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We, the editors of this special combined issue of the Journals of Gerontology, Series A (Biological and Medical Sciences), hope that the following papers will provide readers with fertile ground for the generation of new ideas and opportunities concerning extreme longevity research. The presentation of these varied scientific approaches to the study of a single theme will, hopefully, allow readers to explore topics outside of their own immediate areas of expertise. The contributing authors were asked to write with the aim of speaking to an audience that was likely to be unfamiliar with their research topic or methodology. Thus, they included more background and provided more context than they might when addressing fellow members of their own subdiscipline, so that researchers working in other biogerontological subdisciplines might gain insights and inspiration from work tangential to their own.  The common thread throughout this issue is “extreme longevity,” but this phrase is used differently by those studying a single species versus those comparing different species. What may at first blush be perceived as a nuance is actually a key question for this special issue and for comparative biogerontology generally—namely, whether organisms achieving extreme longevity within a species versus between species do so via some common mechanisms. If mechanisms do differ between long-lived individuals within a species and long-lived species themselves, what may we learn from such a diversity of mechanistic routes to extreme longevity?  Some of the research presented will elicit controversy. Already in this brief introduction, several controversial words and phrases have been introduced. As Hayflick (1) noted: “Communication in the field of biogerontology is a minefield because all of the commonly used terms have no universally accepted definitions.” Along these lines, the proper way to measure something can be a major bone of contention. For example, in their neuropathological and neuropsychological investigation of dementia among centenarians, Leonard Poon and colleagues (2) discuss how disparate reported estimates of dementia prevalence among centenarians are a consequence not only of the vicissitudes of subject selection but also of the different tools used to assess cognitive function, different cutoff measures for naming categories of dementia, and different causes of dementia in different populations. Thus, we feel it worthwhile to briefly digress and define a few terms as we use them in this editorial. These definitions are our own working definitions so that, for the duration of this paper at least, ambiguity may be avoided.  It will come as no surprise to most readers that “biological aging” and “senescence” are slippery words with many somewhat conflicting definitions. For the purposes of this special issue, then, we define our terms as follows. First, borrowing very heavily from Arking (3): “Aging is a cumulative, progressive, intrinsic and deleterious alteration in physiological state which increases vulnerability and culminates in death.” This definition separates progressive increases in vulnerability from the probability of encountering specific challenges during windows of susceptibility. Second, extreme longevity is defined in one of the following ways, as appropriate: (a) an exceptionally long life span based on comparisons within a single species, (b) an exceptionally long life span based on comparisons between mutant and wild-type animals, or (c) an exceptionally long life span based on comparisons among species predicted to have similar life spans because of shared physiological measures, such as size and metabolic rate.

Within- and Between-Species Study of Extreme Longevity--Comments, Commonalities, and Goals