Arguing for Some Negligible Senescence in the Wild to be an Artifact of Data Collection Methods
A negligibly senescent species exhibits little evidence of age-related degeneration or increased mortality risk over much of its life span. The term is becoming a little overused, attached to species that are merely resilient rather than exceptional, but it nonetheless appears that some higher animals, such as naked mole-rats, age little until very late life. Further, there is very good evidence for some lower animals, such as hydra, to be functionally immortal. Are all of the species thought to exhibit minimal degenerative aging in fact doing so, however? Researchers here argue that much of the evidence for negligible senescence gathered from wild populations suffers from methodological flaws, and that these species are not in fact negligibly senescent.
Negligible or negative senescence occurs when mortality risk is stable or decreases with age, and has been observed in some wild animals. Age-independent mortality in animals may lead to an abnormally long maximum individual lifespans and be incompatible with evolutionary theories of senescence. The reason why there is no evidence of senescence in these animals has not been fully understood. Recovery rates are usually very low for wild animals with high dispersal ability and/or small body size (e.g., bats, rodents, and most birds). The only information concerning senescence for most of these species is the reported lifespan when individuals are last seen or caught.
We deduced the probability density function of the reported lifespan based on the assumption that the real lifespan corresponding to Weibull or Gompertz distribution. We show that the magnitude of the increase in mortality risk is largely underestimated based on the reported lifespans with low recovery probability. The risk of mortality can aberrantly appear to have a negative correlation with age when it actually increases with increasing lifespan. We demonstrated that the underestimated aging rate for wild animals with low recovery probability can be generalizable to any aging models.
Our work provides an explanation for the appearance of negligible senescence in many wild animals. Humans attempt to obtain insights from other creatures to better understand our own biology and its gain insight into how to enhance and extended human health. Our advice is to take a second glance before admiring the negligible senescence in other animals. This ability to escape from senescence is possibly only as beautiful illusion in animals.