The Measurement of Post-Reproductive Lifespan
Advancement in the study of PRLS has been hampered by differences over terminology, the use of a wide range of non-comparable measures and the failure to put measures of the scale of PRLS in the context of the time scales on which the organisms live.
Some authors have used the term "post-fertile" rather than "post-reproductive" arguing that anything that an organism does that increases her genetic representation in future generations is a form of reproduction, and that "post-reproductive" is therefore an inaccurate term to apply to post-fertile individuals who are still caring for their young (REFS). Indeed Hamilton (1966) argues that, "if the organism practises parental care 'birth' should be considered to occur... at the age at which the offspring becomes independent." While not disputing the biology behind this argument, we feel that the term "post-reproductive" is deeply enough ensconced in the literature on this topic that the use of alternative terminology to convey the same concept may tend to muddy communication. For this reason we use the term "post-reproductive" to refer to life after direct reproduction (fertility), excluding indirect reproduction (care of young and indirect fitness benefits).
Beyond semantic disagreements, so many methods have been used to calculate the scale of PRLS that efforts at comparisons across species and studies have been few and confusing. For example, XXXX and ZZZZ (REF) present a table of PRSL for 12 primate species, all given in units of years, but calculated in six different ways. Disagreements exist as to how to define the end of reproduction, how to determine the end of survival, and which individuals to include. The measures vary because the type of data used vary, and the interests of the authors vary, figuratively leading to comparisons of the shelf-life of apples to the refrigerator hardiness of oranges. The effect of sample size on these estimates is generally not addressed.
Even when these drawbacks are not found, authors generally fail to correct for the overall longevity of the species in question. One should expect a species that lives 100 years to, on the average, experience more years of PRLS than a species that lives 20 years. Without a denominator related to the time scale of the organism's life history, the numerator of PRLS is fairly uninformative.
In this study, we use a type of data that allow for broad comparability: age specific mortality and fertility figures as calculated in standard demographic methodology. Because the form of the data is highly standardized, the same measures can be calculated across taxa, for males and females, and in a wide range of environments. The use of data sources as information rich as are age specific mortality and fertility tables allows for the use of multiple measures which illuminate different aspects of PRLS, but which need not be falsely compared to each other, because we can calculate every measure for each population for which these data are fully available. Furthermore, the use of age-specific demographic tables allow us to put our measures of PRLS in the context of the reproductive and actuarial longevity of the organisms, allowing for meaningful comparisons between populations with very different lifespans.
Saturday, January 17, 2009
Very rough section of a very rough draft.
Today between 11 and midnight I wrote a very rough section of a section of a rough draft of one of the several papers that will go into my thesis. Feels like progress. I don't expect to have much time for blogging over the next few months, but I will try to post bits like this that are potentially interesting, and that show what efforts keep me from having time for blogging.