In this dataset, the maximum longevity is estimated from record longevity. Many factors can affect biases in longevity records, such as population size and whether animals are kept in captivity or not. Because it is the objective that maximum longevity is to be a reliable term for comparison between species, biases have been minimized.
A great effort to obtain the original source of each longevity record and verify its authenticity was applied; anecdotes are not used to estimate maximum longevity, though they are mentioned in the observations section; and species for which maximum longevity is suspicious of being significantly underestimated have generally a maximum longevity classified as “not yet available”. In addition, whether the maximum longevity of a given species comes from a specimen in the wild or in captivity is indicated for the vast majority of species.
All species have an estimate of the sample size to allow researchers performing comparative longevity studies to minimize the bias of sample size on longevity records. For longevity records obtained from species in captivity, estimates of sample size were obtained from the International Species Information System (ISIS). Estimates of wild-derived records were typically obtained from the sources of the longevity data, such as banding studies in birds.
Sample sizes reflect differences in orders of magnitude in the number of specimens for each species and are classified as ‘tiny’ (fewer than 10 specimens), ‘small’ (10-100), ‘medium’ (100-1000), ‘large’ (over 1000) and ‘huge’. Human beings are the only species with a sample size classified as ‘huge’ and this classifier was included to mark the special status of the human species in this context.
Each entry has a qualifier of the confidence placed in the longevity data. This qualifier is based on the reliability of the original reference from which maximum longevity was obtained, sample size, whether a given species has been studied and reproduces in captivity, and whether there are any conflicting reports. Confidence in the longevity data is hence classified as: ‘low’ (only used for species without an established maximum longevity in AnAge), ‘questionable’, ‘acceptable’ and ‘high’.
Entries in AnAge can be useful for researchers to learn more about the aging process of a particular species. Species with unique aging phenotypes or of special interest to gerontologists, such as species with negligible senescence and commonly used model organisms, are included. Apart from longevity, observations about physiological and pathological changes with age in animals are featured where available.
Although demographic measurements of aging are included in AnAge, these require detailed animal studies which are rarely available and thus represent only a small fraction of the data in AnAge. If possible, the mortality rate doubling time (MRDT) was determined for a given species using the Gompertz equation.
Typical values of major life history traits such as adult body size and age at sexual maturity are also featured in AnAge, at least for most mammals. Estimates of metabolic rates, such as resting or basal metabolic rate, are also featured for some species. Nonetheless, while consultation was done, the original source regarding longevity records (as described above), for other life history traits and metabolic rates, reviews were considered for large-scale datasets. Errors were minimized, although observed discrepancies (e.g., between male and female ages at sexual maturity or between inter-litter interval and litters per year) still reflect inconsistencies in these large-scale data sources.
For mammals, also included is the maximum longevity (tmax) residual, expressed as a percentage of the expected maximum longevity calculated from the adult body size (M) and derived from the mammalian allometric equation: tmax = 4.88M0.153. This is useful to identify species that live longer than expected for their body size. Cetaceans were excluded because there is less confidence in their longevity records, obtained from studies in the wild often using indirect methods, than in those from other mammalian taxa.
Included for some mammals and birds are growth rates. These values represent postnatal growth rate and are expressed in days-1. They were calculated by fitting empirical data taken from published growth curves to sigmoidal growth functions and are considered appropriate for comparative analyses within the same taxonomic class. Please be aware, however, that growth rates for mammals were derived from the Gompertz function while growth rates for birds were derived from the logistic function, so comparisons between the two classes need to take this into account.
Often, additional information relevant to a particular species can be found in the higher taxa of that species. Therefore, users are encouraged to refer to the observations related to their species of interest as well as the taxa it belongs to.
Because the ultimate aim is to help understand human aging, priority is given to species evolutionary closer to humans. Though there is a special focus on mammals and mammalian entries tend to include more information, other taxa are also represented, including some non-animal species. Species are classified according to Kingdom, Phylum, Class, Order, Family, Genus, and Species. The taxonomy of AnAge follows that of the Integrated Taxonomic Information System (ITIS).