Journal Club: Telomere length: a new measure of chronic stress in wildlife?
SUMMARY: Two independent studies find a positive relationship between social environment and telomere length. The first study -- that nearly everyone has heard about -- is in children. The second study -- that few have heard about -- is in pet grey parrots. The second study raises the question: might telomere length be developed as a new way to measure chronic stress -- in animals?
Telomere caps (white) on the ends of human chromosomes (grey).
Image: U.S. Department of Energy Human Genome Program (public domain).
Telomeres, the DNA-protein caps that prevent chromosomal fraying, are positively affected by social stress, according to two independent studies that were just published within days of each other. One study -- which has received widespread media coverage -- found a positive relationship between social environment and telomere length in children, adding support to previous work in people. A second study -- which few have heard about -- found that accelerated telomere erosion is associated with social isolation in pet grey parrots. This then raises the question: might measuring telomere length be developed as a new way to monitor chronic social stress in birds and other animals as well as in people?
If you've been following news in science recently, then you have probably heard about a recent study that found a correlation between chronic social stress and telomere length in children. That study, which was just published in the peer-reviewed journal PNAS, found that African-American boys exposed to disadvantaged environments -- low income, low maternal education, unstable family structure, and harsh parenting -- end up with shortened telomeres by just nine years of age [doi:10.1073/pnas.1404293111].
This study adds to our knowledge in two ways. First, previous investigations in adult caucasians found that shortened telomeres are associated with chronological aging as well as with smoking, obesity, depression, illness and other chronic stressors. Second, this newest study makes some progress towards identifying whether selected genetic variations may make some people more sensitive to chronic stress.
Telomere shortening and chronic social stress
Telomeres are found at the ends of linear chromosomes in all eukaryotes (most bacteria and other prokaryotes lack telomeres because their chromosomes are circular). Telomeres "caps" are comprised of repetitive DNA sequences bound to and stabilised by a specific protein complex known as shelterin. Telomeres prevent chromosomal erosion when they are duplicated during cell division, and thus, telomeres preserve valuable genetic information against loss.
In the past decade of intensive research, scientists learned that some cell types, such as germ cells, have longer telomeres than others. Cell types that divide frequently, such as the cells of the hippocampus (a region in the brain), and blood and bone cells, generally also have longer telomeres.
Although the enzyme, telomerase, maintains telomeres and thus slows cellular aging (senescence), telomeres progressively shorten throughout an individual's life. But more interesting, accelerated telomere erosion is associated with chronic stress -- and even newborns who have been subjected to prenatal stress may have shortened telomeres [doi:10.1016/j.psyneuen.2013.03.010].
Chronic stressors include behaviours that threaten health, particularly smoking and obesity, and social stress, such as childhood trauma, and chronic medical conditions, particularly depression and other mood disorders. Overall, telomere length serves as an early predictor of the onset of disease and mortality. Thus, telomere length may potentially be used as a biomarker of exposure to chronic stress in humans.
Arwen the Grey (Portrait). A pair-housed subadult female African grey parrot, Psittacus erithacus.
Telomere shortening is linked to social isolation in pet parrots
Despite the media focus upon ageing research and telomere studies, there is another study, published a couple days ahead of the PNAS study, that I'll bet none of you have heard about (yet). What makes this study so unusual is not its findings, which are consistent with previous work, but its subjects. This is the first study that has ever examined telomere length in birds. Specifically, the study focused on pet African grey parrots, Psittacus erithacus.
Grey parrots are long-lived birds that are popular pets throughout the world. In the wild, they roost in flocks that may number as many as 10,000 conspecifics and they forage in social groups that number around 30 individuals (Juniper & Parr. 1998. Parrots: A Guide To Parrots Of The World). But when kept as captivebred companions, grey parrots are often kept either as lone pets or sometimes in the company of a second grey parrot. Considering these birds' highly social nature in the wild, it is logical to ask whether pet grey parrots that live in isolation from all other grey parrots might experience chronic social stress.
This was the question asked by Denise Aydinonat, who is a doctorate student at Vienna's University of Veterinary Medicine. She designed a study to address this issue in collaboration with several other researchers at the university. Basically, the team examined telomeres from blood samples collected from 45 pet grey parrots during routine check-ups. The parrots ranged from 1 year to 45 years of age.
As expected, they found that telomeres of older birds were shorter than those of younger birds, regardless of how they were housed. But when the team compared relative telomere length based on the birds' housing situation, they found that single-housed birds had shorter telomeres than pair-housed individuals of the same age.
"Based on our model, the relative telomere lengths of single-housed birds at nine years of age were comparable to pair-housed birds 23 years older than themselves", the authors write.
Quite astonishing, I think.
This is a preliminary study, so the authors could not find a relationship between pet grey parrot mortality and telomere length -- although this may change if their sample size could somehow be increased.
They also found no significant difference in the rate of telomere erosion between single versus pair-housed birds, indicating that isolated birds' telomeres shortened in early life and were not repaired later. The team was unable to determine whether the negative effects of chronic stress on the telomeres could be slowed or reversed by transferring the parrots from a solitary to a social environment.
Unfortunately, the team did not compare their telomere data with plasma corticosteroid concentrations -- corticosterone is the "stress hormone". Although there is no one standard plasma corticosterone measure for response to chronic stress -- such a test must be developed and standardized for each individual species before it can be adopted -- this is the standard method for assessing stress in wildlife.
Additionally, since telomere length varies by cell type, this method must be properly developed too before it can be generally useful.
"Further ... studies are needed to test whether and how [telomere length] can provide an additional biomarker for identifying exposure to chronic social stress", the team writes in their paper.
Although this is a preliminary study, telomere length appears to be a promising biomarker that could be developed as a valuable conservation tool to assess and monitor chronic stress in individual animals and possibly even in wild populations -- probably alongside a hormone assay and other relevant tests.
"[Telomere length] should be more useful as part of an overall composite index, included in addition to other biomarkers of oxidative stress and aging."
Aydinonat D., Penn D.J., Smith S., Moodley Y., Hoelzl F., Knauer F., Schwarzenberger F. & Saretzki G. (2014). Social Isolation Shortens Telomeres in African Grey Parrots (Psittacus erithacus erithacus), PLoS ONE, 9 (4) e93839. doi:10.1371/journal.pone.0093839.t001 (OA)
Also mentioned (and worth reading):
Shalev I., Entringer S., Wadhwa P.D., Wolkowitz O.M., Puterman E., Lin J. & Epel E.S. (2013). Stress and telomere biology: A lifespan perspective, Psychoneuroendocrinology, 38 (9) 1835-1842. doi:10.1016/j.psyneuen.2013.03.010 (free PDF)
Mitchell C., Hobcraft J., McLanahan S.S., Siegel S.R., Berg A., Brooks-Gunn J., Garfinkel I. & Notterman D. (2014). Social disadvantage, genetic sensitivity, and children's telomere length, Proceedings of the National Academy of Sciences, doi:10.1073/pnas.1404293111 (OA)
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When she's not socially stimulating her parrots, Grrlscientist can also be found on on her eponymous Guardian blog, and she sometimes lurks on social media; facebook, G+, LinkedIn, Expatacular!, Pinterest. She's quite active on twitter: @GrrlScientist