Life Expectancy: Age is just a number

The naked mole rat defies the Gompertz law and shows no sign of increased mortality risk as it gets older.
  1. Hiram Beltrán-Sánchez
  2. Caleb Finch  Is a corresponding author
  1. The University of California Los Angeles, United States
  2. University of Southern California, United States

Bald and with wrinkly skin, the naked mole rat may not be the picture of youth, but these creatures continue to amaze us with their exceptionally long life. Naked mole rats are documented to live at least 30 years in captivity. But, they are not yet in the ranks of ‘gero-elites’: certain bats, for example, can live beyond 40 years (Podlutsky et al., 2005). Despite this longevity, naked mole rats show hardly any signs of getting older, such as problems with the heart, bones or metabolism. Females do not go through the menopause and continue to reproduce into their 30s. Even their cells have a remarkable resistance to oxidative damage (Edrey et al., 2011), and age-related chronic diseases, such as cancer, are rare.

So far, most of what we know about this animal is based on studies with small sample sizes, making it difficult to determine how long-lived they really are. Now, in eLife, J. Graham Ruby, Megan Smith and Rochelle Buffenstein of Calico Life Sciences report how naked mole rats never cease to surprise (Ruby et al., 2018).

A mathematical model called the Gompertz-Makeham law of mortality – which states that the risk of death increases exponentially with age – can be used to assess how long species live and what factors contribute to the mortality risk. Ruby et al. used this model to analyze an existing data set of 3,299 naked mole rats across a 30-year timespan and found that they did not conform to the Gompertz-Makeham law. In fact, their mortality hazard did not increase as they got older. This is unprecedented for mammals – one would not expect a small rodent such as the naked mole rat to live for more than six years, let alone show the first signs of aging at a time double its predicted maximum lifespan.

Previous studies suggest that aging nonetheless creeps in: naked mole rats can accumulate oxidative damage in their cells and tissues (a sign of aging) and experience muscle wasting, and there is also some evidence for cancer (Edrey et al., 2011; Andziak et al., 2006; Taylor et al., 2017). This motivates further consideration of Ruby et al.’s demographic criteria so that we can understand why their data show an absence of Gompertz mortality accelerations.

It can be assumed that when no deaths are observed in a group, there is still a risk of mortality – it may just be very low. However, when the number of deaths is low, errors in the sampling method could bias the estimates of the Gompertz parameters (Promislow et al., 1999). At its minimum measurable value, the death rate in a population is either 0 (no one dies) or 1 (a single individual dies). For the data studied by Ruby et al., this means that the minimum rate of mortality that can be accurately reported is 1/3,299: a baseline mortality of ~0.0003 per day. However, the baseline mortality reported in Ruby et al. is even lower (1/10,000 per day). This would indicate that their naked mole rat population is too small to correctly estimate the true mortality rate or the Gompertz parameters. In fact, sample sizes much larger than 3,299 would be needed to detect aging mortality acceleration.

Another component of the Gompertz-Makeham model is the rate of aging, from which the mortality rate doubling time (that is, the time required for the mortality rate to double) can be calculated. For example, assuming that the mortality rate begins to speed up when the naked mole rat reaches maturity at six months of age, the rate of Gompertz aging can be calculated to be about 0.006 per year of age. This is far lower than values found in modern human populations, where the rate ranges from 0.07 to 0.09 per year of age (Finch et al., 2014). Based on this, the mortality rate doubling time would be 115 years for the naked mole rat, compared to eight years for most human populations (Finch, 1998; Finch et al., 1990). This would suggest that unlike any other mammal, the naked mole rats have an extremely low rate of aging.

Its minimal age-related problems and long life-span make the naked mole rat an ideal candidate to study ‘negligible senescence’, a phenomenon that has been observed in a few species that reach advanced ages without increased mortality or disability (Finch, 2015). The study of Ruby et al. furthers our knowledge and provides a good framework for future studies to build upon.

References

    1. Finch CE
    (1998) Variations in senescence and longevity include the possibility of negligible senescence
    The Journals of Gerontology Series A: Biological Sciences and Medical Sciences 53:B235–B239.
    https://doi.org/10.1093/gerona/53A.4.B235
    1. Finch CE
    (2015)
    Molecular and Cellular Biology of Aging
    Negligible Senescence, Chapter 7, Molecular and Cellular Biology of Aging, Burlington, MA: Jones and Bartlett Learning.
    1. Podlutsky AJ
    2. Khritankov AM
    3. Ovodov ND
    4. Austad SN
    (2005) A new field record for bat longevity
    The Journals of Gerontology Series A: Biological Sciences and Medical Sciences 60:1366–1368.
    https://doi.org/10.1093/gerona/60.11.1366

Article and author information

Author details

  1. Hiram Beltrán-Sánchez

    Hiram Beltrán-Sánchez is in the Department of Community Health Sciences and California Center for Population Research, The University of California Los Angeles, Los Angeles, United States

    Competing interests
    No competing interests declared
  2. Caleb Finch

    Caleb E Finch is in the Leonard Davis School of Gerontology and Dornsife College, University of Southern California, Los Angeles, United States

    For correspondence
    cefinch@usc.edu
    Competing interests
    No competing interests declared
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0002-7617-3958

Publication history

  1. Version of Record published: January 24, 2018 (version 1)

Copyright

© 2018, Beltrán-Sánchez et al.

This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited.

Metrics

  • 4,701
    Page views
  • 298
    Downloads
  • 6
    Citations

Article citation count generated by polling the highest count across the following sources: Crossref, PubMed Central, Scopus.

Download links

A two-part list of links to download the article, or parts of the article, in various formats.

Downloads (link to download the article as PDF)

Open citations (links to open the citations from this article in various online reference manager services)

Cite this article (links to download the citations from this article in formats compatible with various reference manager tools)

  1. Hiram Beltrán-Sánchez
  2. Caleb Finch
(2018)
Life Expectancy: Age is just a number
eLife 7:e34427.
https://doi.org/10.7554/eLife.34427
  1. Further reading

Further reading

    1. Ecology
    2. Genetics and Genomics
    Franziska Grathwol, Christian Roos ... Gisela H Kopp
    Research Advance

    Adulis, located on the Red Sea coast in present-day Eritrea, was a bustling trading centre between the first and seventh centuries CE. Several classical geographers--Agatharchides of Cnidus, Pliny the Elder, Strabo-noted the value of Adulis to Greco--Roman Egypt, particularly as an emporium for living animals, including baboons (Papio spp.). Though fragmentary, these accounts predict the Adulite origins of mummified baboons in Ptolemaic catacombs, while inviting questions on the geoprovenance of older (Late Period) baboons recovered from Gabbanat el-Qurud ('Valley of the Monkeys'), Egypt. Dated to ca. 800-540 BCE, these animals could extend the antiquity of Egyptian-Adulite trade by as much as five centuries. Previously, Dominy et al. (2020) used stable istope analysis to show that two New Kingdom specimens of P. hamadryas originate from the Horn of Africa. Here, we report the complete mitochondrial genomes from a mummified baboon from Gabbanat el-Qurud and 14 museum specimens with known provenance together with published georeferenced mitochondrial sequence data. Phylogenetic assignment connects the mummified baboon to modern populations of Papio hamadryas in Eritrea, Ethiopia, and eastern Sudan. This result, assuming geographical stability of phylogenetic clades, corroborates Greco-Roman historiographies by pointing toward present-day Eritrea, and by extension Adulis, as a source of baboons for Late Period Egyptians. It also establishes geographic continuity with baboons from the fabled Land of Punt (Dominy et al., 2020), giving weight to speculation that Punt and Adulis were essentially the same trading centres separated by a thousand years of history.

    1. Ecology
    Jingxuan Li, Chunlan Yang ... Zhong Wei
    Research Article Updated

    While bacterial diversity is beneficial for the functioning of rhizosphere microbiomes, multi-species bioinoculants often fail to promote plant growth. One potential reason for this is that competition between different species of inoculated consortia members creates conflicts for their survival and functioning. To circumvent this, we used transposon insertion mutagenesis to increase the functional diversity within Bacillus amyloliquefaciens bacterial species and tested if we could improve plant growth promotion by assembling consortia of highly clonal but phenotypically dissimilar mutants. While most insertion mutations were harmful, some significantly improved B. amyloliquefaciens plant growth promotion traits relative to the wild-type strain. Eight phenotypically distinct mutants were selected to test if their functioning could be improved by applying them as multifunctional consortia. We found that B. amyloliquefaciens consortium richness correlated positively with plant root colonization and protection from Ralstonia solanacearum phytopathogenic bacterium. Crucially, 8-mutant consortium consisting of phenotypically dissimilar mutants performed better than randomly assembled 8-mutant consortia, suggesting that improvements were likely driven by consortia multifunctionality instead of consortia richness. Together, our results suggest that increasing intra-species phenotypic diversity could be an effective way to improve probiotic consortium functioning and plant growth promotion in agricultural systems.