Negligible Senescence in a Number of Reptilian and Amphibian Species

As scientists note here, a number of reptilian and amphibian species exhibit negligible senescence, in that their mortality risk does not increase with age, at least not until very late life. The question has always been whether there is anything that can be learned from the cellular biochemistry of these species that can serve as the basis for enhancement therapies in mammals. There is no assurance that the basis of negligible senescence in any given species is simple enough to be useful. There is no assurance that even a simple difference could be safely ported over into mammalian biology given the biotechnology of the next few decades. Nonetheless, it is a topic of interest in the research community, a way to broaden the understanding of how differences in genetics and metabolism give rise to sizable differences in shape and length of life between species.

Researchers have documented that turtles, crocodilians, and salamanders have particularly low aging rates and extended lifespans for their sizes. The team also found that protective phenotypes, such as the hard shells of most turtle species, contribute to slower aging, and in some cases even "negligible aging" - or lack of biological aging. In their study, the researchers applied comparative phylogenetic methods, which enable investigation of organisms' evolution, to mark-recapture data in which animals are captured, tagged, released back into the wild and observed. Their goal was to analyze variation in ectotherm aging and longevity in the wild compared to endotherms (warm-blooded animals) and explore previous hypotheses related to aging, including mode of body temperature regulation and presence or absence of protective physical traits.

The thermoregulatory mode hypothesis suggests that ectotherms, because they require external temperatures to regulate their body temperatures and, therefore, often have lower metabolisms, age more slowly than endotherms, which internally generate their own heat and have higher metabolisms. The findings, however, reveal that ectotherms' aging rates and lifespans range both well above and below the known aging rates for similar-sized endotherms, suggesting that the way an animal regulates its temperature - cold-blooded versus warm-blooded - is not necessarily indicative of its aging rate or lifespan. The protective phenotypes hypothesis suggests that animals with physical or chemical traits that confer protection, such as armor, spines, shells or venom, have slower aging and greater longevity. The team documented that these protective traits do, indeed, enable animals to age more slowly and, in the case of physical protection, live much longer for their size than those without protective phenotypes.

Interestingly, the team observed negligible aging in at least one species in each of the ectotherm groups, including in frogs and toads, crocodilians and turtles. "It sounds dramatic to say that they don't age at all, but basically their likelihood of dying does not change with age once they're past reproduction. Negligible aging means that if an animal's chance of dying in a year is 1% at age 10, if it is alive at 100 years, it's chance of dying is still 1%. By contrast, in adult females in the US, the risk of dying in a year is about 1 in 2,500 at age 10 and 1 in 24 at age 80. When a species exhibits negligible senescence, aging just doesn't happen. Understanding the comparative landscape of aging across animals can reveal flexible traits that may prove worthy targets for biomedical study related to human aging."


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