Good studies of basic aging patterns and processes in wild animals in nature depend on the ability to quantify and repeatedly measure reliable biomarkers of aging in substantial numbers of individuals. The statistical approaches currently used by avian demographers are in many cases robust and sophisticated, taking into account potential biases inherent in this type of data. As we have illustrated above, bird studies have other advantages as well. On the other hand, studies of the effects of aging in wild animal populations have some drawbacks for investigating basic mechanisms of aging. The natural habitat of a wild bird is far removed from the pampered and protected environment enjoyed by inbred rodents, and carefully controlled longitudinal studies of aging in wild animals are much more difficult. Intrinsic causes of mortality (endogenous aging-related physiological declines) in these studies will be in many cases indistinguishable from extrinsic causes, like predation, disease, and other environmental stressors.
Wild bird populations are outbred and genetically variable. As in aging studies employing mammalian models (including humans), wild birds that live to older ages and survive repeated measurement of biomarkers may not be genetically representative of the population as a whole. Even in laboratory studies, the longest-lived animals in the population have passed through a significant selective "bottleneck"; this effect may be even more pronounced in the wild under natural selective regimes. Nonetheless, studies of exceptionally and naturally long-lived animals in the wild can contribute in important ways to our understanding of basic aging processes, providing important links between biogerontology, evolutionary and population biology, and natural history.
Birds as Models for Aging Studies: Special Challenges and Recommendations
Until recently, the selection of animal models in biogerontology has been driven primarily by economy, feasibility, and simplicity. Most key advances in our understanding of fundamental molecular and cellular aging processes have resulted either from the use of shortlived, rapidly aging species—inbred laboratory rodents, roundworms, flies—or highly specialized cell lines. But the most popular biogerontological animal models not only have life histories quite dissimilar to those of humans and most domestic animals, they also lack the very trait biologists most wish to understand and emulate: the ability to resist aging. The vertebrate animal models currently best developed for studies of aging, laboratory rats and mice, are highly suitable for use in controlled experiments, but, ironically, are poorly adapted for aging. As the field of aging has evolved, so has our understanding of the promise of carefully selected "nontradi-tional'' vertebrate models for aging, including certain primate species, exceptionally long-lived animals like naked mole-rats, bats and birds.
Other recent reviews have emphasized the importance of a judiciously applied comparative approach to selecting animal models for aging studies, employing a variety of distantly related animal species, and identifying a range of potential molecular "solutions" to the problem of long-term somatic maintenance and repair (Finch, 1990; Austad, 1993; Austad and Holmes, 1999). As we have illustrated in this chapter, a number of bird species are poised for use as the kind of exceptionally long-lived laboratory models needed to apply this approach.
There are additional, obvious challenges to the development of avian aging models. Although some cage birds have been domesticated for over a century, and some basic genetic information is available on the species we have highlighted here, genetic resources for experimental bird models (including information about genetic variability, commercially inbred or isogenic strains, genetic markers and gene sequence libraries) lag far behind those available for use with traditional laboratory animals. It requires a concerted effort by the biogerontological research community and funding agencies to direct resources toward developing new animal models. Since birds typically are significantly longer-lived than laboratory rodents, well-conceived, longitudinal avian aging studies will inevitably take longer and require substantial financial investment if they are to be worthwhile. Accepted techniques for the measurement of standard aging biomarkers will need to be adapted and calibrated for new avian models. This effort will require careful collaboration and communication between biogeronto-logists and other zoologists, as well as a synthetic and interdisciplinary dialog among scholars in the field who are well versed in accepted methodologies in the field of aging, as well as evolutionary principles and comparative zoology.
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