in common). Here is my first attempt at an Abstract that ties it all together:
Humans are a demographically unusual species in many ways, but perhaps the most unusual thing about our demography is the huge portion of our adult females who are post-fertile. This thesis, in four chapters, explores the evolution of post-fertile survival, attempting to understand, from four different angles, how unusual women are in this respect and how they come to be that way.
Chapter 1 is a methodological and comparative study of post-fertile survival in primates. Post-fertile survival is most frequently measured as post-reproductive lifespan, the length of time between reproductive cessation and death. I show that post-reproductive lifespan is not a useful measure for comparative studies and use demographic life-table methods to create more useful measures of post-fertile survival. I then calculate these measures for several human populations and a large group of primate species. These results indicate that women in all populations experience post-fertile survival which greatly exceeds that in other primates under all circumstances. Non-human primates under natural conditions do not experience significant post-fertile survival, while human hunter-gatherers do.
Chapter 2 arises from the question of whether selective pressures associated with being a care-giver tend to increase longevity, potentially partly explaining women's longevity and therefore their post-fertile survival. The chapter focuses on the tradeoff between providing care to existing offspring and competing for matings so as to produce additional offspring. Data on male primates, in which variation in care provided is much greater than in females, are used in a comparative study. I ask whether these data support the assumption of a tradeoff between male care and male mating competition, and if so if one strategy or the other is associated with greater longevity. I find strong support that such a trade-off exists (males in most primate families invest significantly in one or the other, but not both, and care and competition coevolve in a phylogenetically robust pattern). However these data do not support the prediction that level of male care and degree of sex-bias in longevity coevlove meaningfully.
Chapter 3 is an allometric study of brain size, body size, age at reproductive cessation and longevity in primates, in which I ask if human post-fertile survival is predictable based on primate patterns. Again using life-table methods to create parameters more appropriate for comparative study than those used in the literature, I show that while women's age at reproductive cessation can be fairly accurately predicted based on primate scaling patterns, their longevity cannot. This result indicates that the selective forces which regulate these scaling patterns in primates have been altered or amended in humans.
Finally, Chapter 4 is an experimental evolution study. Using rotifers, a short lived microscopic metazoan, I experimentally make the survival of young depend on the continued survival of their mothers and grandmothers in a species which has no natural care of juveniles. I show that under this regime those familial lines which are longer lived, and which bear a larger portion of their young before mother and grandmother die, increase while others die out. However because of low heritability of demographic traits in this population, the experiment does not demonstrate adaptive change, but rather differential success based on stochastic variation.
Taken together these four papers serve primarily to underscore the uniqueness of post-fertile survival in women. Some have argued that human post-fertile survival is either an artifact of social rather than biological evolution, or a widespread trait in female primates simply exaggerated in human females. These studies make clear that human post-fertile survival must be considered as a novel trait, and its evolution explained as such.